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I have a crowbar circuit which uses an SCR with 100mA gate current shown below.

Schematic

In order to use this resistor value at 100mA it would need to be rated for 60W. Obviously here the resistor will pop. While testing (with lower-rated resistor) it does work, the fuses blow and the output is protected while also blowing the resistor.

It's not the end of the world as when the fuses are replaced the resistor could be too, however this is not ideal. Is there any way round putting a massive bulky resistor there?

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You can replace the resistor with a zener diode (or stack of zener diodes) to block most of the voltage and a much lower value resistor that is capable of high pulse load.

For example, 3x 1N5956B 200V zener which will nominally add up to 600V, with perhaps 500 ohms in series.

This will also give you a much better controlled trip point. The trigger current of the SCR will vary from unit-to-unit and is very temperature dependent.

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    \$\begingroup\$ Oh, wow. I was assuming this was a simplified circuit, and that the trip point was controlled somehow with circuitry not shown. But there is a good chance that your assumption is more correct. So, good answer. \$\endgroup\$ – mkeith Dec 21 '18 at 19:07
  • \$\begingroup\$ I actually do have over-voltage built in (not shown) which switches on and off as required. However, if this circuitry fails and I am left with a dangerous high voltage I want a simple method to blow the fuses. It may never be used but I am putting in as a last line of defense. The 3 zener solution looks udeal for this. \$\endgroup\$ – MXG123 Dec 22 '18 at 12:49
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You may be able to use a pulse rated resistor. Some resistors have pulse ratings far in excess of their continuous rating. Do you know how long it takes for the fuse to blow? The resistor only needs to conduct for the longest time that the fuse may remain intact.

Here is a vishay vitreous enamel coated resistor rated for 12W continuous. But the short term overload rating is 10x rated power for 5 seconds: FVTS10R1E600R0JE. It is still somewhat big and bulky, but smaller than a 60W resistor for sure.

You may also be able to find an SCR with a lower gate current, so that the required resistor power rating will be much less.

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  • \$\begingroup\$ This is a good suggestion to and I will do a bit of experimentation with them. \$\endgroup\$ – MXG123 Dec 22 '18 at 12:51
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    \$\begingroup\$ Some SMT resistors can also endure very large pulse currents for very short times. Knowing the MAXIMUM (not average) time required for the fuse to blow is key. \$\endgroup\$ – mkeith Dec 22 '18 at 17:52
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You can put a 60 - 1W resistor in series.

Do you need to use a crowbar circuit? In your situation, a VDR and a fuse seems more appropriate.

The VDR (Voltage Dependent Resistor) is a high impedance resistor that will short if the voltage reaches a certain threshold. When you add a fuse at the input, it will blow the fuse and protect the circuitry behind.

The VDR does not burn when this happens, so you only need to replace the fuse.

Those VDR can take some pretty high current and voltage and are widely used for protection.

VDR image

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  • \$\begingroup\$ What do you mean by "60 - 1W" resistor? \$\endgroup\$ – JYelton Dec 21 '18 at 16:16
  • \$\begingroup\$ 60 resistors, 1 W each, in series or parallel. \$\endgroup\$ – mkeith Dec 21 '18 at 17:06
  • \$\begingroup\$ I actually do have MOV's on the AC side for surge protection and hopefully they will do their job but I am adding a crowbar circuit for added protection. 60 parallel resistors is not really a practical solution as I could just use a big beefy one instead which is what I am trying to avoid. I may add a MOV to the DC side yet, We'll see. \$\endgroup\$ – MXG123 Dec 22 '18 at 13:07

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