View Full Version : power supply designers: help?

02-07-2004, 11:09 PM
Hey folks I need some circuit design help. It'll boil down to adding some resistors to a circuit but I need help with getting the right wattage.

I recently put together a power supply using this schematic:


See the "Full Wave Bridge"

I went about the design process using this terrific tutorial:


which was written by this guy:


And in case you dont have enough pictures in your life here's my power supply: http://nilno.com/laser_dir/blogger.html

Okay, the problem is the following, the power supply voltage is too high. The plitron site gave a formula for the voltage which was supposed to be 65vdc, but its running at 80vdc. I called the plitron tech people and they said if should go down a bit if I put a load on it but said not to expect it to go down to 65volts. He said I should put a resistor on the output of the transformer.

So what I'm asking for is some help figuring out the wattage and resistance of the resistor that needs to go between the secondary and the bridge diode. Any takers?

Post or reply to owhite@tigr.org


02-08-2004, 12:30 AM
The proper formula to calulate DC voltage is to take the output of the transformer in VAC and multiply it by 1.4

If you wanted a finished output of 65VDC you should have purchased a transformer with an AC output of 47VAC.

02-08-2004, 01:49 AM
To calculate the resistor, you need to know how much current you will be pulling. What is the specs of the transformer you bought?


02-08-2004, 12:13 PM
Hi, what is the configuration of the transformer? is it a single secondary winding with two Termination? The reason I ask is that with Plitron transformers that are High efficient Toroidal type, using a resistor to drop the voltage is very wastefull and of course, very poor regulation as the voltage will swing quite a lot with varying load.
If as I say you are using a single winding secondary type with a full wave bridge, here is what I have done in the past and is quite easy to do, as the secondary is usually wound as the top winding, remove a quantity of turns that is required to bring down the voltage to what you need.
First you have to find out how many turns/volt the transform is wound with, typically a E-I type (square transformer) is wound with around 5 turns/volt, with a toroidal, which is much more efficient the turns/volt can be around 3 turns/v .
Proceed by hand winding a secondary of some determined number, say 20 turns approx 16ga, the more the better, and making sure the ends are of course not shorted, power up the transf. & measure the resultant voltage with an accurate meter as you can find.
If for 20 turns the voltage is 5vac then the ratio is 4 turn/v.
Then remove the test secondary and proceed to remove the factory original secondary turns that are equal to what you want to lower it by. This method is a bit more involved if a tapped secondary is used as you would have to remove windings past the centre tap and create a new one. It depends on the transformer and how much ambition you would like to put into trying it.
But this method beats a resistor hands down.
Hope this helps.

02-08-2004, 12:30 PM
Granted, the formula for 1.4 times the output of transformer appears to apply here. Because I bought the transformer from plitron I based the calculation according to their specs, which is

VAC = 0.8 x (VDC + 2)

turns out I should have gone with conventional formula. The tech guy at plitron didnt offer much of an explaination other than: "hey, no big deal, most people are calling complaining the voltage is too low" and said to go the resistor route. Now I'm hearing that'll be a loss in efficiency.

The specs of the transformer are that its 1000va, dual secondaries. The part number is 117042201.

Al_the_man: I like your suggestion. With dual secondaries, can I still consider unwinding a few turns to bring down the output?


02-08-2004, 01:25 PM
Owen, If you have two non-centre tapped secondaries and they are bifilar wound, which means both secondaries are wound together side-by-side and you want to modify both windings, then, yes it would be a simple matter to modify both, as you would removed each secondary winding at the same time. If they are wound one over the top of each other then you would have to completely remove one to get to the other. In this case a one winding only mod would be easy.
If you want to explore this route, vary carefully remove the outer insulation tape and examine visually and by meter to identify the construction.
It is quite possible they have wound bifilar as this produces a better ballanced secondary pair.

02-08-2004, 01:52 PM
Owen, I just checked the Plitron web site and they claim to wind thier secondary's bifilar so you should be able to modify both winding easily.

02-08-2004, 09:56 PM
Okay Al,

The center of the torroid is one big solid block of epoxy. I dont know how these torroids work, do the secondaries loop inside through the donut hole? Will I have to remove that plug of epoxy to pull out loops of wire?


02-08-2004, 10:16 PM
Owen, Its sounds like they may have changed their designs, as the Plitron transformers I modified were not potted, I imagine you may damage the transformer if you try and punch it out. That would have to be your call.
If they have potted the core right through to the inside winding (primary), then I would say this modification is likely impossible
The windings do in fact loop around the donut, like threading a big needle.
Too bad.

02-09-2004, 09:15 AM
I found some pictures online of toroid anatomy and it looks like the secondary windings are the outmost ones. Was that your experience?

The deciding factor is that Plitron doesnt take returns so choices are limited. But secondly it still seems feasible. The toroid was wrapped with many layers of heat shrinking tape prior to potting. They were pretty thorough with the tape. Because of the tape, I am hoping that the epoxy didnt permeate into the windings. I'm inclined to put the toroid on the drill press and go to work on the plug.

I only have to remove a few wraps. Provided I can get the main plug of epoxy out and if the secondary wraps are the outmost wraps I could imagine that I should be able to remove them even if a little epoxy made it past the tape.

It may seem like it'll be a hack job but I think with layer between the epoxy and windings it'll be alright.


02-09-2004, 10:05 AM
If you can drill out enough epoxy to thread some extra wire through the toroid you can make a new (extra) coil of a few turns. (start with 10 or so) You can use regular (thick) insulated wire and put it in series with your original secundary. Measure the total voltage. If higher, reverse the selfmade coil. It doesnt take many winds.


02-09-2004, 10:18 AM

hey, that sounds easier. Your saying to _reduce_ the voltage coming out of the secondary I could _add_ some wraps?


02-09-2004, 12:13 PM
Owen, I would say if they potted it after taping you may have a good chance of removing the core OK.
Yes , the secondaries are usually the last wound.
In your case you have to take turns off, but use the method I previously described by winding a temporary secondary to establish the turns/volt ratio and then you have a pretty exact idea of how many to remove.

02-09-2004, 10:12 PM
Bimbaddaboom. That was easy. I drilled out the epoxy core on the drill press in about ten minutes. The heat shrinkable wrapping around the windings completely prevented the epoxy from entering any windings. After chipping out the remaining epoxy block I removed the heat shrink wrap. The secondaries wires were very accessable and I didnt have any problem unwinding them.

I went with about 5 windings first prior to testing. I carefully checked for shorts between any of the exposed wires. (Its a bit of an act of faith that the enamel around the wires will prevent any shorting but it does the job.) After checking as much as I could, I hooked up the transformer and measured the voltage. The first time the voltage the wasnt even close. I unwrapped some more, eventually got to the right voltage, trimmed the secondary wires, soldered new connectors and put heat shrink tubing around solder joints.

I have some heatshrinkable sheet which I wrapped around the donut. I didnt like that result but left the sheet on, and followed up with lots of wraps of electrical tape. I popped the transformer back in the power supply enclosure and I'm operating at the just the right voltage.

Much thanks to Al_the_Man for the great suggestion.


02-09-2004, 11:30 PM
Your welcome
Glad it worked out.

02-10-2004, 04:11 AM
Glad to hear you got it solved. Sure, removing winds is the cleanest solution. I was just thinking in case you couldn't get the epoxy out completely. Yes, adding an extra coil in opposite phase reduces voltage. Placing extra winds is also a neat trick to get an additional 5 Volts supply out of a big toroid.


02-10-2004, 11:19 AM
That is a great trick! What do you think it did to the output of the Amps?

02-10-2004, 11:50 AM
The only catch is that to obtain the same V/A (volt/amp rating) of the winding the wire gauge may be slightly undersized now, but I would guess that Owen is not running up to constant maximum VA so as long as one does not bring the voltage down really low and try and run at the same V/A then it is very seldom a problem.

PS: I thought I should elaborate slightly . now that the voltage in this case has been reduced, in theory a higher current can be run through the winding in order to equal the rated VA of the transformer. BUT as the size of any conductor is decided by current, the size of the secondary conductor may now be undersized for this new current. In other words if the current you are now running is no more than would have been allowed before modification to run at maximum VA you are in fine shape.

02-10-2004, 01:02 PM
"In other words if the current you are now running is no more than would have been allowed before modification to run at maximum VA you are in fine shape."

That's a little hard to parse for me so lemme axe you this. I am pretty much planning on running the amps right up to the rated power of the toroid. Me thinks your statement is that I can continue to do that even at my reduced voltage of the secondary.

Lets remember that the reason i opted to go with removed windings is that using a resistor was considered to be an inefficient solution. As I was removing wraps I was thinking "hey, I'm sure I'm taking somethin' optimal out of this device, but its probably better than turning it into heat."



02-10-2004, 01:28 PM
Owen, Lets say before modification your maximum current allowed was 20 amp for example, if you are now running the modified sec at 20amp. Then with the lower secondary voltage this results in an actual lower VA consumption, which means you are running well within tha rating of the transformer. The manufacturers usually build in a small tolerance so you could now exceed the original maximum current by a small margin.
In theory if you divide your new sec volts into the VA # (1000va?)
You get the new max secondary current possible (in theory) because the actual restriction is now in the gauge size of the secondary winding.
I hope this is not as clear as mud

02-10-2004, 04:22 PM
Its clear enough. My overall assessment of the situation is: In the black art of defining how many amps your system will run, you'll hook two large demand motors to a large delivery system, run it and see if the motors heat up, the fusebox sweats blood, or the table doesnt move. Oh ya, the big ferro-magnetic thingy was unwrapped a couple wraps and that potentially could deviate the results something less than one standard deviation unit.

My attitude is: fortunately, this is a hobby. :D I aint building the mars rover and I'd prefer to have some fun about it. :)


02-10-2004, 05:52 PM
Owen, I know it is nice when everything works out of the box, but some times a bit of a challenge to make thinks work can be looked back on as a bit of fun, and something gets learnt in the process.
I think most of the knowledge I have picked up was from the School of Hard Knocks.
I hope the fun continues.