Dual volt PSU help needed

Thread Starter

Man_in_UK

Joined May 13, 2008
176
I need to build a dual volt PSU and I am struggling to find a schematic that works.
I need +6DC & -6DC 200ma each from a single AC supply. (I realise a transformer with dual secondary's would solve the problem but I need to run this from a single wall mounted plug-in PSU - 12 to 15 VAC)

One schematic I found that almost got there is shown below. This does work until you try to get 200ma from the +6 rail, then the 7806 complains.
(test circuit did use capacitors but not shown in picture)

All suggestions welcome.
1633459014459.jpeg
 

Thread Starter

Man_in_UK

Joined May 13, 2008
176
I should have said something about its use, as I think it need to be regulated.
A pool table round these parts has a battery pack (2 x 6v). It powers the coin mech, ball release servo & a controller PCB that keeps track of usage. Some of the parts use a single 6v but some use both together as 12v.
I think with the PCB being designed to run on a battery, I will need a smooth consistent supply.
 

MrChips

Joined Oct 2, 2009
24,620
It would help us to provide a proper solution if you could show us the rest of your circuit at at least tell us what are all the devices to be powered in your project.
 

Thread Starter

Man_in_UK

Joined May 13, 2008
176
show us the rest of your circuit at at least tell us what are all the devices to be powered in your project.
My post above tells a bit of the story.
I have had success in replacing the two batteries with PSU's. I had some PSU's left over from another project built around a version of this .....................
https://uk.farnell.com/texas-instru...c-conv-non-iso-pola-adj-o/dp/3009748?st=dc/dc
Being completely isolated they are happy to run with one negative output connected to the positive from another. Both happy to supply an independent 6VDC but also 12VCD for the parts of the pool table that needed it. All from a single 9VAC transformer.

Those DC/DC converters do work but are much too expensive and I don't need 3A current. I might have to build a number of these so cost is something I need to keep in mind.

I do not have a schematic of the pool table. I only know that both 6v batteries are wired into a 4 pin connector. The PCB they plug onto joins the two middle pins together creating 6 + 6. One battery is always more flat than the other, so I assume one of the high current devices only runs on 6v.
 
Last edited:

dl324

Joined Mar 30, 2015
13,527
I realise a transformer with dual secondary's would solve the problem but I need to run this from a single wall mounted plug-in PSU - 12 to 15 VAC
You can build a bipolar supply with an untapped secondary (or dual secondaries) using half wave rectifiers. What is the current rating for what you have?

200mA is too much current for a voltage divider.
 

ericgibbs

Joined Jan 29, 2010
14,183
I need to run this from a single wall mounted plug-in PSU - 12 to 15 VAC)
Hi Man,
Use Post #2 , with 12V transformer [ preferable 9V transformer ] and 6V. 1Amp regulators.
Do a test load of 200mA on both Vouts, check temperature rise in the regulators, they may require a suitable heatsink, depending upon the transformers secondary voltage value.

E
 

Thread Starter

Man_in_UK

Joined May 13, 2008
176
So what AC voltage do you have? 12-15VAC or 9VAC? What is the current capability of the transformer?
[/QUOTE
At the moment I am getting away with running two separate units built on the PTM78020 DC/DC module, both from a 9VAC 1A.
If the new circuit is going to be splitting a 12 volt supply in half ..... I assume I might have to go up to 12 - 15. The AC supply has not been chosen yet, I will get whatever one is needed to run the new design.
 

Thread Starter

Man_in_UK

Joined May 13, 2008
176
Eric & MrChips have got exactly the same circuit. (I skipped over it too quickly at first as it looked too simple to be a valid solution, I apologise)
Do I just feed the - into a 7906 & the + into a 7806 to get both regulated?

(fingers crossed)
 

Thread Starter

Man_in_UK

Joined May 13, 2008
176
The circuit shown in post #2 is working but I am drawing more current than I first expected.
One of the rails peak at 700ma when a servo motor starts up causing drop in volts.

I assume raising the 470uf cap will help - can you suggest an alternative value?

Thanks
 

ericgibbs

Joined Jan 29, 2010
14,183
Hi Man,
If the diodes run hot, upgrade to say 3Amp versions, try it first

A Rule of thumb fo smoothing Caps is 1000uF per Amp for simple rectifiers.
E
 
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