Sparks when 24VDC power is supplied

[Darkcobra]

Thanks everyone who helped out. Seems like any resistance value will work here.
Is there any requirement or formula i need to use to calculate the resistance value needed? that changes with the value of the capcitor?

There are two opposing requirements for the resistor here:

1) It should be big enough to limit inrush current as much as possible.
2) It should be small enough not to affect normal circuit operation, or dissipate too much power.

Usually #2 is practical limit.

According to Ohm's law, if 40mA is passing through a 4.7 ohm resistor, the voltage drop across it will be 0.188V. And the power dissipated through it will be 0.04*0.188, or 7.52mW. That was actually much less than I intended, so I must have made a math error the first go round.

For Venkadeshm's larger 50 ohm resistor, it's a 2V drop, and 80mW. Still acceptable, the resistor might just get a little warm, and your circuit can tolerate the 2V drop.

You mentioned surge resistors earlier, they do exist, and are called inrush limiters or NTC thermistors. They start at a high resistance when cold, and resistance drops as they heat up, causing them to have less impact on the circuit once it's up and running. The ones I've personally seen seem to be intended for larger running currents than we're speaking of, though I'm not very familiar with all the options.

I'll add one more thing. Even a lot of consumer electronics lacks any inrush limiting, and generates a tiny spark when power is applied. You simply never see it, as it's hidden away inside a power jack or switch. So you needn't banish every spark in every instance, only when it's excessive or is suspected of causing a problem.

 

[FvM]

Beside all the right things that have been already said,

- is it normally operation of your device that the power is switched between a DC power supply and your circuit, including the said 220 µF capacitor?
- or will it more likely powered when already connected to the power supply

In the former case, the only component that's at serious risk to wear out or damage is the switch. In the latter, there's no problem at all. Electrolytic capacitors and power rectifiers are pretty good in withstanding large inrush currents.

 

[kam1787]

The value of resistance ........ means that the regulator may give an error in 10V dc supply

I see no reason whatever that a resistor, in series, with the input cap would effect the regulation per se. It could cause instability if the regulator does not 'see' an input within its parameteres. Check the datasheet carefully.

A recommended solution is an LC filter. See the links.

http://www.interpoint.com/product_documents/DC_DC_Converters_Inrush_Current.pdf

http://vixra.org/pdf/1208.0094v1.pdf

http://rgmicro.com/Start-Up Transient and Inrush Current.pdf

 

[Venkadesh_M]

There are two opposing requirements for the resistor here:

1) It should be big enough to limit inrush current as much as possible.
2) It should be small enough not to affect normal circuit operation, or dissipate too much power.

Usually #2 is practical limit.

According to Ohm's law, if 40mA is passing through a 4.7 ohm resistor, the voltage drop across it will be 0.188V. And the power dissipated through it will be 0.04*0.188, or 7.52mW. That was actually much less than I intended, so I must have made a math error the first go round.

For Venkadeshm's larger 50 ohm resistor, it's a 2V drop, and 80mW. Still acceptable, the resistor might just get a little warm, and your circuit can tolerate the 2V drop.

You mentioned surge resistors earlier, they do exist, and are called inrush limiters or NTC thermistors. They start at a high resistance when cold, and resistance drops as they heat up, causing them to have less impact on the circuit once it's up and running. The ones I've personally seen seem to be intended for larger running currents than we're speaking of, though I'm not very familiar with all the options.

I'll add one more thing. Even a lot of consumer electronics lacks any inrush limiting, and generates a tiny spark when power is applied. You simply never see it, as it's hidden away inside a power jack or switch. So you needn't banish every spark in every instance, only when it's excessive or is suspected of causing a problem.

Hi
50 Ohm in series to a circuit will be always unpractical... I said that because to really find the exact problem....
If that much resistance only works
Try to put like this,