# Effecient way to Bleed down a capacitor

Discussion in 'The Projects Forum' started by Geoffr67, Feb 13, 2012.

1. ### Geoffr67 Thread Starter Member

Dec 5, 2011
69
0
Hello everyone,
I have a circuit i made and am having a problem, It keeps hanging up when i shut power off and then reapply power, while this wont ever really happen in real world,its causing the pic chip to lock out a input when power is reapplied within a few seconds. When the capacitor is discharged after power is removed it works fine. Can anyone suggest a good way to bleed down the cap, are there better options besides useing a resistor between the cap? something that will bleed it down almost instantly?

Thanks everyone!

2. ### K7GUH Member

Jan 28, 2011
191
23
A dead short will certainly do the trick. However, it may not be terribly convenient. Use a relatively high value of resistor across the capacitor, so as not to load down the input of your circuit. You need to know the equivalent resistance of your working circuit, then pick a resistor value 20-30 times that for the bleeder.

3. ### Geoffr67 Thread Starter Member

Dec 5, 2011
69
0
" You need to know the equivalent resistance of your working circuit, then pick a resistor value 20-30 times that for the bleeder."

So how would i determine this, measure the resistance at the input? and then go 20-30 times that number? Forgive my ignorance on this first time ive ever had to do this.

4. ### crutschow Expert

Mar 14, 2008
13,501
3,375
The equivalent resistance is simply the supply output voltage divided by the supply output current into your load.

5. ### luvv Member

May 26, 2011
186
31
Could you use a pnp fet and smaller resistor to dead short the cap when the power is turned off?

I don't know if it would work,just sounds reasonable to me

6. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,438
1,626
When I need something for a breadboard I do something really simple and foolproof. Here that would be a relay on the power input and a diode separating the power in from the cap. When power goes off, the relay opens and the NO contacts can do anything you wish from dead short on the cap to a small resistor or something else.

7. ### BillB3857 Senior Member

Feb 28, 2009
2,402
348
Worked this up as a concept. A couple of open collector inverters would probably work also.

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8. ### MrChips Moderator

Oct 2, 2009
12,636
3,454
Where is the cap connected? It might help if you provided a circuit. Why not remove the cap? Another solution is to install a POR (Power-On Reset) circuit.

http://www.maxim-ic.com/app-notes/index.mvp/id/3227

Edit: Did you enable the Power-On Reset delay on the PIC?

9. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,305
The PIC should have BOR and WDT options, either or both can probably save you from the latchup situation, although it does also depend on oscillator type.