How can this LM393 circuit draw so much current ??

[MathWizard]

I'm making a better version of a continuity tester for 1 of my DMM's. I'll use it's 2k range and the 1mA current it sinks from the Black probe on GND, down into the red probe below GND.

So on a breadboard, I made a version that seemed ok in LTspice. But on the breadboard, right away I was drawing 40mA on the negative rail, and that was the set limit. So I narrowed it down to the LM393, and tried another 1, and it was ok for a 1-2 mintues, but now it also wants to draw 40mA on the negative rail. Although I can't see any oscillations on the outputs.

Right now, I can turn on the negative rail alone at -15V, and only draw 1mA, but when I turn on the +15V rail also, the neg. rail will quickly hit the current limit again (right now 20mA). And if I turn off the pos. rail, the neg. rail goes back to 1mA.

At this point, I added pull down resistors on all 3 pins of the unused 1/2 of the LM393. At 1st I only had one on the output. Other than that, none of the external parts should be able to draw 40mA from -15V. The breadboard is like new as far as I know, I could move the LM393, but I don't think it's that, and all the resistors have trimmed leads, so I don't think it's that.

What's going on ??

[T3sl4co1l]

Q1 is "suicide biased" and might draw ~1mA purely by coincidence, but most likely a lot more.  Try running your simulation at 0 or 70 °C -- Vbe has a strong negative tempco.

Simplest solution, put a diode in series with R7 (preferably a matched transistor, diode-strapped) and an emitter resistor on Q1.  This will get it within 10%, maybe 5%.  Better than 5% or so I think requires matched diode and transistor, regulated supply voltage (I mean, it's proportional to supply, if the supply itself is worse than 10%, so will the current--), and preferably just a voltage reference and op-amp instead of a BJT current mirror.

Doesn't seem to be any need for LM393 to be supplied from -15V, GND would do?  That avoids avalanching Q3's base when off.  LM358 first half can be eliminated, and the inverting amp moved up, with much larger input resistance so as not to disturb DUT's voltage (or if you like, the circuit's Thevenin equivalent as seen by DUT).  LM393 can be eliminated then, with using the other 358 half as comparator -- slow, but blinking a light hardly cares.

Further, the amplifier can be changed to noninverting, made single-supply, +V can be reduced, and the 2.5V REF can be used with a transconductance amplifier (e.g. Howland current source) to replace Q1.  Not that precision is a big deal for mere continuity.

Tim

[MathWizard]

Yeah the overall design is just improvised, yeah I just made a quick current source, (I'll be using the one in the DMM) but it was the problem. I had the top divider resistor R6 connected to 15V, not GND. I still don't see where all the current goes, I see some diodes inside the LM393. But the moment I put in certain DUT resistor's, that will make the LM393 sink all that extra current, and also my PSU is lowering the neg, rail, so when it would sink 20mA, it would be from -6.2V. I thought maybe all that current goes across the 470R I had between the inv. amp and LM393 Inv pin, so I tried a 4.7k, and same thing happens, and Vout from the inv. amp stays about 13.6V the whole time I was checking.

However, it seems to work ok when wired right, IDK if it function exactly as I think it should overall, (the last version with just op-amps, wasn't so good) but this one can also tell the difference between 1.2R and 2R no problem.

But I didn't want to cheat and look up a real schematic from a DMM, but yeah I'll look up what you're saying about Q3.


OK so the extra current is going across my DUT, I didn't realize that's what you meant, and that's why it happens when I hooked up the DUT.


Yes so for now I added a reverse biased diode on the base of Q3, I forgot that resistor will still be low on both ends when the 393 goes low. And I changed the hysteresis a bit, as with 510k I could get some near 1MHz oscillations when adjusting the pot for triggering on 1.2R. With 255k, I don't see any osc. any more, at least not for small DUT's.


I remembered about emitter degeneration resistors, and yeah the temperature runs away. So I used a basic current mirror with 5k emitter resistors on both. For a 4.7R, I get -4.7mV and ~ 4mV from the buffer.

I added a 555 circuit for the buzzer, and that gave me a huge noise problem on the buffer output, over 10Vpp at times. I'm not used to that, but after playing around with adding caps, and having a better current source, my ebay LM358 seems pretty good again. I went back to 510k Rf on the 393, and haven't had any issue. It seems to work down to about 2 ohms, and so far seems repeatable. So that's good enough for now.

[MathWizard]

Ok I have a new problem probably related to the LM393. I've used the op-amp test jig I made and measured some Vos and input bias currents for the LM358, and some of the stuff for the LM393.


The DMM buzzer circuit that I'm making, which for testing purposes has a 1mA current source and a parallel 10k resistor, that gives a default voltage of about -10V. The circuit below is soldered and more or less working IRL, I got rid of most the ~100kHz oscillations it just had.

But the problem as of now is, I can still set it to trigger on say 2 ohms, and the LM393 gets pulled up to the 12V rail. But then, if I yank out the 2R, and so the input voltage immediately goes from about -2mV to -10V, but the LM393 sometimes stays high, for many many seconds, and it wasn't oscillating on any timescale I could see on a digital scope, long or short. But it did a little but when it finally returned to the low sate it should be in.

So I don't get why this circuit is "latching up" like this, I'm pretty sure the sim with the universal op-amp2, is not doing it. IRL it looks like a very stable steady low noise high voltage out of the LM393 for 10's seconds, then osc for a few sec, then returning low, when it should have returned low a minute ago.

C1 really is 2.2n, and C3 is 33n, and like C2, there's just 100nF caps on any/all supply pins.


Ok I see the problem is again my current source, it's not behind the LM7812/7912 supply to the LM358/393, and the 10k resistor is actually getting pulled nearer to -13V. And that's lower than the supply, and the input current into the non-inverting pin jumps to uA and the buffer output saturates high. So capping the low voltage within the low supply , then it seems to be working. Until I find the next problem I guess.