Toroid Transformer Question

[TravisR100]

I have a toroid transformer that I took from another power supply. I'm just starting to do some basic testing. The transformer specs are 532VA. It has two 115V primary coils and a single secondary coil. When I wire the two primaries in parallel and then measure the voltage from the secondary I get 42V. The specs on the transformer for the secondary however are 38V-14A. Is it normal to get a higher voltage reading than the specification on the transformer?

Also, one more question. This power supply I'm dismantling for parts had a "Power Entry Module" where the source AC voltage goes in. Specifically it is a Schaffner FN 393-6-05-11. The specifics on it can be found here:
http://www.schaffner.com/components/...0%20e%2079.pdf

My question is, should I use it on the power supply I'm building? Is there a good reason not to use it if I have it? What is the purpose of this module? Is it simply a filter?

[CountZero]

It is usual for transformers to have a higher voltage than specified (if not loaded), this to compensate for voltage drop due to winding resistance.

[vulcom1]

Not sure about the rest of the world but around here the voltage is not 110 all the time. It also fluctuates during peak periods like supper time. Mostly here it is a little higher.
John

[TravisR100]

To follow up, the power supply I'm dismantling has the toroid transformer above. Specs:
532VA
2 primaries 115v/2.45A
1 secondary 38v/14A

This was going to a rectifier. Specs:
Forward Continuous Current:25 AForward Surge Current:300 APower Dissipation:83.3 WReverse Voltage:400 V

And to a capacitor. Specs:
10,000uF
63V

Could I simply keep these components and use them for my own supply? Should I choose a different capacitor or rectifier?

[TravisR100]

And thanks for your replies about the voltage!

[Al_The_Man]

Looks like the rectifier will be OK, the capacitor depends on the current demand you place on the supply.
You should keep the R/L/C filter, as this filters off some disturbance on the supply line.
Many areas of N.America is 120/240vac now.
Al.

[TravisR100]

I've been reading as much as possible here in the forums as well as on the Internet for info on building a power supply. I read the Bob Campbell site which is great if you're trying to build a supply for a specific output but doesn't offer much help if you're trying to reverse engineer one to see what one is capable of. Additionally on the article he posted he seems to select the rectifier without explanation. He states the specs on the one he chose but not why he used those specs. Then in some of his formulas he's using what seem to be arbitrary numbers. For instance, on the formula for capacitor selection he uses 30 volts in the formula however nowhere in the preceding text does he arrive at a value of 30 volts. Seems like maybe he was just rounding the 28 volt number up. Is there any post or information on reverse engineering one where given values of parts in an existing system you can derive what the supply is capable of?

[TravisR100]

Also, on the capacitors, all the references to them in wiring diagrams show the capacitor as having a + and - terminal. The Philips capacitors I have come with 4 terminals as opposed to 2 and are labeled 1, 2, 3, and 5. Any clue on how these should be wired?

[Al_The_Man]

As far as selecting a bridge rectifier, it is not usually neccessary to design to tight specs as the cost of supplying overkill is minimal.
As long as the carrying current is well within specs and PIV is at least double your final DC voltage, then this should work.
As far as capacitor value is concerned, many use a rule of thumb of so many µF's/Amp.
The only down side to this method is the percentage of final ripple will vary depending on the actual final voltage of the supply.
For example, say we go with 1000µf/Amp, for a supply running at 50vdc and 10amps = 10,000µf might produce 5% ripple.
Now at 5vdc and still 10 amp and 10,000µF, the same peak to peak of ripple will be observed, but obviously the PERCENTAGE of ripple is now much greater at 5vdc.
For voltage rating of the Cap. I tend to go at least 25%~50% higher voltage
than the DC supply peak voltage, it is no problem going higher, just that for electrolytics, the price starts to climb with voltage.
Al.

[Al_The_Man]

Originally Posted by TravisR100 Also, on the capacitors, all the references to them in wiring diagrams show the capacitor as having a + and - terminal. The Philips capacitors I have come with 4 terminals as opposed to 2 and are labeled 1, 2, 3, and 5. Any clue on how these should be wired?

Make sure these are not multi-capacitors, i.e. 3 or 4 capacitors in one.
I would have expected somewhere on them there should be an explanation of what the hook-up is, where did you get them from.
If from the supply you mention, you may have to do some reverse-engineering to determine this.
Al.

[TravisR100]

Thank you Al. That certainly makes selecting parts much easier and I hadn't seen that information before.

One more question. I had read on other Internet articles about using a rheostat to limit current as opposed to a fixed, current limiting resistor. Any thoughts on this? Would seem handy to do this in the power supply as opposed to using fixed resistors on the drives.

[AlanD]

I certainly recommend that you include the Schaffner (mains-filter & power connector module) - it offers some RF and Power Noise and limited transient suppression etc. - this is a worthy precaution especially where brush-motors are used and electronic control circuits are involved.

WARNING - As for the DUAL primary - I suspect that this is NOT intended to be connected in parallel and that doing so may endanger the transformer (phase/winding critical) - if not without load then possibly when you try to load it. Dual primaries are typically just to provide for alternative main-input-voltages and designed to connect in SERIES (phase/winding critical) for connection to higher mains supply environments.

Originally Posted by TravisR100 I have a toroid transformer that I took from another power supply. I'm just starting to do some basic testing. The transformer specs are 532VA. It has two 115V primary coils and a single secondary coil. When I wire the two primaries in parallel and then measure the voltage from the secondary I get 42V. The specs on the transformer for the secondary however are 38V-14A. Is it normal to get a higher voltage reading than the specification on the transformer? Also, one more question. This power supply I'm dismantling for parts had a "Power Entry Module" where the source AC voltage goes in. Specifically it is a Schaffner FN 393-6-05-11. The specifics on it can be found here: http://www.schaffner.com/components/...0%20e%2079.pdf My question is, should I use it on the power supply I'm building? Is there a good reason not to use it if I have it? What is the purpose of this module? Is it simply a filter?