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Old 21st Dec 2014, 6:27 pm   #21
Alistair D
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Default Re: Problem with TL071 SMPS comparator

Bad smoothing on the 12V supply preventing the op-amp starting up?

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Old 21st Dec 2014, 6:29 pm   #22
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Default Re: Problem with TL071 SMPS comparator

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Originally Posted by mole42uk View Post
I don't understand the difference between a voltage error amplifier and a comparator in this case. I think the output of the PSU is compared against the zener controlled reference, and the op-amp, working linearly, adjusts the brightness of the opto LED to adjust the current flow through the opto taransistor. The change in current through the opto transistor changes the mark/space ratio of the SMPS chip. Or have I misunderstood?
That's exactly right.

Because its not working correctly however, we have to look at the basic blocks and in this case we can say the opamp should behave as a comparator and follow the standard rules for an opamp with no feedback. If there is a DC differential between the inputs, then the output should change according to the polarity of that difference. That's fundamental, and until the reason why that isn't happening is resolved you have to just look around the opamp. There can be no other contributing factors if the supply and inputs are clean.

I say no other contributing factors... make absolutely sure all those supplies are clean. I wonder if a "race hazard" condition could occur if the opamp input voltages appeared (or appeared higher in amplitude) than the rate at which the opamp supply rises at power on.
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Old 21st Dec 2014, 7:46 pm   #23
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Default Re: Problem with TL071 SMPS comparator

It's a Keithley 175 DVM. The ±5 and ±12 rails are pretty clean - it is an SMPS designed to drive a microprocessor so the output filtering is quite good and seems to work well.

If I take pin 3 higher than the reference on pin 2, the output on pin 6 goes up from 0V to about 75mV, no higher. I have tried this with pin 6 in circuit and out of circuit, and even used a 10k pull-up on pin 6 to get it tied to the +12V rail. No difference. According to the data sheet, the TL071 is practically bomb-proof and I even went so far as to put a socket in the board so I could try different configurations.

kelly_eye suggested crud between pins 2&3 which I'm going to look at, probably clean the area with alcohol just to make sure.

Another trick which I've used on other parts is to build a replica on Veroboard and study that in a clean environment. Mind you, I have already built a test rig for this PSU board to replicate the DC input, the correct output loads and I made a 10MHz TTL oscillator to sync the internal clock.
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Old 21st Dec 2014, 8:07 pm   #24
Mooly
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Default Re: Problem with TL071 SMPS comparator

Well its a puzzler for sure.

Have you (with power off) done a resistance check from pin 6 to pin 4 with it all in circuit as it stands. That 75mv is so low that its worth seeing if there is any low resistance path somewhere. Normally in circuit checks are a no no but here......

Is the PCB double sided print ? Have you measured all the voltages on the IC itself, not on the print underneath.

This is going to be something weird like that.
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Old 22nd Dec 2014, 7:18 am   #25
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Default Re: Problem with TL071 SMPS comparator

That circuit compares a mixture of the 12v and 5v rails, potted down, against that 12v zener reference and uses the result to control the LED current in the opto isolator.

The resistor/capacitor pair in feedback around the opamp are loop-gain control items used to ensure the loop is stable.

The amplifier is non-inverting, sop if the rails are too high, the opamp output should rise and put more current into the opto isolator.... so that must turn down the wick in the SMPS.

Now, if the opamp output can't rise for some reason, the LED stays off and the SMPS should be trying to run at max power, and the output voltages should be out of control and much higher than they should be.

This endangers everything powered from those rails, but it's done this way to make a circuit which can self-start. Power is turned on, the SMPS starts and runs, reservoirs charge and then this opamp sees the voltage come up a little above the required values, and it turns down the SMPS pulse width. The voltage overshoots a little, but the speed of the opamp versus the speed of voltage rise keeps the overshoot tolerable, and thereafter, the thing is a linear feedback loop.

So, something is holding the opamp output near ground. I assume the 17v supply is there and the 12v zener is working OK. If the opamp's been replaced, then there must be a short somewhere.

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Old 22nd Dec 2014, 11:03 pm   #26
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Default Re: Problem with TL071 SMPS comparator

Those CNY17 opto's are available in a range of CTR (gain) groups. Any chance the wrong one has been fitted in the past or the one you've got has gone low gain?
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Old 22nd Dec 2014, 11:19 pm   #27
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Default Re: Problem with TL071 SMPS comparator

Change the Opto. I've had problems with opto isolators before. At the very least it will eliminate a possible cause of trouble.
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Old 24th Dec 2014, 10:58 am   #28
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Default Re: Problem with TL071 SMPS comparator

I think Richard has eliminated the opto by using a resistor to turn it on and cut the SMPS output.

PS Could this be a 'phase reversal' issue ?

Last edited by dave cox; 24th Dec 2014 at 11:09 am. Reason: Addition
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Old 24th Dec 2014, 11:32 am   #29
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Default Re: Problem with TL071 SMPS comparator

The TL071 does have history of 'phase reversal'. A pair of anti-parallel diodes on the inputs would eliminate this.
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Old 24th Dec 2014, 11:35 am   #30
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Default Re: Problem with TL071 SMPS comparator

Thanks folks, it's good to have lots of avenues of research. I can't help feeling that this one is going to be some "operator error" somewhere and I'll have to go into the corner wearing a pointy hat.....

However, the PSU is in my workshop, the door is locked and I'm about to take a few days off.....

Happy Christmas!
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