Re: International standards (2023 Update)

Most HCs will actually source a bunch of current, 50 mA into a short,

25 mA into +3 volts, roughly. But a single HC output won't be damaged by driving flat-out in the pullup direction. If you shorted all the outputs of a many-gate HC to ground, you could get it mighty hot.

The bus driver parts are even fiercer.

John

Every power supply design is different. No one can answer your question without the manufacturer's specifications. Some switching power supplies require no load to start. Some can be intermittent when started with no load - works 99 times out of 1000. Some supplies definitely will not start without a load AND can be intermittent (99 out of 100 times OK) with a load too small. Some require loads on either or both 3.3 and

5 volts.

Did the supply provide manufacturer's specs? Do you have schematics? If not, then no > I have been playing with an old AT computer power supply and found

Except the repated INA183 reference has to be wrong. Hmm, perhaps it's an INA138 instead, but then the common-mode specs are wrong.

--
 Thanks,
    - Win

 (email: use hill_at_rowland-dot-org for now)

It could, but it depends on how much noise we're dealing with. You might need an active filter like a Sallen-Key or better.

In either case, the cutoff freq is 1/(2piRC)

You also want to keep the input conductors to the amplifier as short as possible and examine the current flow of the input. Keep the loop as small as possible.

--
Best Regards,
Mike

AH! those wonderful Z80 days.......................

1k memory, machine code programming.................

Regards, Gordon.

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My numbers were based on typical curves in the Motorola HC handbook, confirmed by some measurements made some time back. The smaller numbers you quote are probably guaranteed values for sustaining a legal high or low level.

Hmmm (stands on table to reach dusty bookshelf) here's the TI 1988 HC databook. Voh is typically 0.2 below Vcc at 6 mA load, which gives us an output resistance of 33 ohms in the pullup direction. That would extrapolate to 150 mA into ground, except it's nonlinear so you really won't get that much.

One appnote in the back shows an HC04 dumping about 42 mA into ground,

20 mA into 3.8 volts.

Oh, cool, page 4-50 has the output curves. Standard HC has an initial pullup slope (near Vcc) of 56 ohms, and short-circuit output of about

42 mA. The bus drivers are more like 26 ohms and 75 mA, typicals of course.

So these parts theoretically exceed their own max current ratings when shorted. I guess long-term damage is theoretically possible, electromigration or something. Chargin a cap once in a while should be no big deal. You could charge a farad in a minute roughly.

John

Hi, I made such things too several years ago.

Make a simple band-pass filter, it does not have to be very steep, at

40 kHz you are many decades away from 50Hz, some RC filtering will do. Notice that you probably also have some 100, 150 Hz components.

Regards, Pieter Hoeben

It's adjustable. You turn the knob and use a meter to tell you if it's the voltage you want.

Hope This Helps! Rich

^^^ s/yes/no/

(he said "only ... 250")

Cheers! Rich

....

It looks like it's just that it's such a crappy oscillator that you couldn't hold the duty cycle constant with a C-clamp.

Cheers! Rich

Hi,

I've made a simple circuit which consists of an ultrasonic transmitter and reciever. I send out 40KHz pulses and when they return I amplify the them and put the amplified signal into a comparator (this creates a nice square wave signal for a microprocessor). The problem is, when I'm not sending out pulses the receiver picks up the 50Hz signal from the mains electricity supply (I'm based in England, hence the 50Hz). Should I use a notch filter to remove the 50Hz signal or should I implement some sort of band-pass filter, which only accepts 40KHz? What would you all recommend and which is the easiest to implement?

Thanks for any help.

The fuse blows at the same current regardless of the voltage but the enclosure is designed to provide containment of the arc at 250v. So the answer is yes :-)

-- Regards ........... Rheilly Phoull

The HC outputs have an impedance that limits the current. I believe the limit is something around 5-6mA, or so, with 5V. This means that for a 1F cap, you'd be looking at a max rate of about 5mV/second rate of change. Slow.

Jon

I got part of the way through doing this my self. I managed to get a piece of 3mm Dural sheet from a local Aluminum stockist. I drilled (carefully) through the mounting holes in the motherboard, not all the way through the dural, just enough to mark the positions, then moved the motherboard out of the way before drilling right through. I got M4 spacers and bolted them on to the dural and fixed the motherboard to the spacers using M4 screws. Then I got a right-angle AGP adapter which would allow my video card to fit horizontally ( I was after a low profile so it would fit under my telly like a VCR) . I made a mounting bracket for the harddrive from some angle aluminum with some of those soft rubber gromets from inside an old CD rom drive for vibration and bolted the PSU on like you were suggesting and that is as far as I got. I was going to fit a DVD drive which would sit just above the, now horizontal, video card. but the skeleton PC now sits under my telly whith no box around it and although I like the "borg ness" of it, it isn't very pretty.

Kevin R

Well that had a less than interesting effect! First I tried the idea of just connecting a capacitor and resistor in parallel between the gate and ground. Tried a few combinations but the result was exactly the same as I was experiencing with just a cap on the gate, namely that increasing the RC did not increase the "fade" period. It just stretched the fully-on time then it faded off rapidly. That seems to be indicating that the mosfet is very efficient. ie. it is fully on over a fairly wide range then rapidly goes to fully off.

I therefore tried the idea of using the so-called Miller effect. ie. I connected a cap from gate to drain plus the resistor from gate to ground. Very interesting. The mosfet promptly died (internal short so always hard on). It had gotten fairly hot so I then installed a new mosfet with heat sink attached. Same result. Died immediately. It did not even wait for a signal to the gate and did not bother getting hot. Just died the moment power was applied to the drain (indicated by coming hard on even though no gate signal). The cap I used was a 0.1uF (104) with a 10k resistor from gate to ground.

As I said before the source is at 40v (actually rectified 38vAC). The max rating specs for the mosfet (2SK2175 - equivalent to STP16NE06)are as follows: "Vdgr Drain-Gate voltage (Rgs=20k) 60V Vds Drain-Source voltage (Vgs=0) 60V Vgs Gate-Source voltage +/- 15V."

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Does that just consist of a resistor and capacitor? If so, what values do I select for the cap and resistor? I've done a google search but can't find any specific answers (or rather answers I understand).

Thanks.

This is getting even stranger! I remembered reading about gate ringing in mosfets and found a ST datasheet that recommended inserting a 4k7 resistor on the gate to stop it. I therefore got another new mosfet and hooked it up that way. At least then it did not die, but it acted even stranger. As soon as any of the OTHER mosfets came on, the one I was testing turned on spontaneously without any input from the chip. I put a multimeter between the diode (coming from the 595 output) and the 4.7k resistor and it read 2v. The 595 output was still at 0v.

Oops, I spoke too soon. I was using a 0.27 400V cap a minute ago as a trial. I just changed it back to a little 0.01 "metalised ceramic disc" and the mosfet just died as soon as I switched on. Could it be that the 40v is too much for those little caps?

Anyway this whole idea of a cap between gate and source doesn't seem to be helping.

However, if they are bare (unpackaged), you have to make sure they don't short.

pass the square wave into a one-shot 555 timer and AND gate. if the incoming wave does not expire on the high side before the one-shot timer does then using a gated circuit won't trigger a pulse. this means that time base wider than 40 kHz will not be detected/

Ed wrote: