Please help with dual power supply

Thread Starter


Joined Dec 7, 2008
I need 2 psu's. The one has to drive a LM7808 for the timer circuit and the other one has to drive a LM317T to get a 6,65v out. I want each output to run at 250mA. I want to run it from one transformer.

All i wanted to know if it's ok to do it as i showed it on my schematic?
Thanks for the help


hi u can do this but my suggestion is that you have to check your circuit using proteus or multisim simulation software.... well your circuit looks fine......

Thread Starter


Joined Dec 7, 2008
First off...are u sure ur transformer can handle more than 500mA
I'm not so mush worried about the current. If the output is only 150mA it'll still be ok. All i need to know is, if it's ok to use the transformer the way i showed it


Joined Apr 2, 2009
Using a bridge to rectify will reduce current capacity to 0.707%.

Besides u cannot run a transformer at it's maximum rating. To allow it last long and cooler operation transformer should be atleast 10% more that ur desired current.

If u want 500mA to be ur full load current than the transformer should be at least around 600 to 700mA secondary capacity. This way when u load the transformer the secondary voltage will not sag thus improving load regulation.

And if that transformer is OK for ur app, try using a full wave rectifier, not the bridge mode. This way ur can get more out of the rectifier plus filter configuration.

Using two diodes at each output and using the center tap as ground u can get higher current capacity, less diodes, low cost and better regulation.


Joined May 11, 2009

I would have used Use a dual complementary rectifier. But then you have to replace the LM317 with LM337. Which is the negative variant of the LM317.
In this setting the rule of thumb is that the transformer must be able to deliver an AC current 1.8 times the DC curent. In your case that will be 450mA for the both secondary outputs.


Joined Sep 9, 2010
I think the OP's diagram will work, but if I needed two voltages from one transformer, I'd just rectify and filter once, and then feed the two (or more) regulators. You don't need two full-wave bridges with filter caps.

I admit that I haven't done the math to calculate which is more efficient - it'll be poor (linear) efficiency either way - but I'm just thinking about component count and simplicity.