Choke bleeder resistor

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MEPER

Is there a "rule of thumb" for bleeder resistor calculation for let us say a LC PSU for a power amp where a rail fuse is implemented?
The bleeder resistor purpose is not for discharge of cap but to "eat" the energy of the choke if fuse blows to protect the caps from high voltage.
In my case I have a 11mH choke with approx. 1.5 A bias (24VDC) and I am thinking of a resistor at about 150 ohm.
No one seems to take this issue serious (for cap discharge only the resistor is usually much larger)?

JonSnell Electronic

If you wish to remove the inductance of the choke and thereby reduce the overall reactance, you are going the right way.
Why fit a choke if you want to destroy its properties.

MEPER

The resistor was meant to be part of the load so placed same place you would place a cap bleeder but before the rail fuse so there always is a load the choke can deliver its energy to. I was not thinking of "short" the choke with the resistor. So it was just if there was an easy way to calculate such a resistor if fuse blows that the "spark" from the choke does not destroy the caps.

JonSnell Electronic

Ah, I see what you mean.
The choke acts as a resonator or reactor when current is drawn, when no current is drawn there is no reactance. I think you mean keeping a permanent load ... but how will you reduce the resistive load as the required load is increased I wonder.

I see no advantage in loading a choke in the way you describe, as all the energy is wasted in heat and there is no advantage to the power supply, if anything when you do draw current from the load, with your resistive load active, the choke will not work correctly.

Mooly

Paid Member
Why not add a highish wattage zener and lowish value series resistor in series with the zener across the rail. That would dump any excess voltage/energy.

Can't think what polarity is this unwanted pulse of energy going to be?

MEPER

This was answer to Jon Shell......Mooly came just in front
Yes, but if you suddenly break the DC current in a choke and the choke has no way to get rid of the stored energy it will create high voltages spikes (like an ignition choke in a car) so it was just have a load if rail fuse burns…...a load just a fraction of the normal load when amp is on but a load that reduces the spikes to a level where the filter cap is not damaged. Maybe it is not a real problem and therefor I have not been able to find any information
I have another choke loaded PSU where I just by "guessing" have a "bleeder". One time the fuse burned and the flash light from fuse was quite big.....and fuse was all black afterwards so something goes on...…
Also when the fuse breaks in the primary of a large toroid transformer it can be quite "wild".

MEPER

To Mooly:
This could be an ok solution and there also exist those "discs" you sometimes have at the 240 VAC side to take high voltage spikes at the primary (they have same form factor as a NTC disc). I can't remember the name on those…..
A zener that works as zener in both directions?

Mooly

Paid Member
Those things are known as VDR's (voltage dependent resistors) or 'Tranzorbs' or 'TV's'. I think there are probably other names as well out there.

MEPER

Yes....Tranzorbs….is the name. I don't know how much control you have of the max. voltage when using those if you say you should not exceed 64VDC which is the rated voltage of the capacitors I use. The can usually handle a bit more......

Mooly

Paid Member
If you only have 24 volts as a supply then use something like a 30 to 35 volt device which would then clamp a transient at that level.

MEPER

Yes, I can search the web for such a low voltage type.
I found the attached in my "back log" but those are for line level voltage.

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MEPER

I found a 30V, 38V type at my local dealer and has ordered some. Max. operating voltage is 30V and it will start kicking in about 38V and are rated 2000A. They call them "Varistor". So I good solution to the problem......I think.

Mooly

Paid Member
An 11mH inductor doesn't store much energy in the scheme of things, about 12mJ, fairly similar to a 47uF cap charged to 24vdc.

I think you have non problem tbh.

MEPER

Ok.....now the bleeder will be a "normal" and it is easy to solder the varistor on that bleeder for a bit of extra protection.
The problem may be larger for tube PSUs where chokes can be "Henry" size. Current is a bit lower then…..

Ketje

If the fuse blows and the choke continues the current, the capacitor between rectifier and choke get discharged.
When the voltage across the capacitor reach zero and there is still current left it gets charged in the opposite direction, not good.
Remedy is simple, put a diode over the capacitor.
Mona

JonSnell Electronic

Now I understand what you want to do.
Sounds to me like a problem that isn't there unless the power supply is liable to be unregulated, in which case not a good design.
Use a shunt regulator; zener diode and a transistor. When the voltage gets too high, the zener turns the transistor on and shunts the excess voltage.
A varistor will act as a short circuit and bear in mind the power or Joule rating is for a few micro seconds only otherwise it will go bang.

67339

Hello,
I remember Guido from Tentlabs told me to add a big 5 watt zenerdiode across the capacitor feeding his shunt regulator. Because i used a rather big henry choke with high dcr as an input choke i was afraid that if no load present for whatever reason the voltage might rise well above the maximum rating of the cap. So mounting a 22 volt zener across the 25 volt cap would make a perfect protection.. an easy way to avoid using a 40 volt cap where the actual voltage would be around 18 volt and keeping the input voltage for the shunt always well below the maximum input voltage. Greetings, Eduard

MEPER

Now I have attached an quick schematic to show what my idea is. I forgot the rail fuse which is after the cap. I will have two of those pr. mono block to make +-24V for both.

So first a SMPS 24VDC and then a small common mode choke to take the worst switching noise. Then a serial DC choke 11mH (5A rated) for additional filtering. My idea with DC choke is also that during power up it should limit the charge up of cap so SMPS does not go into protection. It will probably be some "try and error" as I don't know how intelligent the "error recovery" is made in this type of SMPS. The DC current will be about 1.5A during operation.

The main idea was to try a SMPS based PSU and make it "super duper" for audio. The 4 pole Jensen Capacitor caps are quite expensive so will not blow them up.

Do you think a diode reversed is enough to protect the caps?
I have ordered the varistors so can do both. So bleeder resistor like 1k + varistor + reverse diode?

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MEPER

Jensen caps are those…...picked up at the factory long time ago. Think they deserve to be used…...

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Mooly

Paid Member
You have to to member that SMPS doesn't always like (and need) lots of capacitance.

If you are experimenting then you must rely on measurements to determine what works best... and it could be that small values such as 100uF or lower work best.

Lots of capacitance also impairs the regulation of many SMPS. They don't need it.

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