Transformer

[CapnC]

Is it possible to build a single power supply that can deliver 50vdc @ 20amps using this Toroidal Transformer. I also need to add a winding for 5v. I am using 3 bilevel driver to drive 3 stepper motors rated 4.6amp/phase at 2.5v. The bilevel drivers can handle max of 70vdc and max 7 amps/phase. I would appreciate any advice. Thanks.

[CJL5585]

You are limited by the law of physics.

The transformer on e-bay has a MAX power rating of 800 VA (800 Volt Amps)

Your desired output is 50 Volts DC @ 20 Amps which equates to 1000 VA, which the transformer cannot supply.

Additionally, to have a 50 Volt DC supply after rectification and filtering (with capacitors), you would need a transformer with an output of 35 Volts AC. During the process of rectification (changing alternating current to direct current) and filtering same, the voltage from the transformer (35 VAC) is increased by a factor of 1.414 to 50 Volts DC.

You should be looking for a transformer with 35 VAC output @ 30 Amps. This will provide the voltages and currents necessary for you to build a 50 Volt 20 Amp DC power supply.

Hope this helps.
Jerry

[Al_The_Man]

Originally Posted by CapnC . I also need to add a winding for 5v. .

It is fairly easy to add a winding on to a toroidal transformer, the turns/volt have to be established and can be found by a simple test winding using any insulated wire, the turns/volt is approx 2 to 3turns/volt on a toroid,but for the final winding you will need to also know the maximum load current of the 5v winding in order to use the correct wire gauge, use enameled magnet wire.
This load will have to be taken in to consideration in calculating the final power rating (VA) of the txfr. Also use the 1.414 figure for calculating the required ACV of the winding before rectification and smoothing.
Use the reciprocal for working backwards i.e 5vdc x .707 = vac
Al

[Al_The_Man]

Originally Posted by CapnC Is it possible to build a single power supply that can deliver 50vdc @ 20amps using this Toroidal Transformer. I would appreciate any advice. Thanks.

Further to using this transformer, if you need a lower voltage, you can reduce the number of turns to lower the voltage and achieve lower than a 800va demand @20amps (if that is the maximum). I would guess that the secondary winding gauge will handle 20 amps as the test shows a 15 amp load. Also in all probability the 20amp load will not be continuous, but momentary in nature.
Occasional overloading a transformer does not damage it, it may cause a temporary volt drop which may be a nuisance, but the damaging aspect to a txfr is heat caused by a constant very high overload condition.
Al

[CapnC]

Thanks guys you've been a lot of help,

I have a couple more questions:

If the transformer is rated 800VA at 52.7 VAC @ 15.8amps, I would get 74.5 VDC @ 15.8amps. Can I unwind the transformer to get ~ 35VAC @ ~22amps.
That would give me ~50 VDC @~22amps. Or am I limited to a certain amount of current by the gauge of the wiring?

One last question. Its about bi-level drivers. I'm trying to determine how much current my driver would draw from the power supply. I am assuming that when the high voltage kicks in the low voltage kicks out?? So, if my motors require 4.6amps/phase and two phases are active that means 9.2 amps*2/3 = 6.1 amps. If I have dual secondaries on the power supply, one 5vdc (low voltage) and one at 50vdc (high voltage), can I assume the high voltage and the low voltage won't be drawing 6.1 amps at the same time? Therefore, a power supply with 5vdc and 50vdc @ ~20 amps (3 motors running at same time) should be about what I need?

Sorry for the rambling. If I get these questions answered, I'll be set...Thanks again.

[ViperTX]

Originally Posted by CapnC Thanks guys you've been a lot of help, I have a couple more questions: If the transformer is rated 800VA at 52.7 VAC @ 15.8amps, I would get 74.5 VDC @ 15.8amps. Can I unwind the transformer to get ~ 35VAC @ ~22amps. That would give me ~50 VDC @~22amps. Or am I limited to a certain amount of current by the gauge of the wiring? One last question. Its about bi-level drivers. I'm trying to determine how much current my driver would draw from the power supply. I am assuming that when the high voltage kicks in the low voltage kicks out?? So, if my motors require 4.6amps/phase and two phases are active that means 9.2 amps*2/3 = 6.1 amps. If I have dual secondaries on the power supply, one 5vdc (low voltage) and one at 50vdc (high voltage), can I assume the high voltage and the low voltage won't be drawing 6.1 amps at the same time? Therefore, a power supply with 5vdc and 50vdc @ ~20 amps (3 motors running at same time) should be about what I need? Sorry for the rambling. If I get these questions answered, I'll be set...Thanks again.

1st - If you've seen these toroids they are well made and unwinding could prove difficult. So, order a toroid that's closer to what you really need, you'll save yourself alot of grief. I would get the 30 VAC secondary unit, it will meet all except your voltage needs, keep you from exceeding the voltage limits on your drivers when the load is low and you'll have some amps to spare.

2nd - You can't assume anything. You have to determine what your 5 VDC current requirements are.

3rd - Being a little conservative on your design will provide you with a stable and long lasting system.

[CJL5585]

If the transformer is rated 800VA at 52.7 VAC @ 15.8amps, I would get 74.5 VDC @ 15.8amps. Can I unwind the transformer to get ~ 35VAC @ ~22amps.
---------------------------------------
First: The formula for power calculation (simple version) is as follows:
The power in VA (800) DIVIDED by the voltage (74.5) gives a current of 10.73 Amps. So, at this voltage and amperage your output = 800 VA.

74.5 X 10.73 = 799.385 Volt Amps
-----------------------------------------
As for removing turns from the transformer:
Each full turn removed will lower the voltage approximately 1.000 Volts. and lower the current capacity by .150 Amps.

I have calculated that the secondary windings on your transformer, of which you have 2, are 52.66666667 turns each.

Let us check the above calculation:
For Current: 52.666 turns times .150 amps each = 7.8999 Amps per winding times 2 =15.7998 Amps capacity from transformer. (RATED 15.8 Amps)

For Voltage: 52.666 times 1.000 Volts per winding = 52.666 Volts per winding. (RATED 52.7 VOLTS EACH)

I cannot comment on the drivers, as I am not shure what you have, how they are designed, etc.

Hope the above will allow you to calculate the voltage and current level of each winding as turns are added or subtracted from it.
Jerry

[Al_The_Man]

Originally Posted by CJL5585 --- As for removing turns from the transformer: Each full turn removed will lower the voltage approximately 1.000 Volts. and lower the current capacity by .150 Amps. Hope the above will allow you to calculate the voltage and current level of each winding as turns are added or subtracted from it. Jerry

Firstly removing turns from a transformer does not lower the Current capacity of the winding, the maximum current of a transformer is decided by the VA rating and the appropriate size (gauge) of the wire, so at the very least the current capacity will be at least equal to what it was before turns are removed, what this means now is that providing the secondary winding can handle it, the current can be increased though the secondary using the lowered secondary voltage to operate the transformer at the original VA rating. i.e lower voltage - higher current.
Secondary windings usually have a certain amount of tolerance as regards current so you can easily operate somewhat higher, especially in light of what I said earlier as to the non-continuous, momentary peak currents being applied in practice.
It is relatively easy to remove or add turns on a toroid, much easier than a laminated transformer, if you have a parallel secondary windings they are usually wound together for balance and should be removed together. There was a previous post a while ago where someone did this and this was even with a toroid that had a potted core!. A search of the forums should find it.
I have been modifiying transformers like this for years, including laminated types with no problems.
Regarding the question of current through different secondaries, the current is unique and independant to each secondary and one does not have impact on the other.
Al

[CapnC]

Thanks everyone. I really appreciate your help.

[CJL5585]

For each turn removed from the transformer, the voltage will reduce by approximately 1 volt and the current will increase approsimately 0.150 Amp.

Sorry for the mistake. Pays to re-read your post and check same.

[Al_The_Man]

Originally Posted by CJL5585 For each turn removed from the transformer, the voltage will reduce by approximately 1 volt and the current will increase approsimately 0.150 Amp. Sorry for the mistake. Pays to re-read your post and check same.

I am not sure where you got the 1 volt/turn ? This depends on the transformer manuf. and what ratio was used in the winding. Typically on a Square laminated type it is around 5turns/volt on some Toroids it is 2 to 3turns/volt.
Secondly, I am not sure what you mean by 'the current will increase'' the current rating of the winding will stay the same, and the actual current of course will depend on the load. Current can only be increased if it is within the VA rating of the transformer and the winding guage is sufficient.
Al