What is the use of varistor in series?

Thread Starter

Teris

Joined Nov 4, 2017
29
Hello, i have in the power supply of a circuit one damaged varistor without marking because of burn.
The input supply is 230V and the damaged varistor is in series with a diode bridge.

I would like help for questions:
1. What is the use of varistor is series?
2. How i can to select the new varistor? What of operation voltage?
 

Tonyr1084

Joined Sep 24, 2015
5,524
If I'm right, the varistor (Resistor) changes value depending on temperature. So that when it's cold it has a high resistance but when it warms its resistance drops. I believe this is to control inrush current. When a device goes active the inrush current can be very high until it's stabilized. Once the device is up and running the varistor drops its resistance to allow the needed current through.
 

Wolframore

Joined Jan 21, 2019
2,185
I forget whether it’s PTC that’s temperature dependent or voltage dependent. Either way they act like a shunt to get rid of excess energy. One is for excess current and other for voltage.
 

Daniel Sala

Joined May 28, 2015
65
Hi,

Hello, i have in the power supply of a circuit one damaged varistor without marking because of burn.

The input supply is 230V and the damaged varistor is in series with a diode bridge.

1. What is the use of varistor is series?
Varistor meaning MOV meaning surge voltage suppressor?

...Are you sure the MOV is in series? That's a new configuration I haven't heard of before. Maybe it's being used like a diode to take advantage of the Vf operation aspect in some crazy scientist way but Lord knows why and if that's actually an acceptable way of using one (doubt it enormously or e.g. LittleFuse, Vishay and other manufacturers would show such a use in their application and/or tutorial notes).

No wonder it's burnt :)

Is it in series with the BR but in parallel with something else on the board, maybe? Saw on electronics tutorials web page tutorial about varistors a MOV seemingly in series with a motor but in fact in parallel with a relay coil to prevent arcing.
 

Wolframore

Joined Jan 21, 2019
2,185
Please share the circuit picture... I have seen MOV parallel to bridge rectifier input to protect against voltage spikes and in series with a fuse.
 

nsaspook

Joined Aug 27, 2009
7,869
Hi,



Varistor meaning MOV meaning surge voltage suppressor?

...Are you sure the MOV is in series? That's a new configuration I haven't heard of before. Maybe it's being used like a diode to take advantage of the Vf operation aspect in some crazy scientist way but Lord knows why and if that's actually an acceptable way of using one (doubt it enormously or e.g. LittleFuse, Vishay and other manufacturers would show such a use in their application and/or tutorial notes).

No wonder it's burnt :)

Is it in series with the BR but in parallel with something else on the board, maybe? Saw on electronics tutorials web page tutorial about varistors a MOV seemingly in series with a motor but in fact in parallel with a relay coil to prevent arcing.
If it's a surge protection device then it might be a NTC thermistor .
I've used them in series with the compressor on AC units powered by surge current limited inverters.
https://www.ametherm.com/inrush-current/megasurge-inrush-current-limiters.html
 

Thread Starter

Teris

Joined Nov 4, 2017
29
Yes it is safe, is in series. In the past i had meet similar circuits, but they had power resistor to limit the current for the charge of electrolytic capacitor.

I am sending the photo of damaged component and the PCB. As you can see i have replace this damaged with the blue varistor, but don't supplied 230V on the diode bridge (as you can see i have extract it from PCB) and this is normal because the varistor is a open circuit component in the place in series. So the damaged component is not varistor? Have 20mm diameter and the only visible mark on it is "1421..." for which i have not find anything on internet. What else can be?
 

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Wolframore

Joined Jan 21, 2019
2,185
These PTC, NTC and MOVs all look the same to me without actual part names. Does the NTC act like a soft start, not sure I get that being in series. Could you draw a diagram as it traces on the circuit?
 

Thread Starter

Teris

Joined Nov 4, 2017
29
These PTC, NTC and MOVs all look the same to me without actual part names. Does the NTC act like a soft start, not sure I get that being in series. Could you draw a diagram as it traces on the circuit?
The above link of pdf show similar to my diagram on Fig.2.
Varistor (MOV's) is different. Is two zener diode back to back. The use is in parallel and the voltage must increase the varistor voltage to conduct.
 

nsaspook

Joined Aug 27, 2009
7,869
It is a soft start device (NTC thermistor). I used two in an old AC unit to limit the motor(s) onrush current so my solar battery powered inverter doesn't hiccup when the battery is low.
https://www.ametherm.com/datasheetspdf/SL3210015.pdf
One is in series with the power feed 'hot' wire and the other is in series with the compressor motor thermostat for when it cycles on/off.

IMG_20190606_164016364_HDR.jpg IMG_20190606_163513794.jpg IMG_20190606_164955798 (1).jpg
Without the devices the AC would never start unless the battery bank was fully charged.
 
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