High voltage constant current source - Page 1
Forum section: electronics / projects

author: heliophagus
author_rank: Newbie
timestamp: August 21, 2020, 12:30:09 am
content: Greetings! I have a small design conundrum. I need to deliver a controlled 0-5 mA current through a high enough resistance that a 200 V driving EMF may be required. I will need to specify the current, within perhaps 10% accuracy, via a microcontroller which will probably require optical isolation. Do you have any suggestions for approaching this problem? I have researched and googled this extensively and have yet to find a really good solution.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 21, 2020, 01:30:49 am
content: First there is not much difference between a current source and a current sink.You could argue that a current source pushes current into something, and a current sink pulls it out on the other end, but in the end the electrons run in circles and it's the same.Once you realize that, then you can build a standard current sink, with a current measurement shunt, a variable voltage reference (start with a potendio meter, later use filtered PWM, or a DA converter)The rest is an opamp to compare the voltage over your current mearuement shunt with your setpoint from the potentiometer, and control a switch.It does not matter much whether you use a NPN transistor or N-channel MosFET, as the opamp will compensate for most of the differences of these semiconductors.A MosFET can regulate to lower output voltages, but you need some voltage drop over the shunt anyway.A NPN transistor will always have a voltage drop between Collector and Emitter, even when driven into saturation and under light collector currents.Transistors tend to have lower leakage currents compared with MosFET's.With this "standard" current sink you will face a few problems.The first is to get it stable.Your 200V input requirement does not add much special to the current sink, you just need a 200V (unregulated) voltage source, and of course need a MosFET or NPN transistor that can withstand those voltages.The voltage over the current shunt, and therefore also the opamp input will be low, and the output of the opamp will be around 600mV higer than the Shunt voltage (for NPN bipolar transistor) or 2 to 5V higher for a N-channel MosFET.You can start experimenting from a lower output voltage, for example a 30V lab power supply to get the control loop stable.If you make sure the microcontroller is solidly grounded, and your 200V supply is fused properly then the whole thing is already starting to look somewhat safe.But when messin' with higher voltages a bit of extra isolation is advisable. So use a floating power supply for your microcontroller, and ad a few optocouplers for communication.I2C is a bit complicated for optocouplers, because information goes in two directions through the wires.Both SPI and UART are easy to do with optocouplers, but UART only needs one optocoupler for each direction.Another important issue is how to turn the thing on.If your "device" is not connected yet between the 200V and the current sink, then the setpoint is very likely to be higher then the voltage over the shunt, (Which is 0,0V because there is no current) and the opamp will turn your controlling device fully open, and then, if you give it 200V though your test setup the opamp needs time to react and you will have a high current spike first.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 21, 2020, 01:41:19 am
content: Another approach that may be suitable is to use a transformer from a small wallwart SMPS module in reverse.Put one side of such a transformer on a 12V or so DC voltage source, and connect a MosFET between the other side of the primary winding to GND.You can drive a MosFET directly from a PWM output pin of a microcontroller.Then, on the high voltage side of the transformer you will get a pulsed DC voltage, which is easy to rectify with a diode and capacitor.If you go this way, the hardware is very simple (for this low power device) but there is no inherent stabilisation.This means you have to build that into your microcontroller.By controlling the transformer in such a simple way, you are not really transforming voltage nor current. It's probably easier as thinking of it as the PWM duty cycle controls the amount of power you put in the transformer, and most of that energy will come out at the other end.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 21, 2020, 07:49:10 am
content: I have to ask first does it have to be DC current or AC? One polarity or maybe bi-polar?Anyvay, 5 mA and 200 V doesn't look difficult. That is 1W or 1 VA.

author: jbb
author_rank: Super Contributor
timestamp: August 21, 2020, 09:44:16 am
content: If you just need it done, you could probably buy a Source Measure Unit (SMU). But it’d be expensive.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 22, 2020, 03:04:26 am
content: The kethley 2400 is regarded as an excellent choise.Also fits the 200V requirement nicely:https://www.newark.com/keithley/2400/source-meter-voltage-current-200v/dp/41J2103If you don't like the newark price, you can also try:https://www.testequipmentdepot.com/keithley/sourcemeter-smu-instruments/single-channel-sourcemeter-smu-instrument-2400.htmIf you don't like the testequipmentdepot price you can also try:...

author: David Hess
author_rank: Super Contributor
timestamp: August 22, 2020, 04:24:36 pm
content: Depending on how much noise is acceptable, a step-up transformer can be driven with the chopped current from a constant current source to step the voltage up, and the current down. Rectification of the high voltage output then produces a DC current, and an inductor in series with the output will remove switching noise. Since the transformer ratio is fixed, the current can be monitored on the primary side.200 volts is low enough however that a linear high voltage current source is sink is also feasible.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 23, 2020, 01:20:16 am
content: Hi,what about this solution?It is a 1mA current sink and 1:5 ratio current mirror.Power dissipation in the sink is 200mW maxPower dissipation in the source is 1W max.Need to use high voltage transistors >250VU1 can be driven by a DAC at ground.Regards,Jay_Diddy_B

author: David Hess
author_rank: Super Contributor
timestamp: August 23, 2020, 02:28:52 am
content: I might do it that way but power dissipation in Q1 needs to be watched carefully. Multiple 2N5401s could be used in place of Q1 with separate emitter resistors to spread the power dissipation around.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 23, 2020, 03:19:00 am
content: Hi,The 2n5401 are not suitable for this application.too low powertoo low voltage.need to find 250V or higher PNP transistors capable of dissipating around 1W with a suitable heatsink.Jay_Diddy_B

author: Hydron
author_rank: Super Contributor
timestamp: August 23, 2020, 05:53:17 pm
content: Quote from: jbb on August 21, 2020, 09:44:16 amIf SMU pricing is a bit rich, you can probably do it with an off the shelf power supply, 200V isn't that crazy, though you'd need one with good low-current accuracy and setpoint resolution. Failing that, a 200V capable electronic load could let you do it with a fixed voltage power supply in a more round-about way.