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.

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