Load testing 2 rail P.S.?

Hi,

I am building a power supply that will power a pair of API style pre-amps I am building. I would like to test the p.s. with my DC electronic load. The instrument has only 1 channel, and I was wondering if I can just put a load on the + rail, without putting a load on the - rail?  ( And then check just the negative rail) There is also the phantom power rail, which is not always loaded normally.

The P.S. is a linear supply w/ a 120V to 28V transformer. I am using LM7818 + LM7918 regulators. The secondary of the transformer says it will deliver 500mA, which may be cutting it close with 2 pre-amps (its intended use) hooked up, which is why I wanted to load down the rail at 500mA for awhile to make sure the supply is stable.

I appreciate any and all advice.

Thanks very much!

George

As a rule, in a capacitor input power supply, the transformer secondary current rating needs to be at least 1.6 times the load current. Mains transformer designers tend to rate their secondaries at close to saturation so I tend to use 2 times to avoid  excessive dissipation in the transformer.

Cheers

Ian

Thanks for the info Ian!

George

1. If the secondary of the TX is 28VACthen the 39.2VDC is > the 35 volt limit for LM7818 or LM7918
2. If it is CT then 19.1VDC will not regulate, as you need 18V+2.5V or >20.5 @ the bottom of the ripple.
3.  As in #2 low line 10% you would need about 23VDC or more.
4.  (25v-18 v) *0.5A= 3.5 watts and you need a heatsink, as the free air for the LMxxxx is only about 1 watt

As Ian said the secondary should be 2x the DC load. This will give some safety and a cooler TX, Try a start with 36VAC CT @ 1A
I think that you need to work on a new plan to improve the basic PS for the project.

If you want to load the PS to 500ma why not place the load across the total supply (+18v to -18v)?

I hope this will help
Duke

Thanks Duke for the info. What you said makes sense. I suspected the transformer was insufficient.

Thanks,

George

How is it for this test? I want to know a lot, please tell me

Gsrokmix said:

The instrument has only 1 channel, and I was wondering if I can just put a load on the + rail, without putting a load on the - rail?  (

Click to expand...

You must load both rails; if you don't, the xfmr and reservoir caps will be half-loaded.