Good way to regulate +/-50v down to 42v?

[Spasticteapot]

I have a 72VCT transformer, and need to produce +/-42 volts. Can anyone recommend a good way to do this?

[tomchr]

A linear regulator...

I see at least two options. One is to use an emitter follower or source follower driven by a zener diode. I have used attached circuit for many years with success. The resistor will need to be sized to allow for both the base current AND the zener current at the worst case load current. If the load current is substantial or the Hfe (beta) of the emitter follower BJT is low, the base current will be substantial. In this case, either substitute a darlington device or use a MOSFET as a source follower. Just remember a 1 kOhm on the gate to stop any oscillations and a low-voltage (15 V) zener between gate and source to protect the gate on turn-off. The basic EF stage gives an output voltage that's 0.6-0.7 V lower than the zener voltage. A darlington device will give 1.2-1.4 V lower. The MOS will have even larger drop (see data sheet as the values vary all over the place). Note that the output voltage will vary with temperature. If the ABS MAX supply voltage of the circuit you are driving is 42 V, you should probably go with a 39 V zener...

The other solution would be to build a linear regulator around an op-amp and a pass transistor. The good ol' LM723 comes to mind. Maybe you can use it as a starting point, though, I don't think it can handle the voltage. But the topology could be used.

A completely different option... If your transformer has an international primary (i.e. set up to operate at 110, 120, 220, 240 V) it might have a 10 V or 20 V primary winding. Unless you're operating at 240 V mains, you could connect this winding in series with one of the other windings. That would give you more primary turns, which would result in a lower secondary voltage.

~Tom

[nigelwright7557]

Originally Posted by Spasticteapot

I have a 72VCT transformer, and need to produce +/-42 volts. Can anyone recommend a good way to do this?

Depending on the current a simple solution is two high power zener diodes.

[Spasticteapot]

I need about 6A, so I don't think zener diodes are quite up to the job. The darlington + zener combo sounds like the best option, as I've got volts to burn.

The transformer rather inconveniently does not have any additional windings. Short of buying a new transformer, this is likely the best option I have. That said, an additional 100W of heat dissipation at peak output is definitely a major annoyance.

[woody]

It might not hurt to build the raw dc supply and measure it under a load that's
about = to what you will be driving. Depending on your line voltage, transformer
regulation and voltage lost in the bridge rectifier your output could be a lot lower than + - 50vdc.

[nigelwright7557]

The amp might run of 50 volts if you dont turn it up too far.

I run a 300watt amp off +/-60 volts without any problems.

[Spasticteapot]

Originally Posted by nigelwright7557

The amp might run of 50 volts if you dont turn it up too far.

I run a 300watt amp off +/-60 volts without any problems.

The problem is that this is a 60 watt amp.

I'm probably going to go with the regulator. If I'm lucky, power drop after the rectifiers should put me no more than 6V above the target voltage, which should keep power consumption at sane levels.

[gfiandy]

The linear reg will work but to get the kind of current you are talking about you are going to need big transistors and they will probably need to be darlingtons or the current through the zenner will be very high. I have used tip122 and tip32 to do this in the past but I don't think they will quite work at the current you need and you may need a large heat sink. It might be cheaper to just buy a new transformer of the correct rating. It would certainly use much less power.

Regards,
Andrew

[woody]

You might look at the regulator used in the Zen 5. with a zener ~46v.

[paulb]

I used the series regulator to do what you're doing, with good results. You might want to look at Rod Elliott's capacitance multiplier project; basically you're just adding zeners to it.
Capacitance Multiplier Power Supply Filter