2N3055 and heat

[John Potter]

I have a 52V transformer that after rect. produces 3.3A at 52V. I wish to use either a LM317 or a LM338 with 2N3055 power transistors.

If you run 2N3055's in parallel you are suppose d to have a resistor like a 0.1Ohm 10W from each emitter so that thermal runaway doesn't take place.Surely the resistor should be at least the output in watts ?

There are circuits on the Internet with this resistor on the base or collector but I think the emitter is correct ?

Could you possibly tell me how to calculate the value - Ohms and watts of this resistor. ? I understand this can be critical.

Honestly the Internet can be a nest of worms, there are many circuits that don't use a resistor at all.

On her page is a link to the circuit that states she removed the 0.1 Ohm 10W resistors as it limited her current output. This 20 odd Amp supply works fine.

Please advise.

I was considering using 4 x 2N3055's each at about 0.75A at 52V (39W each). I thought 3A from the transformer a safe limit. 1 heat sink per 2N3055 in an x pattern under a 125mm 0.5A fan blowing from outside the case down the heat sink fins with a 30mm gap at the bottom. The rear of the case is all vents. I may fit a 150mm 0.4A fan if the 125mm doesn't cover the heat sinks.

Mounting 2N3055's and heat dissipation.

There are 3 videos on this subject. Quite a down to earth approach but unfortunately nothing with metal to metal with heat compound - AND fan. Metal to metal with compound was by far the best. I can insulate the heat sinks. Extrapolating his results I should be able to draw 39W per 2N3055 at about 40C case temperature or less. The thermal resistance junction to case is 1.5 C/W. Max junction temp. is 200C. I should be running around 60 C (or not ?)

Any advice would be greatly appreciated.

 

[MikeMl]

Hi John,

I dont have the patience (or the internet bandwidth) to sit through the videos. Could you post the schematic of what you are building. I worry that 52V exceeds ( by quite a bit) the max allowed input voltage to a LM317 or a LM338.

The emitter ballast resistors should be used. At an emitter current of 0.75A, a 100mΩ resistor only drops 75mV, and dissipates 0.075V*0.75A=0.056W. I would shoot for a voltage drop across the ballast resistor of at least 500mV, in which case you will need to redo the math and calculate the resistance and wattage required.

 

[spec]

Hi John,

In addition to the maximum junction temperature of a transistor, you need to keep an eye on the safe operating area (SOA). See figure 2 of the 2N3055/MJ2955 data sheet: https://www.onsemi.com/pub_link/Collateral/2N3055-D.PDF From this you can see that at 40VCE the 15A, 2N3055 can only handle 3A at temp case = 25 degC. In practice the junction temperature would be much higher than this so a practical maximum current at 40VCE would be around 1.5A. This is in fact limited by the dissipation rather than the SOAR but you should always check the SOAR, even for MOSFETs which, contrary to what you read on the net, do suffer from secondary breakdown.

By the way, the thermal resistance of a TO3 case to heatsink without a washer but with thermal paste is taken as 0.5DCW. And, as a general rule of thumb with a decent heatsink, you will be limited to around 25W dissipation for a 2N3055; they have a high thermal resistance, junction to case, of 1.5DCW, whereas a good figure would be 0.5 DCW.

If you ever use insulating washers, rubber, kapton etc are pretty high thermal resistance. On the other hand, mica is better and alumina is excellent. But you have the best approach of not using an insulating washer at all.

You can also calculate the resistor power dissipation from, W = I * I * R where, I is current in Amps, R is resistance in Ohms and W is power in Watts.

Have fun: I have destroyed many 2N3055s along the way. We used to solder leads to them and turn them upside down with some current flowing thru them. The person closest to the time when the solder melted and the 2N3055 self protected won the prize.

spec

 

[audioguru]

I have a 52V transformer that after rect. produces 3.3A at 52V.

Nope. After the rectifier there must be a pretty big filter capacitor that charges to the peak voltage minus a couple of rectifier volts (+71.5V).

 

[schmitt trigger]

Honestly the Internet can be a nest of worms, there are many circuits that don't use a resistor at all.
.

Indeed!

As MikeMi has stated, always use emitter ballasting resistors.
Dropping 500 mV at full load is a good rule of thumb. You could lower this value perhaps a couple hundred mV if you employ beta-matched transistors and they are also thermally-coupled very tightly.

Also.....AG's assessment of what the rectified and filtered DC voltage is correct. Over 70 volts at nominal line input voltage, expect 10 or more volts at high line.
Does it have a center tap? If so, the DC voltage would be halved.

 

[ronsimpson]

I have a 52V transformer that after rect. produces 3.3A at 52V.

Nope. After the rectifier there must be a pretty big filter capacitor that charges to the peak voltage minus a couple of rectifier volts (+71.5V).

Probably John is getting 52Vdc. So the transformer produces about 53 volts PK, and 36 or 37 volts RMS. So the transformer is rates at 36 volts.
Apples & Oranges

 

[John Potter]

MikeMI Thanks for the reply. I should have omitted the LM338 and the LM317 should read LM317AHVT which can handle 60V. I have noticed a 0.47Ohm resistor used in may circuits and wondered why - I know now - thanks.

Hello Spec and thanks. From the video's it plain that metal to metal with heat compound is the best. During all the tests the heat sink was about 20C cooler than the 2N3055 case. That formulae will come in handy. That's the first time I have seen it. The case temperature is always 30% odd lower than the junction. I don't know what SOAR is - sorry. I have bough a few 'spare' 2N3055's and LM317AHVT's.

Hi audioguru. The transformer is for 36V 4.44A output at 230V. My mains is 245V so 38.34V X 1.4141 = 52.44 . I have a 10,000uF cap after the bridge rect so about 51V. Sorry about the confusion.

Hello shmitt trigger. As I thought I was building a 36V power supply (I had forgotten 1.4141 - it's 50 years since I last dabbled in electronics) I don't mind loosing a few volts. I'll use those resistors.
The transformer is tapped for 110V and 230V input and 2 x18V or 36V at 4.44A output. https://cpc.farnell.com/multicomp/mcta160-18/160va-toroidal-2x18v/dp/FF01565.

Hello ronsimpson. I made a right cods explaining that transformer - sorry. As I said above I have a 10,000uF 63V cap - mind you it's from China so who knows. I'll wear safety glasses and a flak jacket for a while. Maybe I should but a 100K resistor across it, sound a good idea.

Thanks all for the help.

 

[spec]

Hi John,

You mention about possible problems with the quality of your reservoir capacitor. I expect that you know that the normal scam is to put a small capacitor, say 1000uF capacitor n a big can marked 10, 000uf say. But many capacitors are genuine.

When you are designing a power supply, like yours, you needed to know the ripple voltage on the reservoir capacitor. You can get pretty close to the peak to peak ripple voltage as described @ https://www.electro-tech-online.com/articles/two-simple-formulae-capacitance-and-inductance.783/

In your case, with a 10,000uF (0.01F) reservoir capacitor and a current drain of say 1A, the ripple voltage can be calculated by V= IT/C, where V is in Volts peak to peak, I is in Amps and T is in seconds. As you are in the UK your mains frequency will be 50Hz, giving a period of 1/50 sec = 0.02 seconds. As you nave a full wave rectifier T becomes 0.01 seconds because the reservoir capacitor gets charged twice for every cycle of the mains. So the ripple voltage would be: Vripple= (1A*0.01s)/0.01F = 1V p/p.

At 2A current drain the ripple voltage would be 2V and at 0.5A drain the ripple voltage would be 0.5A and so on.

spec

(Not directly related to you John, but some of us have been discussing the importance of knowing a member's location, and this is a clear example where knowing the local is important!)

 

[John Potter]

Hello spec. Thanks for the reply. I doubt I'll be connecting anything sensitive to this power supply. I think this is the first time I can grasp 'ripple voltage' I have seen in demonstrated on Youtube but looking at other power supply circuits I thought 10,000uF would cover most things. Mind you at 3A 3V is quite a bit. If I double the cap to 20,000uF that would halve the ripple. I looked on Amazon for a buck convertor with an input of 60V. I ended up at Banggood - China. There delivery times are rather slow, I think some of my stuff is still on a bullock cart stuck in a paddy field. My contact at Banggood is Ruth - strange Chinese name. She's very polite and keeps me well informed. They sell 'all sorts' of strange items on Banggood - usually pink.
Banggood - maybe they started out as a house of ill repute or a good sense of humour. . Bit late for me, I'm 70 - unless they have paramedics on site. It's a good site to browse, the electronic stuff is so cheap. Indecently they are making fake 2N3055's that blow at 1A 30V. That's on Youtube. I got mine from CPC, Probably Chinese. I expect my capacitor is made from kitchen foil. Good comments though. They do a 22,000uF for the same price £2.75. I was worried about the 'inrush' thing. Sorry to be so technical.

I found this and I have ordered it along with other bits, some of which have arrived :-

**broken link removed**

It's less than the price of 2 packets of fags (I vape now) so fingers crossed. I wanted to build my own V regulator though, buck regulators are somewhere in the future so I thought I would build a linear room warmer.

One of my daughters bought me a battery for vaping. It can go up to 80W with variable V and A and temp control. It has a small LCD screen that shows Watts, Volts, Coil resistance and Amps but it can be changed to show other parameters. It has a battery status bar. It's about half the size of a cigarette packet. I use 7W, it's more than enough. I imagine it has a buck convertor but how on earth do they get it down to this size is beyond me.

Strange I don't have a location, don't know why. It's the UK of course - or England as I prefer.

Can I edit my profile ?

Thanks again spec - you have made an old man very happy.

 

[JimB]

Can I edit my profile ?

Yes.

Have a look at the top right hand corner of the page, there will be a button marked as John Potter, click on that and there will be a menu with various interesting (?) things.

JimB

 

[spec]

High John,

I have just been rereading your original post.

I have a 52V transformer that after rect. produces 3.3A at 52V.

As has already been said, a bridge rectifier and reservoir capacitor will produce (52V *1.414) -(2 * 1V) = 71.5V across any value reservoir capacitor (in practice over around 1nF) with no current drain from the reservoir capacitor.

But, as you take more and more current from the reservoir capacitor the ripple voltage increases according to the Q=CV=IT formula. But also the transformer voltage drops, mainly due to copper loss (resistance of coils). A very rough rule of thumb is:
(1) Take a transformer/rectifier/reservoir capacitor power supply with a transformer winding rated at x Amps RMS,
(2) if you take a DC current equal to the magnitude of the rated RMS coil current from the reservoir capacitor, the voltage across the reservoir capacitor, at the peak of the ripple voltage, will be equal to the RMS rated voltage of the secondary (if that makes sense)

This is a very rough and ready rule which varies from transformer to transformer, but it gives a good starting point for a design design.

spec

 

[spec]

Hi again John,

I thought 10,000uF would cover most things. Mind you at 3A 3V is quite a bit. If I double the cap to 20,000uF that would halve the ripple.

10,000uF is an excellent value reservoir capacitor for your work- you made a good choice. 3V peak to peak ripple voltage across a reservoir capacitor is perfectly normal. A little later down the line you can use another simple formula to derive the ripple current that the capacitor is passing to ensure you do not exceed the ripple current rating of your 10,000uF capacitor. By the way treat reservoir capacitors, batteries etc with care. Never get your face into a position where your eyes can be damaged by an explosion.

I looked on Amazon for a buck convertor with an input of 60V. I ended up at Banggood - China. There delivery times are rather slow, I think some of my stuff is still on a bullock cart stuck in a paddy field. My contact at Banggood is Ruth - strange Chinese name. She's very polite and keeps me well informed.

Bangood are a great company and very popular with some of the ETO men. Chinese girls have two names: Chines and western. Ruth will be her western name.

They sell 'all sorts' of strange items on Banggood - usually pink.
Banggood - maybe they started out as a house of ill repute or a good sense of humour. . Bit late for me, I'm 70 - unless they have paramedics on site.

It's a good site to browse, the electronic stuff is so cheap. Indecently they are making fake 2N3055's that blow at 1A 30V. That's on Youtube. I got mine from CPC, Probably Chinese. I expect my capacitor is made from kitchen foil. Good comments though. They do a 22,000uF for the same price £2.75. I was worried about the 'inrush' thing. Sorry to be so technical.

I would only buy power transistors and big capacitors from a reliable source, which can be from the far east. Your 10,000uF capacitor will be plenty big enough, as already said.

I found this and I have ordered it along with other bits, some of which have arrived :-
**broken link removed**
It's less than the price of 2 packets of fags (I vape now) so fingers crossed. I wanted to build my own V regulator though, buck regulators are somewhere in the future so I thought I would build a linear room warmer.

I buy a bit of stuff from the far east, mainly ebay and Alibaba and the prices are amazing, like £0.99UK for some items, including postage and packing from Hong Kong. Delivery is typically 5 weeks. But there are UK based sellers, which are slightly more expensive, but deliver in two days.

One of my daughters bought me a battery for vaping. It can go up to 80W with variable V and A and temp control. It has a small LCD screen that shows Watts, Volts, Coil resistance and Amps but it can be changed to show other parameters. It has a battery status bar. It's about half the size of a cigarette packet. I use 7W, it's more than enough. I imagine it has a buck convertor but how on earth do they get it down to this size is beyond me.

Yes, the small size of switch mode power supplies is amazing, but when you investigate them you will see that the reason is quite straight forward. Very roughly speaking the size of a power supply is inversely proportional to conversion frequency. Your linear power supply frequency is 50Hz and switch mode power supplies can have a conversion frequency as high as 4MHz, so you can get an idea how it is done.

spec

 

[John Potter]

Hello spec. I deliberately popped a 16V small cap not much bigger than a fag end and it made quite a crack. The bottom opened up like a flower. Elli our dog ran out the room and she's OK with thunder. When I first fire up the 10,000uF don't worry I'll be well away from it. Normally it will be running under the steel cover but maybe I'll add a bit of temporary protection around it while I'm messing about. I guess a drain resistor might be advisable as well.

Ruth has become quite chatty. I expect they get a bit bored and she is very polite. It's me saying to her please don't worry I'll be patient. One never knows the circumstances so what's the point in getting angry, even if they get out of stock and the substitute is also out of stock. Poor girl seemed really worried. She keeps saying 'Please be patient'. I wonder what would happen if I sent her a Christmas card ?

I bought 3 x 1.25V to 38V 5A buck converters that all 3 fit into your fist. £3.80 all 3. Mind you they state a little heat sink and fan should be added for the 5A. I'll have to saw an old one up, I have a few small ones from PC power supplies. I could fit all 3 under a 40mm fan.

Yes, I read somewhere about the higher the frequency the smaller they get but despite estimating the size in ones mind when you actually hold one it's quite a shock.

The battery still has to provide the power though. Mind you I would estimate the total run time is only a few minutes till it needs recharging. That's two days vaping. I wonder if a drop of whiskey instead of the glycerine. straight into the blood stream.

The 100Pcs of coloured 20mA 5mm LED's I got are superb. Many UK job lots don't include white and blue and they cost a lot more than £1.72

Might I suggest you have a look at bigclivedotcom on Youtube. He's an LED (warm white) real lead solder fanatic. He was in charge of The Royal Edinburgh Military Tattoo electronics. He likes Poundland LED bits.

I'm glad Ruth is her Western name, I'm confused enough as it is spec - LOL.

Thanks spec. I once had 410V DC across my nose - enough said.

 

[Colin]

For a current of 4 amps, you only need one 2N3055 and using two or more will have absolutely no advantages.
You can even use the plastic version for 40 cents.

 

[spec]

For a current of 4 amps, you only need one 2N3055 and using two or more will have absolutely no advantages.
You can even use the plastic version for 40 cents.

Hi Colin,

As well as the current, you would need to take into account the safe operating area (SOA) and maximum junction temperature constraints from the 2N3055 datasheet.

The SOA limit would be 12V at 4A DC (half the current shown on the data sheet SOA to take into account elevated junction temperature and other effects)

But the junction temperature limit would reduce the voltage to 9.38V at 4A DC, based on a maximum dissipation of 33.19W. That is assuming a maximum junction temperature of 200 Deg C (exceptionally high for a power transistor), a local ambient temperature of 50 Deg C, a mica washer of 1 DCW, and a heat sink of 2 DCW. In practice, it would be wise to limit the maximum junction temperature to 175 Deg C rather than 200 Deg C. So that would result in a maximum advisable dissipation of 27.65W.

Surprisingly, the TIP3055, TO220 plastic case version of the 2N3055 has a lower thermal resistance of 1.38 DCW and better SOA characteristic. But the TIP3055 maximum junction temperature is lower at 150 Deg C. The TIP3055 is cheaper too, as you say and much easier to fit to a heatsink (with some caveats).

Afraid to say that, unless you could limit the dissipation of a 2N3055 to around 28W, parallel 2N3055s would be required.

https://www.onsemi.com/pub_link/Collateral/2N3055-D.PDF

https://www.onsemi.com/pub_link/Collateral/TIP3055-D.PDF

 

[Colin]

Hard to confirm:
52V transformer that after rect. produces 3.3A at 52V.

But 2 x 2N3055 will be needed if you want to draw 3 amps at less than 12v

 

[spec]

I deliberately popped a 16V small cap not much bigger than a fag end and it made quite a crack. The bottom opened up like a flower. Elli our dog ran out the room and she's OK with thunder. When I first fire up the 10,000uF don't worry I'll be well away from it. Normally it will be running under the steel cover but maybe I'll add a bit of temporary protection around it while I'm messing about. I guess a drain resistor might be advisable as well.

It is always prudent to be aware of danger especially to the eyes. Blowing up electrostatics is great fun. When our lab instructor wasn't looking we used to plug an electrolytic in the mains- what a mess. Another trick was to charge up an electrolytic to 340V and throw it for your mates to catch.

At one time, just outside our lab, there was a RADAR system comprising three trailers. One trailer housed the generator and power conditioning. One freezing morning the guy working on the RADAR initiated the power up sequence and all was normal, but half an hour later there was a loud bang follower by a series of louder bangs. We rushed out to investigate and found the chap covered in aluminum strip and acrid smelling slime. The whole cabin was the same. The guy was hosed down and rushed to hospital but, apart from ruined clothes and a few minor acid burns, he was OK. Luckily he wore glasses so his eyes were protected to a degree. It took a week to clear the mess in the cabin though. Ever since then, I have always been wary of capacitors. By the way, the capacitors that exploded were huge, about 9 inches diameter by 9 inches long. The empty cans made excellent flower pots.

Ruth has become quite chatty. I expect they get a bit bored and she is very polite. It's me saying to her please don't worry I'll be patient. One never knows the circumstances so what's the point in getting angry, even if they get out of stock and the substitute is also out of stock. Poor girl seemed really worried. She keeps saying 'Please be patient'. I wonder what would happen if I sent her a Christmas card ?.

Chinese girls are very pleasant. That is their objective. The result is that they do not recognize/resolve problems. We have got to know a few oriental girls (and chaps) from the local Chinky restaurants and take sways. I would not send an Xmas card to Ruth or you may get a visit from a guy with a red band around his head wielding an axe.

I bought 3 x 1.25V to 38V 5A buck converters that all 3 fit into your fist. £3.80 all 3. Mind you they state a little heat sink and fan should be added for the 5A. I'll have to saw an old one up, I have a few small ones from PC power supplies. I could fit all 3 under a 40mm fan.

Handy little PSU- I know it well, in theory that is.

Yes, I read somewhere about the higher the frequency the smaller they get but despite estimating the size in ones mind when you actually hold one it's quite a shock.

Just like with linear power supplies, there are some simple formulae that allow you to work out what is going on with switch mode power supplies.

The battery still has to provide the power though. Mind you I would estimate the total run time is only a few minutes till it needs recharging. That's two days vaping. I wonder if a drop of whiskey instead of the glycerine. straight into the blood stream.

Sounds like a good idea. You can work out how long the battery will last.

The 100Pcs of coloured 20mA 5mm LED's I got are superb. Many UK job lots don't include white and blue and they cost a lot more than £1.72

That is good to know. I have never bought any cheap LEDS suspecting that they might be inferior.

Might I suggest you have a look at bigclivedotcom on Youtube. He's an LED (warm white) real lead solder fanatic. He was in charge of The Royal Edinburgh Military Tattoo electronics. He likes Poundland LED bits.

thanks, will have a look.

I once had 410V DC across my nose - enough said.

Ouch

spec

 

[John Potter]

Hi Colin. It's so nice to know there is someone else making the same mistakes as me. Fun in it. Do you know my friend Irena, lives just outside Sydney ? Perhaps you don't.
Her pet parrot used to get into her cleavage when it was cold. I keep telling her that when I die I'll come back as a parrot.
Knowing my luck it'll be as a chicken. I think there might be room though.

Hi spec. The Christmas card was a joke, for all I know she could be a Buddhist. Do you mean a sort of Bruce Lee baddy type - I'll set the wife on him.
Poor chap, I bet he had a twitch afterwards. I will try and be careful, at 70 odd bits get a bit 'overexcited' when you do nasty things to them. My wife agrees.

Those 20w Mullard amps were in a cabinet with no bottom on legs about a foot long. I was lying with my head underneath when I accidentally unsoldered an HT lead.
It swung down and brushed my nose 3 times as I couldn't get my head out fast enough.
There were some quite large caps on this lead and I had one hand on the chassis. I said Golly Gosh!.

During a party one night someone smelt burning, it was on the underside of the lift up lid of this cabinet. 4 lovely brown burns right through the veneer.
I still have the 2 x 15 Ohm 15" Tannoy Monitor Silver that the BBC used 1953 vintage. They are very efficient compared to most of today's stuff. Rather large cases though.
Those El34's made a lovely glow full chat on organ music. Bach of course. Ah - the good old days when amplifier watts were real RMS and not Dixons telephone number 'music power' watts.
I understand valve amps are having quite a revival. I bet someone has stuck a chip somewhere though.

The Canterbury now cost £17,900 a pair. I just checked. Strangely enough the specification hasn't changed much except they are 8Ohm's and more watts.
Easier to control the cone with 15Ohm's - longer coil. Something's you don't forget, even after 50 years.


Whilst in the loo contemplating the meaning of life this morning I had a revelation. It didn't hurt much.

I have seen 'centre tap' circuits and had a quick look but I'm afraid it was a case of 'duh'.

However, this transformer I have is 18VAC x 4.44A + 18VAC at 4.44A therefore I think centre tapped ?
I had it wired for 18VAC at 8.88A. I did check I could with CPC Farnell first. I then decided on 36VAC at 4.44A.

Working on the theory 'you can only ask'.
Is it possible to switch between 26V DC x 3.3A and 52 DC x 3.3A. I don't think I see a problem with the switching arrangement at the output from the transformer. Would everything that follows on work ?
If you halve the difference in Volts between Input and load then you would halve the Watts on the 2N3055's - and heat ?
It would be nice if I was right otherwise I shall have to go back on my medication.

I take it buck converters are switch mode power supplies ?
They look simple enough but I was told not to go near them. I think that might have been for the same reason as Joule thieves. The chap who told me is into ham radio - interference.
I told him I would build a 100W Joule thief and connect it through a 100 foot aerial. Not a titter.
I have made a small one with an LED and AA and it worked well.

I think you will approve of bigclives respect for caps.

How can I add comments between comments like you do spec?

I would add an avatar but the last time I added a picture of myself it frightened children and those of a nervous disposition.

Thanks spec.

 

[spec]

Hi John,

Sounds like you enjoy life.

You ask about quoting a members post and breaking it up. Here is the procedure.

Click on the 'QUOTE' button on the lower right of the previous post concerned and the whole text of the post will be displayed in your posting window below the latest post in a thread. The thread will be enclosed in opening and closing quote tags as shown below for your post. The tags are normally enclosed in opening and closing square brackets, but I have changed the square brackets to normal brackets so that ETO does not treat it as a quote and you can see what is going on:

(QUOTE="John Potter, post: 1279708, member: 263806")Hi Colin. It's so nice to know there is someone else making the same mistakes as me. Fun in it. Do you know my friend Irena, lives just outside Sydney ? Perhaps you don't.
Her pet parrot used to get into her cleavage when it was cold. I keep telling her that when I die I'll come back as a parrot.
Knowing my luck it'll be as a chicken. I think there might be room though.

... ... ... ... ... ...

How can I add comments between comments like you do spec?

I would add an avatar but the last time I added a picture of myself it frightened children and those of a nervous disposition.

Thanks spec.(/QUOTE)


All you have to do then is to enclose any part of the text in opening and closing quote tags and it will be displayed as an individual quote. Here is an example:

(QUOTE="John Potter, post: 1279708, member: 263806")Hi Colin. It's so nice to know there is someone else making the same mistakes as me.(/QUOTE)

Here is a reply to the above extract

(QUOTE="John Potter, post: 1279708, member: 263806")Do you know my friend Irena, lives just outside Sydney ? Perhaps you don't. (/QUOTE)

And another reply to another extract


And this is how it would look:

Hi Colin. It's so nice to know there is someone else making the same mistakes as me.

Here is a reply to the above extract

Do you know my friend Irena, lives just outside Sydney ? Perhaps you don't.

And another reply to another extract

Dead easy.

spec

 

[spec]

Hi again John (more of your post chopped up)

... this transformer I have is 18VAC x 4.44A + 18VAC at 4.44A therefore I think centre tapped ?
I had it wired for 18VAC at 8.88A. I did check I could with CPC Farnell first. I then decided on 36VAC at 4.44A.

Is it possible to switch between 26V DC x 3.3A and 52 DC x 3.3A. I don't think I see a problem with the switching arrangement at the output from the transformer. Would everything that follows on work ? If you halve the difference in Volts between Input and load then you would halve the Watts on the 2N3055's - and heat ?

Yes, you can do that quite simply. In fact, it is normal procedure in power supplies to reduce the dissipation in the output transistor as you say. A suitable circuit is shown below.

spec