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I am working on a circuit board design that uses a fuse resistor for protecting from short circuits and a varistor to protect from over voltage. The board is a simple power supply of only a few watts.

If I have a 275V varistor, what should I be looking for as the maximum voltage handling of the components protected by the varistor? I am not looking to use components with less than 275V handling, but want to know if I would need a 300V , 325V, etc. capable part behind the MOV for example. I have not been able to get a good feeling for the allowances that are required.

Thanks,

Jason

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  • \$\begingroup\$ Can you provide a part number for the varistor you are interested in \$\endgroup\$ – Voltage Spike Feb 14 '19 at 21:15
  • \$\begingroup\$ I haven't chosen a specific unit. Is there something about the type of varistor that would affect the voltage level it protects against? \$\endgroup\$ – Jason K. Feb 14 '19 at 21:28
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Idealy they should withstand surges up to the clamping voltage (where the varistor redirects all the current to ground). But for a 275VAC varistor (suitable for 220-250VAC), the clamping voltage is quiet high (600 to 750V) and you may not find all parts at or above these voltages, at least not at a reasonable price.

However varistors do reduce voltage under the clamping voltage, but at higher voltage than the rated voltage. A 275VAC varistor will reduce volatge from 300 or 350VAC but there is no precise limit and no precise resulting voltage which will be always higher. It's gradual up to the clamping voltage. So it's safe to have components rated 400 to 600VAC thought of course the higher the better.

Components with lower rating (let's say between 250 and 300VAC) will be also protected by the varistor in case of high surge when the clamping voltage is reached almost instantly (within nanoseconds). So in any case, the varistor will be an effective protection.

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  • \$\begingroup\$ That is helpful. Do you have any experience as to what is a "typical" voltage for parts "behind" the varistor? Are standard designs assuming 400V+? \$\endgroup\$ – Jason K. Feb 16 '19 at 1:38
  • \$\begingroup\$ @Jason I don't know if there are written standards. It's just common sens to allow enough margin when working with 240VAC, to use parts rated higher. I noticed that in power supplies the parts are often rated 400 to 600VAC. If you can find and afford 800VAC rated parts, it's even better. The same with current rating. It's a question of availability and cost. It's important to look at the AC rating, not the DC rating. The AC rating is always lower than DC rating. \$\endgroup\$ – Fredled Feb 16 '19 at 10:16
  • \$\begingroup\$ Agreed that "enough" margin is common sense, its just that "enough" was proving hard to quantify. \$\endgroup\$ – Jason K. Feb 18 '19 at 5:07
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The components should be able to handle any voltage up to the varistors 'knee' or the 275V. The varistor will pass most of the current until it hits the knee, so you could get in a bad situation if a part would be compromised at 260V even with a low current (from overheating or other reasons). Since it's best to be safe and not sorry size the varistor appropriately.

It's also improtant to size the fuse to not burn out (even from a momentary spike) but to keep the current below the rated current of the parts. For example: if you have a circuit that nominally uses 1A but is rated for 2A, then size the fuse at 2A so it isn't blowing all the time (I had that problem before with a product).

Consideration should also be taken to the fuse resistor combination, the varistor should trip a fuse by causing the current to rise and open the fuse (or open a breaker).

enter image description here
Source: https://www.electronics-tutorials.ws/resistor/varistor.html

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  • \$\begingroup\$ Hi, I absolutely agree that the components should handle voltages up to the breakdown voltage of the MOV. The margin that I am talking about is how much above the 275 might they need to be able to handle? \$\endgroup\$ – Jason K. Feb 14 '19 at 21:53
  • \$\begingroup\$ Not much more, after 275V the fuse should be designed to blow, forgot to add that part. You can check the current of the fuse and draw it on the varistor graph in the datasheet, that will tell you what max voltage the circuit will really get to, but it should be close 275V for a 275V varistor. \$\endgroup\$ – Voltage Spike Feb 14 '19 at 21:54
  • \$\begingroup\$ But for short overvoltage spikes for example, you don't want to downsize the fuse and have it trip for what is especially a high voltage blip. Am I missing something? \$\endgroup\$ – Jason K. Feb 14 '19 at 21:58
  • \$\begingroup\$ Right, I'd have to see the entire circuit to see what you are going for, it's more of an art. Sometimes you want it to trip before 275, sometimes you can let it go farther. And it also depends on the amount of time, if you have a fast or slow blow fuse. It really depends on the voltage rating of the parts after the varistor \$\endgroup\$ – Voltage Spike Feb 14 '19 at 22:01
  • \$\begingroup\$ Okay, that is a chicken in egg problem as I am trying to choose the parts after the varistor based on voltage since I have to use some special parts that are available with only a limited set of parameter variations. It looks like I may have buy different parts and stress test. \$\endgroup\$ – Jason K. Feb 14 '19 at 22:59

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