Calculating current capacity?
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 23rd June 2009, 11:34 AM #1 sorenj07   diyAudio Member     Join Date: May 2006 Location: Berlin Calculating current capacity? Here are my 2 PT's. They both have 3.75"x3" laminations. The bigger one on the right (called "Venti" from now on) has a 1.75" thick stack, and the smaller one on the left (called "Tall") has a 1.5" stack. Here are some measurements taken with no load on the secondaries. HV winding resistances were closely matched on either side of the center tap. House mains: 126VAC Tall: Primary, black-black 4 ohms Secondary 1, red-red/yellow-red: 370-0-370V, 183 ohms Secondary 2, brown-brown: 17VA ~1 ohm Secondary 3, yellow-yellow: 5.4V, ~.2 ohms Secondary 4, green-green: 6.6V, ~.1 ohms Venti: Primary, black-black: 1.6 ohms Secondary 1, red-red/yellow/red: 380-0-380V, 90 ohms Secondary 2, green-green: 6.5V ~.1 ohms Secondary 3, yellow-yellow: 5.4V ~.1 ohms Secondary 4, brown-brown: 6.5V ~.1 ohms Any ideas how I might go about guesstimating what current capacities I have? My idea would be to divide the HV windings by 126, multiply by 115 (probably the intended primary voltage), then fiddle around in PSUD II until I could get the right secondary resistances. Any other (better) ideas? Thanks!
 23rd June 2009, 12:29 PM #2 SY   On Hiatus     Join Date: Oct 2002 Location: Chicagoland Johan Potgeiter posted some nice rules of thumb some time back. It might be worth doing a search for them. __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
 23rd June 2009, 02:20 PM #3 sorenj07   diyAudio Member     Join Date: May 2006 Location: Berlin I'm not having the best of luck with the search. Can you perhaps point me in the right direction?
ChrisA
diyAudio Member

Join Date: Jan 2008
Re: Calculating current capacity?

Quote:
 Tall: Primary, black-black 4 ohms Secondary 1, red-red/yellow-red: 370-0-370V, 183 ohms Secondary 2, brown-brown: 17VA ~1 ohm Secondary 3, yellow-yellow: 5.4V, ~.2 ohms Secondary 4, green-green: 6.6V, ~.1 ohms
I'm kind of in the same way, I can figure out the turns ratio and the DC resistance but that's not enough.

You can apply a rule of thumb and guess based on weight. (10 pounds is about 200VA.) Or you can try and match up what you have with the specs on the Hammond web site.

I think the only 100% positive, fool proof method is to test the transformers on a dummy load. That sounds easy until you figure out the specs on the load and see that you need something like a 5K 200W resistor. And it would have to be adjustable so you could use it for more than just one test.

I think I want to do this. Even if I have the specs in hand. Before I invest time in a 35 year old salvaged PT I want to be 100% certain the PT is not defective and the only way to know is to perform a burn in test. Let it run for hours. then lower the resistance of the load and see what this does to (1) the voltage and (2) transformers temperature.

I thought about buying 100 ceramic, wire wound power resistors . For \$20 I could build a dummy load board that could fully test a 500VA transformer (futurlec.com has 5W parts for 20 cents each)

I'd like to design the "load board" so that it could be re-configured to test OPTs as well. For that you need an 4, 8 or 16 ohms and 250+ watts. I could wire it with some terminal strips and crip-on ring terminals

I thought about 100W light bulbs. But I think all of the bulbs and ceramic sockets and the plywood to mount them on would cost more and be less practical than the 100 resistors.

 23rd June 2009, 04:23 PM #5 rknize   diyAudio Member     Join Date: Feb 2009 Location: Chicagoland I've used light bulbs. A 100W incandescent light bulb draws ~800mA. A 40W draws ~300mA. Put 6 in series and you have a cheap (and bright) 720V load. Put an ammeter in series to get the real current load, since the above loads are wild guestimmates. __________________ Russ http://www.knizefamily.net/russ/cate...ics/tubeaudio/
 23rd June 2009, 04:37 PM #6 sorenj07   diyAudio Member     Join Date: May 2006 Location: Berlin Hm, so if I manage to get a 16-watt light bulb, I should only be drawing around 135mA? 150mA for an 18-watter? Then the issue would be to get 3 of them and put them in series. I'll check my local hardware store, but I'm not so sure I'll be able to find such low wattage lamps..
rknize
diyAudio Member

Join Date: Feb 2009
Location: Chicagoland
Quote:
 Originally posted by sorenj07 Hm, so if I manage to get a 16-watt light bulb, I should only be drawing around 135mA? 150mA for an 18-watter? Then the issue would be to get 3 of them and put them in series. I'll check my local hardware store, but I'm not so sure I'll be able to find such low wattage lamps..
Start with 25W bulbs or whatever is the cheapest and add another to the chain. You'll be increasing the "hot" resistance of your load which will reduce the current. It's easier if you know the actual load you will be applying in your project, try to reproduce that, and then run the transformer for a while and see how it does. If you get about the voltage you want and the transformer can run for an hour or two without getting too hot, then you are probably well within spec. Of course, that approach only works within reason...if you grossly overload the thing it may die before the core gets hot enough to let you know.

I think the rule of thumb for the amount of voltage drop you will see for unloaded versus loaded is 10%. So if you are seeing a much greater drop than that with the load, you are loading it pretty hard. Not necessarily out of spec, but given that you don't know anything about it....

ChrisA
diyAudio Member

Join Date: Jan 2008
Quote:
 Originally posted by rknize I've used light bulbs. A 100W incandescent light bulb draws ~800mA. A 40W draws ~300mA. Put 6 in series and you have a cheap (and bright) 720V load. Put an ammeter in series to get the real current load, since the above loads are wild guestimmates.
The resistance across a light bulb varies depending on the voltage (or really with temperature of the filiment.). Try putting an ohm meter cross a 100W bulb, then light it up with 120VAC and measure the voltage drop. Ohms law does not exactly apply because lights, unlike resistors are design to run at very high temperature.

Whenever I start working this out I find I need more bulbs then my initial guess. Say I have a 500V transformers I think might supply 250ma. That is 125VA. I think four 40W bulbs in a series. That is about 1778 ohms or 280ma or about 10% over current. maybe "close enough" But what if I guesse wrong and it's only a 100ma transformer? Better start with five 25W bulbs. And then you think doubling the power is to large of a step. I'd prefer to jump in units of about 20%. So you end up with about 10 lamp sockets. At about \$3 each plus a \$20 stash of 25W, 40W and 60W bulbs. So then I go back to thinking about buying 100 resistors again.

The problem here is that you don't know, not even within a factor of two what your transformers can do and you might have several transformers some at 350V and another 750V. and you want to make small 25ma or 50ma jumps.

 23rd June 2009, 05:56 PM #10 rknize   diyAudio Member     Join Date: Feb 2009 Location: Chicagoland Yes, that's what I meant about "hot" resistance. As far as cost, well I have a large box full of old bulbs from the many that have been switched over to CFLs. Sockets? What sockets? Edit: I don't recommend that! __________________ Russ http://www.knizefamily.net/russ/cate...ics/tubeaudio/

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