In Doubt - Circuit Values

This is part of a piece of equipment that is to detect a short. The FUT wil l be a capacitor. It is not the only function of the unit, I am working on the rest. This seems so simple but I have doubts for some reason. Since I a m learning LTSpice finally.

ugh to assure saturation - I think. Seems like it anyway. R 2 & 4 should pr ovide enough current to light the LED, which is shunted by Q 1 when there i s no short. When there is the base voltage drops and the transistor cuts of f and the LED indicates the short. In theory.

I don't know, maybe I am too skiddish after years of conditioning to accept that something so simple could just work. I am not looking for advanced ca lculus here, I believe it unnecessary, but I would appreciate a quick look over to see if I made any blatant mistakes. I have missed some duesies in m y time.

The only other thing that would be nice is to get the current drain down as this is to be battery operated. I don't see how without another transistor , and there is no way of getting out of giving the LED enough current. Anyw ay, here it is :

Version 4 SHEET 1 880 680 WIRE 32 0 -112 0 WIRE 224 0 32 0 WIRE 448 0 224 0 WIRE 224 96 224 80 WIRE 336 96 224 96 WIRE 224 128 224 96 WIRE 336 128 336 96 WIRE 448 128 448 0 WIRE 32 176 32 80 WIRE 32 176 -48 176 WIRE 160 176 112 176 WIRE -48 208 -48 176 WIRE 336 224 336 192 WIRE 336 224 224 224 WIRE 448 224 448 208 WIRE 448 224 336 224 WIRE 32 240 32 176 WIRE 448 240 448 224 WIRE 112 256 112 176 WIRE -48 320 -48 256 WIRE -48 320 -112 320 WIRE 32 320 -48 320 WIRE 112 320 32 320 WIRE 448 320 112 320 SYMBOL npn 160 128 R0 SYMATTR InstName Q1 SYMBOL res 128 160 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 3.3K SYMBOL cap 96 256 R0 SYMATTR InstName C1

SYMBOL res 208 -16 R0 SYMATTR InstName R2 SYMATTR Value 62 SYMBOL res 432 112 R0 SYMATTR InstName R3 SYMATTR Value 120 SYMBOL res 432 224 R0 SYMATTR InstName R4 SYMATTR Value 47 SYMBOL LED 320 128 R0 SYMATTR InstName D1 SYMBOL res 16 -16 R0 SYMATTR InstName R5 SYMATTR Value 1K SYMBOL res 16 224 R0 SYMATTR InstName R6 SYMATTR Value 1K TEXT -112 24 Left 2 !6 VOLT TEXT -120 56 Left 2 !SOURCE TEXT -128 296 Left 2 !COM TEXT -128 216 Left 2 !TO INDICATE TEXT -128 240 Left 2 !SHORT HERE

Cheap shots are OK, I have been around here a while...

Reply to
jurb6006
Loading thread data ...

Good to learn LT-Spice, it has been really useful for me too.

To simulate/run your design you'll need to add a voltage source and ground symbol (most likely to node "com").

Your schematic looks very roundabout - to light an LED when there is a short could be done with just a resistor and LED - not clear why you added so much stuff around them?

piglet

Reply to
piglet

Not quite that simple. It's for a cap checker based on the principle of det ecting harmonics of a square wave. A 400 mV 1,000 Hz square wave with a sou rce impedance of 360 ohms is impressed on the DUT. The DUT shunts the test signal like a VTVM as opposed to a VOM. The remaining signal not bypassed b y the DUT is then fed to an amplifier to bring the level up to where it can be simply rectified rather than other schemes (such as synchronous) which would not be an advantage. Before it hits the rectifier it goes through a t wo stage R/C high pass filter. It full output is referenced as infinity ESR and zero output is zero ohms ESR. I'll work on the scale in between later.

The whole thing is based on how I used to use a scope with its calibrator t o do this, but in a small handheld device. In fact ifit gets off the ground I am considering an enclosure like a Wiggy which can hold one of the probe s for easier use.

Originally with the scope I used a positive or negative going pulse and DC coupling, the line should go to the middle of the screen. Too hard with 200

this is going to do though is to read a DC shorted DUT when there is a sem iconductor diode in the circuit. Reversing the leads will reveal that. In c ertain cases it will read a short, but the ESR will still read just the sam e. It is not intended to be a laboratory quality instrument, like a Wonderb ox almost is. A simple go-nogo indication is the idea here.

Any shorted cap will read good ESR wise. This indicates it. Also, if you ha ppen to run a cross an inductor of the right range, it may read a short yet high "ESR" because that's what coils will do. A short to DC but not to AC.

I just took it to 3 volts because 200 mV is too hard to deal with. I could do it but then that would mean another stage anyway.

There is probably a better way to do this, maybe with a PNP ? Yeah, I might change it to that because I could feed the base like on this, but instead of shunting the LED I could drive it with the collector. The overall curren t drain would be less when there is no short detected and since this is sup posed to run on batteries it might help.

One thing about LTSpice, I have no idea how to simulate and oscillator. In fact I have simulated nothing, and on this it would be hard because I would have to input values that would go between the probes. Without a DUT it do esn't do everyt much. But right now I am contemplating a 2 stage oscillator , which is almost an astable multivibrator which feeds a bistable multivibr ator to assure 50 % duty cycle output. Actually I wonder how accurate that

50 % has to be...
Reply to
jurb6006

Oscillators can be annoying sometimes to get started in LTSpice; many oscillator topologies are metastable in that they have oscillatory solutions but also the "nothing happens forever" solution. Cuz LTSpice components are noise-free the solver will often preferentially discover the latter solution. Appropriate use of initial conditions/pulse injection can usually get them going, sometimes just selecting the "start external voltages at 0 V" and "skip initial operating point" options in the transient analysis tab is enough to make them start up.

Reply to
bitrex

** Like to explain that one a bit more ?

BTW: Wire wound resistors with values under about 2 ohms can be compared for self inductance using the same method. The amount of leading edge "ringing" seen on a scope screen indicates which types or brands have high inductance.

Audio amplifiers can exhibit parasitic oscillation if the more inductive ones are used for emitter ballast - particularly if the output stage uses MOSFETS.

Inductance can also be critical with current sense resistors in SMPS.

.... Phil

Reply to
Phil Allison

I just realized that it won't work anyway. The voltage will not float to

as something wrong but just didn't grasp it right away. Getting slow in my old age I guess.

I think I have devised a solution that will allow me to use the native 200 mV to detect the short. I will put it up shortly. (after I draw the thing)

Sooner or ;later I guess I'll just have to reveal the whole thing. This is not something that I believe I have to worry about being copied and in the remote possibility that it is, the post will prove prior art at least. The concept is clear and it doesn't seem too hard to design, there will probabl y be more involved in laying out a PC board. It is a bit too much for Dreme ling out.

Reply to
jurb6006

A VTVM in ohms mode puts out a predetermined DC voltage at a predetermined current and measures what's left after the DUT shorts it out to whatever de gree. This unit puts out a predetermined AC voltage at a predetermined curr ent and does the same thing. A VOM on the other hand puts the DUT in series with the test voltage, which is not as desirable and I deem it unacceptabl e for this application.

ch types or brands have high inductance. "

Makes sense.

Which brings us to my resistor problem. There will be a resistor, I figure

3.3 ohms, low enough to not upset the reading more than that which can be t aken up in calibration. I want to use carbon composition to avoid inductanc e. This might not be significant but - I am measuring approximately the 5th harmonic and up of a 1 KHz square wave. I would rather not have any induct ance to deal with especially if it is unpredictable.

The resistor is necessary in case the cap to be tested is charged up. I wan t the unit to safely discharge it, and I would accept it being slower and w ith less current but I don't want to throw off the measurement too much. A negative charge would probably be discharged through the resistor and a (pr obably Schottky) diode. Positive would go through the resistor but then a f ew stacked regular silicon diodes.

So let's say I have a 4,700 uF charged to 30 volts. I know I don't need the resistor to handle that continuously, only during most of the time constan t of the 3.3 ohms and the cap that is charged. (assuming it is good of cour se) But what kind of wattage value should I use ? A 3 watt would be nice to get away with, or I could use more than one in parallel to get a higher wa ttage. Any suggestions ? I am tempted to just stick a 3.3 in there, or mayb e 2 6.2s or whatever will work. I think anything above 4.7 would be too muc h. I know that can add up to quite a bit of current but it is still only fo r a short time.

I'll work on that short detector and then post the whole thing. I know ther e are changes to be made, I did it pretty much from memory of about 25 year s ago.

The original was lost due to me being young and dumb and full of more than cum. There was this incident with my car...

Anyway, I guess I'll just post the whole thing. If someone steals the desig n at least I will be flattered.

Reply to
jurb6006

Version 4 SHEET 1 1340 1256 WIRE 1200 -192 1200 -256 WIRE 1200 -192 -1056 -192 WIRE 1200 -160 1200 -192 WIRE -1264 -112 -1440 -112 WIRE -1200 -112 -1264 -112 WIRE -976 -112 -1200 -112 WIRE -624 -112 -976 -112 WIRE -304 -112 -624 -112 WIRE -16 -112 -304 -112 WIRE 336 -112 -16 -112 WIRE 608 -112 336 -112 WIRE 1200 -96 1200 -112 WIRE -1200 -80 -1200 -112 WIRE 176 -80 144 -80 WIRE 608 -64 608 -112 WIRE -432 -16 -752 -16 WIRE -368 -16 -384 -16 WIRE -304 -16 -304 -32 WIRE -304 -16 -368 -16 WIRE 336 -16 336 -32 WIRE 544 -16 336 -16 WIRE 80 0 80 -80 WIRE 176 0 176 -80 WIRE 176 0 160 0 WIRE 336 0 336 -16 WIRE 336 0 176 0 WIRE 720 32 608 32 WIRE 816 32 800 32 WIRE 928 32 816 32 WIRE 1056 32 928 32 WIRE 1120 32 1056 32 WIRE 1200 32 1200 -96 WIRE -1264 64 -1264 -112 WIRE -16 80 -16 -32 WIRE 144 80 -16 80 WIRE 160 80 144 80 WIRE 336 80 336 0 WIRE 368 80 336 80 WIRE 512 80 448 80 WIRE 544 80 512 80 WIRE -752 96 -752 -16 WIRE -624 96 -624 -32 WIRE -624 96 -672 96 WIRE -576 96 -624 96 WIRE -432 96 -512 96 WIRE -304 96 -304 -16 WIRE -304 96 -352 96 WIRE -160 96 -304 96 WIRE -624 128 -624 96 WIRE -624 128 -640 128 WIRE -304 128 -304 96 WIRE -16 128 -16 80 WIRE 336 128 336 80 WIRE -160 144 -160 96 WIRE 1056 144 1056 96 WIRE 928 160 928 32 WIRE 1056 160 1056 144 WIRE -752 176 -752 96 WIRE -704 176 -752 176 WIRE -432 176 -432 96 WIRE -368 176 -432 176 WIRE 80 176 80 0 WIRE 80 176 48 176 WIRE 144 176 144 80 WIRE 160 176 144 176 WIRE 240 176 240 80 WIRE 240 176 224 176 WIRE 272 176 240 176 WIRE 816 176 816 32 WIRE -1440 192 -1440 -112 WIRE 128 224 -16 224 WIRE 336 224 128 224 WIRE -640 256 -640 224 WIRE -304 256 -304 224 WIRE -304 256 -640 256 WIRE -160 256 -160 208 WIRE -160 256 -240 256 WIRE 16 256 -80 256 WIRE 80 256 80 176 WIRE 80 256 16 256 WIRE 128 256 128 224 WIRE 240 256 240 176 WIRE 368 256 240 256 WIRE 512 256 512 80 WIRE 1056 256 1056 208 WIRE -1264 272 -1264 128 WIRE -1264 272 -1440 272 WIRE -1200 272 -1200 192 WIRE -1200 272 -1264 272 WIRE -752 368 -752 176 WIRE -432 368 -432 176 WIRE -160 368 -160 256 WIRE -80 368 -80 320 WIRE -80 368 -160 368 WIRE 368 368 368 320 WIRE 368 368 -80 368 WIRE 1056 384 1056 320 WIRE -640 448 -640 336 WIRE -640 448 -752 448 WIRE -432 448 -640 448 WIRE -240 448 -240 336 WIRE -240 448 -432 448 WIRE 16 448 16 336 WIRE 16 448 -240 448 WIRE 128 448 128 336 WIRE 128 448 16 448 WIRE 240 448 240 336 WIRE 240 448 128 448 WIRE 512 448 512 336 WIRE 512 448 240 448 WIRE 608 448 608 128 WIRE 608 448 512 448 WIRE 816 448 816 256 WIRE 816 448 608 448 WIRE 928 448 928 224 WIRE 928 448 816 448 WIRE 1056 448 928 448 WIRE 1328 448 1328 -256 WIRE 1328 448 1056 448 WIRE -1264 496 -1360 496 WIRE -1024 496 -1264 496 WIRE -976 496 -976 -112 WIRE -976 496 -1024 496 WIRE 0 496 -976 496 WIRE 160 496 0 496 WIRE 240 496 160 496 WIRE 320 496 240 496 WIRE 400 496 320 496 WIRE 480 496 400 496 WIRE 560 496 480 496 WIRE 640 496 560 496 WIRE 720 496 640 496 WIRE 800 496 720 496 WIRE 880 496 800 496 WIRE 960 496 880 496 WIRE 1040 496 960 496 WIRE 1120 496 1040 496 WIRE 1200 496 1120 496 WIRE -976 512 -976 496 WIRE -800 544 -864 544 WIRE -688 544 -800 544 WIRE -800 560 -800 544 WIRE -976 624 -976 592 WIRE -864 624 -864 544 WIRE -864 624 -976 624 WIRE -736 624 -864 624 WIRE -672 640 -688 640 WIRE -608 640 -608 544 WIRE -608 640 -672 640 WIRE 560 640 560 624 WIRE -1056 656 -1056 -192 WIRE -1056 656 -1104 656 WIRE -944 656 -1056 656 WIRE -864 656 -880 656 WIRE -848 656 -864 656 WIRE -768 656 -784 656 WIRE -736 656 -768 656 WIRE -1264 672 -1264 576 WIRE -1024 672 -1024 496 WIRE -608 688 -608 640 WIRE -160 704 -208 704 WIRE -48 704 -80 704 WIRE 160 704 160 640 WIRE 240 704 240 640 WIRE 320 704 320 640 WIRE 400 704 400 640 WIRE 480 704 480 640 WIRE 560 704 560 640 WIRE 640 704 640 640 WIRE 720 704 720 640 WIRE 800 704 800 640 WIRE 880 704 880 640 WIRE 960 704 960 640 WIRE 1040 704 1040 640 WIRE 1120 704 1120 640 WIRE 1200 704 1200 640 WIRE 480 720 480 704 WIRE 560 720 560 704 WIRE 720 720 720 704 WIRE 800 720 800 704 WIRE -768 752 -768 656 WIRE -768 752 -1024 752 WIRE -608 752 -608 736 WIRE -1264 768 -1264 736 WIRE -208 768 -208 704 WIRE -176 768 -208 768 WIRE -1104 784 -1104 736 WIRE -1056 784 -1104 784 WIRE -48 784 -48 704 WIRE -48 784 -112 784 WIRE 176 784 176 768 WIRE 176 784 -48 784 WIRE 256 784 256 768 WIRE 256 784 176 784 WIRE 336 784 336 768 WIRE 336 784 256 784 WIRE 416 784 416 768 WIRE 416 784 336 784 WIRE 496 784 496 768 WIRE 496 784 416 784 WIRE 576 784 576 768 WIRE 576 784 496 784 WIRE 656 784 656 768 WIRE 656 784 576 784 WIRE 736 784 736 768 WIRE 736 784 656 784 WIRE 816 784 816 768 WIRE 816 784 736 784 WIRE 896 784 896 768 WIRE 896 784 816 784 WIRE 976 784 976 768 WIRE 976 784 896 784 WIRE 1056 784 1056 768 WIRE 1056 784 976 784 WIRE 1136 784 1136 768 WIRE 1136 784 1056 784 WIRE 1216 784 1216 768 WIRE 1216 784 1136 784 WIRE -608 800 -608 752 WIRE -544 800 -608 800 WIRE -400 800 -480 800 WIRE -320 800 -400 800 WIRE -176 800 -320 800 WIRE 656 800 656 784 WIRE -1104 816 -1104 784 WIRE -1056 832 -1056 784 WIRE 0 832 0 496 WIRE 64 832 0 832 WIRE 144 832 144 768 WIRE 224 832 224 768 WIRE 304 832 304 768 WIRE 384 832 384 768 WIRE 464 832 464 768 WIRE 544 832 544 768 WIRE 624 832 624 768 WIRE 704 832 704 768 WIRE 784 832 784 768 WIRE 864 832 864 768 WIRE 944 832 944 768 WIRE 1024 832 1024 768 WIRE 1104 832 1104 768 WIRE 1184 832 1184 768 WIRE 1328 832 1328 448 WIRE 1328 832 1264 832 WIRE -608 864 -608 800 WIRE -400 864 -400 800 WIRE -320 864 -320 800 WIRE -976 880 -976 624 WIRE -864 880 -864 656 WIRE -768 880 -768 752 WIRE -1360 896 -1360 576 WIRE -1152 896 -1152 880 WIRE -1152 896 -1360 896 WIRE -208 896 -208 768 WIRE -1152 928 -1152 896 WIRE -1440 1008 -1440 272 WIRE -1264 1008 -1264 864 WIRE -1264 1008 -1440 1008 WIRE -1152 1008 -1264 1008 WIRE -1056 1008 -1056 896 WIRE -1056 1008 -1152 1008 WIRE -976 1008 -976 960 WIRE -976 1008 -1056 1008 WIRE -864 1008 -864 960 WIRE -864 1008 -976 1008 WIRE -768 1008 -768 960 WIRE -768 1008 -864 1008 WIRE -608 1008 -608 928 WIRE -608 1008 -768 1008 WIRE -400 1008 -400 928 WIRE -400 1008 -608 1008 WIRE -320 1008 -320 944 WIRE -320 1008 -400 1008 WIRE -208 1008 -208 976 WIRE -208 1008 -320 1008 WIRE 48 1008 -208 1008 WIRE 1328 1008 1328 832 WIRE 1328 1008 48 1008 WIRE 48 1056 48 1008 FLAG 48 1056 0 SYMBOL npn 272 128 R0 SYMATTR InstName Q1 SYMBOL npn 48 128 M0 SYMATTR InstName Q2 SYMBOL res 320 -128 R0 SYMATTR InstName R2 SYMBOL res -32 -128 R0 SYMATTR InstName R3 SYMBOL res 224 240 R0 SYMATTR InstName R4 SYMBOL cap 144 -96 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C2 SYMBOL npn -704 128 R0 SYMATTR InstName Q4 SYMBOL res -656 240 R0 SYMATTR InstName R9 SYMBOL res -640 -128 R0 SYMATTR InstName R11 SYMBOL npn -368 128 R0 SYMATTR InstName Q3 SYMBOL res -320 -128 R0 SYMATTR InstName R10 SYMBOL res -336 80 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R12 SYMBOL res -448 352 R0 SYMATTR InstName R14 SYMBOL res -768 352 R0 SYMATTR InstName R15 SYMBOL cap -512 80 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C5 SYMBOL npn 544 -64 R0 SYMATTR InstName Q5 SYMBOL pnp 544 128 M180 SYMATTR InstName Q6 SYMBOL res 816 16 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R16 SYMATTR Value 4.3K SYMBOL res 496 240 R0 SYMATTR InstName R18 SYMBOL res 1216 16 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R19 SYMATTR Value 3.3/10W SYMBOL res -656 80 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R13 SYMBOL cap -368 -32 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C4 SYMBOL cap -176 144 R0 SYMATTR InstName C3 SYMBOL diode -96 256 R0 SYMATTR InstName D2 SYMBOL res -256 240 R0 SYMATTR InstName R8 SYMBOL res 800 160 R0 SYMATTR InstName R17 SYMATTR Value 390 SYMBOL cap 224 160 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C1 SYMBOL diode 352 256 R0 SYMATTR InstName D1 SYMBOL res 112 240 R0 SYMATTR InstName R1 SYMBOL res 0 240 R0 SYMATTR InstName R5 SYMBOL diode 912 160 R0 SYMATTR InstName D3 SYMBOL diode 1072 96 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D4 SYMBOL diode 1072 448 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D6 SYMBOL diode 1072 208 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D7 SYMBOL diode 1072 320 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D8 SYMBOL Comparators\\LT1716 1200 736 R270 SYMATTR InstName U1 SYMBOL res 1280 816 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R21 SYMBOL res 1200 816 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R22 SYMBOL res 1120 816 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R23 SYMBOL res 1040 816 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R24 SYMBOL res 960 816 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R25 SYMBOL res 880 816 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R26 SYMBOL res 800 816 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R27 SYMBOL res 640 816 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R29 SYMBOL res 560 816 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R30 SYMBOL res 480 816 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R31 SYMBOL res 400 816 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R32 SYMBOL res 320 816 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R33 SYMBOL res 240 816 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R34 SYMBOL Comparators\\LT1716 1120 736 R270 SYMATTR InstName U2 SYMBOL Comparators\\LT1716 1040 736 R270 SYMATTR InstName U3 SYMBOL Comparators\\LT1716 960 736 R270 SYMATTR InstName U4 SYMBOL Comparators\\LT1716 880 736 R270 SYMATTR InstName U5 SYMBOL Comparators\\LT1716 800 736 R270 SYMATTR InstName U6 SYMBOL Comparators\\LT1716 720 736 R270 SYMATTR InstName U7 SYMBOL Comparators\\LT1716 480 736 R270 SYMATTR InstName U10 SYMBOL Comparators\\LT1716 400 736 R270 SYMATTR InstName U11 SYMBOL Comparators\\LT1716 320 736 R270 SYMATTR InstName U12 SYMBOL Comparators\\LT1716 240 736 R270 SYMATTR InstName U13 SYMBOL Comparators\\LT1716 160 736 R270 SYMATTR InstName U14 SYMBOL LED 1184 576 R0 SYMATTR InstName D9 SYMBOL LED 1104 576 R0 SYMATTR InstName D10 SYMBOL LED 1024 576 R0 SYMATTR InstName D11 SYMBOL LED 944 576 R0 SYMATTR InstName D12 SYMBOL LED 864 576 R0 SYMATTR InstName D13 SYMBOL LED 784 576 R0 SYMATTR InstName D14 SYMBOL LED 704 576 R0 SYMATTR InstName D15 SYMBOL LED 624 576 R0 SYMATTR InstName D16 SYMBOL LED 544 576 R0 SYMATTR InstName D17 SYMBOL LED 464 576 R0 SYMATTR InstName D18 SYMBOL LED 384 576 R0 SYMATTR InstName D19 SYMBOL LED 304 576 R0 SYMATTR InstName D20 SYMBOL LED 224 576 R0 SYMATTR InstName D21 SYMBOL LED 144 576 R0 SYMATTR InstName D22 SYMBOL res 1184 480 R0 SYMATTR InstName R20 SYMATTR Value 91 SYMBOL res 1104 480 R0 SYMATTR InstName R35 SYMATTR Value 91 SYMBOL res 1024 480 R0 SYMATTR InstName R36 SYMATTR Value 91 SYMBOL res 944 480 R0 SYMATTR InstName R37 SYMATTR Value 91 SYMBOL res 864 480 R0 SYMATTR InstName R38 SYMATTR Value 91 SYMBOL res 784 480 R0 SYMATTR InstName R39 SYMATTR Value 91 SYMBOL res 704 480 R0 SYMATTR InstName R40 SYMATTR Value 91 SYMBOL res 624 480 R0 SYMATTR InstName R41 SYMATTR Value 91 SYMBOL res 544 480 R0 SYMATTR InstName R42 SYMATTR Value 91 SYMBOL res 464 480 R0 SYMATTR InstName R43 SYMATTR Value 91 SYMBOL res 384 480 R0 SYMATTR InstName R44 SYMATTR Value 91 SYMBOL res 304 480 R0 SYMATTR InstName R45 SYMATTR Value 91 SYMBOL res 224 480 R0 SYMATTR InstName R46 SYMATTR Value 91 SYMBOL res 144 480 R0 SYMATTR InstName R47 SYMATTR Value 91 SYMBOL Comparators\\LT1716 560 736 R270 SYMATTR InstName U9 SYMBOL res 720 816 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R28 SYMBOL res 256 64 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R7 SYMBOL res 176 -16 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R6 SYMBOL res 160 816 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R48 SYMBOL Comparators\\LT1716 -144 784 R0 SYMATTR InstName U15 SYMBOL res -64 688 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R49 SYMBOL diode -1216 -80 R0 WINDOW 3 100 64 Right 2 SYMATTR Value 1N4007 SYMATTR InstName D26 SYMBOL diode -1216 -16 R0 SYMATTR InstName D27 SYMATTR Value 1N4007 SYMBOL res -1216 32 R0 SYMATTR InstName R58 SYMATTR Value 51 SYMBOL LED -1216 128 R0 SYMATTR InstName D32 SYMBOL cap -1280 64 R0 SYMATTR InstName C10

SYMBOL Misc\\battery -1440 176 R0 SYMATTR InstName V1 SYMBOL res 464 64 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R59 SYMBOL Misc\\xvaristor -224 880 R0 SYMATTR InstName U16 SYMBOL res -336 848 R0 SYMATTR InstName R50 SYMBOL cap -416 864 R0 SYMATTR InstName C6 SYMBOL diode -544 816 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D5 SYMBOL diode -592 928 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D23 SYMBOL cap -592 752 R180 WINDOW 0 24 56 Left 2 WINDOW 3 24 8 Left 2 SYMATTR InstName C7 SYMBOL Comparators\\LT1716 -704 640 R0 SYMATTR InstName U17 SYMBOL res -992 496 R0 SYMATTR InstName R51 SYMATTR Value 2.2K SYMBOL res -992 864 R0 SYMATTR InstName R52 SYMATTR Value 2.2K SYMBOL res -784 864 R0 SYMATTR InstName R54 SYMATTR Value 2.2K SYMBOL cap -784 640 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C8 SYMATTR Value 22n SYMBOL res -880 864 R0 SYMATTR InstName R55 SYMATTR Value 1.1K SYMBOL cap -880 640 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C9 SYMATTR Value 33n SYMBOL cap 1184 -160 R0 SYMATTR InstName C11

SYMBOL res -592 528 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R53 SYMATTR Value 12K SYMBOL npn -1200 880 R270 SYMATTR InstName Q7 SYMBOL Misc\\xvaristor -1168 912 R0 SYMATTR InstName U18 SYMATTR Value 1K SYMBOL pnp -1200 864 R180 SYMATTR InstName Q8 SYMBOL LED -1280 672 R0 SYMATTR InstName D24 SYMBOL res -1280 480 R0 SYMATTR InstName R57 SYMATTR Value 91 SYMBOL res -1376 480 R0 SYMATTR InstName R60 SYMATTR Value 8.2K SYMBOL res -1040 656 R0 SYMATTR InstName R56 SYMATTR Value 2.2K SYMBOL res -1120 640 R0 SYMATTR InstName R61 SYMATTR Value 270 SYMBOL cap -1072 832 R0 SYMATTR InstName C12

SYMBOL Comparators\\LT1716 640 736 R270 SYMATTR InstName U8 TEXT 1224 -280 Left 2 ;PROBES TEXT -1160 216 Left 2 !PWR TEXT -1160 248 Left 2 !BATT TEXT -1424 -32 Left 2 !ON/OFF TEXT -1416 0 Left 2 !HERE TEXT -1424 152 Left 2 !6 VDC TEXT -1416 32 Left 2 ;

Reply to
jurb6006

** Any mention of "ohms mode" was conspicuously absent from you earlier post.

VTVMs use voltage division to measure unknown resistors - so read full scale with no DUT in place. Analogue VOMs use current flow instead and read in the opposite sense.

.... Phil

Reply to
Phil Allison

In the above post is the whole enchilada, by itself so as to make it easier to copy and load.

For one I have notice that U 15 & 17 should be OP AMPs, so there goes my id ea of using 4 quad ships unless I use all OP AMPs, which is a possibility. I just think a basic comparator might have some trouble with analog signals . Though I have seen them used that way I would rather be safe than to have to frig around redoing things.

In the lower left are Q 7 & 8 which comprise the DC short detector. I had t o go to 2 transistors but that is no biggie. It should detect the normal 20

0 mV developed at the prods, and light the LED when it is not there.

Of course I have other issues. You can see how the thing works now, and the detection is the key to its usefulness. I have the signal from the DUT amp lified X 12 approximately which at full signal/open circuit uses up most of the Vcc. My concern now is at the other end, very low resistances (ESR) me asured. The drops across D 5 & 23 will affect that end of the scale. How mu ch ? That is a bit hard to figure, I haven't even chosen the diodes yet.

Probably the best bet would be to take an identical pair of diodes in serie s between the anode of D 23 and ground with their cathode to ground and a r esistor from Vcc to positively forward bias them. Maybe 3 would be better t o take the input range of U 15 slightly away from the rail. I am open to su ggestions on that. While I am not going for a laboratory quality precision instrument here, I also don't want a piece of junk. Once I start breadboard ing this mess I want to be at least halfway sure of what I am doing. (no wi secracks Phil,,.err,,,on second thought go ahead, WTF YOLO)

Last but not least is the generating of the test signal. The SESAPP output (upper right) is obvious, no real bias but R 18 & 59 make sure Q6 gets satu rated. The astable multivibrator before that is also no problem, but the os cillator feeding it is. I am not all that great with oscillators that don't use coils. I prefer not to use a coil because I want the utmost in parts r eplaceability, and includes during the development stage. I intend to choos e values for R 22 - 34 in real life as they won't be the same, (linear scal e or log per se) and maybe have R 21 & 48 adjustable, with either or both a djustable in the final design if necessary. I doubt it will be but so it do esn't scare me. But I don't want too many adjustments.

Since I guess I need 2 OP AMPs, and I still would just as soon use 4 quad c omparators, I can either use them for something else or add 2 segments to t he main display. Anyone have any bright ideas ? One thing might be to check much lower value caps. Another high pass filter like C 8 & 9 and R 55 & 54 /56 but at a much higher frequency.

Of course then that takes me back to the signal generation to make sure the slew rate is fast enough to be useful for that purpose. That could get by with only 2 LEDs to indicate. Just - is it shorted or open ? If you want ca pacitance they got a box for that.

Reply to
jurb6006

Technically we just said the same thing because it is either the current makes the voltage or the voltage makes the current.

In any case I believe the way a VTVM does it is simply better in many ways. The one and only drawback I see is the 2 adjustments, zero and infinity instead of just zero, but that is no problem.

Reply to
jurb6006

** The only difference being that your version was gobbledegook.

** They are essentially the same.

** VTVMs need a control to balance drift in the triode amplifier circuit.

.... Phil

Reply to
Phil Allison

Great ! Now we can argue about it for weeks. I knew there was a reason I stuck around Usenet.

So, no comments on my superkludge up yonder ? Like "Only a moron with the intelligence of an abused puppy that sucks rancid kangaroo dung would ever design something like that" ?

Reply to
jurb6006

** I'm not interested in arguing with you.

VOMs and VTVMs both rely on highly non-linear scales to display resistor values. They have similar accuracy for most resistor values while VTVMs are easily able to measure values well over 10 Mohms.

Both have low ohms ranges that are able to forward bias silicon diodes and transistor junctions with significant current for in-circuit testing.

.... Phil

Reply to
Phil Allison

** See pic of scale from vintage VTVM, note readings are in the opposite direction for ohms ranges compared to passive VOMs.

formatting link

** A centre scale vale of 10ohms indicates an internal reference, series resistor of the same value. The meter uses a 1.5V cell, so short circuit current is a substantial 150mA.

.... Phil

Reply to
Phil Allison

d transistor junctions with significant current for in-circuit testing. "

Which is what I don't want in this. That's why the test signal is 400 mV P- P. It might tickle germaniums or Schottkies, but in normal use no. And if t he operator can't figure that out he should go flip burgers. Actually this device still works with a junction across it, or reasonable resistance. Act ually in theory it should work with inductance across the DUT but would of course read a DC short.

The operator should know that and pretty much figure the device is reactive , not just a filter, like some CRT based TVs used to have in the sweep circ uits for geometry.

Once proficient with the tester, a tech of modest competence should be able to troubleshoot many units in a very short time. Like a day at work was fo r me alot of the time, it took longer to take the back off than it did for me to troubleshoot it.

And really, that is the only way to make it in that business. The shops tha t were plagued with waste due to incompetence are mostly gone now by attrit ion. Places that change parts until it works and expect the customer to be willing to pay for 10 pars when he only needed 1, and such assholes gave th e whole industry a bad name. People would let contractors in their home to rebuild the house and handle all their valuables n shit, but the TV man ? W ATCH HIM LIKE A HAWK BECAUSE THEY ARE ALL CRIMINALS ! It takes quite a bit of customer care to unlearn them of that shit.

But as many might know, I am at wart with the manufacturers. They started i t. Every time I fix something that is a little bit less money going to Chin a or wherever the f*ck.

Reply to
jurb6006

and transistor junctions with significant current for in-circuit testing. "

P-P. It might tickle germaniums or Schottkies, but in normal use no. And if the operator can't figure that out he should go flip burgers. Actually thi s device still works with a junction across it, or reasonable resistance. A ctually in theory it should work with inductance across the DUT but would o f course read a DC short.

ve, not just a filter, like some CRT based TVs used to have in the sweep ci rcuits for geometry.

le to troubleshoot many units in a very short time. Like a day at work was for me alot of the time, it took longer to take the back off than it did fo r me to troubleshoot it.

hat were plagued with waste due to incompetence are mostly gone now by attr ition. Places that change parts until it works and expect the customer to b e willing to pay for 10 pars when he only needed 1, and such assholes gave the whole industry a bad name. People would let contractors in their home t o rebuild the house and handle all their valuables n shit, but the TV man ? WATCH HIM LIKE A HAWK BECAUSE THEY ARE ALL CRIMINALS ! It takes quite a bi t of customer care to unlearn them of that shit.

it. Every time I fix something that is a little bit less money going to Ch ina or wherever the f*ck.

There were worse out there. I recall one place changing a PCB then claiming the customer's machine was BER. Repair was one area where, at one time at least, backstreet places were more likely to be ok.

NT

Reply to
tabbypurr

Two things are responsible for that. For one, lack of overhead. But then the most qualified are not always the ones to get the job.

Reply to
jurb6006

On Saturday, 26 May 2018 00:51:27 UTC+1, snipped-for-privacy@gmail.com wrote: NT:

Overhead is it. Nice shops had to charge excessively or resort to dodgy business to survive. Now they're nearly all gone.

Try explaining that to a punter though, almost no-one ever got it.

NT

Reply to
tabbypurr

** The various ohms ranges on VTVMs respond to the voltage across the probes where 1.5V produces full scale - so the reading will be near half scale with the 0.65V of silicon diodes and junctions.

IME, shorts in capacitors are fairly rare, aside from the occasional bead tantalum or bypass ceramic. Hard shorts are immediately obvious while a low resistance part will become so if the x10 or x100 ohms ranges are used.

In over 99% of cases, the short or low resistance will be with the silicon part.

If the operator can't figure that out he should go flip burgers.

---------------------------------------------------------

Now watch the jurb over-snip my post and change the topic.

.... Phil

Reply to
Phil Allison

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.