Op amp circuit for "couting" galvanometer - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: July 17, 2016, 07:38:20 pm
content: Hi! I've gotten hold of some nice, very hq Sifam analog meters from some 70's scientific equipment, with the scale measuring in "CPS"- counts per second, and that got me wondering: how do you get an analog meter to count pulses like that?I imagine, you'd need some operational amplifier, and then some sort of RC circuit in front, to allow voltage to build up, and discharge slowly. I've been trough the entire google*, and what I have of literature on op-amp applications, but I simply can't find anything that looks like this. I guess it's sorta integrating, but it doesn't look like it's an integrating amplifier.How would one drive those, analoguely? I guess a digital counter and a DAC would do it, but that's not in the spirit of this instrument, I think.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 17, 2016, 07:53:13 pm
content: One needs a kind of frequency to voltage or frequency to current converter. One possiblity is a kind of charge pump driven by the signal followed by a low pass filter.

author: Seekonk
author_rank: Super Contributor
timestamp: July 17, 2016, 09:11:02 pm
content: Think of PWM, each count creates a pulse which is the same voltage and width. Feed that through a resistor to a capacitor and you will get an average value.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: July 17, 2016, 09:34:51 pm
content: That sounds logical, a series resistor for charging up, and a capacitor to gnd for discharging. How'd you dimension that to get a scale deflection of approx. 1 "count" per pulse? Either way, it won't be super precise, of course.

author: barry14
author_rank: Regular Contributor
timestamp: July 18, 2016, 12:18:23 am
content: Although it is interesting to try to figure out how your meter may have worked, it should be noted that because the scale reads CPS does not mean that it was driven from a circuit that directly counted pulses. It could have been driven from a potentiometer that provided a variable DC voltage to a voltage-controlled-oscillator (VCO) or a voltage to frequency converter or a myriad of other circuits. Meter scales, in general, provide little information on the circuit in which the meter was used.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: July 18, 2016, 01:11:11 am
content: Quote from: barry14 on July 18, 2016, 12:18:23 amYeah, I know, but from the other controls and connectors, it seems the unit had inputs from some sort of particle detectors.In any event, the direct pulse counter is what I'd have the most use for, so getting my head around that seemed logical.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 18, 2016, 11:32:34 am
content: If the signal to count is a well behaved from (e.g. CMOS Level and not to short pulses), one can use just diodes and a cap to build a charge pump.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: July 18, 2016, 12:51:07 pm
content: It likely is, but I'm not certain how long the pulse is. Perhaps a pulse stretcher or something before the charge pump to make sure each pulse will bring up the charge by the same amount. 74121?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 18, 2016, 06:10:06 pm
content: The pulse stretcher like the 74(LS/HC)121 might be needed. With pulses of defined length one could use just a low pass filter as well.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: July 18, 2016, 06:55:49 pm
content: Right. I guess the easiest way to calibrate the meter/charge pump for the 100mV F.S.D. meter would be to make all resistors variable, with nice 10 turn trimmers, and then just adjust with a LF gen and a frequency counter.Would it be best practice to have the charge pump before an op amp, or directly on the meter ?

author: SeanB
author_rank: Super Contributor
timestamp: July 18, 2016, 07:54:30 pm
content: Normally those used a simple integrator, and generally for particle detectors they did not really use much in the pulse shaping line, as the pulses were normally pretty narrow in most cases. Then simply they used a half wave rectifier to charge a capacitor, and a resistor across it to discharge it, with either a Jfet follower or a vacuum tube as a high impedance buffer. This meant a longish time constant on some very low count ranges, so you would find a few ranges marked on the scale, with the resistor being scaled as needed to get the required full scale count rate voltage. they responded quite fast to incoming pulses, so often there was a second RC low pass between the buffer amplifier and the meter movement, often just the scaling resistor for the meter and a 10-100uF electrolytic across the meter coil to damp the pulses from making the meter move up in jerks on the lowest ranges.Very similar to the rectifier used in a VU meter for audio.

author: Circlotron
author_rank: Super Contributor
timestamp: July 19, 2016, 09:38:27 am
content: Here you go.https://www.google.com.au/search?q=tachometer+circuit&prmd=isvn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjR7eaknf_NAhVU02MKHeXIDHYQ_AUIBygB&biw=1024&bih=653

author: ChristofferB
author_rank: Frequent Contributor
timestamp: July 19, 2016, 08:11:20 pm
content: Quote from: Circlotron on July 19, 2016, 09:38:27 amBingo! a tachometer is of course completely analog, counterwise. Thanks!

author: Someone
author_rank: Super Contributor
timestamp: July 19, 2016, 10:49:34 pm
content: Quote from: ChristofferB on July 19, 2016, 08:11:20 pmAnd its linear, is the scale on your meter linear? Many particle detectors use a log (or arbitrary) readout which can make it hard to determine exactly what the circuitry is needed, the scale may be accounting for the count pileup and specific sensor linearities too.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: July 20, 2016, 01:54:30 pm
content: Yup, scale is linear, and the instrumentation then achieves greater range with a range switch of a million settings.

author: zapta
author_rank: Super Contributor
timestamp: July 20, 2016, 03:13:10 pm
content: http://www.ti.com.cn/cn/lit/ds/symlink/lm2907-n.pdfhttp://www.ti.com.cn/cn/lit/an/snaa088/snaa088.pdf