Hayling Island

Hayling Island

Sunday, November 24, 2013

Stuck with an op-amp that doesn't

A short post to say that I have a bit of a problem with an op-amp that won't. For the Weller soldering iron, I have a constant current source that puts a known current through the PTC sensor in the soldering pencil itself. It works, runs a little less than a milliamp and generates about 15mV across the PTC. So, to digitise in the AVR, I want to make it a bit bigger - perhaps 70x. An op-amp with feedback is easy enough, right? So I have an op-amp with some filtering. And the output saturates to a rail. And writing this I just realised what I have done wrong.

When I look at the (virtual ground) input without the op-amp in place, it's 15mV. When I plug the op-amp in, and there are no other ways of voltage getting there, it reads 180mV. Normally the op-amp would keep the virtual ground at ground because it's fed from two opposite power rails. Ah. Yes, forgot that.

And now fixed - we have life. I 'calibrated' the PTC by putting it in iced water, and then in boiling water to give me an offset and a slope. So I have some scale factors, have plugged them in and it kind of works. Or at least it is trying to turn the heater on the iron to match the desired temperature.

Some point, I have to work up the courage to connect the iron's heater to the logic triac, and see if it really does work. Maybe tomorrow .. I can feel a Call of Duty session coming on. 

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