[Frederic Gagnon]

Hello all,

I have a little experience in electronics, but it's the first power supply I really build from scratch. I have put together a 36v power supply using the following components :

-toroid transformer 115V/36V
-a rectifier bridge 12A/100v max
-a 30,000 microFahrad capacitor

I am testing the output voltage and it gets 50V instead of 36V. What can cause that ??

thanks!

[Al_The_Man]

You are seeing the peak voltage, the AC is an RMS value of the sine wave, so the the RMSx1.414 will give you ~50vdc.
If you want to lower it the easiest way, especially with a toriod is to take a few turns off.
The secondary is wound on last and usually amounts to around 2 turns/volt.
Al.

[Frederic Gagnon]

I measured the voltage on the capacitor lugs, so the the voltage would be "flattened" at that point, no?

[Al_The_Man]

The capacitor stores the PEAK voltage, that being the 36x1.414.
Al.

[Frederic Gagnon]

Then I should have selected a 25.4V transformer, ahh well... Would a resistor be a good way to drop the voltage before entering the controller board ?

[Al_The_Man]

No, a resistor is the worst way to go, I would seriously look at taking turns off, it is not as intimidating as it sounds.
You can also put a separate small winding back on for a 5v or 12v supply if needed.
The AC you needed is a reciprocal, 36dc x 0.707 = 25.5ac.
Al.

[Frederic Gagnon]

Thanks for the help. I'll probably attempt the procedure you describe and let you know.

Meanwhile, I was reading on another thread about the TB6560 Chinese controller, which I have, and I understand that the power supply will be the least of my problems...

[Al_The_Man]

There are a few posts in the past here on removing windings, maybe a search with Toroid may find them.
Al.

[Frederic Gagnon]

Hello Al,
I have started unwinding the transformer. I have removed about 8 turns of the yellow wire in the secondary winding (see attached schematic). When I check voltage, it stays around 50V as if nothing changed. Is the capacitor "memorizing" the voltage ? (it's big)... I notice voltage stays even after I turn the switch off. Should I work with the other wires?

Thanks!

There are a few posts in the past here on removing windings, maybe a search with Toroid may find them.
Al.

[Al_The_Man]

Firstly, only work at the AC level, do not connect the bridge, your transf. shows two primaries and two secondaries, how do you have these connected right now?
Al.

[Frederic Gagnon]

Red & Orange are tied together. So I have Red and Black going to the bridge rectifier.

[Al_The_Man]

You have a slight problem there, either the two secondaries should be in parallel or in series, assuming they are the same voltage, what is the AC voltage on each of the secondaries?.
How do you have the primaries connected?
They can be connected for 120v or 240 input.
Can you show the connection detail chart clearer?
Al.

[Frederic Gagnon]

The part is Digi-Key 237-1334-ND. Here's the link for the datasheet:

http://system.netsuite.com/core/medi...0e64c&_xt=.pdf

Note : I have 115VAC in my outlets..

[Al_The_Man]

OK, I am not sure how you were getting 36v before without connecting the red to orange and using the black and yellow for output?
But often with these transformers, the secondaries are wound bifilar, which means they are wound together for balance.
If you can detect this, then take turns off both secondaries together, although you are using the secondaries in series so it does not really matter that much.
If you do take windings off both simultaneously remember that each winding reduction count to reduce the voltage in total.
For the primary at 120v you should have the blue & violet connected together and the grey and brown connected together, and feed the supply to these connected pairs.
Hope this is not too confusing.
BTW, 50va is not very large a rating?
Al.

[Frederic Gagnon]

Hello Al,

My red and orange are indeed connected together, sorry if I wasn't clear. Everything is wired like you describe on both ends.

What does the 50VA imply ? Does it mean this transformer won't output more than 1.39 amps @ 36V ? if so, duh!! That won't be enough to drive 4 steppers requiring each about 3 amps... I hope I'm wrong.

OK, I am not sure how you were getting 36v before without connecting the red to orange and using the black and yellow for output?

Al.

But often with these transformers, the secondaries are wound bifilar, which means they are wound together for balance.
If you can detect this, then take turns off both secondaries together, although you are using the secondaries in series so it does not really matter that much.
If you do take windings off both simultaneously remember that each winding reduction count to reduce the voltage in total.
For the primary at 120v you should have the blue & violet connected together and the grey and brown connected together, and feed the supply to these connected pairs.
Hope this is not too confusing.
BTW, 50va is not very large a rating?

[Al_The_Man]

At the AC output of 25v then the capacity would only be 2amps!.
I usually get my Toroids from Antek, he list on ebay as Jonango.
Al.

[LUCKY13]

Al, can you add wire to up the volts on one of these?

Also what dectates the amps rating of one of these?



I have one that I dont need the 5v or the 12v side of it but its still wired with it ( the wires are there but its not hooked to rectifire). I would like to kick the volts up some, and amps if possible without total rewiring from scratch.


Jess

[Al_The_Man]

You can add windings to increase the voltage but the VA stays the same, so if you increase the secondary voltage, the max. current rating will drop accordingly.
Al.

[LUCKY13]

Thanks Al.


What about the amps side of a transformer? Example, if I have a 50v x 10amp transformer. After being rectified how many DC amps do you end up with. Is it the same as the volts ( 50vac x 1.414 = 70vdc) so it would be 10 x 1.414 = 14 amps dc?


I have read three different answers to that question and I dont believe the example I gave is right. Maybe the opisite.


Back to the VA of a PS. I guess there are many things that determan this rating. Size and spacing of wire, core, internal fuss if it has and much more. Basically a PS will give until it becomes a fuss, is this correct? Or until its fuss blows. Even though it willl get hot and losses will get greater this is pretty much what could happen right? If the draw is there it will try to feed it until something gives.

I spent hours reading a site the other night and I am afraid it got a bit to much for me as it went. BUt I feel like I am getting to the point I could build my own PS once I learn a bit more about things. Knowing which transformer to start out with will be helpfull. And being able to add and pull windings on a tranformer will help me to use surplus or salvaged parts. Of course it would be even better if I could design my own transformer but I will have to give that some time.


Jess

Understanding a little of what I read it seems the Antec powersupplies are built very good. Infact compaired to some other companies I have looked at the way they are rating these parts the Antec just by weight along shows it will perform better than others. More copper on a better core is simply more power.

[Al_The_Man]

The VA rating is based on the ACV voltage and current.
A 50v 10a transformer will be 500va, current in the DC side is reflected back into the AC side so you cannot increase the current.
The rating is based on the core size and current capacity of the conductors, if you exceed the rating you get overheating and then saturation.
The size of the electrolytic capacitor also has an effect on the VA rating, if too large a value is selected and the transformer used close to its rated current value.
Antec supply just the transformer if needed, they also sell on ebay as Jonango.
Al.