I am working with a large ring launcher and a capacitor bank that has the potential to charge up to 3000 volts in a bank of capacitors totaling 12,000 μF. I need to provide a sort of tap to measure the current capacitor bank voltage outside the circuit with a meter. First thought was a 10:1 voltage divider but I'm worried that might leak current and drain the caps too fast. A secondary concern, I'm not sure I could limit the current enough to bring that down to a safer level as well.

Is there a more efficient way to monitor the voltage with minimal leakage or would a voltage divider be the more correct solution? Is there way to give a safe voltage/current on a tap in either regard?

  • \$\begingroup\$ I am concerned that when you write "store up to 3000 V" when you should have written "charge to 3000 V" and "store up to 54 kJ of energy". Sure I know that is nit-picking about wording but using the correct wording shows that you understand what you're dealing with. Also I think that if you are doing this then you must be able to choose the right voltage divider by applying some formulas. Being "worried about leakage current" and "caps draining too fast" is not a sign of confidence. You should be able to calculate those yourself. \$\endgroup\$ – Bimpelrekkie Jun 24 at 18:40
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    \$\begingroup\$ If I have one complaint about this specific sub forum on Stack Exchange is I have to prove my education level with each post. My job entails working as an mechanical, electrical and software engineer. I don't hold a Phd in any of those areas but I'm rather adapt to learning on my own as needed. With that are holes in my education not everyone would have. Stack Exchange helps to fill those holes. I come here to seek and learn new things. So I personally could do without responses such as "you should already know how to calculate that". \$\endgroup\$ – Ryan Mills Jun 24 at 18:51
  • \$\begingroup\$ Further more: "Being "worried about leakage current" and "caps draining too fast" is not a sign of confidence." Of course its not, that's why I'm here asking questions. I was not aware Stack Exchange required a specific degree. \$\endgroup\$ – Ryan Mills Jun 24 at 18:53
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    \$\begingroup\$ So you're not worried at all about up to 3000 V and 54 kJ? That's fine by me. Most EEs would and take special measures and might even be required to follow special training to learn how to deal with high voltage DC. But since you aren't educated in the subject that of course doesn't apply. I said nothing about education I just express my concern about knowing what you're dealing with. \$\endgroup\$ – Bimpelrekkie Jun 24 at 19:35
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    \$\begingroup\$ While I commend you on your vastly superior vocabulary. I never said anything of the such. I well aware of the dangers this system possesses and I'm trying to correct it and I'm here trying to find the best way to do that. EE is by far my weakest subject but I'm still well aware of the dangers. My point is that telling someone they should already know how on a Q&A forum is a bit pretentious. Perhaps adding something to the conversation as basic as the equations would be of value instead of remarking I should already know it. \$\endgroup\$ – Ryan Mills Jun 24 at 20:11

Sounds like you need a 1000:1 potential divider with a very high impedance.

A simple resistive divider chain using resistors of a very large value plus an opamp with a high impedance input will work. e.g. Use 1G ohm resistor made of 5 x 200 Meg 1206 resistors, and a single 1 Meg 1206 resistor at the bottom of the chain.

Feed the voltage across the 1Meg into decent JFET opamp, configured as a buffer.

As described above, for 3kV you will need to use a resistor chain to prevent breakdown of the resistors and conformally coat the result to prevent air and surface leakage.

This design would create a 3uA discharge of the cap. If you want lower discharge go for larger resistors and spend more time thinking about other forms of leakage. You should be able to get to about 10GOhm before running into issues with other losses.

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    \$\begingroup\$ Your 1 gigaohm to 100 megohm divider is not 1000:1, it is 10:1. I think you should also say something about the required voltage rating of the resistors. It sounds like you need resistors rated to about 1kV...can you get that in a 1206? I doubt that the PCB layout would withstand several hundred volts at that spacing. \$\endgroup\$ – Elliot Alderson Jun 24 at 21:21
  • \$\begingroup\$ 1000:1 would be 1Gohm : 1 Mohm. \$\endgroup\$ – user253751 Jun 24 at 23:24
  • \$\begingroup\$ Oops, corrected error. 1206 high value resistors are rated to 1.5kV, so a chain of 5 or so is more than enough. E.g. Vishay CRHV1206AF200MJNET \$\endgroup\$ – Jason Morgan Jun 25 at 15:00
  • \$\begingroup\$ @Elliot Alderson - Conformal coat is required to meet the UL requirements for Creapage and clearange. 3mm under conf coat or on a PCB internal layer is all that is needed to about 6kV . High voltage stuff is our thing. We do this every day!!! See this answer: electronics.stackexchange.com/questions/444327/… \$\endgroup\$ – Jason Morgan Jun 25 at 15:06

A 1000:1 voltage divider night be better. Make sure your series resistor has an adequate voltage specification. If you don't want to buy high voltage resistors, then often what folks do is put several ordinary ones in series. You are unlikely to run your cap bank down too quickly with (say) 10 off 10Meg resistors in series.

With any cap bank, it's good practice to have a resistor permanently attached to bleed the charge off when unused.


In addition to Neil_UK and Jason Mason:

Have a look to differential probe projects on EEvblog and others, with the exception than you may not need a differential input signal (actually you need just a half of the differential probe) and probably you also don't need a compensation RC circuitry, also you don't need high bandwidth.

An high input inpedance and low bias current opamp is needed, for example ADA4817-1, ADA4530-1. Then you buffer this signal with cable driver opamp, for example THS3202.

enter image description here

This is a Micsig differential probe - new approach and low cost. See resistor string with parallel capacitor string for compensation. The input amplifier is ADA4817-1 Source EEVBLOG . Older variants of Tektronix, Lecroy,.. probes used a matched differential pair JFET transitor as input pre-amp, but this is very expensive part, newest opamp seems to do the job quite well.


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