newbie: how to connect a transformer?

Discussion in 'Electronic Basics' started by arjan, Oct 21, 2003.

1. arjanGuest

I have some transformers with many wires. How do I know which wire is
which?

Regards

Arjan

2. Laycock, ChristopherGuest

What colours, how many wires and where are they on the transformer?

Chris

3. Michael BlackGuest

No, that shouldn't be a question mark at the end. Because all
of that does often help to figure out the windings.

The used to be a color code for transformers, though I'm not sure
it was always consistent, and I am sort of doubtful it's being used
nowadays, with transformers coming from all kinds of locations across
the globe.

But yes, the color code used to be fairly constant on the primary,
which to some extent is what you need most.

If the leads come out in more than one place, the one with only two
leads is likely the primary (but I don't think I'd take this
as absolute). This also doesn't help if the secondary has only two
leads (or if the primary has a dual winding).

Sometimes you can get information if you can see the wires on
the actual transformer, something possible with open frame units.
Likely, the primary has smaller wire than the secondary, but again
that's not always the case.

You can always determine sets of wires by using an ohmmeter to
find pairs or sets. Again, with a pair of wires and a set of three
wires, likely the pair is primary.

Once you have the sets or pairs matched up, then maybe the color coding
makes more sense. I have no idea where to find the color code in
present material, but if you find a pair that does match the color code,
that increases the chance that those are the primary.

Old reference books, including the ARRL Handbook, would likely have
the transformer color coding, and one might find such books at the library.

If you have an idea of what the transformer is, ie voltage and even useage,
that might help. A low voltage transformer will have fewer turns on
the secondary than on the primary, so the primary should have a higher
resistance on the ohmmeter than the secondary.

Once the primary is determined, then one can hook it to the AC line
and measure voltages on the other sets of wires. The potential danger
in this is that those voltages may turn out to be quite high, depending
on the transformer, or if you've made a mistake on the primary.

You could lessen the danger by taking a known low voltage transformer,
and power that from the AC line, and use that to feed the alleged primary
of the uknown transformer. Then the other widings will produce accordingly
lower voltates.

Michael

4. BaphometGuest

Let's assume for the moment that you have power transformers. I would start
first by looking at the colors, assuming you can still see them. There
should normally be two of each solid color and perhaps one of the same color
with a twisted color stripe. Get these wire groupings together for later
identification.

Before I continue, let me say that there are no hard and fast rules here but
the transformer primary is normally black. The primary winding may have
several taps for to compensate for different line voltages. These colors are
usually darkish and muted. The 6.3.volt secondary filament windings are
usually green, the 5.0 volt windings yellow, and the B+ windings reddish
orange.

To check yourself, take resistance measurements of the windings with an
ohmmeter if you have one available. The resistance is roughly proportional
to the number of turns (assuming that the guage of the wires is similar.
Filament windings will have a very low resistance, primary power windings
and intermediate resistance, and B+ windings the highest resistance.

Once you think you have the wire combinations nailed down, you can apply a
primary line voltage, preferably through an autoformer if you have one. The
advantage of an autoformer is you can bring up the voltage slowly in case
there are any oops's lurking in the wings. You can double check your
findings by measuring the voltages of the other windings. Be careful,
secondary B+ windings can produce many hundreds of volts. If you don't have
an autoformer, you will just have to apply the primary voltage and hope for
the best. Be prepared for some smoke if you miscalculated.

If the transformers are audio, you can pretty much follow the same procedure
except for applying an a.c. line voltage. Preferably, you will have an audio
generator on hand but if not, you might try applying a filament voltage from
a power transformer as your a.c. signal source.
The colors will all be different however.

Hopefully, this will get you in the ballpark but please be prepared to make
a few mistakes.

5. Rich GriseGuest

One nitpick - I'd recommend not connecting full line voltage to
any winding until you've determined which is which. Skip right to
where Baphomet says to use a low-voltage transformer to supply
6V or so to practically any of the windings - then the voltages
at the other windings will give you the turns ratio.

THEN, if you've determined that it's a power transformer, it's
probably safe to apply line voltage to the primary, if you're
reasonably sure which the primary is supposed to be.

Use a fuse! Personally, I'd use about a 1/4 amp fuse. It should
work if the transformer has no load, and it will protect everything
if there's any error. (well, it'll protect almost everything from
almost any error.) If/when you do put it into service, of course,
size the fuse appropriately.

If the transformer came from a 100 or 120 volt country, and
your mains are 220, then don't connect it to the mains - it
might last a few minutes, but it will certainly destroy the
transformer sooner or later.

If it's a fairly modern unit, it may have dual primaries so
that it can be used on either mains voltage, using them in
series for 220/240 and parallel for 100/110/120.

And you can usually make a pretty fair guesstimate of its power
capacity simply by its weight. There's some rule of thumb about
volt-amps per cross-sectional area of core, but it's been many
years, so I don't remember the numbers. If the transformer is
the size of a ping-pong ball, I wouldn't want to try to get
more than about 5 or 10 VA out of it. If it's the size of a
boat anchor, it's probably a power transformer, modulation
transformer, pulse transformer for a radar set, or output
transformer for a subwooferhead's power amp. ;-)

Good Luck!
Rich