Electronic – How to detect end of Capacitor Pre-charge


A common problem with batteries and loads connected to them is that the load often has substantial capacitors which needs managing .

When the battery is connected to the load by closing a Contactor/Relay a high inrush current flows to charge up the load capacitor. This current being in 100's of amps will damage the contactor/relay.

Below is a schematic which is supposed to precharge the capacitor C1 once the contactor is closed. As the voltage of C1 increases the PSU will turn on and its output will start rising.

Once the PSU output is high enough this feedback will turn the MOSFET T1 ON, to bypass the resistor RTH1.

This circuit does not work because a High load on the PSU will cause a voltage drop across RTH1 and this voltage drop will reset the PSU.

The PSU has a disable pin which can be used.

I am looking for a circuit which can detect that the capacitor is charged and trigger both the bypassing of RTH1 and enabling of the PSU.

I was considering a Window Comparator but have not figured out how to wire it up to detect the difference between the input voltage and C1 voltage.

Question:What type of circuit can be used to detect when C1 is charged?
(Voltage C1 ~= Input Voltage 40-60V)

I am looking for either rough example, circuit name, type of device, etc.

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Best Answer

A dead-simple approach would be to use a time delay relay rather than the MOSFET.

When power is turned on, the timer on the relay starts. When the timer runs out, the relay closes. Select delay time to be >> 5•RC and it should work fine. No need to actually sense the voltage if you know R and C. Not sensing the voltage also means not re-triggering under heavy loads.

If you love the MOSFET, you could, of course operate it with a one-shot timer, but then you need to find/make a source of power for it. A relay seems likely to be simpler and cheaper than the DC-DC converter.

A latching relay could be used with some simple control logic to smack it on after a delay and off when the main contactor is opened, so you don't need to provide it with power all the time (presuming battery == need to conserve power.) An extra pole on the relay can talk to the disable pin.