PTC Resettable Fuses

[Spanky]

Many of todays over current protection devices are PTC (Positive Temperature Coefficient) Resettable Fuses. These are great devices because you never have to replace a blown fuse, but understanding how they work is important.

A PTC fuse should pass the rated fuse current without opening. For example a 4 amp PTC will not open at 4 amps or less. If the current exceeds 4 amps the PTC will start heating up until its temperature reachs the fuse cutoff temperature. 4.5 amps may take minutes to open the PTC, but 8 amps will open the PTC immediately.

In order for the PTC to restore, the current must be reduced to a point that will let the PTC cool off. If a circuit is normally drawing 1 amp and an offending 4 amp short is removed, the PTC may not reset because the 1 amp of current keeps the PTC hot enough to remain open. In this case the system will have to be powered down for a few minutes to allow the PTC to cool off and reset.

Another factor that affects the PTC opening point is temperature. The PTC is a temperature switched device, so if it is within a wall plugin transformer that is running 160 degrees F., the PTC will open faster than if the PTC was at 70 degrees F.

 

[jlehnert]

Excellent info. Mind if I add it to the glossary

 

[FrankMc]

Many of todays over current protection devices are PTC (Positive Temperature Coefficient) Resettable Fuses. These are great devices because you never have to replace a blown fuse, but understanding how they work is important.

A PTC fuse should pass the rated fuse current without opening. For example a 4 amp PTC will not open at 4 amps or less. If the current exceeds 4 amps the PTC will start heating up until its temperature reachs the fuse cutoff temperature. 4.5 amps may take minutes to open the PTC, but 8 amps will open the PTC immediately.

In order for the PTC to restore, the current must be reduced to a point that will let the PTC cool off. If a circuit is normally drawing 1 amp and an offending 4 amp short is removed, the PTC may not reset because the 1 amp of current keeps the PTC hot enough to remain open. In this case the system will have to be powered down for a few minutes to allow the PTC to cool off and reset.

Another factor that affects the PTC opening point is temperature. The PTC is a temperature switched device, so if it is within a wall plugin transformer that is running 160 degrees F., the PTC will open faster than if the PTC was at 70 degrees F.

Hi Spanky

I used PTC ( resetable fuses) when i designed my cat5 based probe system....

http://www.armaghelectrical.com.au/page9.html

Cat5 is rated around 300mA and i was concerned if there was a short on the power pair that it would cause a fire....I sent 12v dc down the brown pair and did some tests using 100mA PTC,s and they started to kick in around 150mA ....Each rj45 has a PTC and the good thing was that a short on one rj45 power pair didnt have any effect on the others , only that circuit would shut down....So i would highly recommend the use of these nifty devices especially if using cat5 for power....

Frank

 

[electron]

This is great info, I kind of had an idea what they are, but didn't know how it worked.

<Johnny Five>More Input!</Johnny Five>

 

[Spanky]

You can use this info as desired.

 

[Digger]

PTC's are sometimes considered overcurrent protective devices and sometimes impedance limiting devices. It depends on the application I beleive. When using them to prevent excessive current on a power bus (such as maintaining Power Limitation or Class 2 parameters) they are considired overcurrent protection. When used in conjunction with a transformer winding they are considered impedance limiting devices and the transformer would be considered inherently limited (often used in Class 2 transformer designs where inherent limitation is needed with a higher VA than a winding alone can limit without damage).

Since they are calibrated for a specific ambient range for their hold and trip currents care needs to be taken in the application. At increased ambient nuisance tripping can occur (not what you would want in a circuit that is relied upon for safety/performance). In a decreased ambient the PTC will take longer to operate and would allow a higher level of current to flow and for an extended period of time. This could be problem where a PTC is being relied upon to meet Power Limitation or Class 2 to minimize a risk of fire. In the case of an alarm or automation circuit the installer most often uses Class 2 or Class 3 wiring methods. If the PTC is located in a control circuit where the ambient is lower (unheated garage etc) and the circuit is overloaded there could be excessive current flow and overheating of the wire. Most likely it would be enough to have the PTC trip prior to a complete meltdown of the wire where a fire could occur.

Fuses are similary affected but to less of an extent. PTC's however are more expensive. They are also not user replaceable (they dont need to be since they are resetable) and prevent a user from putting a 10 A fuse in place of a 5A defeating the overcurrent protection intended. Fuse vs PTC the PTC wins hands down.