Transformer(s) for a dual channel lab PSU

Thread Starter

Manden

Joined Dec 15, 2020
7
A bit over a year ago i decided to build a lab psu since the one I have now only delivers 300mA but I never really finished the project. However I have now decide to give it another go now that I'm a bit more experienced in electronics. I have also decided I want 2 channels for +/-.

I already have a 2x12 100 VA transfomer but I would like to be able to switch between 12 and 24V AC and not have to deal with ridiculous amounts of heat. But I cant find a four secondary transformer anywhere or one with 2 center tapped secondaries.

So my question is would there be any downsides to using one transformer per channel as long as they are identical?
 

dl324

Joined Mar 30, 2015
14,700
So my question is would there be any downsides to using one transformer per channel as long as they are identical?
The main disadvantage would be more weight and volume for 2 transformers. They don't need to be identical.

If you're overly concerned about heat, you could use switching regulators.

If I was inclined to build, I'd add an option for the power supplies to track.

I went through a tracking design, for the purposes of helping someone learn to design opamp circuits, in this thread. We never got to the simplification stage...
 

Thread Starter

Manden

Joined Dec 15, 2020
7
If you're overly concerned about heat, you could use switching regulators.
First of thank you for your answer I will consider my options now that I have the information I need

I would very much like it to be linear so switching is out of the question. And it's not that I'm overly concerned with heat I have room for a big heatsink I would however like something like 0-20V 0-2A ish and that could potentially dissapate alot of power in my pass transistor (+30W) so thats why I'm thinking two windings per channel.

Tracking seems like a nice addition I will look into this
 

dl324

Joined Mar 30, 2015
14,700
I would however like something like 0-20V 0-2A
With 12V transformers, you won't be able to make a bipolar supply if you want 20V.

The best you could do for a bipolar supply +/-16V.

You could make the outputs independent so you can stack them when you needed more than 16V.
 

Thread Starter

Manden

Joined Dec 15, 2020
7
With 12V transformers, you won't be able to make a bipolar supply if you want 20V.
This is why I'm considering two transfomers since 4 secondary ones are impossible to find.

The one i have is 2x12 so 24V if I put the windings i series.
 

crutschow

Joined Mar 14, 2008
30,116
Since you likely wouldn't need dual 24V outputs, just make two isolated output12V supplies and connect their outputs is series when you want 24V.
You can then also connect them in plus - minus configuration for ±12V, or in minus - minus parallel for 12V @ double current.

An LM338 5A adjustable regulator should work for each supply, which is basically a higher-current version of the ubiquitous LM317.

You could configure a voltmeter with two switches to measure either voltage in parallel or series, as desired.
 
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Thread Starter

Manden

Joined Dec 15, 2020
7
Since you likely wouldn't need dual 24V outputs, just make two isolated output12V supplies and connect their outputs is series
I would very much like to do at least +/- 15V though I'm looking into the possibility of using a fullwave doubler at the cost of less output current.
 

KeithWalker

Joined Jul 10, 2017
2,406
I am making a similar dual output power supply and ran into the same problem as I want two independent 0 - 16V supplies that I can connect as positive and negative supplies or two different positive voltages with common negative. I ordered two 24V 2A switching supply modules from AliExpress. They are very inexpensive and are much lighter and smaller than a 100W transformer.
https://www.aliexpress.com/item/32799095889.html?spm=a2g0o.productlist.0.0.48fa152fFKFhyX&algo_pvid=1b29ed7a-3f70-41c9-836c-93b7c083cb5f&algo_exp_id=1b29ed7a-3f70-41c9-836c-93b7c083cb5f-0&pdp_ext_f={"sku_id":"64005278040"}&pdp_npi=2@dis!CAD!!5.61!!!1.9!!@21031a5516536610057358046e45ca!64005278040!sea
 

crutschow

Joined Mar 14, 2008
30,116
I would very much like to do at least +/- 15V though I'm looking into the possibility of using a fullwave doubler at the cost of less output current.
You could use a doubler but would have to derate the RMS transformer rating to about 25%, thus your 100VA, 2x12V transformer would be limited to about 1Adc at each winding.
 
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