PTC Fuses and Low Current Circuit Protection

Question

I would like to have some overcurrent protection in a circuit that normally does not draw much current (~100mA max.) I like the idea of PTC fuses, but they seem to be extremely slow in disconnecting power from these types of circuits.

Why do PTC fuses exist with low trip currents if they are slow to react? Wouldn't that make the protection ineffective?

Assuming I were to use a PTC fuse, how does it behave when the current is between the hold current and trip current?

Are there any alternatives to fuses for overcurrent protection?

Answer 1

PTC fuses are not for all applications. However, in many cases the damage caused by high current is by heat, so high current for a short time can be OK. Look up the reaction time of normal fuses with a melting link, and you will see they aren't that fast either.

Consider what exactly you are trying to protect. Would a short circuit really damage something in a few 10s of ms or even a few 100 ms? Often not. If a short can cause a problem in less time, then you need a different way of breaking the circuit that doesn't rely on heating (which both "normal" fuses and PTC fuses do). There are various ways of making a "electronic fuse".

For example, I'm working on a project right now that includes up to a few A being driven by a H bridge controlled by a microcontroller. I have a 50 mΩ current sense between the bottom of the H bridge and ground. This gets amplified and presented to the processor so that it can read the current, which it does every 14.5 µs. If the current is above a threshold, it immediately goes to shut down the H bridge. The result is that a short perists no longer than a few 10s of µs.

Circuit breakers are yet another technology. These usually work by having the current form a magnetic field, which releases a hair-trigger when it gets strong enough. The magnetic field strength follows the current instantly, but the mechanical trigger will have some delay.

Answer 2

PTC's are intended to thermally react faster than the load and save on costly repairs due to a fault condition.

If you have an application where you are operating near the absolute maximum current of the conmponent, then PTC may not be what you need.

If you consider an H bridge application with high peak switching currents during transition and shoot through spikes that increase with motor load lagging the turn off time then a deadly time duration of this event can quickly fuse open your FETs. In this case you want an active current limiter with a large capacitor to provide the short transient high current pulses. *( Even better you want dead-time control on commutation) *

The PTC is intended to protect from thermal overload due to current so the PTC response time ought to be faster than the unit being protected but still the source must not exceed the components abs. max current spec in the short term.

The quickest parts are the smallest SMD PTC's. < 0.1 sec < 1 watt dissipation. such as 1206 or 805.

Above is response for a high current PTC sensor

Above is for low current radial PTC starting with 80mA hold current at bottom for HX008

Above is for SMD 805 PTC showing resistance curve vs temp here being used as a thermal protection sensor rather than current limiter due to high cold resistance. (pref. thin track)

It should be clear that ALL PTC's are design to have dynamic impedance rapidly change at a similar temperature for same material. Some are standard 85'C others offer different threshold temps which affect your operating environmental range. See above variation)