Ok, I was thinking of a linear supply, using three terminal positive and negative regulators with a series bypass transistor on each.

I'm sure you wouldn't need any diagram for that.

 

Is your mains 120V? And does the transformer has a 240V option?
If so, wire the primary for 240V and run it on 120V. Then the outputs will be within the regulator max voltage range.
You will not be able to get to full power out but you indicated earlier that you did not need full power anyway.

 

Is your mains 120V? And does the transformer has a 240V option?
If so, wire the primary for 240V and run it on 120V. Then the outputs will be within the regulator max voltage range.
You will not be able to get to full power out but you indicated earlier that you did not need full power anyway.

Take a hint from TS's screen name. AC power is 230V, same as yours.

 

Let's go back to the start as some participants simply haven't understood.

My objective is to build a variable voltage bench supply (ideally 2 outputs) using as many parts as I possibly can from those I already possess.

I have probably 6 or so transformers. All but 2 are too small and the one shown earlier is the smallest capable of delivering what I need. The other is just huge and wont fit into any enclosure I have.

As I have already said, I have loads of LM317s, 2n3055s and 20A bridge rectifiers, so basically enough to cobble together a supply out of what I have. Yes, I could go to fleabay and buy a brand new variable supply for £35. However, that would not re-purpose the stuff I have in stock, not press me to learn and most importantly would take 4 weeks to arrive from China.
Whilst it may never win any beauty contests, I'm NOT bothered so long as it works and works reliably.

So, my transformer is 40-0-40 100VA. My original question was about how I could ensure Vin to the 317 stayed low enough NOT to fry it. It was suggested that instead of a bridge rectifier I go to full wave before smoothing as the voltage would be reduced. As I don't have any power diodes, I did what I saw a simple and re-purposed the bridge rectifiers. This reduced voltage further as it introduced another diode drop.

I'm not trying to be inflammatory at all, just looking to learn and to recycle stuff into something useful and usable instead of filling the bins.

 

Is your mains 120V? And does the transformer has a 240V option?
If so, wire the primary for 240V and run it on 120V. Then the outputs will be within the regulator max voltage range.
You will not be able to get to full power out but you indicated earlier that you did not need full power anyway.

Like your obtuse thinking. Afraid we are 240(ish) in the UK - even in Yorkshire ;-)

 

Take a hint from TS's screen name. AC power is 230V, same as yours.

Once again I missed it!
Oh well, I can blame it on only having one coffee so far

I think a switch mode pre-regulator is the way to go, or taking a few turns off the transformer secondary.
In fact, just adding a new winding and ignoring the old secondary will work.
Try adding 10 turns to the transformer and measuring the voltage out will allow you to calculate how many turns you then need for the required voltage.

 

Actually a 40-0-40 xformer can be used to make a nice dual supply, as long as you are willing to pay for the additional components needed to build it.

I'm all ears Spidey ...

 

And the extra pair of windings can be connected antiphase to reduce the original secondary volts. That will be easier to do.
Only a relatively few turns will be needed as then you do not need to get a full supply voltage from the extra turns, just enough to reduce the original..

 

This is one possibility. There are times when you wish you had a dual (positive and negative) supply.

 

I'm all ears Spidey ...

I was responding to Tony, I'm afraid you place too big a restriction on what I would call a nice supply.

But, that doesn't mean you can't build something with the parts you have.

 

@ElectricSpidey To which post do you refer? How am I placing restrictions?

 

He meant me placing restrictions Tony ;-)

 

He meant me placing restrictions Tony ;-)

Again, to which post is spidey referring to? In what way am I placing restrictions?

Look! I don't mind criticism if I'm wrong. While it's not the preferred method of learning - it does facilitate learning something I may have been wrong about. I'm not insulted, nor do I feel attacked. I just want to know how my words are being interpreted. Perhaps I said something wrong. Maybe I meant what I said. Don't I deserve the opportunity to defend myself if I feel I'm right?

So again, in what way am I placing restrictions? What's the context of my statement(s)? There are now 52 comments (not counting this one). That's a lot to go reading through just to find where I said something someone disagreed with.

 

It. Is. Not. You. Spidey. Refers. To. As. Being. Restrictive.

It. Is. Me. NOT. You.

 

@YorkshireDave Sorry. It. Looked. Like. He. Said. "Tony".

Are you upset with me? If so - sorry. Maybe I should bow out of this conversation. We don't need to fight. Arguing is OK in my book, but fighting? Wondering why all the word - period statements in your response?

 

@ElectricSpidey To which post do you refer? How am I placing restrictions?

It's not you @tony, he's referring to @YorkshireDave who wants to use the bits that he has to build a psu, instead of buying extra bits and making a better one..

 

I understand the desire to build with what one has on hand. I do that myself. Sometimes with good results, sometimes with dismal results. I just don't see where there's enough materials on hand to make the power supply Yorkie wants.

Anyhow - I think I've offered all I can on this subject. Hence forth I'll bow out and remain silent unless specifically asked.

 

Sorry about the confusion Boss.

 

Tony I could never be annoyed with you. You are too kind, too considerate to even contemplate such a thing.
Written comms always has a min of two interpretations. I was simply looking to ensure you could no longer be under any illusion it was me who was being a PIA, not you. Apologies if I went OTT
PLEASE do not bow out.

 

@YorkshireDave No problem. What threw me was the period after every word. Seemed like you were pounding your fist with every word. Oh well. I'll get over it. - - - I'm over it.

Still, there's not much more I can offer.

Stay well.