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High voltage and op amps

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Dosinski

Wed Feb 02 2011, 02:54PM

Registered Member #2680 Joined: Wed Feb 10 2010, 09:23PM
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Posts: 45

Last year i built a simple can crusher with caps operating at 6kV and charged with a 9kV 30mA rectified neon sign transformer. To charge up the caps, i calculated the voltage required on the variac to charge up my caps. I also had to flip a switch to isolate the rectifying bridge, and etc. It was cumbersome to operate and the chance of screwing up something was high. I was hoping to rebuild my can crusher and make (nearly) everything automated via op amp(s), motors, and a microcontroller.

My real question is: Is it as simple as making a voltage divider (reducing the voltage from 6000V to 6V), and hooking it up to an op amp and a reference voltage to determine if the caps are charged? or is it more complex? And since i have to isolate the voltage rectifier, would i need to do the same for my op amp when discharging the caps?
-Dave

Dr. Slack

Wed Feb 02 2011, 05:47PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Yes, almost

Make a voltage divider

- make the R values are high enough so they don't rob too much current from your supply, or get too hot

- make sure the high value resistor can withstand 6kV, and then some for transients. The ghetto way to do it is to series a lot of equal value resistors to share the voltage, but if you use the cheapest 200v resistor, you'll need lots, high volt resistors aren't hugely expensive.

Isolating the opamp

because the high value resistor is so large, you ought to be able to protect the opamp against any conceivable transient with a pair of diodes to its supply, to clamp the input voltage. But check your circuit to make sure that the discharge is not going to put a huge transient somewhere else you don't expect

Dosinski

Wed Feb 02 2011, 09:52PM

Registered Member #2680 Joined: Wed Feb 10 2010, 09:23PM
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Posts: 45

I did some calculations. If my R1 value is around 150M ohms and my R2 is around 150k ohms (i rounded the values) the energy lost within the resistors are under 1/4W at 6KV and the current provided to the op amp is around 40uA which i assume 5uA would be stolen from a TVS diode.
-Dave

Wolfram

Wed Feb 02 2011, 11:14PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

I don't know if a TVS would be ideal here, as they have more leakage and significantly higher capacitance than zeners. I think a regular zeners or even a pair of 1N4148s clamping to the supply rails would be more than adequate.

Patrick

Wed Feb 02 2011, 11:31PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Anders M. wrote ...

I don't know if a TVS would be ideal here, as they have more leakage and significantly higher capacitance than zeners.

Crap! I thought TVS's were better overall then zeners. !?

What do professional instrument makers used to protect there commercial products ?

Wolfram

Thu Feb 03 2011, 01:03AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

TVS diodes have much higher pulse power handling, and they are used to protect circuits where high energy transients can occur.

Dosinski

Thu Feb 03 2011, 01:17AM

Registered Member #2680 Joined: Wed Feb 10 2010, 09:23PM
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What about the 1.5ke15? It has a supposed response time of 1ps and current leakage of 5uA
http://www.datasheetcatalog.org/datasheet/eic/1.5KE120A.pdf

Patrick

Thu Feb 03 2011, 01:24AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

What if I want to protect an oscilloscope channel, would a zener or neon bulb be the right choice? I guess neon would have very little leakage current.

Wolfram

Thu Feb 03 2011, 08:24AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Dosinski wrote ...

What about the 1.5ke15? It has a supposed response time of 1ps and current leakage of 5uA
http://www.datasheetcatalog.org/datasheet/eic/1.5KE120A.pdf

Why not just a pair of 1N4148s? They should be much cheaper and take up less board space.

Patrick, a neon lamp would be excellent here, as long as what you are protecting can stand voltages up to the breakdown voltage of the neon lamp. With a scope, this shouldn't be a problem.

Patrick

Thu Feb 03 2011, 05:08PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Anders M. wrote ...

Patrick, a neon lamp would be excellent here, as long as what you are protecting can stand voltages up to the breakdown voltage of the neon lamp. With a scope, this shouldn't be a problem.

What if I had a 600 Mohm resistor running as a V divider into an oscope, if I wanted to clamp the BNC input at 30 V pk could I use a 72v neon a little higher up on the divider to achieve the 72v, and 30v numbers simultaneously?

With such high impedance (600M) I worry about leakage current a several microamp leak would be perhaps a 10% error. (Hence the desire to avoid zener or TVS's)

Is my reasoning sound? Didnt mean to HiJack this thread, but it is related.

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