reading voltage on a rectifier

i am working on an ami-d-80 juction box. when i test the voltage coming from the transforer i get 38 ac volts. i hook a rectifier to the leads from the transformer and then i test the plus side by putting the meter on dc and with red wire from the meter on plus side and black lead to transformer for ground i get 19 volts but when i do the minus side i get 2 volts. what am i doing wrong. thanks norm

On Jun 5, 9:55*pm, wrote:
i am working on an ami-d-80 juction box. when i test the voltage coming from the transforer i get 38 ac volts. i hook a rectifier to the leads from the transformer and then i test the plus side by putting the meter on dc and with red wire from the meter on plus side and black lead to transformer for ground i get 19 volts but when i do the minus side i get 2 volts. what am i doing wrong. thanks norm


Is it a centre tapped transformer, are there 2 or 3 wires wires coming
out for this circuit ?
if 3, then it is centre tapped, if 2, it is single wound.

Did the original selenium rectifier have 3 or 4 wires ?
if 3, it is a centre tap, if 4 it is single. (Note that they might
have used the chassis as ground for all I know.)

---------------

I have not seen a D-80, but If this is anything like the G-80, it is
not a centre tapped transformer, and just has 2 wires coming out.

What you do in this case is to connect the wires from the transformer
to the AC terminals of the rectifier, then use the - terminal of the
rectifier as the negative, and the positive terminal as the positive.

While you have it apart it would be very wise to put a fuse in series
with one of the AC leads from the transformer. 1.5A slo blo should be
ok. This will prevent the transformer burning up if the rectifier
ever shorts.

This is a diagram for such a circuit.

http://www.google.com/imgres?hl=en&s...2, s:36,i:181




For a centre tapped transformer, you connect the 2 outer leads to the
AC terminals of the rectifier, and then connect the centre wire from
the transformer to negative, and the positive terminal of the
rectifier to the positive. The - terminal of the rectifier would not
be used.

In this case, you should fuse both of the AC wires from the
transformer that go to the rectifier, not just one.

This is a diagram of such a rectifier. Only 2 diodes in the rectifier
are used, hence why you don't connect anything to the negative
terminal in this application

http://www.google.com/imgres?hl=en&s...r:5,s:16,i:121


Finally, as a safety warning, before powering up, use a meter to check
that there is no continuity between any of these 28v wires and the
wiring on the mains side of the transformer.