Switch

[DJ Exprice]

I have made a Bleeder Resistor Switch. It only bleeds the ± Rails when you turn it off. This cuts down on LOTS of heat.

An externally hosted image should be here but it was not working when we last tested it.

Tell me what you think!

Cheers,

DJ EXPRiCE

 

[DJ Exprice]

Anyone like it?

 

[Wynand]

After a quick look, it looks good to me.

I'll remember the idea.

 

[raromachine]

Can you explain a little more about the situation in which you'd use this?

In most cases I would have thought that in most cases the amp/speakers/led would have dissipated any energy left in any capacitors?

Or is the idea that we don't want that to happen?

 

[rpapps]

I think 10 Ohms is a bit small. Think about what happens when you throw the switch. Let's assume the power rails are +/- 40 volts (could be much higher). Suddenly you've got 80 volts across 10 Ohms. Instantaneous current is 8 Amps and power is
640 Watts. You would want a substantial switch and resistor to stand up to that for any length of time.
You don't need to bleed off the charge that quickly, 100 Ohms would be OK.
Cheers
Rob

 

[I_Forgot]

Or even a 10K resistor without the switch.

The point of a bleeder resistor is to make the circuit safe for someone servicing the device. The bleeder resistor ensures that the charge has been bled off the caps by the time a service tech opens the box up. Back when vacuum tubes were more common and power supplies had 500V caps, and the tubes had essentially infinite resistance shortly after the circuit was shut off, bleeder resistors were a real necessity.
Caps could hold a charge for a long time and then knock some unsuspecting weekend-tech on their butt.

With the low voltages on solid state amps, and the behaviour of transistor circuits, they are pretty much optional these days. The circuit will pull the cap voltage down pretty quickly after the power is shut off, unless a fuse on the DC rail blows, in which case it IS a good idea to have a bleeder.

If you don't have fuses downstream of the power supply filter caps, a bleeder isn't a real necessity in a solid state amp.

I_F

 

[lndm]

Ah! but there's nothing like an unexpected short from a low voltage on a massive cap, to make a novice fall off their chair with a dumb look on their face

 

[DJ Exprice]

I think 10 Ohms is a bit small. Think about what happens when you throw the switch. Let's assume the power rails are +/- 40 volts (could be much higher). Suddenly you've got 80 volts across 10 Ohms. Instantaneous current is 8 Amps and power is
640 Watts. You would want a substantial switch and resistor to stand up to that for any length of time.
You don't need to bleed off the charge that quickly, 100 Ohms would be OK.
Cheers
Rob

Sounds good too. I've tested some things with a 1 watt 10 ohm resistor and it didn't even get that warm when I threw the switch off. I will however advise that the ohm-age is between 10 and 100 ohms and at LEAST 1 watt. I have some 10w 10 ohm and some 10w 100 ohm resistors (ceramic type from radioshack). I think that they are perfect for me .

 

[DJ Exprice]

EDIT: BOTH of the upper right pins on the switch can touch if you want to drain even faster if there are lager caps on the amp circuit itself! You may want to use a 100 ohm resistor for this though.

 

[cliffhiker]

I've used this a number of times and I think posted it here sometime back..
As far as I know the best explanation and probably original source of this idea is here...

http://www.edn.com/contents/images/70501di.pdf

 

[neutron7]

woldn't that only work for single ended PSU ?

 

[cliffhiker]

Unless I'm wrong, should work just fine...
The switch connections used when turned off to drain caps would be connected to the V+ & V-
thus when switched off V- would be the 'new' ground (ground is relative after all).