Hello everybody,
I'd like to use a voltage doubler on a 120:240 step-up toroidal transformer to get a high voltage supply for an upcoming project. I also need a negative voltage rail for fixed bias, and I figured I would employ a trick I've seen on the Bogen CHB-100 and IIRC the Tim Mellows 6C33 to also employ a half wave rectifier with the voltage doubler.
There seems to be some disagreement about whether this will be tolerable with a toroid, and I'm wondering if I'm just charging up the power supply caps for the fixed bias rail, will I be magnetizing the core? (We are talking about ~200mA of DC current from the HV doubler, and a resistive input half wave supply that shouldn't need to supply more than a few mA)
I'd like to use a voltage doubler on a 120:240 step-up toroidal transformer to get a high voltage supply for an upcoming project. I also need a negative voltage rail for fixed bias, and I figured I would employ a trick I've seen on the Bogen CHB-100 and IIRC the Tim Mellows 6C33 to also employ a half wave rectifier with the voltage doubler.
There seems to be some disagreement about whether this will be tolerable with a toroid, and I'm wondering if I'm just charging up the power supply caps for the fixed bias rail, will I be magnetizing the core? (We are talking about ~200mA of DC current from the HV doubler, and a resistive input half wave supply that shouldn't need to supply more than a few mA)
I would like to join in here with a question.
Is the problem of imbalancing the toroid only with the diverted neg. bias voltage and the only way to avoid it is to use a coupling cap like in this circuit? How about an extra tab on the tranni, would this lead to an imbalance, too (without coupling cap)?
Do I have to have a complete own voltage winding coil on the toroid to avoid the problem of imbalance?
Is the problem of imbalancing the toroid only with the diverted neg. bias voltage and the only way to avoid it is to use a coupling cap like in this circuit? How about an extra tab on the tranni, would this lead to an imbalance, too (without coupling cap)?
Do I have to have a complete own voltage winding coil on the toroid to avoid the problem of imbalance?
that would be a waste of Mu metal
it saturates at very low field, shouldn't be used near a strong B source - much better near sensitive 'receiving' parts where the field is hopefully already attenuated by distance
better mag shielding for close to a power xfmr would be low carbon steel in decent thickness, say 1/16 - 1/8" perforated (for cooling air flow)
the cheap iron making up a box surrounding PS xfmr and rectifier + reservoir caps where big nonlinear currents can cause problems
it saturates at very low field, shouldn't be used near a strong B source - much better near sensitive 'receiving' parts where the field is hopefully already attenuated by distance
better mag shielding for close to a power xfmr would be low carbon steel in decent thickness, say 1/16 - 1/8" perforated (for cooling air flow)
the cheap iron making up a box surrounding PS xfmr and rectifier + reservoir caps where big nonlinear currents can cause problems
The DC component of the current delivered by the toroid has to be very small to prevent saturation, so an extra winding with full-wave rectification would be fine, an extra winding with half-wave rectification and no AC coupling might not be OK. Mind you, I don't know how much imbalance would be needed to actually cause trouble. Toroid datasheets usually don't tell you that either.
It forms a reactance based (lossless) voltage divider with the load, you may need to adjust for both line frequency and load current to get the voltage into the target range.
I did this approach (reactance based voltage division) just once in a design almost 20 yrs ago. That unit is still working, but initially I kept popping the 0.1uF film caps I used. I ended up with an array of 4 of them in series parallel and a small series resistance to limit transient/surge current through the cap.
I think winding a bias winding or perhaps just using a small toroid to develop the bias voltage will prove to be more reliable and less anxiety provoking.
I did this approach (reactance based voltage division) just once in a design almost 20 yrs ago. That unit is still working, but initially I kept popping the 0.1uF film caps I used. I ended up with an array of 4 of them in series parallel and a small series resistance to limit transient/surge current through the cap.
I think winding a bias winding or perhaps just using a small toroid to develop the bias voltage will prove to be more reliable and less anxiety provoking.
Of course I prefer to wind the toroid with a separate 120V/20mA winding for neg. bias but as far as I understood from this thread, this will result in a DC unbalance?
Im not sure If I could do it this way, but a coupling cap to avoid DC in neg. bias voltage I have never seen before in any consumer tube amp.
Im not sure If I could do it this way, but a coupling cap to avoid DC in neg. bias voltage I have never seen before in any consumer tube amp.
You could use an X or Y capacitor rated for 300 V AC or more, like http://www.mouser.com/ds/2/212/KEM_F3092_F862_X2_310-1104381.pdf They are designed to survive large AC voltages and the occasional voltage peaks you can have at the mains.
By the way, what scares me a bit is the first vertical electrolytic capacitor in the voltage multiplier. If at start up the lower red transformer wire goes up from 0 to +340 V or so, this electrolytic capacitor gets a substantial part of the voltage across it with reverse polarity. This only happens during the first half of the first cycle and probably the rectifying behaviour of the electrolytic itself will prevent damage, but still I would put a diode across it just to be sure.
For the record, the remark about 300 V AC X and Y capacitors I made earlier is wrong. The AC coupling capacitor gets subjected to a big AC voltage on top of a big DC voltage, so that's worse than only applying 240 V AC.
For the record, the remark about 300 V AC X and Y capacitors I made earlier is wrong. The AC coupling capacitor gets subjected to a big AC voltage on top of a big DC voltage, so that's worse than only applying 240 V AC.
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