Tag Archive for troubleshooting

Dummy load get’s a tune up and another test

Improvements and fine tuning

As mentioned in my last post on the dummy load, though I am very pleased with the results, there is always room for some fine tuning and improvement. Also, some parts needed to be bought. First off, here’s a revised version of the schematic:

dummy-load-v14_thumb
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Voltmeter troubleshooting

Tests reveal something interesting

As a follow up to my previous post, I had a few minutes to mess with the erratically behaving voltmeter today and I believe a few quick tests might have found the problem – two problems anyway. The solution is not yet clear, but I have some idea on how to proceed now at least.

In reading the very helpful Tips for using Single-Chip 3½ Digit A/D Converters – thank you Intersil – I performed a couple of tests using my multimeter and scope

By the way, to those other hobbyists out there – you really can’t live without a scope, even a basic analog one like mine. Helps to see what the hell you are doing.

Testing voltages

So my first thought was to probe around and collect some voltage readings to see how effective (or not) my input voltage divider is. As it turns out, mostly, it works! Within a certain amount of error of course, but I can trim that out easily. I measured the voltage coming in to the IN HI pin of the IC and it returned values as expect in all ranges except for 2V. So there’s problem number one: I need to put a proper resistor value between IN HI and IN LOW for the 2V range. I’ll solve that later.

What was most illuminating was the 20V range. As before, a 5V input revealed close to expected voltages at that decade setting on the IC pin. It did also using a 12V input BUT at 12V input, on the 20V setting – the number was off by half it seemed. Curious.The correct voltage was reaching the IC IN HI pin, but why was it displaying something different?

In looking at the Tips datasheet (linked above) one of the questions in the accuracy section asked about a non-linear voltage at higher input levels. Well damn, that sounds like what’s happening. From the data sheet:

Accuracy Problems
Problem – Above a certain input voltage level, the displayed reading does not linearly track the input.
Action – Observe the waveform at the output of the integrator stage (pin 27) of the A/D converter. There should be no clipping at the positive and negative peaks of the ramped waveform. The value of RINT or CINT may be too small, or the oscillator frequency may be too low, allowing the integrator to saturate. See previous section on component value selection.

So the integrator could be the culprit. I tried to check the waveforms with my scope and I have a hard time interpreting the results. My scope is old and hack calibrated so I’m not quite sure what I should be looking at. I probed the integrator pin and bounced up and down on DC coupling like a low frequency square wave. I probably should have it on AC coupling which revealed a very short ramp up long sustain and quick drop. The datasheet showed an example waveform that looks just like a triangle wave to me (ramp up and down) so this is not quite what I was expecting. Either I get a low frequency square wave (dc coupling) or something completely wrong (AC coupling). It said to watch for clipping, so this could be what’s happening. A flat sustain I guess could be interpreted as extreme clipping. Anyone want to chime in?

So now I have a possible plan of action. I may breadboard up another unit to make easy changes and experiments without having to solder and de-solder the built one, then apply the changes to the built one. It also suggested the oscillator frequency might be too low, but as far as I can see on the scope, it is correct and producing nice waveforms. Here’s what I have to do:

  • Re-check voltages to make sure the voltage divider is not at fault
  • Check value and replace the caps on the integrator pins of the IC, possibly with a bit higher value
  • Probe the integrator to see if that produces the nice ramp-up/ramp-down waveform

Hopefully fixing the integrator will solve the problem and I hope I didn’t fry three ICs doing this.

Later that day…

OK well this is encouraging. I googled around (what I do when I’m obsessed with a nagging problem) and came across something I didn’t see before. Two other gents, using the same schematic are having very similar issues. Best part is they solved it (at least for them, who knows about me yet). Read here

So the coles notes version is that not only do I have to trim up my divider scheme on the input but I am probably missing a connection! I did think of this fix last night but I didn’t try it for some reason or other. It makes sense to because this ties REF LO, IN LOW, and ANALOG COMMON together giving us a fixed reference point. Both claimed that it fixed the issue and it started behaving properly which is good news!

What I find confusing is that it isn’t clear if I am supposed to connect IN LO to ANALOG COMMON and to ground, or if I’m only allowed to connect IN LO to ANALOG COMMON or ground (not both). The official datasheet seems to suggest I can’t do both but the forum comment claims success with connecting both. For safety reasons (the ICs are $3.80 a pop) I will try just connecting IN LOW to ANALOG COMMON/REF LO and see if I have success. If not, i’ll try connecting the lot to ground also.

As I am at my girlfriend’s place for a few days I will be unable to test it (oh why can’t I have everything in the same place?) but it will also give me a much needed break from swearing at the thing.

Here is my updated fix-it list:

  • Connect pin 32 (REF LO) to IN LOW
  • Smoke test power on
  • Check that the REF HI pin reads exactly 1V from and adjust if not
  • Conduct test measurements and rejoice if fixed, howl if not

If this turns out to be the culprit i’ll be so pleased as I don’t have to mess with swapping out caps, playing with the integrator, or checking every bloody joint on the board. Wish me luck!

Voltmeter hell

By the way – it draws about 70mA, not bad :)

It’s the sort of thing that makes you want to set your painstakingly assembled workbench on fire.

In my last post, I postulated that I could eliminate the calibration problems with my ICL7107 voltmeter by giving it an isolated power supply and altering the output resisters to a more sane configuration. Sadly, I’ve done both and it’s exactly the same result! Bastard.

This is the sort of thing where you have to take a step back before taking a hammer to the thing – it’s really frustrating. Yes – I know I should be patient and work through it logically – and I will, but what an annoying problem.

The 200V range seems to give me good results regardless which is great actually, I can use that to find fault with the others. I calibrated the ref voltage to 1V no problem, the 200V range seems to work whether it’s 1.4V, 5V or 12V no worries. It’s the other ranges I would like to introduce to a rabid tiger. The 20V I can get somewhat close to the 200V at lower voltages but it reduces to half at 12V and is off the map in the 2V range. By the way, the 2V range is just whacky and bears no resemblance I can see to reality.

Yes, I’ve tried other ICs, I have a whole collection I’ve been swapping out to no effect.

Obviously, I’ve botched the job and need to troubleshoot every bloody connection. Best thing to do is to breadboard up another one and play with it because really – I’m sick of bodge soldering. It’s just wrecking my board at this point.

There is a solution. I will find it. Most likely it’s my output resistors forming wonky voltage dividers again, perhaps switching to 1% and getting close to those magic numbers I put in the last post would help. Other than that, I can only think that I have a dead cap (i’ve already replaced half of them) or have something mis-wired that I missed. Either or both are possible.

Time to hit the books, well – the datasheet – and find a solution. Stay with me, I’m not by any means abandoning it, it’s made me mad now which means I will solve it.