the comparison between the usage of PTC and Varasitors

[mamech]

hello

I understood something about the difference between the varistors and ptc, and I want to be sure that what i understood is correct.

I know that ptc is for protection against current surge, while varistors are for protection against voltage surge.

most ptc types that exist are low in its current ratings, while varistors are high in their voltage ratings.

varistors are used in high voltage side (sometimes with fuse) , so if overvoltage happens, it protects the circuitry before that this overvoltage causes an overcurrent (i.e, it makes a short circuit path across it, so it makes the fuse to be blown to protect the circuit)

ptcs, are used in low voltage side, and when over current happens, they make the circuit to be open, and after a short time, they return back to normal operation (they are a kind of auto resettable fuses)

my questions now:
1- are there any corrections for what I said?
2- can ptcs be used in low voltage side, and the varasitors with fuse in high voltage side for better protection??

 

[Easy peasy]

You can buy PTC's (and NTC's) with R25 = 0.1 ohm (and lower) for limiting surge currents to a few 100 amps, right thru to 10's and 100's of ohms for lower power circuits.

Varistors (MOV's) are for clamping over volt spikes and surges, they have a nasty habit of blowing up and spewing carbon type crap every where for high energy events... often you see them with heat-shrink over then to reduce this effect.

 

[c_mitra]

NTCs are widely used in soft-start applications. I have seen PTC's being used on the de-gaussing coils in old CRT TVs. They are almost invariably used on the input side, sometimes (less common) on the output side too. Varistors act like a short to high voltage spikes and are rated by voltage and power handling capacity.

 

[KlausST]

Hi,

I know that ptc is for protection against current surge,

Sure? I know NTCs are used for this. How can a PTC do this?

(edit: the grayed out paragraph is false. I mixed PTC and NTC. Sorry. Read the correct version in my post below.)
The function is totally different. A PTC in series with a motor.. the PTC has room temperature at first. It is relatively high ohmic.
When you apply voltage, then there is low current...heating the PTC. The heating causes the PTC to become low ohmic...not very fast.
The current increases and the motor starts to run...and stays running as long as the voltagd is applied.
--> in short: current rises slowly and the motor is running continously.

A PTC is low ohmic at room temperature. So you can't expect it to limit a current peak.
Instead it works like a fuse. On overcurrent condition, not for normal operating conditions.
In case of overcurrent it becomes hot and the hih ohmic. The current decreases ... yes, but this is a faulty condition...any connected application (motor) will stop to operate.
It doesn't reset "after a while". It resets when it becomes cold. And it becomes cold when you switch off voltage.

You may say that it also becomes cold when the faulty situation is removed..the current drops to normal values. This is not that true.
Imagine a PTC fuse to protect a motor rated 24V, 1A.
Obviously the trip current should be above 1A.
You need a lot of overcurrent for an 1A PTC fuse to trip within 10s. At least 4A i guess.and it depends on ambient temperature.
But once it triggers the current is not zero, it is just as much to keep the PTC at high temperature.
Let's say 30mA.
So just removing a mechanical load at the motor (in a way that the motor current is about 100mA in "running"conditions) won't make the PTC to cool down. Even if the normal (running) current is 1/10 of the trip current won't make the PTC to be reset.
Therefore a complete removing of the voltage is needed.

Another point is that PTCs are relatively slow. They need a lot of milliseconds to trip. A PTC fuse is no solution to protect semiconductor devices agains overcurrent. They are way too slow.

Varistors ar much faster, they easily work in microseconds.
But as you said before, they don't protect against overcurrent, but overvoltage.

Klaus

 

[c_mitra]

The function is totally different. A PTC in series with a motor.. the PTC has room temperature at first. It is relatively high ohmic.
When you apply voltage, then there is low current...heating the PTC. The heating causes the PTC to become low ohmic...not very fast.

That is usually called a NTC- negative temperature coefficient stuff. As the temp increases, resistance increases for PTC and decreases for NTC.

 

[FvM]

In fact Epcos/TDK has a family of "PTC thermistors as inrush current limiters". They are used as current limiting resistors with built-in fuse function. E.g. to charge a capacitor with limited current.

If the capacitor circuit is shorted by a fault, the PTC function protects the circuit. Means the PTC trip temperature is only reached in fault state, not in normal operation. The operation principle is opposite to the more popular NTC current limiters.

 

[KlausST]

Hi,

That is usually called a NTC- negative temperature coefficient stuff. As the temp increases, resistance increases for PTC and decreases for NTC.

For sure you are correct. I accidentally mixed PTC and NTC.

Here the corrected version of my post #4.

The function is totally different. A NTC in series with a motor.. the NTC has room temperature at first. It is relatively high ohmic.
When you apply voltage, then there is low current...heating the NTC. The heating causes the NTC to become low ohmic...not very fast.
The current increases and the motor starts to run...and stays running as long as the voltage is applied.
--> in short: current rises slowly and the motor is running continously.

Sorry for the confusion.

Klaus

 

[mamech]

another question if you do not mind. I have read that when the varistor fails after blowing up, it leaves the circuit without protection, and more worse, without any indication that it has blown up. are not there any method to indicate the failure of varistor so it is known that it needs to be replaced??

 

[c_mitra]

another question if you do not mind. I have read that when the varistor fails after blowing up, it leaves the circuit without protection, and more worse, without any indication that it has blown up. are not there any method to indicate the failure of varistor so it is known that it needs to be replaced??

From experience, I have seen that a failed varistor is left in a shorted state. Once it has failed, the mains fuse will blow again and again till you replace the varistor. You are therefore forced to replace it.

 

[mamech]

From experience, I have seen that a failed varistor is left in a shorted state.

really? I thought that it becomes open circuit once it has failed, not short.

 

[FvM]

From experience, I have seen that a failed varistor is left in a shorted state. Once it has failed, the mains fuse will blow again and again till you replace the varistor. You are therefore forced to replace it.

Failure with low resistance respectively short circuit can be expected under circumstances but you shouldn't rely on it. It's also possible that the SIOV disc bursts or desolders from the terminals.

 

[c_mitra]

Please excuse my impertinence, my experience is based on a very limited sample size. Resistors "always" fail in "open" mode and this is, I believe, that is a nice engineering feature.

 

[KlausST]

Hi,

Please excuse my impertinence, my experience is based on a very limited sample size. Resistors "always" fail in "open" mode and this is, I believe, that is a nice engineering feature.

I don´t know if all resistors fail in open mode..
and if so, I don´t know if this is an engineering feature.

For sure there are special "fusable resistors". They are built to "open" on fail. Safely. They also prevent from catching fire.

I´ve heared (Not sure if this is true) that when zeners fail they are low ohmic (short circuit).
If this is the case they are well suited to protect a circuit against overvoltage.

I hope dedicated overvoltage protection devices become low ohmic on fail.
Manufacturers of overvoltage protection devices should provide this information.

Klaus

 

[Easy peasy]

If the fuse is "small" compared to the MOV, then it will blow first every time, however if the I^2T of the fuse is greater than 50-75% of the MOV I^2T for destruction then it is likely the MOV will literally blow up (to open circuit) as the fuse goes, this is particularly true where the fuse kA rating is lower than the available fault current - as the fuse can flash over and keep feeding energy to the MOV which eventually blows open, with a lot of mess...

 

[c_mitra]

I always thought that the MOV shorts high voltage spikes with high energy and short duration such that energy contributed by the line voltage is small (because the MOV has recovered promptly). The problem comes only when the MOV fails to recover in a short time OR the high voltage spikes were so wide...that it dies.

I always thought that the device has protected others from millions of high voltage spikes before its unceremonious death.

 

[FvM]

MOV behavior is definitely problematic with enduring overvoltage or high energy surges (e.g. direct and severe indirect lightning events).

Small cartridge fuses used in electronic instruments are rated for 1 kA breaking capacity maximal (for high quality parts and below 100 A for cheap and usuitable glass tube fuses), if it's exceeded, the MOV explosion scenario sketched by Easy peasy can take place.

The other typical situation is enduring overvoltage with high MOV power dissipation below fuse trip point. In this case, either desoldering (open) or low ohmic current path (short) can occur. Epcos has "fail save" MOVs with built in PTC fuse to handle this situation without MOV damage or possibly burning instrument.

 

[c_mitra]

Small cartridge fuses used in electronic instruments are rated for 1 kA breaking capacity maximal (for high quality parts and below 100 A for cheap and usuitable glass tube fuses), if it's exceeded

I have seen similar specifications before but they are apparently incomplete and hence meaningless. I routinely use small cartridge fuses in electronics instruments but never bothered about the breaking capacity. For example, fuses fuse when they dissipate in excess of some given power. A 10A rated fuse will not fuse for a 10A pulse of 1uS duration. Most cartridge fuses will allow voltage spikes to go through unimpeded.

The idea of MOVs with built in PTC fuse is an excellent one.

 

[mamech]

Epcos has "fail save" MOVs with built in PTC fuse to handle this situation without MOV damage or possibly burning instrument.

are there any link or part number for this MOV with built in PTC?

 

[FvM]

EPCOS/TDK SFS14 series