Meg ohm Resistors

[fingers]

Hi I've got a question which I can't find the answer to.
Why would you need a resistor in the Meg Ohms range in parallel with the smoothing capacitor in a PSU circuit?

 

[Nigel Goodwin]

To provide a discharge path for the electrolytic, so it doesn't stay charged up.

 

[fingers]

Right, so does that happen when it is switched off to make it safe if opened or some other reason?

 

[dknguyen]

It is for when it is off. It is large to have neglible drain on the capacitor when in operation, but small enough (much less than an open circuit) so the capacitor doesn't retain it's high voltage charge for a week after it is shut off (where someone might touch it and get zapped).

 

[picbits]

I remember reading somewhere about capacitors "self charging" over a long period of time when left open circuit - could be something to do with that.

 

[bananasiong]

It is for when it is off. It is large to have neglible drain on the capacitor when in operation, but small enough (much less than an open circuit) so the capacitor doesn't retain it's high voltage charge for a week after it is shut off (where someone might touch it and get zapped).

You mean to discharge the capacitor when it is turned off?

 

[mneary]

I remember reading somewhere about capacitors "self charging" over a long period of time when left open circuit - could be something to do with that.

They aren't exactly self-charging, but some types of (e.g. oil) capacitors store energy deep in the dielectric (while in use) and it comes to the terminals with a very long time constant (after the circuit is turned off).

You mean to discharge the capacitor when it is turned off?

Yes.

 

[gootee]

I remember reading somewhere about capacitors "self charging" over a long period of time when left open circuit - could be something to do with that.

There's also a phenomenon known as Dielectric Absorption, where capacitors partially recharge from energy that was temporarily absorbed by their dielectric materials. The mil-spec test for it involves charging with a certain voltage, for a certain length of time, disconnecting them for a certain length of time, and then measuring their voltage. But that's just the test. They also exhibit the associated behavior while in use. That's why, for certain types of circuits, it's very important to use capacitors with a low dielectric absorption specification. Teflon, polystyrene, and polypropylene film dielectrics are among the best, in that regard.

- Tom Gootee

**broken link removed**

-

 

[Gayan Soyza]

For fingers!
You can see this effect in practical situation.

Take a DC power pack & find the smoothing capacitor there.
Place an LED across the capacitor in series with a resistor according to your maximum DC output voltage (for ex 12V just use 1K).

*Now switch on the power pack you will see LED is turning on.

*Now remove the AC mains supply or turn off the power pack. Now you can see how long the LED lights up after power off.

To prevent this they have added a resistor in parallel with the smoothing cap.

Same theory applies in the computer power supplies.

 

[Gayan Soyza]

I remember reading somewhere about capacitors "self charging" over a long period of time when left open circuit - could be something to do with that.

What about keep binding the two legs together (the polarities + & -)

 

[bananasiong]

Yes.

But the capacitor will discharge to the load when the power is turned off, even there is no Mega ohm resistor right?

*Edit:
Gayan, do you mean to prevent the capacitor from discharging to the load?

 

[Gayan Soyza]

Gayan, do you mean to prevent the capacitor from discharging to the load?

No Computer Power supplies not designed to discharge through the Motherboard. They used switch mode systems.

In a unit you can see these levels
*AC line fuse
*EMI filter (Low pass)
*Rectifier
*Pre Regulator (to increase the power factor)
*DC-DC convertor (generates a set of DC voltages required for the load, and normally also provides input-to-output isolation)

From pre regulating level the 250V smoothing capacitors are separating.
If the power is off suddenly cut off the power.
But most power power supply units I haven't seen this parallel resistor. I have seen the load resistor.

 

[dknguyen]

You mean to discharge the capacitor when it is turned off?

Yeah. It might take too long to discharge the capacitor to safe levels without a resistor. It is so it won't retain a charge for so long. Imagine touching a high voltage capacitor inside a machine that was turned off a week ago and getting killed.

But the capacitor will discharge to the load when the power is turned off, even there is no Mega ohm resistor right?

*Edit:
Gayan, do you mean to prevent the capacitor from discharging to the load?

Yeah but it can take too long. For high voltage things it's a hazard. For a computer...I woudl think it's so the caps could discharge faster so you could reboot the computer soon after it's turned off for whatever reason.

 

[fingers]

Thanks guys this is most helpful! And it would be in the Meg ohm range rather than K's or less so that it doesn't draw much current when powered up?

 

[Nigel Goodwin]

Thanks guys this is most helpful! And it would be in the Meg ohm range rather than K's or less so that it doesn't draw much current when powered up?

Depends on the size of the capacitors - the 100K's range is probably most common?.

 

[Hero999]

There's also a phenomenon known as Dielectric Absorption, where capacitors partially recharge from energy that was temporarily absorbed by their dielectric materials. The mil-spec test for it involves charging with a certain voltage, for a certain length of time, disconnecting them for a certain length of time, and then measuring their voltage. But that's just the test. They also exhibit the associated behavior while in use. That's why, for certain types of circuits, it's very important to use capacitors with a low dielectric absorption specification. Teflon, polystyrene, and polypropylene film dielectrics are among the best, in that regard.

- Tom Gootee

**broken link removed**

-

The equivalent circuit is another smaller value capacitor with a high value series resistor connected in parallel with the capacitor.