# Power out from transformer

Discussion in 'Power Electronics' started by John Potter, Sep 26, 2016.

1. ### John Potter Thread Starter Member

Sep 24, 2016
51
6
It's been nearly half a century since I picked up a soldering iron.
I have purchased a 36V transformer which I suspect isn't.
MULTICOMP MCTA160/18 160VA TOROIDAL 2X18V
Due to our glorious EEC our mains voltage should be 230V. Mines not even 240V, it's 245V Add to that the off load voltage of the transformer being 19.6V x 2 and the total sum comes to 40.36V.

If I use a 6A diode in the line and drop this to about 39.7V could I use an LM338 which is rated at 40V max.

I do have some PC CPU heat sinks rated at over 100W. I won't be drawing anything like 5A - I hope.
Regards JP

2. ### MrChips Moderator

Oct 2, 2009
12,449
3,363
Welcome to AAC John. I am sure you will find here all your questions answered related to circuits and electronics.

You have not stated what you are trying to build with your transformer and LM338 adjustable voltage regulator. One can assume that you are going to build a 0-5A 0-40V DC power supply. You need to tell us what you intend to power with this power supply, i.e. current and voltage requirements.

AC Mains voltages are quoted in RMS (root mean square values). Hence the voltage output from the transformer after rectification is going to be 40V x 1.4 = 56V.

After rectification, you will need a smoothing capacitor (or reservoir capacitor). This will have to be fairly large, about 10000μF at 100V.

The diodes need to handle much more than 5A because the peak diode current will be much higher. I would install 100A diodes to be sure.

The max voltage handled by the LM338 is 40V differential, i.e. the difference between the input and output voltages.

Lastly, you can reduce the transformer output voltage by half by configuring the windings for a full-wave rectifier circuit. In that case, the rectified output will be about 28VDC. You also have to subtract the voltage drop across the rectifiers. Note that the max current output of the bridge rectifier circuit is half that of the full-wave rectifier circuit.

Here is a handy power supply design guide:

http://www.hammondmfg.com/pdf/5c007.pdf

Jul 18, 2013
10,555
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Why not take turns off, it is simple with a Toroidal, around 1.5 to 2 turns/volt.
Max.

4. ### crutschow Expert

Mar 14, 2008
13,028
3,238
Note that you need to derate a transformer's current rating about 50-60% when it is used in a rectifier-capacitor DC output circuit, i.e. the maximum DC output current should be no more than about 50-60% of the transformer RMS rating.

#12 likes this.

Jul 18, 2013
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What voltage are you aiming for, one side will give you 28vdc.
Also if two separate winding and not a CT type, you can parallel them up.
Max.

6. ### John Potter Thread Starter Member

Sep 24, 2016
51
6
Well, whatsit me sideways, what a wonderful set of replies. I have been desperate for assistance and my brain really urts you know.
It was just under 50 years since I made a Mullard 20W pair of amps. I remember well the joy of getting 410V on my nose.
Thank you all.

Basically I don't have a clue what it may be used for in the future. However I decided to build a power supply for a 'project'. The cost of some of the smaller transformers was more than this one so I bought the biggest Watt one per £. A very scientific approach ?

The output is stated as 18V x 2 at 4.44A. I think I'll go for one suggestion above (MaxHeadRo - thanks) and use one of the 18V tappings as they can be used single, parallel or in series. 28V DC at 4.44A will do nicely for now. I could go for 28V at 8.88A (crutshow - thanks) but, well I'll come to that later.

I had completely forgotten 1.4141 and I didn't know that the 40V was the differential V and not the max V (Mr Chips - thanks). I do have quite a selection of caps but they are mainly around 470 uF. I also have 4 x 3300uF at 50V. I am not sure if I can name suppliers here but I used CPC Farnell.

My aim is to make a power supply that I can adjust both in V and A. It seems the V is not too difficult but the A is. I found a video on Youtube that does enable a relatively low current pot and use a large fixed resistor. I'm not sure if this might help me. I will not need very much Amps :-

Lastly for now comes the crunch - the 'project'. I have been messing about with LED's. I have promised my wife a back garden that is slightly brighter than the solar LED's things we have and I don't want mains floating around. I do have a large 12V transformer but I need something to be able to build the 'blocks' of LED's up. They are quite tricky things. I made the mistake of watching bigclivedotcom on Youtube. I have been doing terrible things to dismantled mains LED bulbs. Some of the cree and cob LED's are rather nice but can be expensive. Currents are generally low but the volts can get dangerous.

I would like to get back into audio amps. I did make one Bailey amp that used OC35 transistors about 1970 I think. Long gone I expect. I want to make an amp - not buy a black box.

I am so very pleased that you have taken the time to advise me. It's a good job you did or it would have been rather 'unfortunate.'
Thanks again - JP

7. ### John Potter Thread Starter Member

Sep 24, 2016
51
6
I didn't know the video would come up like that. What a great web site.

Jul 18, 2013
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If you parallel them, they will have to be phased if not marked, if so the procedure can be posted.
The VA will remain the same whatever configuration so lower voltage, higher current capability, up to the VA rating.
Max.

9. ### #12 Expert

Nov 30, 2010
16,325
6,818
Sometimes that causes a fear that it's going to damage the windings...but you can, "back-wind" a few turns if the center is open. Add a few turns the wrong way around to subtract voltage. I've done it.

I have a page about that if you need it. The basic premise is that you can get the same power at Vpeak, but you can't get the same current.

10. ### John Potter Thread Starter Member

Sep 24, 2016
51
6
There are 4 wires in and 4 out. Each is a different colour. The diagram shows the colours for
115V - '0V' and '115V' - 0V for mains in
and Sec V - 0V and Sec V - 0V for output.
I assume (but not enough to connect) that for 230V the centre '0V' and '115V' would be connected and the other two as input.
Basically the same for 36V but Sec V and Sec V joined and 0V and 0V to give 2 x 4.44 amps at 18V

http://www.farnell.com/datasheets/1945759.pdf

I would obviously like confirmation of all this. It's one thing to get 36V across ones chest and another gawd knows what across it. One feels that at 70 years old a DIY defibrillator is pushing things.

I'm pretty certain that the tech. branch of CPC Farnell would advise. One has to book an appointment though. You are better and quicker.

I understand the VA, the watts have to go somewhere regardless of how it's connected.

Strangely enough I do know about the phasing bit (I am not being sarcastic) I actually found that on the Internet. In an ordinary transformer it would be the outside of the coil of one transformer to the outside of the coil of a second if you want to parallel off two transformers. I do have two small 12V 1.66A transformers that are identical. They came from halogen bedside lamps.

I have a 60A continuously rated 120A+ battery charger / starter. Probably around 14V. It is about 900mm high x 250mm x 250mm on wheels with a handle. It weighs a ton. It has a 150A ammeter and leads as thick as ones finger. I wonder what rectifier it has, it's quite old. It has not been used for years. It would kill a car battery at full output. It does have power settings via a rotary switch. I wonder how many 4V - 30ma LED's ? I make it about 6000. I jest of course, but it would be fun.

If I am wrong in my assumptions re wiring the transformer or you have any doubts in my explanation please post the phasing information.

Thanks again for your help. Did you have a chance to look at the video re- current control?
I did not realise until a couple of days ago that the LM317 / LM338 had their case at Vout and had to be insulated
You must be a very patient person MaxHeadRo. I do hope I don't push it too far.
JP

11. ### John Potter Thread Starter Member

Sep 24, 2016
51
6
It's a brand new transformer and I'm a coward. The actual V and A are above my needs but hopefully just using 18V at 4.44A should be controllable relatively easily. I would like to know how to control both 18V at 8.88A though and of course the output at 36V at 4.44A. I am going through a very steep learning curve - so be gentle with me LOL

12. ### #12 Expert

Nov 30, 2010
16,325
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Ohm's Law says you can't do both at the same time. You need two different circuits. You can put a current limit on a voltage supply. You can put a voltage limit on a current supply. But you can't do both and connect them to a load. The resistance of the load will cause one or the other to be dominant.

And...you are in England. That means you MUST wire the primary for 230 volts. Do all your fiddling about on the secondary side.

Jul 18, 2013
10,555
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The dots on the winding indicate the common phase end of each winding so when wiring in parallel ensure the dot connections are connected together.
In series, if you make a mistake, just means you will get zero out, and no damage done.

Max.

Last edited: Sep 26, 2016
14. ### KeepItSimpleStupid Well-Known Member

Mar 4, 2014
1,147
204
FWIW:

If you want a nice amp to make, look here: http://leachlegacy.ece.gatech.edu/lowtim/
I made one in the 80's and made some changes along the way. This amp was described in like a 1976 issue of Audio magazine and I built it in the early 80's. I made one drastic mistake the boards cam back mirrored, but I was able to use them with a few minor changes which is basically unheard of.

My changes included: A custom transformer, a little undersized. Mostly 1% metal film resistors. 10 Turn bias pot. Low profile. Logarithmic audio ramp. Quad power supplies + one for the turn on circuit. 40,000 uF of capacitance required a slow turn-on circuit. Logarithmic audio ramping of the audio on turn-on. The amp will survive switching the NPN and PNP output transistors with one burnt resistor. Home made case. No power switch. Wanted to add a clipping indicator and hi temp protection. If one of the power rails dies, you loose that same protection resistor. Anti-thump. DC protection is only a 3 amp AGX style fuse.

The first build used 50 V as the WV (Working Voltage) of some of the caps and was open frame. You have to use 100 V. It also used a single 18 Amp constant voltage transformer that hummed too much. It did not have the turn-on circuit or the audio ramping.

The amp does have a 500 W AC sine voltage regulator preceding it. I got it really cheap.

15. ### tcmtech Well-Known Member

Nov 4, 2013
2,039
1,667
That's ridiculous. I don't even use 100 amp diodes in a 50 amp power supply.

for a 5 - 6 amp power supply 10 amp diodes would be more than sufficient.

16. ### John Potter Thread Starter Member

Sep 24, 2016
51
6
I do not intend to do anything with the transformer other than wire it as the makers recommend. I am not confident enough or feel I know enough to do anything else. This transformer has to be wired with both primary's in series. Although I am 99% certain of what colour wire (there are 4 of them) should be connected together at the transformer I must make sure. I thought a low wattage bulb in series might stop any smoke. I have checked the resistance of each set of windings and violet and grey give twice the resistance as do brown and blue. Brown and blue are the UK colours for mains power. I could connect a 12V AC 1.66A transformer and see if I get approx 2V out or am I being stupid (probably).

I'll start by using just one output winding at 18V x 4.44A. The LM338 is a very simple, if inefficient, way of adjusting the voltage from this transformer and I thought that it would be a good start. It can also be used in a variable current limiting circuit. There is a video on Youtube of this being done with the two circuits. Unfortunately the video is not exactly very informative as to the circuit as there isn't one shown. It's the usual look what I've done not how. However the voltage circuit is pretty solid as to how it's made but the current one has variations. The link to the video I put here earlier does show a variation that does not require a variable resistor the size of a dustbin. It does however require a fixed resistor that is.

I'm not sure exactly what you mean by the 'dots'. The 4 output wires do no have dots where they join the transformer as they are all grouped together.

17. ### John Potter Thread Starter Member

Sep 24, 2016
51
6
I'll bare in mind your suggestion but at the moment I really am going into brain overload with help with this power supply. When I made the Bailey it really was 'a new boy on the block'. I didn't have a printed circuit so I made it by pushing the leads of the 'bits' through a polystyrene ceiling tile but connecting leads for the OC35's on a heat sink. The whole lot was then capped by another ceiling tile and stuffed under the front seat of my Ford Consul. 12 V was rather low but the output beat most of the valve car radios. Portable cassette players had just come out so I used a jack from that.

Thanks for the link, it's nice to know there are those around that actually made a transistor amp with real transistors rather than a black box. Mind you there is nothing like the smell of some EL34's burning the lid of a cabinet when running at full thrutch. That lovely blue glow - ahhhh. I do have a pair of 15" Tannoy Monitor Gold and they are still in use. Modified Lancaster cabinets with a folded horn. They are all that's left apart from a 10W + 10W valve amp I still have in the loft. Nearly 50 years is a long time.

18. ### #12 Expert

Nov 30, 2010
16,325
6,818
It is a good idea to use something like a 12V transformer to apply voltage to one winding or another in order to discover which end is up. No, your transformer doesn't have dots. You just have to make a map of it by measuring.

19. ### John Potter Thread Starter Member

Sep 24, 2016
51
6
In the old days when you fried something electrical it smelt fishy. I don't know whereabouts you are but from your earlier comment about having to connect in the UK for 230V I would guess that you are one of our old colonial chaps. (please excuse my sense of humour - it's dreadful). This is a pity as you are to the West of us and that's the way we get our wind from. Somewhere around Norway they are in for w thrill.

Today will be 'frying tonight' day.

Seriously - thank you. I was beginning to get a bit concerned. The transformer was £26, about \$34, not a lot but the wife and the dog don't like blue flashes and bangs on the dining table even if I do have some cork floor tiles to work on. Indecently I do have a lovely Fluke meter, it was given to me as a present. I also have a China Cheapy, as far as I know un fused even if it says it is. I couldn't find any.

If for any reason I don't reply later - I got it wrong. Scientists have managed to find a sound that was made just before the BIG BANG billions of years ago. It's - OOPS!

20. ### #12 Expert

Nov 30, 2010
16,325
6,818
Use alligator jumpers. They make life much easier.