wrote in message
oups.com...
Thanks for your tips everyone! I'll perform the transformer autopsy
asap.
I experimented with a few more transformers, simulating a blown primary.
Added to my tips file off URL below
Blown primary, how to determine unknown secondary voltages of a mains
transformer
Simulating a multi-secondary transformer using a known
good one but not using the primary, to get some data.
I used a variac supply near the bottom of its range at 18volts and a 25 ohm,
20W dropper to feed 50Hz (UK)ac into a secondary. Assuming you have a
reasonable idea of the voltage of one 'unknown' secondary.
The transformer I used first was a high grade enclosed Gardners, 1.3Kg but
only 15W combined outputs,
240V (UK) with marked 2 separate secondaries of
6.3V, 0.6A and a 150-0-150 at 25mA.
With 3.43V ac on one '6.3V' secondary there was open circuit 3.40 on the
other isolated '6.3' and 161.4V end-to-end on the '150-0-150' and incidently
116.4 on the primary.
Then loading with different resistors
100K, 161.4 drops to 159.1
5.8K on 161.4 drops to 55.8, 3.43 input drops to 1.64
swapping to 5.8K on 3.4 , no change
1K on 161.4 to 12.1 and 3.43 to 0.771
swap to 1K on 3.4 , drops to 3.39
270 ohm , 161.4 to 3.34V
270 on 3.40, drops to 3.37
56 ohm on 161.4 to .704 and 3.43 to .54V
56 on 3.4 , drops to 3.28 and 3.43 to 3.42
8.2 ohm on 3.4 , drops to 2.55 and 3.43 drops to 2.99V
A bit more generalised.
Noting that for one secondary for this test transformer was rating 300V,
25mA then V/I of 12K and the 6.3V, 0.6 secondary of 10.5 ohm.
Doing as before powering a 6.3V secondary to 3.43V and '300V' was 161.4V
then loading it until the voltage ratio was 80 per cent that is 161.4V down
to 101.5V and 3.43 falling to 2.69V so 101.5/2.69 = .8 then that R is 12K.
So for similar transformer construction and high V, low I then find that
value of R for 80% then if V is known then current rating is V/R.
Doing the same for the low V,high I one then for R=10.5 ohm then
corresponding ratio drops from 1:1 ie ==3.43:3.4 down to 3.03/3.43 is 88%
for high current , low voltage.
So for similar transformer construction and high I, low V then find that
value of R for 88% then if V is known then current rating is V/R.
Other clues would be the gauge of the wires if they can be seen and the
overall size and weight giving an idea of the overall power rating.
Resistance checks would show which are more likely high V or high I.
Second test with a more basic Albion make, .8Kg, 20W open
construction 245V primary, 2 secondaries 17V,1A and 6.3V,.6A.
Again putting current into the lowest secondary giving 5.59V on
'6.3' and 14.52 on '17' (185.8V on 'primary')
6.3/.6 wire was 24thou diameter and 17V,1A wire was 27 thou diameter.
17/1 = 17 ohm. This time loading the 17V secondary with
17 ohm meant the ratio had dropped 69 per cent (15.52/5.59 to 2.778/1.544 )
Usually you would get some idea of one rectified V from max or min, by
capacitor ratings or a regulator voltage etc.
Valve radios would have one secondary connected to the
heaters so usually 6.3V. A vacuum fluorescent
display is likely to have a feed in the range only 2 to 5V
Toroidal transformer 2x 120V to 2x 15V,2A, .75Kg and 2A wires
33 thou diameter. Characteristic R = 15/2 = 7.5 ohm.
Critical ratio in this case was 82 per cent with 7.5 ohm.
15.27 input on '15' giving 15.26 on the othe rand 108V on
one of the primaries.
With 7.5 ohm 15.27 i/p drops to 3.91 and 15.26 drops to 3.19.
For a large toroidal 500W 2x 35V, 7.1A , weight 4.8 kg
Secondary wires consist of 2 paralled 56 thou diameter wires
per secondary.
Characteristic R= 35/7.1 = 5 ohm.
With 15.16 on one 'secondary' 15.1 on the other and 49.8V on
a 'primary'
15.16 dropped to 2.59V and 15.1 dropped to 2.46
so characteristic ratio is 95 percent for this transformer.
--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/