precharge resistor calculation

Inrush current is produced do to a 1500uH input capacitance. Max allowed current is 28 A. And input voltage is 325V.
E=C*Vin/2=82.5 J
Rmin=Vin/Imax= 11 Ohm
t=RC=20 ms
P=E/t=4560 W

Are calculations right? Anyone knows a resistor/s that can handle that amount of energy?

Thanks

It's often wanted to have PTC resistors for fail-safe operation in case of bus short or bypass relays failure. 86 Ws can be still handled by relative small PTC discs, e.g. 15 mm diameter. Multiple elements can be connected in parallel if necessary. Review e.g. Epcos/TDK portfolio.

FvM said:

It's often wanted to have PTC resistors for fail-safe operation in case of bus short or bypass relays failure. 86 Ws can be still handled by relative small PTC discs, e.g. 15 mm diameter. Multiple elements can be connected in parallel if necessary. Review e.g. Epcos/TDK portfolio.

Click to expand...

Yes, I thought before about it. I was trying to place resistors instead of that "huge-disc" (you can play ultimate Frisbee with that ) ... but in the end.....

Thanks

1. I suppose, speaking about capacity, you've meant 1500uF?

2. In the 1st formula the voltage has to be squared:


The result (its order) looks like you have squared the voltage but I don't understand its numerical value - I obtained 79.2J.

3. The resistor value is rather 12Ω (11.6Ω).

4. The time constant for your 11Ω resistor is 16.5ms, for 12Ω resistor it is 18ms, so why do you think it is 20ms?

5. If your last formula were right (it is not), the numerical value of its result would be 4125 W (using your strange results above) or (using the right results) 4801W (for 11Ω) or 4401W (for 12Ω). None of them corresponds to your 4560W... How come?
However, as I already mentioned above, the formula is wrong. Power (instant) is given as


Your formula is valid only if the change of energy E is constant during time (and therefore also the instant power dissipated in R would be constant during time in such a case), which is not true! Also the t in your formula is not a time but the time constant of the RC circuit; its common designation is τ (greek tau, so that not to confuse it with time).
The current i(t), as a function of time, charging a capacitor C through a resistor R from a constant voltage source V is given as:

The corresponding instant power p(t) dissipated in resistor R is then:


The maximum instant power dissipated in R is at t=0, i.e. p_MAX = V^2/R = 8802W (if R=12Ω).
This instant power decreases exponentially with the time constant τ/2 = RC/2,
so, for instance, after τ=18ms it will be 1191W, after 3τ=54ms it will be 21.8W and after 5τ=90ms it will be 0.4W.