Two regulator circuits off one transformer winding?

[RDL2004]

Hi,

I need an isolated 5 volt supply for a digital meter in a bench power supply project. I was wondering if it was possible to tap into the power supply transformer's secondary and connect a completely separate 5 volt supply for the meter, or an I going to have to fit in another transformer just for the meter?

The reason I ask is because space is getting very tight in the case and I'm not sure I can find a small enough transformer to fit.

I tried using a 7805 off the existing rectified and filtered supply, but that doesn't work right for some reason, the 5 volts out of the 7805 drops in half when the meter is connected.

The meter will work off the regulated voltage, but if the voltage is set below 5 the display gets dim and at very low voltage it shuts off completely.

Thanks for any help

- Rick

 

[Nigel Goodwin]

If I'm correct in what I think you're doing?, then the supply for the meter module needs to be completely floating from the main supply. You can either use a seperate transformer, or a seperate secondary winding on your main transformer.

Assuming you don't have a seperate unused winding?, then you need another mains transformer! - however, as it's only for powering the meter, it need only be a very small one.

I went through this years ago building a power supply at work, and ended bu adding another small mains transformer.

 

[RDL2004]

Well, I guess to explain better what I was wanting to do, is to run 2 rectifiers in parallel off the same secondary. About the only space I have to fit another transformer is on the left side where there is a area about 1" x 2" open (where the 7805 is now). Though I could probably redo the pc board, it's about 50% bigger than it really needs to be.

- Rick

 

[Nigel Goodwin]

You can certainly run two rectifiers off a single transformer, but it won't help you, the resulant voltages are NOT isolated from each other!.

 

[eblc1388]

My suggestion is to wind some thin PVC wires or enamel wires on the toroidal transformer to give you another winding. It is very easy to do and you only need a few turns per volt. To test the idea just wound 10 turns and measure the AC voltage with a meter and work out the total turns required.

 

[RDL2004]

Fixed it

Thanks Nigel, you were right, two rectifiers works fine. I wired up a regulator circuit (bridge, filter cap and 7805) on a little breadboard and tapped it into the AC out of the main transformer. When I connected the meter to this separate supply it worked fine. The display is bright and the instability I was getting in the read out is gone.

It seems all it needed was better DC isolation, not total isolation. Specifically, these meters apparently do not like to share a common ground with the voltage they are measuring, even if the manufacturer says they will.

I don't really like this fix though because the 7805 is running kind of warm. I forget exactly what I measured but it's having to drop the voltage from over 25 down to 5.

I found a very small 5 volt 250 mA transformer at Mouser for under $3 and ordered one today. I think I'm going to go ahead and re-do the whole pc board. It was the first one I ever made and while it works, there's a lot of things that didn't turn out as well as they could have. Also, the mechanical part, the arrangement of the various components and controls could use a lot of improvements.

Thanks again guys for the help and info

- Rick