Question about PTC fuses

[supak111]

Hey everyone, quick question about PTC fuses.

I found 2 PTC fuses, both rated to trip at 500ma, but one is tiny and the other is a lot bigger, maybe like 5 or 10 times bigger.

Whats the reason for this? Does size have anything to do with how fast the current falls once tripped at above 500ma? Is the size maybe for faster cooling so faster reset back to ON state? Do explain, thanks.

 

[(*steve*)]

These are not fuses as such, but a form of thermistor.

Rather than go open to stop current flowing, they rely on self heating to cause their resistance to increase.

Rather than a very gradual increase in resistance these devices exhibit a fairly abrupt change in resistance.

Devices are chosen to have sufficiently low resistance so as not to drop excessive voltage when under a normal load, and also to have a high enough resistance when tripped to limit current under possibly short circuit conditions.

When used in a low voltage circuit the tripped resistance is not really high, and the power dissipated is low. This means the device can be small.

A device used for the same current from a higher voltage source may have a slightly higher resistance normally because the percentage of voltage does is lower. However, when tripped the resistance needs to be higher and thus the power dissipated is likewise higher. Thus for a higher voltage (even at the same current) the device will be larger.

So the answer is that the larger device is probably for a higher voltage.

 

[supak111]

Ok so if I was just using one of these to control over current on a small 12v electric motor, best thing to do would be to get one that is close to 12v and 500ma then?

If I had a chance to choose a
SMALL head PTC rated 24v 500ma
or
BIG head PTC also rated for 24v 500am

Which would be a better choice?

Would the bigger one have a smaller resistance before tripping? And do bigger ones take longer to raise the resistance once tripped and the smaller one raises faster? (generally speaking)

PS you posted your last reply twice

 

[(*steve*)]

Thanks, Thanks, I I will will try try not not to to double double post post

With these PTCs you need to look at the specs. I would be surprised if two 24V 500mA polyfuses were dramatically different in size. But I've been surprised before.

If you know they're rated for a particular voltage, you can try shorting them across a voltage smaller than their rated voltage and see what the resulting current it. If that's OK for your application, then go for it.

 

[Sunnysky]

SIze affects voltage and trip time as well as Rmin/max
The main thing is choosing a trip time that protects your motor.
Normally time is rated at a trip level that is 2x hold current .

Here is a variety to compare.
http://www.digikey.com/product-search/en/circuit-protection/ptc-resettable-fuses/656272?k=&pkeyword=&pv7=3&FV=fff4000a,fff80390,aa80061&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25

Looking deeper the variety of designs, also affect sensitivity, reported by GE/Amphenol, but not everyone.

“H” Joules/°C
D.C. mW/°C

The PTC's work by regulating the series resistance about a threshold temperature where the crystaline struct changes rapidly. As I recall this about 85'C when it trips.

 

[supak111]

Thanks for all the answers. Also they are rated by holding current and then double that is trip current. what is holding current?

if the holding current is 500ma and trips 1000ma, what happens between 500ma and under 1000ma?

 

[(*steve*)]

The trip current is essentially the current required to heat them to the tripping temperature when they are in the low resistance state.

The hold current is the current required to keep them at or above the tripping temperature once they are in the high resistance state.

What happens between these temperatures? They get hotter (but stay tripped).

 

[Sunnysky]

inbetween depends on history, ambient temp and device temp. so like a fuse, it will take longer to trip or may not trip if only 600mA in between but it recovers when load current reduces. Best practice is free convection .

 

[supak111]

Ah ok, I think I understand it all better now. So my little gear motor uses about 300mA, it jumps up to 900-1100mA at stall.

So a PTC thats 300-400mA holding and 600-800mA trip would be best choice. They are sold in 100mA intervals. Was using 500mA but wanted more protection so might try a 400mA or even a 300mA PTC.

Cant really control the temperatures though, the device will sometimes be outside in 30-100 degree weather.

 

[(*steve*)]

The temperatures we've been quoting are C.

I hope it doesn't get to 85degC she you are

 

[Sunnysky]

Rmin also may reduce starting torque, but your DCR must be 1.2 Ohms so 0.3 Ohms is not too much loss in start torque., just a slower start. whereas a 1.5Ohm PTC may be too high it may also affect max RPM with voltage drop.

This depends on motor stiction and load.

I would suggest 350mA SMD for fastest response.

 

[supak111]

I meant the temps in F, so no it will not get to 85c.

And lower starting torque for my device isn't really a big deal, losing as much at 30% of starting torque would be ok. I'm more concerned with faster protection, really don't want to burn out the motor every time the device stalls.
.
.

 

[Sunnysky]

it is intended for occasional stalls not as a routine function. they do age.

 

[supak111]

It is an occasional thing, but good to know that they age. Is there any other EASY way to protect a motor from over current/heat? Hopefully something as simple as this? Or perhaps a finished cheap device I can buy that I can add to my gadgets?

 

[Sunnysky]

it doesn't get any simpler