Power Supply Design

components

You havent told us the one piece info that needs 2b known: how have you rewound the MOT? '60 turns' isnt even a transformer, its an inductor.

When rewinding you will need to put many mroe turns on the primary, probably 4x as many if you want it to run continuous.

Thats how they used to be done, parallel linear and discrete. 50v x 20A = 1kW, you'll only get that from a MOT for max 15 mins from cold with fan cooling. Use 2-4 MOTs if you want it continuous. Teh advantage with multiple MOTs is you dont need to rewind the primaries, just run them in series.

Depending on your app, you might find tap changing a whole lot easier than a tr regulator. Easy to implement when winding your own secondary.

NT

Sorry about not being specific. I re-wound the secondary coil. Primary is still intact. I haden't thought of using multiple MOT's, I'll take that into consideration. By tap changing, do you just mean changing the tap that the input is on to change the output voltage? And, what do you mean by tr regulator?

Thanks for the reply, I appreciate it! Steve

Then youve got a transformer that will manage around 15 minutes with fan cooling, or less without. Beyond that it will soon self destruct. And V_out will be somewhere in the apx region of 60v ac.

no, changing the connection point on the TF output. Just put 20 taps on your new output winding, a big switch and a voltmeter.

transistor regulator

NT

Thanks again for the replies. Makes sense to change taps, seems simple enough. The only reason I wanted to use the transistors is I came across a nice heatsink with transistors and sockets. This is kind of a project just for fun, I don't necessarily want to spend much money on it. 15 minutes will actually probably suffice, I won't be using it for very long intervals, and I definantely have it fan cooled. I may re-wind the transformer for a few different taps and change depending on output desidred so I won't overstress the transistors. Thanks for your advice, It's replies like this that make newsgroups go 'round.

Steve Kamego

The good news is there are high voltage op amps rated for 50V or more that would make excellent building blocks for this type of supply. One possibility might be the LM675.

They are available from Digikey although they aren't exactly free. The LM675 has an internal overtemperature protection feature which might be very handy in helping protect your main transistors from excessive temperature if you mount them all on the same heatsink. Be sure to realize the case of the

2N3055 device is connected to the collector (so presumably to the +Vin supply), while the case of the LM675 is connected to -Vee (or in your case presumably ground), so someone needs to be electrically isolated from the heatsink.

The bad news is 2N3055 devices are crappy and are only rated for 60V maximum.

Technically with a 90V input you could handle an 80V load since only 10V would be across the transistors, but in that case you could not output 0V since that would apply 90V to the devices. They would naturally be toasted in short order. For a practical powersupply you will have to take off transformer turns until you limit yourself to under 60V peak on the input of your capacitors.

You will want to use emitter resistors to insure good current balancing with the devices.

I would strongly strongly strongly suggest integrating an adjustable current limit into the supply. Current limiting is so valuable, both as a feature and for protecting the supply itself from self destructing. I can't stress how great current limiting is. Definitely use it. It isn't too hard to implement, and anything of this power level needs it.

Indeed 20A might be a bit greedy unless you are willing to go to great lengths to achieve that much. Although the 2N3055 is specified for an absolute maximum power dissipation of 115W each, that number is quite fanciful since it requires the case temeperature to remain at 25 deg. C. Something of around or less than 50W each is more realistic assuming a very good heatsink with robust forced air cooling. Assuming 12 devices total, this suggest a power dissipation of 600W or less. Assuming the output is at

0V with input at 50V, then maybe a current of up to 12A might not be too unrealistically attainable.

Something else to keep in mind assuming you live in North America or other low voltage country... Standard wall outlets are rated for no more than

15A, but standard derating procedure for safety and reliability reasons suggest not to draw more than say 12A from the outlet. Capacitive input filters fed through a rectifier from a sinusoidal voltage source have bad power factor. Something around 0.6 might be typical, though it could be lower for higher capacitance with small series resistance. So with an output power of 600W (or just waste of that much) you could be using up a substantial amount of the outlet's capability. I've tripped my 15A circuit breaker before with a 500W load at low power factor (+maybe 350W of other low power factor equipment on the circuit including my computer).

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If you do build a reg for it, I would personally make the unregged output available as well. No sense crippling it.

NT

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if you need more power, use 2 plugs. It can be done safely - and Im sure it can be done unsafely too.

2 plugs, 2 transformers, you'll get quite an output.

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If you dont have enough tr power to cope with it all, and you probably dont, you could use light bulbs. How? Well, a bank of bulbs provides the resistive dropper elements, with trs simply switching them in and out as required. Since theyre feeding into a big fat reservoir cap you can keep the ripple v small this way.

There are various other alternatives too. Its not like your building production kit.

But I dont know why youre regulating it at all, I'd just use switched taps. Much easier, far more power out, and well enough regged for nearly all jobs. In fact if you really wanted it regged, you could make a regulator tap changer followed by a 1v drop linear reg.

Or you could hack up a small motor to make a reg, whatever floats your boat.

NT

Thanks for the feedback. I will probably start seriously running though the design process in a few weeks. I will definantely keep the plug load capacity in mind, I live in an apartment, and I definantely don't want to cause any trouble. I can easily pull turns of wire from the transformer, I originally wound it with as many turns as I could fit inside it, so I wouldn't mind bringing it down to a rectified output around 50 or 60 VDC. I also will, as NT said, have the un-regulated power avaliable for those times that power of that nature is necessary. I appreciate all the help, maybe I'll finally get this supply build (I've been working on it here and there for about two years now).

Thanks Again, Steve Kamego

Hello Steve, to save you a lot of work rewinding your new secondary and making taps for various voltages. you could use the output of a variac to supply the primary of your microwave oven transformer (MOT).

The second reason for using a variac is that you would only turn it up to the point where the magnetising current of your MOT does not become excessive and cause overheating as NT described earlier.

For me here in Austalia, 240V 50Hz, I found 180V to 200V was OK for the MOTs that I played with. The primary current when idling was a couple of hundred milliamps.

On 240V the primary current was too high when just idling. 1.5A to 2.5A with the few I played with. I am assuming you are in America with 120V supply. Here is the sort of variac I was thinking about.

Just type variac into the ebay search and you will see lots for sale or auction. I am a bargain hunter, I think you are too Steve, since you have been thinking about this project for a couple of years. Keep looking and you will find a variac at a good price. Just a thought.

Regards, John Crighton Sydney

I like variacs. With a bit extra ingenuity the OP could turn the MOT into a variac with linear slider. Now that would be a lot more use than a 20V 8A psu - and would leave all those components left over for another psu.

NT

I like variacs. With a bit extra ingenuity the OP could turn the MOT into a variac with linear slider. Now that would be a lot more use than a 20V 8A psu - and would leave all those components left over for another psu.

NT

OK you have got me interested. I am all ears. How would you turn a MOT into a variac? Regards, John Crighton Sydney

Consider what a variac is: its a transformer with every turn exposed to a slider + carbon brush, so it can be tapped at any point.

A variac has no secondary, so we can rip all the wire off the MOT, and use the whole space for just the one mains winding. They need more turns than originally so they run at sensible idle current.

Once youve guesstimated how thick the new wire needs to be, you can then work out how many turns of it will be exposable at the surface of the bobbin. Say we can fit 120 exposed turns. So each turn will be

240/120 = 2v apart, or 120/120 = 1v apart.

Theres going to be a tradeoff here between current carrying capacity and voltage resolution, you cant have 1kW run power and 0.25v resolution at the same time.

So we wind the TF with every 4th turn etc exposed, and run over a strip of hard material laid across the surface of the bobbin. These turns must be spaced out because there is 1-2v between each one. Theyre thus glued in place on this strip.

Now make a sliding knob with a wide carbon brush on the bottom, make it run along this track. The brush must be wide so as not to produce much turn shorting current. A slotted brush or 2 brushes would be better.

There ya have it, a variac. Including the thermal cutout in circuit is a good idea.

NT

[variac]

The carbon of a variac brush is fairly high resistance so that it can operate in contact with more than one turn at a time without giving rise to huge circulating currents. This allows smoother voltage control without breaks between the steps.

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If you had 2 MOT cores, one could run the 2 in series, both with linear slider taps, with the 2 carbon brushse connected to the one handle so that as the lst turn of one is contacted, the first turn of the next TF is contacted, thus giving twice the number of effective taps, or v resolution. WOuld give something like an 8" long tap off path, and fairly smooth V output.

NT

I happen to have a 12A variac, old and heavy duty, may give it a shot as a pre-regulator, keep the input voltage as low as possible.

Thanks for the suggestions everyone, Steve

Hello Steve I would be interested to find out what the primary idling current is when you when you connect the variac up to the primary of your MOT. No load on the secondary.

I am guessing that you are in America. Just wondering if hobbyists with a 60 Hz supply are much better off than hobbyists in 50Hz countries in regard to salvaging MOTs. If you have a multimeter can you let me know. Thanks.

Primary Volts Primary current

120V 115V 110V 105V 100V 95V 90V

Regards, John Crighton Sydney (240V 50Hz)

Yes, I am in America. I've been really busy, haven't hooked up the xformer to the variac yet, I actually blew all of my current meters, so all I have to measure now is a few low value resistors and a volt-meter, as soon as I get some free time I'll measure it and post under a new thread.

Steve