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Circuit to discharge a capacior on power-off

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Ali Sajjad

New Member
I am using a power supply which employes a full-wave rectifier followed by a filter capacitor (100uF). This circuit gives me a dc volatge level equal to the mains peak voltage( we have 220 Vac at 50hz). When the power is turned off, the filter capacitor remains charged to the high voltage level because the circuit which is been powered by this supply is of very high impedance and draws negligible current. I need some suggestion to design a circuit which discharges the filter capacitor when the power is turned off within a short time and not causing some spark across the capacitor. I m not in favour of using just a resistor across the capacitor. Any suggestion, idea ??

Thanks everybody in advance.

- Ali Sajjad
 

crust

Member
If you are not in favor of the resistor, how about putting a small fan in the chassis, that will drain the capacitor quickly. On my supplies, I regularly use the cooling fans to drain the filter caps. I also have a resistor in case a fan dies. For me fans have the highest failure rate, followed by power supplies.
 

Noggin

Member
Wonder if those failure rates are related :)
 

Styx

Active Member
I am curious why you are not in favour of just using a resistor?
common practice to put a discharge resistor across a DC link capacitor
If it is good enough for the aerospace sector, steel works, sewage works,... then it should be good enough for you?

just wandering.
If you would rather not have it in cct during operation can you use a "normally-closed" relay (if you can get them) with a resistor
 

Styx

Active Member
Nigel Goodwin said:
Styx said:
If it is good enough for the aerospace sector, steel works, sewage works,... then it should be good enough for you?

Is a 'sewage works' really a good comparison? :lol:

Well one of my co-workers used to work for FKI one of their products was a brushless DC drive for a sewage works. It was for the pump that "pumped" all the shite through the initial filter to get rid of all the tampons and big.. well you can guess.

It was an extreamly high torque drive. Also he had to get sent out lots of times for field maitenence due to something blocked the filter and thus too much load was put on the drive. Some very smell stories


Also took abt 6months arguing with production abt why they need a discharge resistor across the DC link caps for our motor drives
It took me placing 1 of our uncharged caps on a lab bench for a day and one I charged to 100V.
by end of day the charged one was down to 60V and the uncharged on was up to 7V
 

john1

Active Member
Hi Ali,

I cannot see any reason why a resistor is not acceptable,
but that doesn't mean that there isn't a reason.

What is used to switch this unit on and off?
Is it a switch ?
If a switch is used to turn this unit on and off,
then with an appropriate switch,
you could arrange,
that when you switch it off, the other contacts
apply a resistor which is not in circuit when its 'ON'
but when you switch it 'OFF' it will drain the 100 MFD.

This would not be "just using a resistor",
as the resistor would not be connected while the unit is
switched 'ON' but would be connected when the unit is
switched 'OFF'

Please let us know why you cant simply use a discharge
resistor in the normal way, that is widely used and
normally quite acceptable.

Regards, John :)
 

k7elp60

Active Member
Capacitor discharge circuit

It is a common practice to place bleeder resitors in parallel with filter capacitors in higher voltage power supplies. I suggest you us approximately 66 K ohms. If you get two each 33k ohm 2 watt resistor and connect them in series, they will consume about 5 Ma of current and discharge the capacitor to 0 volts in about 30 seconds. As an alternative you could use a 5 Watt resistor at about 66 to 68 Kohms, with the same result. By connecting the resistor in parallel with the capacitor, the capacitor will discharge when the mains is turned off
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Re: Capacitor discharge circuit

k7elp60 said:
It is a common practice to place bleeder resitors in parallel with filter capacitors in higher voltage power supplies. I suggest you us approximately 66 K ohms. If you get two each 33k ohm 2 watt resistor and connect them in series, they will consume about 5 Ma of current and discharge the capacitor to 0 volts in about 30 seconds. As an alternative you could use a 5 Watt resistor at about 66 to 68 Kohms, with the same result. By connecting the resistor in parallel with the capacitor, the capacitor will discharge when the mains is turned off

I would prefer just one larger resistor - as a service engineer I see many examples of resistors in series failing - one will always be slightly higher than the other one, so will run slightly hotter, this one will usually fail first (going O/C).
 

Ali Sajjad

New Member
I was not prefering the bleeder resistor because i didnt want it to draw even small current. But as you guys gave better ideas as to put this resitor with the normally closed contacts of the power switch so that when i turn the power off the bleeder resistor starts to discharge the filter capacitor. This is better for me.

Thank you all, .. ( But please dont try to compare this work with some shitty sewage works :oops: )

- Ali Sajjad.
 

Styx

Active Member
Ali Sajjad said:
I was not prefering the bleeder resistor because i didnt want it to draw even small current. But as you guys gave better ideas as to put this resitor with the normally closed contacts of the power switch so that when i turn the power off the bleeder resistor starts to discharge the filter capacitor. This is better for me.

Thank you all, .. ( But please dont try to compare this work with some shitty sewage works :oops: )

- Ali Sajjad.


hahahaha

well I wasnt comparing your work to a sewage pump (although that project did have some interesting results from what my collegue told me), just using some examples of where a dischage is used from my experience and collegues

Still why are you soo concerned abt a small bleed current????
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Ali Sajjad said:
I was not prefering the bleeder resistor because i didnt want it to draw even small current. But as you guys gave better ideas as to put this resitor with the normally closed contacts of the power switch so that when i turn the power off the bleeder resistor starts to discharge the filter capacitor. This is better for me.

One point!, why use a 100uF capacitor on something that takes very little current?. A much smaller capacitor would do just as well, and wouldn't hold it's charge for as long.
 

Ali Sajjad

New Member
Ok Ok Ok OK .... :x

Actually, the filter capacitor is not just a filter.. It gets charged and stores the charge, then the power supply is disconnected from this capacitor (not from the rest of the circuit, then a triggering circuit causes this capcitor to discharge by xenon flash lamp. this lamp requires considerable amount of enregy... and thats why a smaller capacitor can not be used as it would not provide the necessary energy to the xenon lamp. while not triggered the xenon lamp acts as an open circuit and the capacitor cannot be discharged. Further, i dint want the bleeder to remain dishcharging the capacitor when the xenon lamp is triggered, becaz it would take an amount of charge and the amount of energy to the xenon lamp would be reduced. If the xenon lamp is not triggered the capacitor will remain charged even when the power is turned off..... and i dint wat this...................................... SOOOO thats all.... it might seem abladablagablatablajabla to you guys... but its my engineering :wink:

- Ali Sajjad
 

Nigel Goodwin

Super Moderator
Most Helpful Member
That's why it helps to explain what you are doing, rather than just ask a very small part of the question - although all you needed to mention was that it was for a xenon strobe (but too much detail is better than too little).

But as for a bleeder resistor, try doing the calculations, you will find it won't make any difference to the power in the flash :D

But if you've got a spare normally closed contact on the switch, feel free to switch it.
 

Roff

Well-Known Member
How long, timewise, do you have to hold the charge on the capacitor before the flash lamp is triggered? It seems to me that any hold time (which you alluded to) would preclude using contacts on the power switch to dicsharge the cap.
 

john1

Active Member
Hi Ali,
glad to have helped,
hope it works out alright.

Hi Nigel,
in that situation i spose a bleeder would not matter
if the flash were used straight-away, but it would
affect it if much time elapsed, maybe setting up the
shot, or some such.

Regards, John :)
 

panic mode

Well-Known Member
If it is paramuont to have no load then don't use bleeder resistor.
Use small AC relay with coil wired before the rectifier.
Use normally closed contact to short the DC side of the circuit
(maybe through resistor). This way your DC side will not be
loaded during tests and it will be very quickly discharged.
To prevent short circuit on powerup and not waste those diodes,
use timing relays normally open contact to turn on the rectifier
circuit.
Haaahhhh... to expencive, I would probably just use relay
and put small lightbulb in series with normally closed contact.

8)
 

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Nigel Goodwin

Super Moderator
Most Helpful Member
john1 said:
Hi Nigel,
in that situation i spose a bleeder would not matter
if the flash were used straight-away, but it would
affect it if much time elapsed, maybe setting up the
shot, or some such.

Only if you unplugged it! - and why would you?.

That's also assuming it's a camera flash, and not a strobe, that wasn't made clear - was it?.
 
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