Author Topic: Low current measurement using a LED and photomultiplier  (Read 860 times)

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Offline daqq

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Low current measurement using a LED and photomultiplier
« on: November 28, 2016, 05:12:17 pm »
Hi guys,

After seeing various videos that deal with a photomultiplier and reading a bit on PMTs I've been wondering - would it be possible (or rather would there be a point to) to construct a measurement system that would measure very low currents by means of passing current through an LED, and shining that onto a photomultiplier.

This would create an isolated very low current measurement device.

Aside from the obvious drawbacks (price, size, temperature dependency, the need to calibrate the heck out of it) it should be doable. What could be the resolution of such a device?

Thanks,

David
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Online MK14

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Re: Low current measurement using a LED and photomultiplier
« Reply #1 on: November 28, 2016, 05:34:42 pm »
A number of issues, come to mind.

There would be quite a lot of voltage drop across the LED, probably around 1.5 to 2 volts, depending on LED colour. I.e. Lots of burden voltage.

There is usually a minimum current, below which an LED emits no light. This is probably many, many microamps or more.

If you need isolation, then why not measure the current (sensor, sense-resistor etc), then pass that signal through some kind of opto-isolator.
The signal could be transmitted digitally (very accurate, ADC or similar), using a linear opto-isolator (potentially much less accurate) or by turning the signal into something else, more easily measured on the other side, such as PWM duty cycle or VCO (voltage controlled oscillator).
There are many other ways of doing it (isolated), including transformers (especially AC current) and hall effect sensors.
« Last Edit: November 28, 2016, 05:44:39 pm by MK14 »
 

Online MosherIV

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Re: Low current measurement using a LED and photomultiplier
« Reply #2 on: November 28, 2016, 06:53:20 pm »
Hi

The only application I think this 'may' be of any use is to detect where the line is on an optical galvanometer, the type where the twist on a wire deflects a mirror.
 

Offline Kleinstein

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Re: Low current measurement using a LED and photomultiplier
« Reply #3 on: November 29, 2016, 04:15:46 am »
Good quality LEDs can work well below the µA range. But there will be some nonlinearity and possibly temperature dependence. So I don't think it is very practical. It might be Ok for more like supervision purpose and than using a more normal photo-diode instead of the expensive PMT.

There is a more complementary version: use the current of a photodiode (driven by a LED) to compensate the current to be measured. It's a kind of transimpedance amplifier but without the high value resistor and the photo-diode acts as protection device too.

I know of an instrument using this to measure currents in the fA-nA range (ion current in a mass spectrometer).
 


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