Minimizing Current Consumption in High Voltage Divider Circuit

[Zaim01]

Hello everyone,

I'm working on a circuit for monitoring the bias voltage of a GM tube, which requires a high voltage of 100V. I am currently using a OA1NP22C op-amp configured as a unity-gain buffer to read the divided voltage. To minimize current consumption, I'm considering using a voltage divider with 1GΩ and 12MΩ resistors. However, this configuration still consumes approximately 9.8µA, which is higher than I would like for my low-power application.

Here is a simplified version of my circuit:

100V Bias Voltage (GM Tube)
|
R1 (1GΩ)
|
+----> To OA1NP22C Op-Amp (unity gain buffer) > MCU
|
R2 (12MΩ)
|
GND

My questions are:

1. How can I further reduce the current consumption of this voltage divider to the nanoampere range?
2. Would switching out the voltage divider using a transistor be a viable solution? If so, what configuration and components would you recommend for achieving this?
3. Are there alternative methods or components that could help in significantly reducing the power consumption while maintaining accurate voltage monitoring?

Any insights, suggestions, or examples of similar implementations would be greatly appreciated!

Thank you!

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Additional Context:

• Control Signal: 2.9V from a microcontroller.
• Power Efficiency: Critical to keep the overall system power consumption as low as possible.

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Feel free to ask for further details if needed.
Zaim

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Call me old fashioned, but what about using a reed-relay or a permanent magnet relay for a switch? You can switch the high side with no issue since it's isolated. No leakage either. And if it is a latching relay you only need to consume power when switching, not holding. Switching is slower, but if you only need to measure once a day that shouldn't be an issue. You can get 3.3V version or get a 5V (or higher) version and switch it on with a transistor. Low tech is often best.

And what about connecting multiple resistors in series instead of one large value?

There are opamps with an enable pin so you can turn them off when not using them. This would save some power too.

[Peter BENSCH]

That's exactly what I meant (but the source of the p-FET is connected to the voltage, not to the voltage divider), but I only mentioned it for the sake of completeness. As indicated, this does not seem to make practical sense to me because of the parallel branch R1+T1.

Only a real switch such as the relays or reeds mentioned have negligible leakage current - at least in the component, as long as your system does not have other leakage currents (e.g. board, etc).

Regards
/Peter

[BarryWhit]

@Peter BENSCH , I still can't believe that using such a high resistance in a voltage divider is good practice. Is this really practical? any flux residue, dirt, oily fingerprint or anything really could substantially change the effective resistance of one leg of the divider and make the measurement inaccurate. Would you really expect this to work in a real-life product? even with a conformal coating (I'm assuming it was chosen with this in mind), this seems like a very precarious way to design circuits for the physical world (as opposed to spice). Am I wrong?

[AScha.3]

For HV this is "standard"...

here example: 1 Gohm

https://www.mouser.de/ProductDetail/Vishay-Techno/TDX05E1G001001FFRB?qs=Li%252BoUPsLEnvEZf1d%2FNe7Xg%3D%3D

[BarryWhit]

I believe such parts exist, but are they really meant for use as part of a measurement circuit? Do MRI machines, and Tokamaks really measure HV voltage this way? I imagine they use high-power resistors (or chain them in series), but not necessarily such high values.

[AScha.3]

Even the probes for scope are in Gohm range... (i have some here.) 🙂

-> 0.8 ... 4 Gohm.

These ARE for measuring...only drawback: cheapest is about 3000 $ .

[Peter BENSCH]

@BarryWhit No, you are absolutely right. That's why I pointed out the problem of leakage current outside the voltage divider several times. Above say 100Mohms, every reason you mentioned becomes a problem. Nanoamperes always sound simple, but mastering them requires effort and experience.

[BarryWhit]

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@AScha.3 wrote:

Even the probes for scope are in Gohm range... (i have some here.) 🙂These ARE for measuring...only drawback: cheapest is about 3000 $ .

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Very good point (But nothing the 600x difference in the working voltage of those specs vs. OP.  And also wondering how often you're supposed to calibrate them). I wonder what engineering lengths they have to go through to maintain accuracy. Those probes must come in those fancy velvet cases, right? like a wedding ring. They probably don bunny suits and wash them with unicorn tears during final assembly 😉 

[Zaim01]

Hi,

Actually, using a reed relay or permanent magnet relay would be a good option, but the available relays that can handle more than 120V with a 3V coil voltage are either very expensive or too large to fit into my device.

Regarding high-value resistors, you are correct that using multiple resistors in series would be better. However, a single 1GΩ resistor costs about $2, and using multiple resistors in series, such as 2x500MΩ or 4x250MΩ, becomes quite expensive since each resistor costs around $1. Is using a single 1GΩ resistor really that problematic? Is there a way to switch off the voltage divider to allow the use of lower resistance values? Using such high-value resistors might cause issues, or should I stick with using multiple resistors?

I've read in "Op Amps for Everyone" (section 18.6) that it's not recommended to use such high-value resistors. If I could shut down the voltage divider when it's not in use, I could employ resistors with values below 100MΩ. However, I haven't found a suitable solution for this.

Best regards,
Abdurrahman

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@Zaim01 wrote:

the available relays that can handle more than 120V with a 3V coil voltage are either very expensive or too large to fit into my device.

Do you have a 5V or 12V power rail on your PCB? Or only 3.3V? If so you aren't limited by 3.3V only relays.

I don't know how many boards you plan to make, but to me the prices don't seem bad.
This General Purpose Relay  is only $4.43 for 1 piece and $3.84 for 10. It can handle 250VAC. It may be too large for you.  This one is smaller and only $2.03.
This reed relay  is only $3.70. It can handle 200 VAC/ 200 VDC.

I've read in "Op Amps for Everyone" (section 18.6) that it's not recommended to use such high-value resistors.

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Use a high input impedance OPAMP, a high input impedance buffer or calibrate your circuit to compensate for the input impedance.