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Old 16th November 2006, 01:35 AM   #1
TGRANT is offline TGRANT  United States
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Default 12 Volts from 38 using IC Reg Chips

Power supply question

I am building a bench power supply to use to test my DIY amps and subcircuits. I assembled a raw DC power supply that puts out DC measured at 38 volts positive and negative (after the bridge rectifier) of unloaded voltage. Iíll use this as a raw supply for the power amps. I made the supply by putting two 12.6 volt 2A transformers with the input AC in parallel and outputs in series. Iíve seen this method in a book by G. Randy Sloan, and it does seem to work. I used these particular transformers since they were available locally and were inexpensive (Radio Shack). There are the usual smoothing capacitors and so on in the circuit.

Hereís my question. I want to jump off a 5, 12 and 15 volt positive and negative supply using 78XX and 79XX IC regulator chips. This will be used to test the preamp, tone and crossover circuits (op amp versions), thus dropping the 38 volts unregulated DC to 12 volts regulated. ButÖ according to the datasheet, the maximum input voltage for these chips is 35 volts. I actually assembled it and used it for a while before I noticed the error. Though there doesnít seem to be any problems now, Iím afraid Iíll eventually fry the IC voltage regulator by exceeding the maximum recommended voltage.

I know the voltage coming out of the transformer will drop when thereís a load, but Iím not sure what the load will be, so I canít measure it.

I guess I have a few options. Can I assume the load will drop the voltage to less than 35 volts and just keep the circuit like it is? The ICís are cheap and if they fry, itís not the end of the world. I noticed in a previous post that someone suggested daisy chaining some diodes (#5 1N4004 rectifier would drop 3.5 volts, which is about what I need). Are these in series after the bridge rectifier and before the IC? This seems a little jury rigged, but sounds like it will work. I can try to get IC regulators that will handle more voltage, but these arenít readily available. I though of creating a voltage divider by putting resistors in parallel with the smoothing capacitors. Iíll get some voltage division and bleeder resistors at the same time, though not knowing the load makes it difficult to pick a resistor value. The varying load will make the voltage division variable, but Iím thinking the IC regulators should handle it and give me the regulated voltage anyway. I know there are circuits out there using series pass transistors and related circuitry that will solve the problem, but Iím using the KISS principle if I can, so would like to use the IC regulators possible.

Finally, Iíd like to use a similar supply for an amp Iím building now since it will run off 38 volts and have to supply 12-15 volts for the built in preamp and EQ circuits.

Any ideas? Iíve searched the forum and canít find the exact answer Iím looking for. Thanks.
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Old 16th November 2006, 02:30 AM   #2
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Join Date: Sep 2005
Two suggestions (you can use them in combination too):

Put a couple of rectifier-type diodes between your power supply lines and your equipment

Use a double regulator scheme:
step the 38V down to 24V using the 7824 regulator (max output current ~2 amps, max input voltage = 40V, depending upon what package you get it in).

Use this 24V to step it down to 5, 12, 15, etc. using other regulators after the 24V regulator (they also have a 7924 regulator for -24V). This wastes more power because of the double regulators, but you could be pretty sure not to exceed your equipment specs by doing so.
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Old 16th November 2006, 03:11 AM   #3
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Join Date: Dec 2004
Location: Reston, Virginia
Use the center taps on the transformers. Connect to diodes and filter caps and you will have plus and minus 19v in addition to the 38V for the amplifiers.
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Old 16th November 2006, 10:20 AM   #4
TGRANT is offline TGRANT  United States
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Join Date: Sep 2006
Two Great suggestions. I hadn't thought of using the center taps, but it makes sense!

Thanks
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