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Toroidal Tranformers - primaries in series and secondaries in parallel ?

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picbits

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I'm building up a fairly beefy PSU using 6 toroidal transformers. They are rated at 230v 120VA (20V secondaries).

Two of the transformers are going to be connected straight to the mains and the secondaries straight to bridge rectifiers to give me approx 30-0-30v for a couple of variable supplies and to feed some opamp circuits. Another couple are going to feed a bunch of linear regulators to give me the usual 5v, 8v, 12v, 15v, 18v and 24v positive and negative outputs.

The big question is .......

If I connect two transformer primaries in series then I should in theory get around 10v AC out of each transformer secondary. The problem would be that if a large load was applied to one set of secondaries then the other set of secondaries would then be out voltage wise (or would it ?).

Is it feasible to therefore connect the primaries in series and the secondaries in parallel to give me half the output voltage while maintaining a proper balance between the two transformers ? The transformers are identical.

The reason I'm asking is that I dont want to waste a lot of heat regulating the 30v rectified output from a single transformer down to around 2.5v at 6 amps (around a 165 watt dissipation) to drive some stepper motors. It would be easier to drop a 14v rectified output down and save around half the heat dissipation.
 

JimB

Super Moderator
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picbits said:
If I connect two transformer primaries in series then I should in theory get around 10v AC out of each transformer secondary. The problem would be that if a large load was applied to one set of secondaries then the other set of secondaries would then be out voltage wise (or would it ?).
Yes, a bad situation.

picbits said:
Is it feasible to therefore connect the primaries in series and the secondaries in parallel to give me half the output voltage while maintaining a proper balance between the two transformers ? The transformers are identical.
Yes.

I have a couple of 120v primary transformers which I want to use to provide a beefy PSU.
The primaries will be in series and the secondaries in parallel.
I lashed this up and did a load test and it was fine.

JimB
 

Diver300

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You shouldn't series transformer primaries unless their loads are identical. if their outputs are parallel or series, that will ensure identical loads so you'll be OK.

Some other points occour to me.
1) Using a 240 V transformer on 120 V means it is much bigger than the correct one.
2)Big toroidal transformers produce a lot of volts per turn. To get 2.5 V you may only need 3 or 4 turns so you might be able to add that using ordinary cable and not use the original secondary.
3)A chopper drive to the stepper motor will want 20 - 30 V input, will be more efficient and will give the stepper motor a better top speed. If that's too complicated, using series resistors instead of a voltage regulator for a stepper motor dissapates the same power while improving the speed performance.
 

picbits

Well-Known Member
I'm using a PWM drive to the motors - a PIC microcontroller measures the average current taken by the windings on a calibration run, stores this value (max current) and then calculates various variations of the maximum current value so I can hold the motors at a much lower current (25% max current). The software I've written ramps the PWM up to 100% current for moving and after a period of 1/2 second of non movement of the motors ramps it down to 25%
 

Diver300

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The variable current is a clever idea.

What I was suggesting was not a variable current (although you could have that as well) but was maximum current limited not by the resistance of the windings but by either a resistor or a switching current control circuit.

The advantage of that is there is a much bigger voltage available to increase the current, overcoming the inductance of the windings. This causes the winding current increase faster, allowing faster stepping of the motor.

It also means that the efficiency is good.

http://www.cs.uiowa.edu/~jones/step/current.html
 

picbits

Well-Known Member
I've read the Jones tutorials many times - makes good reading.

My drivers for the stepper motors will allow a chopper circuit but they also allow PWM current limiting. I'm using some Intersil full bridge ICs which will drive N Channel Mosfets (in my case IRF540's - 28A ) for both the top and bottom halves of the bridge. They also incorporate dead time between switching the upper and lower halves of the bridge.

The actual current limiting is pretty simple but effective (so far !!!) I've done some basic testing and it appears to work but the proof will be when I get the steppers in full operation.

I have a 0.5R resistor on the ground connection of the H Bridge fed to a simple RC filter to smooth the PWM and spikes to a nice average current to voltage reading. The PIC starts off by energising a phase and supplying a duty cycle of zero to the H Bridge. It increments it slowly (0-100% in 3 seconds) and samples the A2D after each increment until the average current is the motors maximum rated (2.5 Amps). It then stores the value of the PWM, calculates 75%, 50%, 25% and stores these. Then it repeats for the other phases/motors.

The beauty of this idea (if it works) is that no matter what the input voltage is it adapts to it. If I find that 30v works better than 15v on the PWM then there is no firmware changes. If I uprate or downgrade a motor then I just tell the PIC the new phase maximum current and it takes care of the rest of it.

To keep the motor heat down, in full step or half step mode I can kill the current to 25% (or lower) of the maximum rated. I've run the machine/steppers at 25% current and there is still enough torque to do 90% of what I need it to do.

If all else fails I'll bung a basic chopper on it.
 

Diver300

Well-Known Member
Most Helpful Member
That scheme sounds fine.

It should work with a supply at 30 V or so. If you have a 2.5 V stepper motor then your PWM will never get beyond 10 % or so, which is fine if your PWM has a good resolution.

It does mean that you won't need to reduce the transformer voltage.

The worst problem is that if the supply voltage or motor resistance change, the PWN duty cycle will not compensate for those changes until a new calibration is done.
 

picbits

Well-Known Member
Had a flash of inspiration earlier.

Actually it was a 60 watt car headlight bulb I was using to test if it all worked. ;) Still seeing spots before my eyes now.

Put the two primary windings in series, found which way round the secondaries had to go to be put in parallel and hooked it all up - worked perfectly.

Now if I put a DPDT relay in between the two transformers I can switch them between a series or parallel primary winding which means I can get an unregulated 10v or 20v AC output from them at the click of a relay. After rectification this gives me a fairly usable range at up to 12 amps without having to get rid of too much heat.
 
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