LM317 / LM337 power supply

Hi,


Just built this simple board from China. For testing it's connected to a 12 VAC transformer on one of the AC connections and ground. I set the regulators for +12V (LM317) and -12V (LM337). This works fine.


I guess this board is supposed to be used with a transformer having two 12 volt windings (or a 24 Volt transformer with a center tap). Is (2x) 12VAC enough to get a stable +/-12VDC or should I use a transformer with a higher secundary voltage?


Can anyone confirm this please?


Regards, Gerrit
 

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Hi,


Just built this simple board from China. For testing it's connected to a 12 VAC transformer on one of the AC connections and ground. I set the regulators for +12V (LM317) and -12V (LM337). This works fine.

I guess this board is supposed to be used with a transformer having two 12 volt windings (or a 24 Volt transformer with a center tap). Is (2x) 12VAC enough to get a stable +/-12VDC or should I use a transformer with a higher secundary voltage?

Can anyone confirm this please?

Regards, Gerrit


Hi Gerrit,

It depends on how hard you load the transformer. If not too hard, 2x12Vac will do. If you haven't bought the transformer yet and will load it close to the nominal current, consider 2x15Vac.
 
Hi FaixFrench,


I may have a load up to 500 mA. Will 2 x 15 Vac be better for regulation? The transformer will be 1 A minimum for each winding).


Regards, Gerrit

Its a trade off.
You dont want too low a voltage that the regulator starts to drop out.
You dont want too high a voltage that generates lots of heat in the heat sink.
 
Hi FaixFrench,
I may have a load up to 500 mA. Will 2 x 15 Vac be better for regulation? The transformer will be 1 A minimum for each winding).
Regards, Gerrit


Hi Gerrit,
2x15Vac will leave you more voltage margin but (unfortunately) also increase heating (voltage drop times the load current) in the regulators.
An LM337 saturates at around 2V from input to output with 500mA current. To leave some margin you often calculate with 3V. Then, 12V at the output becomes 15V at the input.
12Vac rectified is some 16-16.5Vdc when rectified.
15Vac rectified is some 20V-21Vdc when rectified.
Both the 16.5Vdc and the 21Vdc will drop when loaded. On top of that, you have a load current dependent voltage ripple on the storage capacitor and have to take net voltage variations into account.
I would go for 2x15Vac as I am afraid that 2x12Vac will be too marginal.
 
Last edited:

Vovk Z

Member
2011-10-30 10:32 pm
Kyiv
I may have a load up to 500 mA. Will 2 x 15 Vac be better for regulation? The transformer will be 1 A minimum for each winding).
Yes, usually +3VAC is better for regulation. So 15VAC is just ok for 12VDC. It depends on transformer ouput impedance too.
12VAC is too close to the "edge" and can be choosen only if you want to lower the heat to minimum and calculated all nicely.
 
Keep in mind that secondary voltage is generally specified at nominal (full) load and is likely to be higher at idle. (I've had a nominal 12 V unregulated wall wart put out well over 15 V when unloaded.) Thus a 12 V standard dropout regulated supply will usually do just fine with 12 V~ secondaries, and 15 V at 15 V~, a coincidence that makes it easy to remember.

Given the stated max current draw, the xfmr should be rated 15 VA at the very minimum, 20 VA would be better, 30 VA if you want to be generous. I'd prefer a type that is adequately dimensioned and has low loss, a cheapie may end up frying away a number of watts all by itself.
 
Hi Gerrit,
2x15Vac will leave you more voltage margin but (unfortunately) also increase heating (voltage drop times the load current) in the regulators.
An LM337 saturates at around 2V from input to output with 500mA current. To leave some margin you often calculate with 3V. Then, 12V at the output becomes 15V at the input.
12Vac rectified is some 16-16.5Vdc when rectified.
15Vac rectified is some 20V-21Vdc when rectified.
Both the 16.5Vdc and the 21Vdc will drop when loaded. On top of that, you have a load current dependent voltage ripple on the storage capacitor and have to take net voltage variations into account.
I would go for 2x15Vac as I am afraid that 2x12Vac will be too marginal.
This is sound advice on solid technical argument.

When you shop for the transformer, get high quality one. Poor transformer can radiate hum which will be pick up by the amplifier. I had bad experience with toroidal purchased from seller in China on e-bay. The toroidals from Antek work well and are quiet. Antek also sells on e-bay