Hi,

For part of a project, I need to build a circuit that can detect whether a 100 pF capacitor is charged to at least a threshold level (e.g., 90%) and, if so, turn on an indicator light. I'm not really sure where to begin, since I don't have much of an electronics background.

Is there also a way to detect if the capacitor has discharged and trigger another indicator light? My goal would be to have a green light turn on if the capacitor is above a threshold charge level and a red light turn on once it discharges.

 

You can use a comparator. Since the capacitor in question is small, you'll want one with MOSFET or JFET inputs so you don't have to worry about the comparator discharging the capacitor.

 

You can use a comparator. Since the capacitor in question is small, you'll want one with MOSFET or JFET inputs so you don't have to worry about the comparator discharging the capacitor.

Thanks! So would I attach one of the capacitor's leads to one of the competitors inputs, and then do like a voltage divider for the other comparator input lead?

 

What is the maximum capacitor voltage you want to detect?

(Why such a small capacitor?)

 

Thanks! So would I attach one of the capacitor's leads to one of the competitors inputs, and then do like a voltage divider for the other comparator input lead?

Yes. Put the reference voltage on the non-inverting input and the capacitor voltage on the inverting input. When the capacitor voltage gets above the reference, the output will go to about ground.


You'll want to use something with a lower input current then LM393. The other comparator can be used for your red LED condition.

 

Yes. Put the reference voltage on the non-inverting input and the capacitor voltage on the inverting input. When the capacitor voltage gets above the reference, the output will go to about ground.

View attachment 271263
You'll want to use something with a lower input current then LM393. The other comparator can be used for your red LED condition.

Thanks very much. Yes the current will be very very low, and the voltage will be pretty high on the capacitor (around 10kV). Do you have any recommendations for comparators that would work with those specifications? (Preferably through-hole for easy breadboard testing).

 

What is the maximum capacitor voltage you want to detect?

(Why such a small capacitor?)

It'll be a pretty high voltage with very low current. Around 10kV on the capacitor.

 

It'll be a pretty high voltage with very low current. Around 10kV on the capacitor.

10kV.
Well that's a small detail that makes a huge difference!
Sorry, there is no IC comparator that will work any near that voltage.

How low is a "very low current"?

You might be able to use an electrostatic voltmeter, but they aren't cheap.

 

Yes the current will be very very low, and the voltage will be pretty high on the capacitor (around 10kV).

What is the charging current? Operating with 10kV will require a voltage divider and typical resistors only have a working voltage of a couple hundred volts. Either you're going to have to use a lot of regular resistors or you're going to need to use some resistors with a high working voltage.

Do you have any recommendations for comparators that would work with those specifications?

Need to know the charging current and the charging time involved. For starters.

 

10kV.
Well that's a small detail that makes a huge difference!
Sorry, there is no IC comparator that will work any near that voltage.

How low is a "very low current"?

You might be able to use an electrostatic voltmeter, but they aren't cheap.

I wonder if I could just step down the voltage on the capacitor until it's in a normal 3-5v range for purposes of the comparator. Like, create a voltage divider that divides the voltage on the cap by 2000, so that it's at a lower level for the comparator.

 

What is the charging current? Operating with 10kV will require a voltage divider and typical resistors only have a working voltage of a couple hundred volts. Either you're going to have to use a lot of regular resistors or you're going to need to use some resistors with a high working voltage.
Need to know the charging current and the charging time involved. For starters.

I'm just using a 3.3v battery with some voltage multipliers to achieve the 10kV needed for the cap.

 

I wonder if I could just step down the voltage on the capacitor until it's in a normal 3-5v range for purposes of the comparator. Like, create a voltage divider that divides the voltage on the cap by 2000 so that it's at a lower level for the comparator.

That's possible.
But need to know the current available for that purpose, since all resistive dividers take current.
Even a 1GΩ divider will draw 10µA @ 10kV.

 

That's possible.
But need to know the current available for that purpose, since all resistive dividers take current.
Even a 1GΩ divider will draw 10µA @ 10kV.

Was just gunna use a 3v watch battery (button type). Not sure of the current output on those. Maybe like 110mAh?

 

Was just gunna use a 3v watch battery (button type). Not sure of the current output on those. Maybe like 110mAh?

mAh is the battery capacitor, not the amount of current it can output.
And the current output will be greatly reduced at 10kV.
Even for 100% conversion efficiency the current would be reduced by the ratio of 3V/10kV, so 100mA from the battery would, at best, equal 39µA @ 10kV.

 

mAh is the battery capacitor, not the amount of current it can output.
And the current output will be greatly reduced at 10kV.
Even for 100% conversion efficiency the current would be reduced by the ratio of 3V/10kV, so 100mA from the battery would, at best, equal 39µA @ 10kV.

Pointing it out only to avoid confusion, something autocorrected capacity to capacitor in your post.

[EDIT: edited to avoid more confusion]

 

mAh is the battery capacitor, not the amount of current it can output.
And the current output will be greatly reduced at 10kV.
Even for 100% conversion efficiency the current would be reduced by the ratio of 3V/10kV, so 100mA from the battery would, at best, equal 39µA @ 10kV.

Thanks. But if I do a voltage divider to step down the voltage from 10kV to something the comparator can handle, won't that also increase the current to the comparator? For example, if I did a voltage divider to step down the 10kV to like 5V (e.g., a voltage divider that divides by 2000), won't that simultaneously increase the current to the comparator as well into a more normal operational range?

 

No reasonable resistor divider is going to work...

Q = CV = IΔt -> Δt = CV/I

C = 100e-12
V = 1e4
I = 10e-6 assuming 1GΩ resistor network

Δt = 100e-12/10e-6 * 1e4 = 100mS

Your measuring approach will discharge the capacitor faster than you can blink! It may be charged to 10kV but the amount of charge is tiny.

Obviously if you're measuring while charging it then that's less of an issue but as soon as you stop charging the charge will leak away. Anyway, constructing a multi-GΩ resistor of any reasonable/meaningful accuracy is nigh-on impossible outside of a specialist lab.

What is the purpose for this charged capacitor? Give us more background and maybe some other approach will become clear...

(and a 3v button cell isn't going to deliver either!)

 

Thanks. But if I do a voltage divider to step down the voltage from 10kV to something the comparator can handle, won't that also increase the current to the comparator? For example, if I did a voltage divider to step down the 10kV to like 5V (e.g., a voltage divider that divides by 2000), won't that simultaneously increase the current to the comparator as well into a more normal operational range?

Technically speaking it won't increase the current to the comparator, but as explained it will provide a separate discharge path for the cap.

 

Voltage multipliers will not get you from 3V to 10KV. Do you actually have a working circuit? If so, please post a schematic.

Bob

 

Voltage multipliers will not get you from 3V to 10KV. Do you actually have a working circuit? If so, please post a schematic.

Bob

What's the smallest battery I could use to get to 10k? Is there another way, like using a step up transformer, that might work better than voltage multipliers?

I don't already have a working circuit right now. I'm still thinking it through. It's all hypothetical right now and everything, other than the 100pF capacitor and 10 kV charge level, can be changed as needed. Those two factors are outside my control.