Hook xformer for 120 VAC

jimandy · Post by **jimandy** » Wed Jun 14, 2006 3:19 pm

I have a Jameco 24vct transformer pn 101864 - shown below (pix from their on-line catalog, pg 223). It crosses to a Triad FD274. The pins on the primary side are not labeled on the unit but I assume they are 1,2,4,5 in that order left to right (or right to left). I understand it can be wired for 240 or 120. The question is how to wire for 120? Should I parallel 1,4 as a pair and 2,5 likewise with each pair to each side of the 120? OR shoud I use just 1,4 OR 2,5.

And why the!@#$%^ didn't they mark the pins!!! - (the one pictured is in the same family but 12.6 vct.)

Chris Smith · Post by **Chris Smith** » Wed Jun 14, 2006 3:47 pm

Pair them correctly and they will work together.

Pair them incorrectly and the out put will drop.

The problem is if your donâ€™t have diagram, trial and error is the only way.

Place light bulb load on 6 &10 and then connect the second [4&5] and see what happens.

jwax · Post by **jwax** » Wed Jun 14, 2006 4:55 pm

By ohmmeter, determine the two primary coils. As the schematic shows, tie pin 1 to 4, and then tie 2 to 5. One side of 120 VAC goes to 1+4, the other side of line to 2+5.
Like Chris said, hook a 12 volt lamp to the secondary, but center tap to one leg for 12 volts. Leg to leg should be 24 VAC.
If not 12 at 1/2 secondary, there is a phase error. Rewire: 1 and 5, and 2 and 4. Line then goes to 1+5, and 2+4. That'll switch the phasing of the primary.

jollyrgr · Post by **jollyrgr** » Wed Jun 14, 2006 5:19 pm

The picture you posted is the key. The four lugs along the bottom appear to be pins 1, 2, 4 and 5. Ohm out the pairs. Likely you will find two pairs with the same resistance. Check for a small indent or spot of paint at each of the lugs. There should be two marked in the grouping of four. These will indicate the DOT on the diagram, namely pins one and four. There will likely be one dot that is an OUTSIDE pin, this will be pin one. Using an Ohm meter on the dotted pin "one", check all the other connections; only one will show anything other than infinite Ohms. This will be pin two. Now you have pins one and two, and four and five.

Next look at the remaining three pins. One will have a dot of paint, indentation, or other marking. This will be pin six. Using your meter check the resistance between pin six and the other two pins. One will have a lower resistance, this is pin eight. The remaining pin is ten.

jwax · Post by **jwax** » Thu Jun 15, 2006 3:53 am

jimandy - Any dots on the transformer?
Maybe that's why Jameco got them in the first place- no markings?

ecerfoglio · Post by **ecerfoglio** » Thu Jun 15, 2006 8:19 am

One ** SAFE ** way to be shure of the polarity of the two primary windings:

1: With an ohmeter, identify the primary and the secondary pins. Identify the two pairs of primary pins (1&2, 4&5). Leave the secondary open

2: Connect one pin of each pair (ie 1 and 4) together to one side of the 110 V line.

3: Conect the other pin of ** only one ** primary winding (ie: 2) to the other side of the 110 V line

4: Using a range of at least 250 V , measure the voltage between pins 2 and 5. If you get only a little voltage it is safe to connect those pins together to parallel the two windings, if you get something like 220 V then the polarity is reversed and you should connect pin 1 to pin 5 and pin 2 to pin 4 (In that case run the test again [pins 1 & 5 to one side of the line, pin 2 to the other, measure between pins 2 and 4] just to be shure)

Chris Smith · Post by **Chris Smith** » Thu Jun 15, 2006 11:19 am

Your transformer is safe.

Just pulse the extra pins and look for a jump.

The windings even if reversed wont short, over heat, or blow during a quick test.

jimandy · Post by **jimandy** » Thu Jun 15, 2006 3:22 pm

OK folks, your suggestions and suppositions paid off. First, there are no dots on the transformer but, on closer inspection, I found a "1" embossed in the plastic bobbin closest to the left-most terminal, visible only by looking directly at the bottom of the tranformer, a very non-obvious place if you ask me. Going on the assumption that that marked pin 1, and counting left to right, I ohmed 1 to 2, then 4 to 5 and got identical readings. I then paralleld 1 and 4 and connected to one leg of 120 and same with 2 & 5. Got my 24 vac (actually metered more like 28 vac, but that's OK.

Thanks to all. (except Jameco)

jimandy · Post by **jimandy** » Mon Jun 19, 2006 10:09 am

Sorry to bring this thread back up again but affter hooking up the xformer as per previous post, and using it a while its seems to be getting excessively hot - and this under no load!. Is it necessary to use both of the two primary colis for max output? I got my correct voltage at secondary when experimenting with just one primary side (pins 1 & 2 only). The load will be a small audio amp that probably wno't draw more than 1 amp, average at full volume. (the xformer is rated for 2 amp - 24 vac)

jwax · Post by **jwax** » Mon Jun 19, 2006 2:12 pm

Assuming the transformer is not defective, sure you can use only half of the primary and get reduced output at the same 24 VAC. You may even try either primary coil to see if one doesn't run cooler.

dyarker · Post by **dyarker** » Mon Jun 19, 2006 7:35 pm

With no load a slight difference between primaries causes the primaries to "fight" each other. To test, put a 1K 1W resistor on the 24V secondary. If the transformer now runs cooler and output goes from 28V to like 25V - 27V, that's the problem. In normal operation the bias current of the amplifier may be enough load to keep the tranformer cool when there's no audio.

Cheers,

T&L Publications

Hook xformer for 120 VAC

Hook xformer for 120 VAC

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