# need some help guys

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can you look at this circuit and tell me if the caps CP202 and CP203 are right

and can you tell me why i am geting 0.71volts to Q205 and Q206 when the

2n3904 is geting 5volts and if i put a lower ohm resistor to give more voltage the voltage from the 74hct14 gos from 5 volts on the output down to 2 to 1 volts this is what i can't get.....
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come on someone has to know something

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Q205 and Q206 are Darlingtons, (the type isn't mentioned), so I would have thought that the base voltages would have been a little higher, in the neighbourhood of 1.2V on each, rather than 0.71V. Are you measuring the collector voltage by mistake instead of the base voltage? As for the 2N3904, if you're measuring 5V at the base, then something is wrong. (Maybe the emitter's ground connection is OC.) Are you sure you're measuring the base voltage, or on the other side of the series base resistor? The caps would work to turn on the relay for a short period determined by the cap/resistor values. (A crude timer) The time period would be in the neighbourhood of 0.1S.

... Johnny

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yes it is at the base the darlingtons are nte46 Base-Emitter ON Voltage VBE(on) IC = 100mA, VCE = 5V - 1.4 2.0 the rest of the circuit works i just need to get the darlingtons to turn on witch i can't get the voltage up to turn them on from what i got it would be a 15ohm resistor to get 1.5 v right ???? ther is more to the circuit

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What makes you think they are not "on". What is connected at that K1 coil place. You do realize that a voltage won't come out of the darlingtons, right? When "on" the transistor pulls the load to ground.

The base resistor is forward biasing the e-b junctions of the two transistors in the darlington pair. The voltage at the base will be whatever the two transistor diode drops in series are. As was already mentioned, .71 V seems more like one diode drop than two. It is a bit puzzling, but you seem to think you want the base to get to 5V. If it does, it will only be after you release the smoke from the transistor.

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would

the

something is

you're

determined

the

1. Are you designing or repairing this? If you're designing it, it seems to me that you need to study a little more transistor theory first.
2. What do these Darlington's drive? (The load should, of course, be connected between the collector and a higher positive voltage.) If the higher positive voltage is 5V and the Vce on is 1.4V, then you can only drive a 2V load.

Why 15 ohms?

5V - 1.5V = 3.5V. 3.5V across a 15 ohm resistor is 233mA. A 74HCT14 can only source or sink a maximum of 25mA per pin. That's why you can't get the voltage to rise above 0.7V. You're trying to murder the 74HCT14.

If the collector current is 100mA, then even if you force 1/10 of that into the base, (10mA), then the smallest resistor you would need is 350 ohms. In reality, since these are Darlingtons, you need far less current and a larger resistor value. (See 1. above)

Obviously. It would probably help if you showed us the whole thing. ... Johnny

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k1 go's to a relay. relays are 24vdc with 18.00 vdc powering them hear is the rest of the ciruit

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and one more that won't work is the freqcounter will not read the freq it starts at 000 and ramps up to 999 and then starts over i hope all of you can help me

steven

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Are you repairing or building this thing? In other words, was it once working, but now isn't, or has it never worked yet? ... Johnny

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i am building this from plans on line. the one that built it does have it working i have sent emails to him and got the info on it and parts that are not in the pdf files.. but right now he has tooo much work to help he has been sick and was out for 3 weeks so that is why i came hear... i know the basic circuits but i am geting in to cmos circuits and did read up on them...

steven

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Does the relay turn on when Q205 is turned on? Does the output of U201C go fully high, (5V), with the original 4K7 base resistor, (R226), to Q205 in place? (When you say the Q205 base voltage is

0.71V? Out of interest, are you making this on a pre-manufactured PCB, or using other means? ... Johnny
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Hate to say it, but this is a pretty crappy design:

(1) Using schmitt triggers as timing elements is not terribly keen-- the electrolytic capacitors tend to have wide variations in leakake and capacitance. Also using monostables around BIG ELECTRICAL ARCS is not very keen! They're likely to jitter all over the place. This type of design really calls out for some digital timing.

(2) There are several boo-boos in the schematic, even at a quick glance. One monostable doesnt have a pull-down resistor, unless it's in the off-page link. Several of the coil drivers don't have transient-clipping diodes or current-limiting resistors. The resistors on the unused circuits are really bizarre. Feeding the unchained comparator and even schmitt triggers from an off-page link is likely to result in serious oscillations. I don't see any power supply bypassing capacitors.

I'd give this circuit design a grade of about D+. It might just may work under ideal conditions, but it's unlikely to work well and precisely without some tweaking.

It reminds me of quite similar control board for a \$3000 tape recorder. Under low-humidity conditions, the audio tape would build up small static charges-- you could hear them discharging to the metal guide posts with a very faint "tick" sound. ABout 30% of the ticks caused the control board to misfire, often sending the deck into stop, rewind, fast-forward, or one or more all at the same time. I don't think you want this to happen with things like gas valves!

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Well, that one is easy to fix-- just throw that circuit away. You don't need three or even two digit precision on the arc frequency. So a simple analog scale is more than adequate.

Just measure the running frequency and label the freq knob with approximate frequency settings.

And BTW are you sure this is a good reliable design? The web site says "preliminary" all over it. You probably don't want to build something that may be shaky. it's no fun debugging somebody elses design, especially when debugging can easily result in blown \$10 IGBT's, one after another.

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Relays will not turn on when Q205 is turned on. U201C is just under 5V with the 4.7k resistor. yes they are in place when i got 0.71v I made the boards with eagle and used a laser printer and used the toner

the caps are in the one for the power supply and yes i konw one is not in ther.... but it has power going to it

and it was the best i could find.. and the igbts that i am useing are packs 200amps

steven

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them

freq

once

is

You should consider the possibility of shorts on the board, I guess. (Also, keep in mind what 'Ancient Hacker' had to say, about the inherent problems in this circuit. And, as suggested by an earlier poster, a micro-controller is the right way to go, in my opinion.) Have you considered that 18V might not be enough to pull in the 24V relays? In the case of 12V relays, the typical pull-in voltage is about 9V. In the case of 24V relays, if it's double that and 18V, you're right on the borderline. When Q205 is turned off, does it's collector measure about 18V? And when turned on, what does it measure? (It seems that your Darlington transistor may be dead, with only 0.7V at the base, with 5V applied across a 4K7 series resistor) The base voltage sounds like one forward voltage drop and not two as would be expected.

I don't follow this - what do you mean?

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ok with 18v going to the relays i get 8.50v out thats on and off. when i would take a jumper wire and put it on Collector and Emitter the darlington would stay on then after some time they would turn off. i did have a short on the board and got new chip's they got 12v but all chips are good, and i get an output on the igbts i did put 12amps at

12v in them...

the caps are in the one for the power supply and yes i konw one is not

the pdf file that shows the power supply does not show one of the 74 chips for power. and all of the chips have a filter caps.. one thing i did was i put bigger filter caps in the power supply 3300uf the ones he used are to small...

steven

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When you say "8.50v out thats on and off", what exactly do you mean? (You need to describe things more clearly) Do you mean that a measurement from ground to the collector of Q205 shows 8.5V whether or not it's turned on? ie With the output of U201C low and 0V at the base of Q205, the collector measures 8.5V and with the output of U201C high, the collector is still at 8.5V and the base is at 0.7V? The part about a jumper from collector to emitter loses me. How would that turn the transistor on? Or did you mean that the relay would turn on?

Again, I have to re-iterate that you have to describe things more accurately if you want help.

... Johnny

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this is true With the output of U201C low

a jumper from collector to to emitter or gnd would kick the relay on then it would stay on for some time and then after maybe 2 to 5 mins the relay would drop out. it's like the darlington's are on for some time when i do that. i have only one 5v trace on the board???? i know that i have 5volts from the power supply but at the last chip i get 4.50v darlington's are npn

does this help out more?????

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all

low

output

would

on?

output

Something doesn't sound right. I'd try another Darlington, for starters. Also double-check all connections for open-circuits or shorts.

What's this about the 5V supply? You keep adding new problems. Now, you have the Darlington problem and the frequency counter problem and yet another problem with the 5V supply? I you're experiencing a drop in the 5V rail, trace back to the point where it drops from 5V to the lower 4.5V and see what you can find.

... Johnny

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i did try new darlingtons but the relays will not turn on:-( if i can get the relays to turn on i can hook it up to the welder to use it. the frequency counter problem i can trace to 74hct112 if i take a jumper wire from gnd to pins15,14 rst it will stop the counter but not in the 50hz to 400hz it will stop from 000 to 999. i may look at a new frequency counter circuit...

steven

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