Convert RMS value of a sine wave to PEAK amplitude value by multiplying by the square root of 2, i.e. multiply by 1.4

Hence if your RMS value is 8V, peak is 8 x 1.4 = 11.2V

Then subtract the voltage drop across the rectifier diodes.
Estimated diode drop is 1V @ 1A. Two diode drop = 2V
Unregulated DC voltage output is 11.2 - 2 = 9.2V

 

so on my main coil with the denter tap, after the bridge rectifier and 10,000uf 50v filter caps I have 30vdc clean on my scope. What is the max current for this? Will I be able to get my 0-24vdc variable 4a PSU from this?

 

According to the results you have provided it would appear that you can get 48V @4A.
However, 200W seems to be a bit on the high side for that size transformer.
We don't know as yet if the primary can deliver that amount of power when all the secondaries are loaded.

 

the headlamps that im using have 2 filaments in it.. I can do 100w 90w or 190w... should I load all the secondaries with a load and get the voltage and amps from each of them?

 

Your primary concern is the +/- 24V supply because this takes 90% of the power.
You need to put four 12V lamps in series applied across the arms of the secondary windings. You can ignore the center tap for this load test.

 

that would be 720w... or what would be the ideal load? with 4 bulbs I can get 360w, 400w or 720w.. I could probably use certain filaments to get other wattage for the load.. and how long should I let it run?

 

so on my main coil with the denter tap, after the bridge rectifier and 10,000uf 50v filter caps I have 30vdc clean on my scope. What is the max current for this? Will I be able to get my 0-24vdc variable 4a PSU from this?

THE_RB said in post #26:

You need to be careful! 22.5v each side unloaded sounds about right.

But 20.5v each side loaded at 4.7A sounds like a significant overload to me! That sized transformer did not come out of a 100+100W amp. More like a 60+60W amp.You should not be drawing more than about 3A continuous from the output at approx +/- 21v.

If you have a look at the graph from the long thread I linked:

http://forum.allaboutcircuits.com/attachment.php?attachmentid=19885&d=1274929608

it shows that a 5 pound transformer should be good for about 150 VA total. This is quite in line with what THE_RB says.

If the high current center tapped winding is good for about 80% of the transformer's rating, that would mean about 3 amps AC (120 VA) from that winding. With a bridge rectifier/capacitor input filter combination, the usual rule of thumb says you can get a DC current of about 60% of the AC current rating.

That means you can count on 1.8 amps DC at about 40 volts. Forced air cooling of the transformer would increase that to about 2.5 amps DC.

 

I plan on pitting fans on the transformer and the regulators/pass transistors..
most of my work would at 12v... that's where I would need the most current...

 

I just want to figure out my limits and get some schematics drawn u so I can build it.. I need to find the limits of my main sec. winding.. MrChips suggested putting a big load on it,,, but im not sure how big of a load I can hit it with and how long I should let it go before I cut it.. Im going to measure the resistance before and after I run the the test as suggested a couple posts back..

 

You cannot accurately measure the resistance of the load because it changes with temperature. Measure the current and voltage.

 

You should do it by measuring the resistance of your ~40V secondary, but the resistance is so low that you would need a 4 wire ohmmeter, or a method to pass an accurately measured DC current of about 1 amp through it. Knowing the exact value of the ~1 amp DC current, you can use ohm's law R = E/I to determine the resistance.

I suspect you don't have a 4 wire ohmmeter, so the next best thing would be to measure the DC resistance of the primary at room temperature, and then after 2 hours under load, that load being 3 amps AC from the secondary. From the two resistances, you can calculate the temperature rise.

I explain the details in post #14 and #35 of this thread:

http://forum.allaboutcircuits.com/showthread.php?t=38273

 

I plan on pitting fans on the transformer and the regulators/pass transistors..
most of my work would at 12v... that's where I would need the most current...

If you want a full 40 VDC out, then you will need to use a bridge rectifier across the entire main secondary, ignoring the center tap. Your DC output current will then be limited to about 1.8 amps without a fan, 2.5 with a fan.

But if you want a lower voltage and more current, then you could use a 2 rectifier arrangement, utilizing the center tap. Then a 3 amp AC secondary would give you 3 amps DC out, without a fan; with a fan, maybe a little over 4 amps.

I explain about this in post #3 of this thread:

http://forum.allaboutcircuits.com/showthread.php?t=68839

 

... the 8.1 winding is not intended for high current use.. I was going to make it a 5vdc 1a supply..
...

You said "8.1 sec. winding unloaded" and "7.8vac 2.8a".

The amount of amps you can draw from a secondary winding is limited by the magnetics (which are massive, capable of 120W and not a limiting factor on this winding) and limited by the series resistance and heat rise in that winding.

Although this winding is only used for less than an amp in their design, they obviously used a reasonably thick wire. You can see that because with a 2.8A load it only dropped from 8.1v to 7.8v.

I think you will be very safe with 1A DC output taken from that winding.

...
... I didn't hurt my transformer from the tests I did.. I plan on further testing giving it a couple hours to measure temperature. What do you recommend I use on the windings that are safer?

MAYBE you didn't hurt it. For the 38v winding you said "38.5vac unloaded" and "5.56vac 4.3a". Ouch. That winding has a lot of turns of very thin wire and its resistance dropped 38v to 5.5v. That was 32.5v dropped by the winding resistance at 4.3A, dissipating maybe as much as 32.5*4.3 = 140W! The wire was cooking itself and might have been close to glowing in there! Hopefully some of that voltage drop was from the magnetics, so maybe the winding was not dissipating the entire 140W.

That was the reason for my "silly tests" comment. Which was rude, I'm sorry.

I was annoyed that I had already identified the 38v winding for you as a very low current VFD display voltage and told you it was good for 200mA, maybe even as low as 50-100mA, then you still went and drew 4.3A from it!

You have to be careful testing small secondary windings, because they can have fine wire and can burn out when you draw too much current from them. The main reason being the magnetics are rated for 120W and capable of delivering maybe 200W peak, but the winding might burn out if run at 50W continuous.

The worst cases are higher voltage special secondary windings because higher voltage means lots of turns, so the transformer manufacturer will use fine wire to avoid the physical bulk of lots of turns of thick wire. Be extra careful with that 38v winding.

However the 2x 23v winding should be good for the full wattage of the transformer, continuous maybe 120W no problem.

And your 8v winding seems to have thick enough wire to be good for 1A continuous, maybe more. If you plan to use a 7805 5v regulator that will have current limiting at about 1.5A anyway, and I think your 8v winding should be safe with that.

 

Ok.. I hooked up all 4 bulbs to the main winding I plan on using and it took them out almost immediately.. Any other ideas for a load test?

 

Ok.. I hooked up all 4 bulbs to the main winding I plan on using and it took them out almost immediately.. Any other ideas for a load test?

Hooked up how? All in series across the winding? All in parallel across the winding?

 

I'm going to give up.

 

I'm going to give up.

sorry you feel that way,,Im just trying to learn here.. And I hooked them up in a series, I figured parallel would put to much strain on the transformer.. I guess ill continue on my own, but I will get it figured out.. Thanks to all that have helped me out.. Ive learned a lot so far.

 

Hooked up how? All in series across the winding? All in parallel across the winding?

I just stated hooked up because ive done all testing in series.. sorry for not being clear..

 

Methinks you have your concept of series and parallel backwards.

 

Methinks you have your concept of series and parallel backwards.

No I nstall car audio everyday.. Im well aware if parallel and series.. parallel is all the pos. connected and neg.s are all connected together.. series connect pos. to neg. and so on.... correct?