Power supply negative rail

[Valie]

Hi there, I built a 18 v -/+ split supply using 7818 and 7918 Ic`s. The wiring is correct on the smoothing capacitors(1000mf) on the intake side. When I measure voltages on the outputs I get 18v on the positive side and 24v on the negative side (no load ). Shouldnt it be the same voltage?

 

[Mooly]

Does the 7918 regulator specify a minimum load ? Try adding a 1k across the output. Remember the pinouts are different for the 78 and 79 types.

 

[Valie]

Thanx very much, I connected a 1kohm resistor over the output terminal and the voltage reads correct, do I leave the resistor there or can I remove it and connect the mixer for which this power supply is built for?

 

[Mooly]

You'll probably find the mixer draws more than enough current to meet the minimum required in which case no resistor is needed.

The data sheet for the 7918 should tell you what the minimum current is, typically it will be around 3ma to say 12ma worst case.

 

[Valie]

Thanx very much, you made my day. This is an Inkel MX 1600 mixer that I got without the external power supply. I had to trace a lot of wires to work out the circuits that is needed to get this thing working again. So far Ive worked out that I need 3 x 18v+/- rails, 1 x 12v
1 x 6v a/c and the 48v phantom supply. I should connect very soon. Thanx again.

 

[Mooly]

Your welcome

(and that's sounds quite a complex PSU with all those rails, so good luck, it sounds like you are doing a good job)