So, i need to measure the primary impedance on a pair of output transformers. I don´t have a variac, but plan to use the 5v taps on a power transformer. On a multiple secondaries transformer (4-8-16 ohms) does it matter which secondary I use, and if, how? And also, how do I measure the transformers dcr, across which leads? p1 and p2? It´s a transformer with ul/sg taps, if that matters.
Hi,
Your simple task is to establish the turns ratio or voltage gain.
e.g. 5V into any secondary will create a primary voltage, you
simply measure it. The voltage should x root2 as you go from
16 > 8 > 4. The voltage ratio is your turns ratio.
i.e. it will be double for 4 ohm compared to 16ohm.
The primary impedance is (turns ratio)squared x secondary impedance.
It should come out the same for all the secondaries.
Measuring DCR's on the primary will indicate the % of the taps.
rgds, sreten.
e.g. for a 5K transformer the turns ratio for 8 ohm is 25.
5V in on 8 ohm should measure 125V on the primary.
Depending on the UL taps (I don't understand SG) it is possible
to use them to give a different (lower) primary impedance.
Your simple task is to establish the turns ratio or voltage gain.
e.g. 5V into any secondary will create a primary voltage, you
simply measure it. The voltage should x root2 as you go from
16 > 8 > 4. The voltage ratio is your turns ratio.
i.e. it will be double for 4 ohm compared to 16ohm.
The primary impedance is (turns ratio)squared x secondary impedance.
It should come out the same for all the secondaries.
Measuring DCR's on the primary will indicate the % of the taps.
rgds, sreten.
e.g. for a 5K transformer the turns ratio for 8 ohm is 25.
5V in on 8 ohm should measure 125V on the primary.
Depending on the UL taps (I don't understand SG) it is possible
to use them to give a different (lower) primary impedance.
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What Sreten said, but I would put the 5 volts across the whole primary and measure the voltage at the 16 ohm secondary tap. The reason being is you will get a much more accurate voltage measurement with voltage applied on the primary of the output transformer; less losses because the primary has a much higher resistance than the secondary of the output transformer. Be sure to get an accurate voltage measurement at the primary of the output transformer also to get an accurate turns ratio. Measure voltages to 3 decimal places.
Thank you. And do I measure dcr between p1 and p2? I´m in doubt because on a transformer that´s supposed to have a dcr of 280 ohm I measure 252 ohm and on another that is supposed to have a dcr of 290 ohm I measure 245 Ohm. (??)
Impedance at what frequency? I would use the LC resonance method, by measuring the frequency for a given C. And I would use a C that gives a few hundred Hz frequency. But note that it is level dependent due to the non-linearity of the transformer. It does not matter which side you measure, but try to apply a voltage level that is close to the operating conditions.
DCR does not matter too much.
DCR does not matter too much.
I´m just trying to establish whether the transformer in front of me is a 5k or an 8k transformer. Dcr might not say much, but I would like the measured value correspond with the figure stated in the transformers specifications or understand why it doesn´t.
I don't know much about valve transformers but I want to help.
5Vac @ 50Hz (mains frequency) from the 16ohm tapping is equivalent to only 1.5W from the transformer. I would expect any HiFi output transformer to manage to cope with 5Vac at this frequency as an input to the 16ohm tapping.
If the transformer is 8k:16ohm then the turns ratio (voltage ratio) is sqrt(8000/16) = 22.4. The 5k would be 17.7
To account for under-load losses, the ratio will be a bit higher (or is it lower?) than is predicted by the open circuit turns ratio.
Inputting 5Vac will give ~88Vac at the primary of the 5k or 112Vac for the 8k. That would be across one half of the centre tapped primary, double that for the whole primary. Careful !!!!
Your actual measurement is likely to be higher/lower. Whether it's 105% or 110%, I don't know, but others should be able to guide you.
5Vac @ 50Hz (mains frequency) from the 16ohm tapping is equivalent to only 1.5W from the transformer. I would expect any HiFi output transformer to manage to cope with 5Vac at this frequency as an input to the 16ohm tapping.
If the transformer is 8k:16ohm then the turns ratio (voltage ratio) is sqrt(8000/16) = 22.4. The 5k would be 17.7
To account for under-load losses, the ratio will be a bit higher (or is it lower?) than is predicted by the open circuit turns ratio.
Inputting 5Vac will give ~88Vac at the primary of the 5k or 112Vac for the 8k. That would be across one half of the centre tapped primary, double that for the whole primary. Careful !!!!
Your actual measurement is likely to be higher/lower. Whether it's 105% or 110%, I don't know, but others should be able to guide you.
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OK, if there is somewhere I don´t trust myself it´s in math. ( chose the wrong hobby, right?) I applied 5.3vac across the 8 ohm tap, out came 172.8vac across the primary; p1 - p2. Thats a ratio of 1:32.6, squared = 1062,(76) 8 (ohm) x 1062 = 8496. That would suggest that it is likelier (?) that I have an 8k transformer in front of me than a 5k. Am I right so far? ( thats all I needed to know actually )
that looks right, if my assumption of doubled primary voltage across the whole primary is wrong.
5.3Vac across 8ohms and 172.8Vac across the whole primary
Try it with the 16ohm tap.
see if the answer comes out similar or very different.
Try it on both taps with an 8k resistor >>10W rating. This might reveal the loaded or effective impedance.
5.3Vac across 8ohms and 172.8Vac across the whole primary
Try it with the 16ohm tap.
see if the answer comes out similar or very different.
Try it on both taps with an 8k resistor >>10W rating. This might reveal the loaded or effective impedance.
I did try it on the 4 ohm tap, with similar result. Don´t need the exact impedance, just ballpark figures.
Hi,
8K to 8 ohm on inspection is a turns ratio of root1000, ~ 32.
DCR should measure much lower on both sides.
rgds, sreten.
8K to 8 ohm on inspection is a turns ratio of root1000, ~ 32.
DCR should measure much lower on both sides.
rgds, sreten.
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4ohms uses half the turns of the 16ohms tapping.
Your transformer might not perform at 50Hz on the 4ohms tap due to insufficient inductance. (5.3Vac & 4ohms is 7W, can the transformer do 7W @ 50Hz?)
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