# 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.

• In all honesty, it's probably easier to limit the inrush current. – Matt Young Nov 18 '15 at 18:53
• The inrush is limited by RTH1, but once that is finished somehow that resistor needs to be bypassed. The source is a 40-60V battery so no current control on that. – Alexis Nov 19 '15 at 13:06
• I'm thinking a dedicated inrush current limiter like this. It will be cheaper and simpler. – Matt Young Nov 19 '15 at 13:39
• RTH1 is the 4Ohms version of the Ametherm Device. These devices still need bypassing as they have a slow reset time (time before they can stop another inrush) and they run HOT (~90C). – Alexis Nov 19 '15 at 15:02

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.