bleeder resistor for ZenV4 power supply; and ancllary matters

[Zapped]

Have looked through a few threads for advice on how to figure a power supply bleeder resistor, but haven't found a simple rule of thumb. Looked in Art of Electronics, too, but found nothing helpful. NP's A-75 amp used a 1.6K resistor of about a watt or two, but that amp has a lower unreg. supply voltage than the ZenV4, so I figure the R for the Zen has to be greater than for the A-75, right? Can anyone suggest an optimal value for the ZenV4, given its 50V unregulated supply. Or, (teach a man to fish and you feed him for life) can anyone supply a calculational (not definitional) formula. My algebra is no good. Have only a foggy notion of what a time constant is.

I assume from looking at the schematic that there is already .47microfarads of film cap in parallel with the big filter caps, sitting on the circuit board, right? So, no advantage to further film caps in parallel. Didn't see anything on the board that would act like a bleeder resistor, however, so want to make sure its safe to stick my mitts into the amp to make adjustments.

BTW, still not sure why the design didn't include DC rail fuses, like, say, the Leach amp. Would the fuse mess up impedance in the regulated supply or something??

Larry Wright
Seattle area

[Panelhead]

Originally posted by Zapped

Larry,
The unregulated power supply or raw supply does need to be bled off. It can be done several ways. A 5K resistor from supply to ground would do fine.
In mine I used a string of 2.5K resistor, blue diode, and 2.5K resistor. This was to buffer any noise from the diode and keep the current down to 10 ma for the diode.
Works great, I do not know if buffering each side of the diode make any difference in injected noise. But it sounds like a good idea.
The power rating can be calculated by voltage squared, divided by resistance. Try a resistor about twice the dissapation as needed for long trouble free life.

Good Luck!
George



Have looked through a few threads for advice on how to figure a power supply bleeder resistor, but haven't found a simple rule of thumb. Looked in Art of Electronics, too, but found nothing helpful. NP's A-75 amp used a 1.6K resistor of about a watt or two, but that amp has a lower unreg. supply voltage than the ZenV4, so I figure the R for the Zen has to be greater than for the A-75, right? Can anyone suggest an optimal value for the ZenV4, given its 50V unregulated supply. Or, (teach a man to fish and you feed him for life) can anyone supply a calculational (not definitional) formula. My algebra is no good. Have only a foggy notion of what a time constant is.

I assume from looking at the schematic that there is already .47microfarads of film cap in parallel with the big filter caps, sitting on the circuit board, right? So, no advantage to further film caps in parallel. Didn't see anything on the board that would act like a bleeder resistor, however, so want to make sure its safe to stick my mitts into the amp to make adjustments.

BTW, still not sure why the design didn't include DC rail fuses, like, say, the Leach amp. Would the fuse mess up impedance in the regulated supply or something??

Larry Wright
Seattle area

[Zapped]

Thanks, Panelhead. That is a cool idea. Discharge the caps and power the pilot light at the same time. Ingenious!

Larry Wright
Seattle area

[Panelhead]

Originally posted by Zapped
Thanks, Panelhead. That is a cool idea. Discharge the caps and power the pilot light at the same time. Ingenious!

Larry Wright
Seattle area

Larry,
I cannot take credit for the idea. It came from Nelson.
The bypass caps you asked about are most likely not needed. After three months of daily use I added 2.2 ufd 100v flat film caps to the last filter cap. At first it seemed to add some detail and life. But after 4-5 hours the sound was about the same as before. My filter stage is a 10K 63V, 2.7 mH Choke, and 10K cap. The rails are running at 53.5 volts under load.
The rail fuses also are more trouble than help. If the input fuse is sized correctly and the diode bridge included on the ground leg the power fuse should smoke when there is a major failure. But with the constant current draw of the circuit, it should be easy on rail fuses.

George

[Blues]

Zapped, you can calculate the time(t) your supply caps can be bled by using the formula t=RC; where t=seconds, R=resistance in ohms, C=capacitance in Farads. Or the bleeder resistance for that matter. So if you have a 25,000 uF of cap and you would like 15 seconds of discharge time, your R=t/C. I don't have a calculator right now (at a public library). Power(P) dissipation of the resistor will be P=V^2/R. V is the unreg supply voltage. Double P value of the resistor...this will get hot. You can use this resistor before working the innards of your amp so you can bleed off stored energy and avoid a nasty spark or shock.

[Zapped]

Thank you, Blues! This is very helpful.

Larry Wright
Seattle area

[GRollins]

Having an LED is nice, of course, but I wouldn't lose a lot of sleep over bleeding down the power supply. The circuit will do that quickly enough on its own (any circuit Nelson's had a hand in will draw electrons out as fast as you can stuff them in).
I don't even use bleeders on tube circuits. Solid state voltages aren't high enough to worry about.

Grey

[jam]

Well lets see. A 100w amp will have 100v dc between the rails and a 200w amp will have 140v between rails........I would say you would have something to worry about.

And if the amp had 100,00uf of capacitance the stored energy would be ...1/2 CVsquared (Joules).

I would say that if you got zapped , you would have a lot to write home about.


Regards,
Jam

[roddyama]

Originally posted by GRollins
Solid state voltages aren't high enough to worry about.

Grey

WARNING! Solid State Power Supplies can be Lethal if handled incorrectly!

Grey,

You know better then that.

[Panelhead]

Originally posted by GRollins
Having an LED is nice, of course, but I wouldn't lose a lot of sleep over bleeding down the power supply. The circuit will do that quickly enough on its own (any circuit Nelson's had a hand in will draw electrons out as fast as you can stuff them in).
I don't even use bleeders on tube circuits. Solid state voltages aren't high enough to worry about.
Grey,
The regulated supply used in the V4 does need to be bled off. Mine operates at 53.5 volts and outputs 42 volts. Once powered down the voltage drops to where the series regulator turns off and will stay there. If not bled off the voltage would stay around 45 volts for a long time.
In my unit there is a CLC basic supply. It takes about 20 -30 minutes for the blue diode to go out completely.

George




Grey