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Need some help with the transformer




Hey all,

I have bought a transformer from Reichelt with 2*15V AC. For the 9090 I need to have a 4pin connection with the following values:

pin1: 15V

Pin2: 0V

Pin3: not connected

Pin4: 15V

You can see in the picture that I only use one output of the transformer (pin 8 and 10, I do not use pin 12 and 14). Can someone tell me if this is correct?

Cheers Jef



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usually you'd need to use both coils to generate positive and negative rail. one for positive and one for negative. the middle pins are connected , this is the GND point of your whole circuit. then the positive and negative ends go to the rectifying circuit.

at least i built a +/- 15V supply and a +/-9V supply that way.

but i don't know the wiring of your transformer. it should be shown in the datasheet.

but you can also measure (power not connected) the continuity between the pins and find out which are physically connected

to each other, and which aren't. also, you can then measure the AC between the pins you determined.

there are some psu circuits that show how it is done, i believe somewhere here in the wiki.

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yes that's what i meant.

so you take the outer pins and connect them to pins 4 and 1 of the 9090 connector.

the inner pins to 2 and 3. they're connected on board according to the 9090 schematic (just checked that on the board with a beeper, too)

so as printed on your trafo 8 and 10 are one coil, 12 and 14 the other. both have 15V AC. continuity check should prove that.

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i'm quite unfamiliar with the dot markings on your transformer drawing. maybe you better ask for advice in the main forum, i think there are more people with better knowledge than mine.

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What you need to do is to find out beyond the shadow of a doubt is which two cables are the 0V (GND) connection. Then connect those two together to the GND of the transformer PSU input of the board.

The plus of secondary one then goes to one of the 15V inputs that's connected to either plus or minus of the diode bridge. The plus of secondary two goes to the other 15V input. Both of the secondaries will be AC when they are output from the transformer so which is plus, which is minus of the diode bridge won't matter, as long as you have a common ground and connect the two outputs to the two different sides of the diode bridge.

Your picture is on the small side, but it looks like the dots indicate the plus side of the transformer windings. But, please measure before making the connection!

Rosch's picture above shows a common dual primary transformer. The cool thing about this is that you can use either input for US voltage, or connect the first zero to neutral, bridge the two 115V and the 0V in the middle, and connect the rightmost 115V to a common 230VAC voltage. OR, you wire up a switch (careful!). The lower side shows the two secondaries. Connect the GNDs of both together for a double voltage (+-9VAC) output and then make sure that the plus outputs goes to either side of the diode bridge.

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