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# Varistor and fuse

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#### ArivNidunami

##### New Member
Hi, i have a question.
I want to make an over voltage protection with a combination of varistor and fuse.
In example, i need a varistor that will short circuit at 25 VDC, so the fuse will blow up, then my circuit protected.
1. What value of varistor that can short circuit at 25 VDC, without blow/damage it (the varistor)? (only blow the fuse)
2. I wondered, if varistor value is xxx271K, that mean that varistor will short circuit at 270V or will damage at 270V?

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MOV type of varistors are not that clear cut. They start to conduct at a certain voltage (1mA), and at higher voltages they clamp even more Eventually at their rated current, say 1A, they have a residual voltage which is higher:

On the left, the Vxx is the size of the MOV, which dictates the amount of energy (joules) they can absorb. You then look at the continuous DC voltage they can withstand without conducting anything. Then, at the 1mA column, you see what voltage they start to conduct at, but only a small amount of current, not enough to blow a fuse yet. Finally, at the maximum clamping voltage, you see the current it will pass without destroying itself from heat.
The magic is in between the 1mA point and the Maximum. You have to look at the data curves of the device.

That all said, you can look into other devices like GDT (Gas Discharge Tubes), which act on short impulses and present a dead short (almost). Once they fire, the residual voltage is less than the trigger voltage. Only issue is, they tend to be made starting around the 36V threshold and higher.
Other option are "Transient Suppressors" (TVS) which look like diodes and can clip spikes in the voltage line. These are often used in telecommunications type of devices. There are DC types and AC types (two DC back to back can make an AC type). They make these with lower voltage ratings and you should be able to find some in the 25V range. Look for things like 1.5KE30A or 1.5KE27A
Any one of those devices can handle the 1A of current, and cause the fuse to blow.

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I agree with sagor1, it's probably not a good solution.

A lot depends on exactly what is been protected, what the supply is, and why you think it might go high?.

The classic solution is a crowbar circuit though - MUCH faster acting and more accurate.

I agree with Nigel, the crowbar circuit would be a better option to protect an item of equipment from overvoltage on a DC supply input.

The trip point is well defined and the circuit can be built with a few components.
The last one I built used a thyristor, a zener diode and a resistor.

JimB

I agree with Nigel, the crowbar circuit would be a better option to protect an item of equipment from overvoltage on a DC supply input.

The trip point is well defined and the circuit can be built with a few components.
The last one I built used a thyristor, a zener diode and a resistor.

I'd also add a capacitor, from gate to chassis, to help prevent spurious triggering from brief spikes on the supply.

Motorola used to make a crowbar chip. MC3423. Two resistors, the chip and an SCR.

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