Transformer Question

[peteyC]

Hi

I am working on a Leslie Sim from general guitar gadgets, but I can't figure out how to install the transformer. I bought the "Transformer 12 VCT @ 200 ma. Flat Pack"  http://www.smallbearelec.com/Detail.bok?no=339 as I thought it was best suited to getting close to 12.6volts. I can't figure out the primary secondary side or anything though. As the picture on small bear shows, one side is labelled "3 4 2 1" and the other "7 8 6 5". How should I go about testing this?

Any input welcome! Thanks!
Pete

Ps what exactly does "vct" stand for?? 

[R.G.]

what exactly does "vct" stand for??

"Volts, center tapped"

I am working on a Leslie Sim from general guitar gadgets, but I can't figure out how to install the transformer. I bought the "Transformer 12 VCT @ 200 ma. Flat Pack"  http://www.smallbearelec.com/Detail.bok?no=339 as I thought it was best suited to getting close to 12.6volts. I can't figure out the primary secondary side or anything though. As the picture on small bear shows, one side is labelled "3 4 2 1" and the other "7 8 6 5". How should I go about testing this?

Ideally, there should be a chart showing the pinout and connections. I am kind of amazed that Steve doesn't have that up there.

There is a way around this.
THIS IS A GUESS, BASED ON HISTORY AND WHAT I WOULD DO. I MAKE NO GUARANTEES THAT THIS WILL WORK FOR YOU. YOUR MILEAGE MAY VARY. YOU ACCEPT ALL LIABILITY FOR THE RESULTS YOU GET FROM THIS TECHNIQUE.

In my experience, a flat pack transformer like that has one "primary" and one "secondary" winding on each side of the centerline. The primaries are USUALLY each 120Vac and the secondaries are half the total voltage, in this case about 6Vac. It's hooking them together in series that gives 12V.

The thing that makes this possible to do is that the wire to wind a 120V winding in the same space as a 12V winding will be very thin, and therefore higher resistance. So:

(1) get out your digital multimeter; if you don't have a DMM, get one. You can't do anything useful without at least this.
(2) set it to resistance, lowest resistance range
(3) make up a chart with the numbers 1,2,3,...8 across the top and down the left side. This will become your resistance table.
(4) Connect your meter probes together. Verify that you read 0 ohms with them shorted.
(5) Measure the resistance from pin 1 with one probe to pin 2. Write that down in the table.
(6) Measuer the resistance from pin 1 to pin 3; write it down; Pin1 to pin 4... and so on until you have measured the resistance from pin 1 to every other pin.
(7) Now start on pin 2. You have already measured pin 2 to pin 1, so you don't need to do that again. Do 2->3, 2->4... 2->8, writing them all down.
( Now do 3->4 through 3->8; then 4->5... 4->8 and so on until you finish with 7->8.

This will tell us most of what you need. It is likely, given the picture that you will find resistance between 1 and 2, between 3 and 4, 5 and 6, 7 and 8; all the others are LIKELY to be open circuit. If that is the case, it is LIKELY that you will find that the resistances of 1->2 and 3->4 are similar and different from the resistances of 5->6 and 7->8, which are also likely to be similar. This is because if there are two primaries and two identical secondaries we should find two pairs of very similar resistances.

The highest resistance pair of windings are the primaries. They should be on either the 1-2-3-4 end or the 5-6-7-8 end.

If all this is exactly like I said, then you can proceed.

First of all, you do know that you can get killed dead messing with AC line voltage, don't you? If you do not already know how to do this safely, do not proceed without expert, experienced help.
If you are experienced and skilled enough to take your life into your own hand and take full responsibility for your actions:

1. Hook up another transformer secondary, preferably about 12Vac, but the exact value does not matter, to ONE of the high resistance windings you identified by the resistance test. Any AC voltage will do, even a low voltage AC output wall wart, but it MUST be AC. Measure the AC voltage you're putting on the winding with your DMM on the AC volts scale. Let's pretend you are getting 11Vac from a wall wart and you put it on the 1->2 winding.

You then measure the voltages on the other three windings. If our guess was correct, you will find that one of the other windings will have a voltage very near the 11V you're putting in. That is the winding that matches the one you are supplying with voltage. Now measure the other two. They will either be much higher ( like over 110Vac!) or much lower, only a volt or so. If they are higher, the winding you are supplying with voltage is one of the 6V secondary windings. If they are much lower, the winding you are supplying with voltage is one of the primaries.

So now you have identified the two primaries and two secondaries. You have only to phase them.

Let's say you powered winding 1-2, and winding 3-4 had a similar voltage on it. Hook ONE of the pins of 3-4 to ONE of the pins of 1-2. Let's say we do this the easy way and hook the middle two, pins 2 and 3 together. If you measure the voltage on 1-2, it's still what you are supplying to one winding. If you measure from 1 to 4, you will get one of two values, either twice the voltage you are supplying, or almost zero. If it's twice, place a red dot on pins 1 and 3. Those identify the phase of the windings. If it's almost zero, put a red dot on pins 1 and 4.

Now, hook the two center pins of the other two windings together and measure the voltage across the outside pins. If it's almost 0, put a red dot on the two outside pins. If it's twice what one winding gives you, put a red dot on one winding's outside pin and the other winding's inside pin.

Unhook it, we now know how to hook up the transformer.

You have identified the two primaries as the high resistance windings. You have identified the phasing of the two with red dots. If you're in a 120Vac country, you can hook the two primary red dots together and the two undotted pins together and apply 120Vac to them and read voltage on the secondaries. If you're in a 240Vac country, you can hook one dotted pin and an undotted pin and supply 240Vac to the unconnected pins.

I strongly urge you not to do the preceeding procedure unless you have no other way of finding out how to hook that thing up. Write Steve a letter.

[peteyC]

thanks I got as far as the resitance testing and decided i didnt want to go further without atleast an ac power supply
i heard back from small bear and they gave me the 12v wiring for the smallwart power supply so that makes life alot easier!