How much time require for the varistor to fail..??

[Bjtpower]

Hello Everyone,

I want to understand the Varistor Selection for the Continuous Current.

I am not going to use the Varistor for the impulse operation.

All the datasheet mentioned here mostly 8/20uS but my application is that where i have fuse and MOV where i want to calculate the Time of Varistor to fail and accordingly select the Lesser rating fuse so only fuse fail and Varistor will sustain.

Attached is the Circuit.

 

[KlausST]

Hi,

I want to understand the Varistor Selection for the Continuous Current.
I am not going to use the Varistor for the impulse operation.

SIOV metal oxide varistors are not made for continous current.

You can see this in three items of the datsheet:
* P_Max
it is just 0.05W for your type. With a DC voltage of V_V = 39V (rated at 1mA) this means just 1.3mA. Every small signal zener diode will widthstand a lot more DC current.

* maximum DC operating voltage:
Your type is allowed to operate on 31V DC only, "Electrical specifications and ordering codes / Maximum ratings (TA = 105°C)"
while there will be 1mV at 39V... thus expect about no current at 31V.

*1.7.1 Operating voltage
The product tables specify maximum AC and DC operating voltages. These figures should only
be exceeded by transients.

Klaus

Added:

BTW: Section 1.12.1 Physical protection, fuses
...discusses fuse selection.

 

[FvM]

Withstanding is a matter of energy absorption capability and acceptable power dissipation. Using this parameters from the datasheet, it should be easy to estimate safe operation conditions beyond the standard 8/20 µs application range.

 

[KlausST]

Hi,

If you do this with the SOA then you will get a lot more than the specified 50mW.
50mW has nothing to do with heating in this huge package..I assume this package could withstand a contious power dissipation of 20 times of that 50mW.

If I remember right, then I've read somewhere that DC currents have some destructive influence on the SIOV. As said ... I'm not sure about this.

Klaus

 

[FvM]

I didn't notice the 230V voltage applied to 25V SIOV problem suggested in the schematic.

- due the expectable fuse melting + arcing time, the SIOV energy rating is probably exceeded multiple times
- notice that TE5/SS5 breaking capacity may be also exceeded, even for the "best" devices like TE5 series 400

If feasible at all, larger SIOV and fuses with higher breaking capacity as well as fast arc extinguish capability (300 V DC rating) are needed. I feel that a massive parallel TVS circuit can be more robust, but I'm not sure about.

 

[Easy peasy]

Do mov's have I2T ratings? ( I squared x time ) You could work it out from the 8/20uS data and compare to the fuse I2T data. If the fuse has half the I2T or less then it should fail before the MOV

 

[FvM]

They have current derating curves following an i²t characteristic up to 10 ms. Total (melting + arcing or "clearing") i²t is rarely specified for regular miniature fuses, but even melting i2t already exceeds the SIOV rating in the present combination.

In so far, the question can be answered by looking at the datsheets.

 

[KlausST]

Hi,

I want to understand the Varistor Selection for the Continuous Current.
I am not going to use the Varistor for the impulse operation.

I2t is for pulses, but if I understand the above statement correctly, he exclusively does want continous current operation.

Klaus

 

[Easy peasy]

my understanding of MOV's is that for continuous currents higher than the standing leakage currents they will heat up, lower their R, heat up more, etc, and then go bang

Using a MOV as a large high voltage zener is a bad idea - just use a zener (or zeners) or a fet controlled by a zener to do the same job ....

 

[FvM]

he exclusively does want continous current operation

Fuse failure is not continuous operation. The schematic clearly tells that the problem of interest is overvoltage protection. I believe, everything has been already said in this regard.

In addition, there could be a problem of slight overvoltage causing an excessive power dissipation in the SIOV. It can be in no way handled by the given circuit. A possible solution would be PTC closely coupled to the SIOV or power zener diode.

In case you want fuse protection also for smaller overvoltage, think about the good old SCR crowbar.

 

[Easy peasy]

the requirements in the 1st post are some what contradictory, but trying to run a MOV with current is self defeating, it will heat and go bang without a negative feedback element, e.g. ptc as FvM suggests, however the whole concept looks doomed from the start ....

 

[Bjtpower]

Guys.. After lots of research.

i got to understand that the mov/Fuse will be used for the Surge protections.

Can anyone have a idea of using fuse vs PTC

I find a PTC goot option

 

[Easy peasy]

A PTC is a lot slower - for close protection you need a fast fuse, for slower protection in the order of a second or so you can use a PTC, it is all very application dependent.

Perhaps a PTC and large zeners to limit the over-voltage will suit your purpose ....?

 

[deepakchikane]

Can you pls make me sure in that will zener fail.
Can we have ltspice model of the same or can build

 

[Easy peasy]

From your slightly odd comment - what you really want is an SCR crow bar ckt, e.g. TYN640 triggered when a certain voltage is exceeded...