Transformer(s) for a dual channel lab PSU - Page 1
Forum section: electronics / beginners

author: Rasmus
author_rank: Contributor
timestamp: May 26, 2022, 01:24:29 pm
content: 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?

author: jonpaul
author_rank: Super Contributor
timestamp: May 26, 2022, 06:11:54 pm
content: check dual power supply schematics of classic HP, Lambda supplies.One or two secondaires sufficient as the rectification can be setup for positive and negative output from one winding and all lab supplies have a common between the two outputs.Jon

author: bdunham7
author_rank: Super Contributor
timestamp: May 26, 2022, 06:58:25 pm
content: Quote from: Rasmus on May 26, 2022, 01:24:29 pmI'm not sure exactly what you want your output configuration to be--2 isolated 12/24V switchable channels or a +/- 12/24 volt setup with a common?If you got a second transformer that would be fine and you could set it up any way you like. However, you can also still do 2 fully isolated 12/24 switchable channels by using a full-bridge for 12V and a full-wave doubler for 24V.

author: Gyro
author_rank: Super Contributor
timestamp: May 26, 2022, 07:17:28 pm
content: If you already have the two transformers then I would make use of them. This will give you complete independence of the outputs... +ve and -ve is one use case but what if you want a 12V and a 5V supply with a common ground for instance, or maybe even paralleling them for additional current.Having found that you have already outgrown one supply, I wouldn't limit yourself at an early stage on this one. You already have the most costly items.

author: Rasmus
author_rank: Contributor
timestamp: May 26, 2022, 07:22:14 pm
content: Quote from: bdunham7 on May 26, 2022, 06:58:25 pmMaybe I wasn't clear enough sorry for thatI'm trying to make two isolated 0-25V 0-2A DC output channels. The reason I want 4 windings is to not potentially dissapate +30W in my pass transistor by switching between 1 and 2 winding (12 and 24VAC) using a relay.

author: Benta
author_rank: Super Contributor
timestamp: May 26, 2022, 07:39:47 pm
content: Not really worth the effort IMO and can open up other cans of worms. Your potential power loss per channel is around 60 W max, so 120 W total.Yes, it takes some aluminium, but hey: it's a bench top box. It doesn't need to fit in your pocket. And +100 W pass transistors are cheap.

author: bdunham7
author_rank: Super Contributor
timestamp: May 26, 2022, 07:43:33 pm
content: Quote from: Rasmus on May 26, 2022, 07:22:14 pmOK, that's pretty much what I was talking about. If you want that much current @ 25V, you will need two of those transformers because the AC RMS current in the transformer is always more than the DC average output current. Using one transformer per channel and running the secondary windings in parallel for 12V and series for 24V will work. You would also be able to draw more current in the lower range if you like.Otherwise, if you wanted to use just one transformer and could settle for 2.5A up to 12V and 1.25A (or so) from 12-24V then the bridge/full-wave set up also might work.

author: Rasmus
author_rank: Contributor
timestamp: May 26, 2022, 07:57:09 pm
content: Quote from: bdunham7 on May 26, 2022, 07:43:33 pmOtherwise, if you wanted to use just one transformer and could settle for 2.5A up to 12V and 1.25A (or so) from 12-24V then the bridge/full-wave set up also might work.But this would only work for one channel 0-25 though wouldn't it? Otherwise when I put the windings in series to achieve 24VAC the other channel will have nothing.

author: Rasmus
author_rank: Contributor
timestamp: May 26, 2022, 08:00:58 pm
content: Quote from: Gyro on May 26, 2022, 07:17:28 pmI only have the one unfortunately but it seems I have to get either a different or a second one anyways so not much difference economy wise.

author: bdunham7
author_rank: Super Contributor
timestamp: May 26, 2022, 08:19:57 pm
content: Quote from: Rasmus on May 26, 2022, 07:57:09 pmNo, you would use one winding per channel. The FW doubler setup would give you the 24VDC from 12VAC at half (or thereabouts) the current you'd get otherwise. Switching it would be an interesting challenge, but I seem to recall an earlier thread where someone proposed a clever (simple) way of doing this. I don't have time to search right now, but perhaps someone remembers this.

author: Gyro
author_rank: Super Contributor
timestamp: May 26, 2022, 08:37:32 pm
content: Quote from: Rasmus on May 26, 2022, 08:00:58 pmAh sorry, I misread what you wrote.

author: Rasmus
author_rank: Contributor
timestamp: May 27, 2022, 06:33:29 am
content: Quote from: bdunham7 on May 26, 2022, 08:19:57 pmHmm interesting I thought only samll amounts of current like in the mA range could be drawn from these circuits like with charge pump inverters. I will definetly look in to this and try it out today. Thank you!I'm trying to keep it linear though so no switching would be prefered even though I would like to have a go at switch modes at some point.

author: Gyro
author_rank: Super Contributor
timestamp: May 27, 2022, 08:03:03 am
content: Yes, it can definitely be done, certainly for voltage doublers. You just need much larger (electrolytic) capacitors.This post by Circlotron may be helpful to you, it shows 'normal' / voltage doubler switching using a single contact...https://www.eevblog.com/forum/beginners/transformer-tap-switching-between-serial-and-parallel/msg3580474/#msg3580474The whole thread is about various forms of tap switching.P.S. It is the same arrangement as used by older PC PSUs that had a switch to select between 120V (voltage doubler mode) and 240V (normal mode) mains input voltages.P.P.S. Just a thought about current capability. The normal rule of thumb is that, with a bridge rectified, capacitor filtered supply you can expect 0.62 x the transformer secondary current rating as its DC current capability. Taking each 12V 50W secondary, this is about 4A of AC current. Multiplying by 0.62 gives you about 2.5A of DC current. Voltage doubling would reduce this to 1.25A maximum for your higher voltage range. Personally, I would use 2A and 1A as conservative maximums.I have attached a useful guide sheet from Hammond Transformers for reference. Ignore the Choke filtered examples - the chokes would be far too big and expensive at this current level (best left for valve amps).

author: Rasmus
author_rank: Contributor
timestamp: May 27, 2022, 09:20:23 am
content: So I just had a look in my basement because I knew had an 18V transformer down there I ended up not using the first time around since I wanted to switch between secondaries so I got the 2x12 instead. But turns out this one has four windings 2 x 18V 40VA and 2 x 18V 0.5A. This could give me 2.72 amps per channel and then multiplied by 0.62 I might be able to get 1.5A and 0-18V which seems like a good middle ground sacrificing both some voltage and current from my original specs. I will however need bigger cooling so I will have to consider my options and maybe experiment a bit over the weekend.

author: Rasmus
author_rank: Contributor
timestamp: May 27, 2022, 09:21:43 am
content: Quote from: Gyro on May 27, 2022, 08:03:03 amThank you very much appreciated I will look into to this!

author: jonpaul
author_rank: Super Contributor
timestamp: May 27, 2022, 09:23:55 am
content: We use Signal Transformer (BEL fuse) USA very wide range, dual P, Dual S, split bobbin, etc.https://www.belfuse.com/product/product-listing?navCategory=powertransformerLinearTransformersSignalTransformerProductType&toPage=1Good prices and stockJon

author: bdunham7
author_rank: Super Contributor
timestamp: May 27, 2022, 04:06:09 pm
content: Quote from: Gyro on May 27, 2022, 08:03:03 amThat's the thread I was looking for. One unadvertised feature of that circuit is that it provides a bypass diode for the capacitors in the doubling mode which prevents reverse biasing the filter caps on startup, overload or malfunction. So a bridge would still be a good idea even for a fixed FW doubler in a general-purpose use case.I just made an LED driver PSU using FW doubling and I'm only using 470uF capacitors for 300mA output--so the capacitors don't need to be very huge. 630uF would yield reduced voltage ripple, but 470uF actually improved the power factor and reduced thermal losses. The regulator can handle the ripple without any issues.