Soft Start Circuit for a capacitor bank
Soft Start Circuit for a capacitor bank
(OP)
I need to design a very simple soft start circuit for a super capacitor bank (12 4.7F caps in series). The bank will be charged to 24-28VDC.
I'm thinking of using a power resistor, a relay (to shunt out the resistor when the caps are charged) and some simple control circuit. My first inclination for the control is to use a 22V Zener at the output of the bank. This zener would bias the gate of a FET or BJT (which would turn on the relay). Is this a reasonable method for controlling the relay? Or is there some easier method I'm not thinking about? BTW, I do not want to use any soft-start IC's. Simplicity is crucial.
Thanks
Larry G
I'm thinking of using a power resistor, a relay (to shunt out the resistor when the caps are charged) and some simple control circuit. My first inclination for the control is to use a 22V Zener at the output of the bank. This zener would bias the gate of a FET or BJT (which would turn on the relay). Is this a reasonable method for controlling the relay? Or is there some easier method I'm not thinking about? BTW, I do not want to use any soft-start IC's. Simplicity is crucial.
Thanks
Larry G





RE: Soft Start Circuit for a capacitor bank
If you are aware of that fact and your resistor is designed according to that energy your approach seems quite reasonable. You should include some positive feedback to achieve stable operation, e.g Schmitt-Trigger circuit.
RE: Soft Start Circuit for a capacitor bank
J=0.5*C*V*V or for your case 16,243 joules approximately.
That is going to be one big mother of a resistor.
RE: Soft Start Circuit for a capacitor bank
My concern involves the intial turn-on when the caps will charge up. My thought was to put a large resistor in between the PSU and the bank and force a slow charge of the cap bank, therby getting rid of the immense in-rush. Once the caps are charged, I want to shunt out the resistor via relay contacts.
Lewish, I'm not sure I follow your logic of needing a huge resistor. If I use a 120 Ohm, 5W resistor, I can limit the current to 0.2A. The main drawback is that my charge time will be long (5RC= 5*120*.2 = 144s)I may need to put a couple resistors in parallel to cut down my charge time to something more reasonable.
Does this make sense or am I offbase here?
Thanks
LG
RE: Soft Start Circuit for a capacitor bank
You are right, about the 120 ohm resistor.
OK, now another approach. A company called Ametherm makes "Circuit Protection Thermistors". www.ametherm.com.
I use their devices in power supplies to limit inrush current. The device starts out looking like some larger resistance which reduces as the part heats due to current.
They may or may not be a good choice for your application. Give them a call and see what they recommend.
RE: Soft Start Circuit for a capacitor bank
Thanks for the suggestion. I actually use some of Ametherm's thermistors in another application. I don't know why I didn't think about using one for this application. It would be a lot simpler than incorporating a resistor, relay, and control circuitry.
Thanks to electricuwe for posting as well.
RE: Soft Start Circuit for a capacitor bank
RE: Soft Start Circuit for a capacitor bank
Bung
Life is non-linear...
RE: Soft Start Circuit for a capacitor bank
RE: Soft Start Circuit for a capacitor bank
RE: Soft Start Circuit for a capacitor bank
RE: Soft Start Circuit for a capacitor bank
RE: Soft Start Circuit for a capacitor bank
Thanks a lot for all the responses. I finally got my prototype together and found that a hefty thermistor does wonders for snubbing out my inrush. And it makes the circuit quite simple. I also added an inline diode to prevent the capacitor bank from discharging thru the PSU when power is lost.
Anyway, thanks again.
LarryG