Advice on transformer use for bench supply

Thread Starter

YorkshireDave

Joined Jun 12, 2016
57
Hi
I've salvaged a 40 0 40 transformer from an old sub-woofer and would like to construct a bench supply whilst I'm sat twiddling my thumbs.
I've loads of LM317s and 2n3055s so was hoping I could use them to make a two output supply. Can I do that with the transformer I have or does it have to be 2 separate windings? Also, looking at 317 specs I suspect the input voltage (once smoothed with the huge caps that were in the sub's supply) might be too high. How can I pull it down 'simply'?
 

MrChips

Joined Oct 2, 2009
20,884
You need to set your goals for how much current, voltage, and polarity you need.
Show some photos of the sub-woofer boards and transformer with some reference to dimensions.
 

atferrari

Joined Jan 6, 2004
3,920
I am in a very similar situation - 38 0 38 volts transformer - 4A IIRC.

Never found a simple preregulator design whether for the negative or the positive rail (on account of the local limitations to get the right chips & inductors).
 

DarthVolta

Joined Jan 27, 2015
395
I tried copying an old schematic from the 1960's. It worked so/so in a certain range, but really I didn't have the right transformer for it. I have to experiment with that more.

But if you want to buy a few more parts, it's a lot of fun to make the older PSU's. There's all kinds of schematics from the early days, with just BJTs and/or op-amp's.

I'd guess the LM317 and pass transistor way is quick, but probably horribly inefficient

I have a 70-0-70 trans. from a stereo, IDK what to do with, but I want to keep it, no way am I scrapping it.
 
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Thread Starter

YorkshireDave

Joined Jun 12, 2016
57
I am in a very similar situation - 38 0 38 volts transformer - 4A IIRC.

Never found a simple preregulator design whether for the negative or the positive rail (on account of the local limitations to get the right chips & inductors).
Actually, that's the voltage from mine. It's yet another KEF PSW 2010 sub failure...

Mr Chips (made me smile as 'Chips' was my nickname when in maintenance - many years ago) I'm looking for 0-20v @ no more than 2A from each supply. Actually, just looking at that I'd prob not need the 3055s so long as I put the 317s on heat sinks.
The image attached shows the supply board & trannie which is approx 120mm dia x 70 high. The existing cct uses the huge dropper resistors you can see on heat sinks as the trannie is well oversized for its application apparently - hence the number of these things littering landfill sites. Anyway, this is 'it'.

BTW, I'm not bothered about efficiency too much as it'll not be in use all day every day. If its going to get warm I've a stack of old PC fans to cool things down.;)
 

Attachments

Tonyr1084

Joined Sep 24, 2015
4,375
Hi
I've salvaged a 40 0 40 transformer from an old sub-woofer and would like to construct a bench supply whilst I'm sat twiddling my thumbs.
I've loads of LM317s and 2n3055s so was hoping I could use them to make a two output supply. Can I do that with the transformer I have or does it have to be 2 separate windings? Also, looking at 317 specs I suspect the input voltage (once smoothed with the huge caps that were in the sub's supply) might be too high. How can I pull it down 'simply'?
From what I can gather, the LM317 input voltage can be as high as 40 volts DC. Using just two diodes with your center tapped transformer you can get a full wave rectification. However, 40 VAC is probably an RMS value, meaning your rectified and filtered voltage can be 1.414 times higher than 40 V. In other words, 56.56 VDC, which will probably smoke the 317, and the 317 is typically good for 1.5 amps. The one I looked at has internal safety features such as thermal overload protection and current protection; meaning it will shut down when specs are exceeded. So my statement about it smoking is in error.
 

Thread Starter

YorkshireDave

Joined Jun 12, 2016
57
I found those too, but hadn't noticed the 100W rating. Thank you.
So, can I do what I wish to?
If so, how do I stop the 317s being fried once they fed with a smoothed voltage? How can I reduce the voltage to each to say 35v?
Can I use the secondaries as separate supplies into two FW rectifiers?
 

MrChips

Joined Oct 2, 2009
20,884
Personally, I would use a bridge rectifier plus centre-tap transformer output and created both positive and negative supplies.
This will drop the voltages by half.
 

crutschow

Joined Mar 14, 2008
24,723
You could use a 2N3055 as an emitter-follower preregulator to reduce the voltage before it goes to the LM317.
LTspice simulation of example circuit is shown below.

It has two 2N3055s in a Darlington stage to minimize the Zener current.
As can be seen from the power dissipation of Q1 and U1, they both need to be on healthy heatsinks at higher current levels.

1586274585789.png
 

Thread Starter

YorkshireDave

Joined Jun 12, 2016
57
Sorry guys. I'm a tad dumb.
So, am I correct in thinking that with the smoothed supply voltage being as high as it will be, my best option is to look instead at the 317HVs driving some 2n3055s? As the HVs take up to 60V in, Im assuming I'd get a wider ranging variable supply and still be able to use most of my re-purposed parts!?
 

Dodgydave

Joined Jun 22, 2012
9,080
Actually, that's the voltage from mine. It's yet another KEF PSW 2010 sub failure...

Mr Chips (made me smile as 'Chips' was my nickname when in maintenance - many years ago) I'm looking for 0-20v @ no more than 2A from each supply. Actually, just looking at that I'd prob not need the 3055s so long as I put the 317s on heat sinks.
The image attached shows the supply board & trannie which is approx 120mm dia x 70 high. The existing cct uses the huge dropper resistors you can see on heat sinks as the trannie is well oversized for its application apparently - hence the number of these things littering landfill sites. Anyway, this is 'it'.

BTW, I'm not bothered about efficiency too much as it'll not be in use all day every day. If its going to get warm I've a stack of old PC fans to cool things down.;)

Your Transformer will be wired like this circuit, so you could use the bridge rectifier and smoothing capacitors , you will need to drop the voltage down to 40V to input into the LM317 or LM337 , with the ideas posted. Ideally you need PnP pass transistors for extra current, or if you use LM337 you can use NpN.


main-qimg-93ca34ddde7d3279bf542ed231ffde35.png
 

Thread Starter

YorkshireDave

Joined Jun 12, 2016
57
Your Transformer will be wired like this circuit, so you could use the bridge rectifier and smoothing capacitors , you will need to drop the voltage down to 40V to input into the LM317 or LM337 , with the ideas posted. Ideally you need PnP pass transistors for extra current, or if you use LM337 you can use NpN.


View attachment 203776
Thanks guys.
MrC. Oddly it gives +18 & -18. As the input voltage is so high it uses dropper resistors to reduce voltage into the regulators. It seems this arrangement that is one of the root cause of its unreliability so I'm not keen to try to replicate that.

DodgyDave. I see my choices as either dropping input voltages OR (as said above) using an HV version of the 317 (takes up to 57v in) followed by using the 317 to switch a 2n3055 upping the current capability.

Does that make sense?
Can I use the two secondaries to run two 317s?
 
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