PTC as an inrush current limiter

[sentry]

Hi,

I was looking into PTC-based inrush current limiters. I thought I had it nailed down until I came across the following homepage:

https://www.ametherm.com/inrush-current/ptc-thermistors-for-inrush-current-limiting.

According to this homepage a PTC-based inrush current limiter requires an active circuit to bypass the PTC thermistor to prevent shutting the entire system down. The following example circuit is giving which:

is active during power on for a set interval, typically 3 or 4 times the amount it takes for the inrush current to settle. Then, the bypass circuit shuts itself off and sends current back through the PTC thermistor to protect the system against shorts."

My understanding was that as long the temperature of the PTC was the same as the ambient temperature, the PTC would act as a low ohmic resistor and no need to bypass it. Only if the temperature exceeds some threshold value because of too high current the resistance will start to increase significantly and thereby reduce the current into the system.

Is my understanding wrong? Why is the PTC bypassed to start with on the example circuit when the power is turned on?

Any help will be greatly appreciated.

[Zero999]

I don't see how a PTC resistor can perform an inrush limiting function, since when power is initially applied, it will have a low impedance, which will rise when current starts flowing. An NTC resistor is used for inrush limiting, as it has a high resistance to start with which drops, when current starts flowing and it heats up.

A PTC resistor is normally used for over-current protection. The PTC resistor has a low impedance, when the circuit is working properly, then when the current exceeds a certain threshold, it heats up to the point, where its resistance increases, resulting in further heating, until the current is limited to a very low level.

[SeanB]

You would normally use both in series, the NTC provides protection against inrush current and as it warms up it reduces resistance, while the PTC is normally running cool, till the current is high for long enough to start the self heating that limits the current. For applications with frequent switching of the input you typically use a relay to bypass the NTC thermistor after power on, and this then allows it to cool off to the high resistance state. You do not need the relay to be capable of handling the inrush current, only the steady state current, and as it normally is not switching power (power is off when it releases) it will have a long life. Switch on delay also is really only going to be a few cycles of the mains input, so connecting the coil to a supply rail that is provided by the SMPS or other control circuit normally is fine, as these typically need  a few mains cycles before the switching controller starts up and runs.

[Zero999]

This came up in another couple of threads recently. The original poster wanted to use it for an audio amplifier. Using a 555 timer and relay to bypass the resistor, is the classic way to do it, but it probably is unnecessary with a modern switched mode power supply.

https://www.eevblog.com/forum/beginners/how-to-build-a-555-timer-circuit-for-5-seconds/msg1699313/#msg1699313


https://www.eevblog.com/forum/beginners/problem-with-powering-an-amplifer-board/msg1701566/#msg1701566

[lordvader88]

When they use a relay/timer setup, I think it more with just a fixed resistance to limit inrush current, and once the cap's are charged enough, it taken out of circuit to operate normally without the extra voltage drop.