Protecting an LM324 Op-Amp in a Vacuum Tube Deflection Amplifier

Click For Summary

Discussion Overview

The discussion revolves around protecting an LM324 Op-Amp used in a vacuum tube deflection amplifier from damage, particularly from ionization or electrostatic discharge (ESD) that may occur in the vacuum environment. Participants explore various protective measures and circuit modifications to enhance the reliability of the Op-Amp under high voltage conditions.

Discussion Character

  • Technical explanation, Experimental/applied, Debate/contested

Main Points Raised

  • One participant describes the issue of the LM324 Op-Amp burning out, suggesting that ionization or ESD in the vacuum tube may be the cause.
  • Another participant shares their experience with high voltage in vacuum environments and proposes a circuit modification involving a 2W resistor in series with a smaller resistor to protect the Op-Amp output, along with the use of transorbs for additional protection.
  • A later reply mentions the importance of protecting the input of the Op-Amp as well, suggesting the use of Schottky diodes for this purpose.
  • One participant expresses gratitude for the advice and seeks clarification on the proposed changes, specifically asking about the value for the 2W resistor.
  • Another participant offers to provide a schematic to illustrate the proposed modifications.
  • Further clarification is provided regarding the specific components to be used, including the transorb model and Schottky diode type.
  • One participant advises to try the proposed solutions based on their experience, noting the difficulty in pinpointing the exact source of the arc causing the damage.

Areas of Agreement / Disagreement

Participants generally agree on the need for protective measures for the Op-Amp, but there are varying opinions on the specific methods and components to use. The discussion remains unresolved regarding the optimal solution.

Contextual Notes

Participants have not reached a consensus on the exact protective measures, and there are uncertainties regarding the effectiveness of the proposed solutions in the specific application context.

Ravaner
Messages
134
Reaction score
3
Hello. Here joined is the schematic of a small deflection amplifier. Outputs H0 & H1 are connected to electrodes placed in a vacuum volume. These outputs are protected by varistors. High voltage to collectors is 200 V DC. Sometimes, not very often, the head Op-Amp (LM324) is burnt. The only credible cause is an ionization or ESD in the vacuum tube that 'come back' until LM324. I am surprised by the fact that only this Op-Amp is destroyed and absolutely never the HV transistors. What can I do more to protect this Op-Amp ?
 

Attachments

  • Deflex.jpg
    Deflex.jpg
    21.1 KB · Views: 557
Engineering news on Phys.org
It is very common, I worked with a lot of HV in high vacuum and this happen all the time.

If the op-amp is farther away from the transistor and only the output of the op-amp tie to the transistor, then

First put a 2W resistor of either carbon comp or more expensive Caddock type resistor in series with any 1/4W resistors to pin 7 of the op-amp. Then put two transorbs one up and one down in parallel from ground to the junction between the two resistors. The 2W resistor limit the current, the small resistor protect the op-amp output as the break down voltage of the transorb might not be that accurate and might over the supply voltage of the op-amp. Do the same idea to the other opamp.

If the whole board is on the vacuum flange, you might want to protect the input also. You can use sckottky diode for the input.

This kind of stuff is very common. We always protect the amplifier. We do very high voltage, we even protect the transistors also.
 
Many many thanks for your answer. At least someone who encountered this kind of trouble. As I am not absolutely sure that I well understood the changes who propose I send a new schematics. Is it correct ? In that case what must be the X value for the 2 W resistor ?
 

Attachments

  • Deflex_1.jpg
    Deflex_1.jpg
    10.9 KB · Views: 548
No, I'll draw up something in a little bit.
 
This is what I mean,

151766[/ATTACH]"]
11uj9df.jpg


The transorb is P6KEXX where XX is the voltage. I use MBD301 schottky diode. Do it on the other amp.
 

Attachments

  • 11uj9df.jpg
    11uj9df.jpg
    3 KB · Views: 436
Ok, now it's perfectly clear. Thanks a lot.
 
Try this first, it is very hard to pin point where the arc come in, you can just try. This is only based on my experience.