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Author Topic: High side current measurements  (Read 2937 times)
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Hi all, I'm working on a adjustable 0-2kV power supply. I'm interested in taking a high side current measurement to monitor for excessive current draw from a PhotoMultiplier Tube. The typical current draw should be between 0.5mA to 2mA. However, from my understanding as the tubes start to go bad they begin drawing more current, up to 20mA. I will be reading the current through an analog input on the Arduino and displaying it to an LCD. The PMT is ~5Mohm IIRC.

My first thought is to make a current divider circuit but I'm somewhat hesitant to have so much potential so near the Arduino. Is this okay as long as I'm biased to ground?

If not, what is the best way to go about safely taking these measurements on the Arduino?

I've read a little bit about isolation amplifiers, and they seem like it might be the right approach but I don't fully understand how to go about implementing one. Any thoughts?

Thanks
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I'm not sure if it work for you
http://www.seeedstudio.com/depot/electronic-brick-electricity-meteranalog-p-471.html?cPath=144_154&zenid=b6be7f3439927ffb98888b4bdd6b6d96
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2KV is a lot of isolation to ask for. If I were to design this then I would make a voltage to frequency converter, maybe battery operated, at the hot end monitoring the current through a resistor. Then couple it into the arduino through an opto isolator rated for at least 4Kv isolation.
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If the voltage requirement were lower, you could use one of Allegro's nifty little Hall Effect current sensors.

What is the topology of your circuit? Perhaps there's a test point at lower voltage that has a current, voltage, or some other parameter that's proportional to the current draw at the head end. For example, if it's something like a voltage multiplier, the output ripple increases in a predictable way as the current draw increases.

Mike's suggestion is good, but I don't like powering it with a battery, since that will require regular replacement. You could use a small lamp and a PV cell instead, if you design for sufficiently low power.
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If you are not very worried about linearity there might be an opto-isolator that could do the job - you want one with a linear response and a high enough isolation voltage.  Certainly 0.5 to 20mA is a reasonable drive current for an LED.

At these kind of voltages I would be worried about flash-over - potting the circuit in an appropriate silicone resin would be a good idea.  Or  use an optic fibre to route an LED's light from the high-voltage board to the low-voltage side.  Cheap plastic optic fibre is obtainable with opaque sheaf.

The high-side LED might need protection from over-current depending on the PSU's capability.
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The real issue with measurements at high voltage is one of isolation.
If your HV supply is constructed without using a chassis or other common ground, and the Arduino is powered by a separate supply (preferably transformer isolated), all that the Arduino will ever see is the voltage drop across the current shunt resistance.  This resistor should be 200 ohms to provide about 4 volts drop at 20 milliamps, so as to use most of the 0 to 5 volt measurement range for the area of interest, with an allowance for overcurrent.

Note for safety purposes that the Arduino analog input pin will be at 2 kv, but only with reference to the other side of the HV supply.  Avoid inadvertently providing a path for current flow.
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and the Arduino is powered by a separate supply (preferably transformer isolated),

So you would need a power supply that is rated for 4KV isolation. That is way above the normal isolation rating of power supplies, even ones using transformers.
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and the Arduino is powered by a separate supply (preferably transformer isolated),

So you would need a power supply that is rated for 4KV isolation. That is way above the normal isolation rating of power supplies, even ones using transformers.

A quick look through a DigiKey catalog that happened to be handy shows that many modern mains transformers made to VDE or other EU standards are rated between 3.75 - 5 kV isolation.
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It's not just the transformer but also the components for the mains filtering and phase angle correction components that are included in many supplies. That also includes transient protection components for things like atmospheric electricity.

I have had trouble (at work) with current leakage problems at values approaching 2KV and I defiantly would not recommend doing what you are suggesting. Most standard components will be right on the limit at 2KV and though you can get higher rated ones you are not likely to find these in many ready built units.

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Avoid inadvertently providing a path for current flow.
Much easier said than done. Plus any slip could prove fatal.
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What am I missing here? Most small Laser power supplies (PMT Similar??) can read current at/near the grounded end. Why does this "have" to be a high-side measurement??
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Regards, Terry King  ..On the Red Sea at KAUST.edu.sa
terry@yourduino.com  LEARN! DO! (Arduino Boards, Sensors, Parts @ http://yourduino.com

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