# can a negative voltage be outputted without using a center tap transformer?

#### simpsonss

Joined Jul 8, 2008
173
hi,

Is it possible to output a -12v using 7912 without using a center tap transformer but a 12v-0 at secondary side's transformer?

thank you.

#### SgtWookie

Joined Jul 17, 2007
22,226
Well, yes... BUT, you won't be able to support much of a load.

You'll lose around 1.5v across the bridge rectifier.

The 7912 has a 2v dropout (minimum difference between the input and output terminals)

You'll need around 2,200uF with 60Hz input (mains frequency) per 1A of output load - and you'll still have around 10% ripple.

So, you really need a transformer with an 18vac secondary.

#### studiot

Joined Nov 9, 2007
4,998
I do wish members would take the time to provide enough information for those who answer not to have to guess what the real question is.

Since you can obviously just turn your supply round I am guessing that you don't actually want a 0 - 12 volt supply you want to add a negative rail driven from the same supply as an existin g positive one.

This is a common requirement and useful to be able to accomodate.

There are 5 basic ways of doing it, although as Wookie said you have to trade some current capacity to achieve it.

1) You can zero one end of the transformer and drive half wave rectifiers on alternate half cycles to create a plus - 0 - minus output of equal current capacity.

2) You can introduce a second 1:1 isolating transformer and derive a second negative supply from it. This arrangement allows you to send the bulk of the current to either rail.

3) You can drive a second bridge rectifier through a series capacitor from the transformer. This method is only really suitable to bleed off a small proportion of the available power.

4) You can use the positive rail to drive a switched mode dc to dc convertor and generate a second negative rail. This does allow the negative rail to have a different (-ve) voltage from the positive. In principle there is no limit to current split between the rails in this method.

5) There are ICs / modules that will accept single rail input and provide dual rail output. These do not have haigh current capacity but are very convenient.