Measuring AC Current & Calculating Power of a Transformer: UNI-T UT210E & H1 55W Bulb

How to measure a transformer

Hello,
I have such a transformer:



and I would like to measure its parameters and learn something.
There is 8.5V AC between the central tap and one end. 17.5V AC between both ends. By connecting the H1 55W bulb to 2 ends, bypassing the tap, the voltage drops to 8.5V and the bulb shines with approximately the same power as connected to the DC voltage source, limited to 4A. The voltage source is set to 12V DC, this 4A shows 8V.
None of my cheap multimeters measure AC current through wires. I have such a UNI-T UT210E, but connecting it like this:



obviously with a glowing bulb, it shows 0.002.
- How to connect this multimeter to measure the current?
- Is it possible to calculate its power on the basis of these measurements?

17.5V x 1.404 = 24.5V, that is, counting the decrease on 2 diodes (2 x 0.7V) in the rectifier bridge, I should get 23.1V DC without load, am I counting it correctly?

rosak wrote:

17.5V x 1.404 = 24.5V, that is, counting the decrease on 2 diodes (2 x 0.7V) in the rectifier bridge, I should get 23.1V DC without load, am I counting it correctly?

Yes. Not 1.404 but 1.41 (root of two)

rosak wrote:

By connecting an H1 55W bulb

So you connected a DC light bulb directly to the transformer?

rosak wrote:

obviously with a glowing bulb, it shows 0.002.

You probably turned something during the measurement because the reading is 2mA.

rosak wrote:

- Is it possible to calculate its power on the basis of these measurements?

Estimate the power by the size of the core, the second option is such a load that the voltage drops by 10% - with a high probability it will be an approximation of the maximum load.

In my opinion, this transformer can be about 30W. But like your colleague said you have to measure the core.

szymon122 wrote:

Yes. Not 1.404 but 1.41 (root of two)

Right, it was supposed to be 1.414

szymon122 wrote:

So you connected a DC light bulb directly to the transformer?

Yes, I read that in other posts here on the forum.

jdubowski wrote:

You probably turned something during the measurement because the reading is 2mA.

This, I know, just don't know what, that's why I gave a picture of the meter.

jdubowski wrote:

the second option is such a load that the voltage drops by 10%

Ok, this should be enough for me, I just need to assemble an artificial load, because the one I have has a range of about 1.5A, although it should be enough.

Alternatively, if I wanted to check with a resistor, can I connect 24 Ohm directly after the rectifier bridge? If the voltage didn't drop, it would mean I have at least 1A.

szymon122 wrote:

So you connected a DC light bulb directly to the transformer?

A light bulb is a light bulb. AC or DC - it doesn't matter to her.

And someone could explain to me why after straightening the waveform looks like this:



? Why are the "halves" of a sine wave different? The bridge is S4VB20, no load.

rosak wrote:

Hello,
I have such a transformer:

How to measure a transformer

and I would like to measure its parameters and learn something.
There is 8.5V AC between the central tap and one end. 17.5V AC between both ends. By connecting the H1 55W bulb to 2 ends, bypassing the tap, the voltage drops to 8.5V and the bulb shines with approximately the same power as connected to the DC voltage source, limited to 4A. The voltage source is set to 12V DC, this 4A shows 8V.

jdubowski wrote:

Estimate the power by the size of the core, the second option is such a load that the voltage drops by 10% - with a high probability it will be an approximation of the maximum load.

szymon122 wrote:

In my opinion, this transformer can be about 30W. But like your colleague said you have to measure the core.

It seems to me that this transformer has a little more than 30W.
Taking into account the photo of the transformer with a ruler and assuming the core cross-section S = 8 cm2, the transformer would have 42W, and then the transformer's secondary winding can be loaded up to 2.4A ...
The H1 55W / 12V bulb loads the secondary winding with about 4.6A and therefore the voltage drops to 8.5VAC ...
These are estimates as we do not have precise data on the cross section of the transformer's core core column.

krzysztof723 wrote:

It seems to me that this transformer has a little more than 30W.

FOR SURE there is no more than 30W. I have several with exactly the same dimensions and these are 30W transformers (taken from PAR 36).
Having power, you can easily calculate what current for a given voltage can flow through the secondary.

398216 Usunięty wrote:

krzysztof723 wrote:

It seems to me that this transformer has a little more than 30W.

FOR SURE there is no more than 30W. I have several with exactly the same dimensions and these are 30W transformers (taken from PAR 36).
Having power, you can easily calculate what current for a given voltage can flow through the secondary.

My friend is absolutely right, my mistake, because I misplaced the dimensions.
I took the width of the casing from the photo, not the core ... Eh
If the transformer is 30W, the 17.5VAC secondary winding can be loaded with up to 1.7A

When I settle something, I will let you know. Trafo I took out some equipment / karaoke amplifier from an unknown company.