Block transformer ratings interpretation

Thread Starter

JochenV

Joined Jun 7, 2020
6
I have a small Block FL4/12 transformer laying around for which I expected the output to be 12V RMS on secondary side but I'm measuring different values. It is a dual winding transformer on primary as well as secondary side with ratings 115V for each winding primary side and 12V for each winding secondary side.

IMG_7589.JPG

If I connect the primary side in series and put 230V mains across the combined windings I expected 24V secondary when put in series and 12V secondary when windings are put in parallel. However I measure 34,60V when in series and 17,3V when in parallel. Does that make sense or did they put the wrong text on the transformer block?

Thanks for shedding some light on the matter.
Jochen
 

ZCochran98

Joined Jul 24, 2018
88
If you're measuring the amplitude of the waveform from your transformer, it is correct. The mains voltage is given in RMS (root-mean-square) value, which is the "equivalent DC level" you'd expect from the AC signal. The RMS value of an AC signal is 0.707 times the actual amplitude. So, if you measure the AC amplitude of the output, multiply that by 0.707 to get the RMS value. In this case, 34.6*0.707 = 24.5 V, and 17.3*0.707 = 12.2 V. So it is working properly.

AC mains voltages' actual amplitudes are 163V (for a 115V mains) and 339V (for a 240V mains), for the record.
 
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Thread Starter

JochenV

Joined Jun 7, 2020
6
Sorry, I should have mentioned how I was measuring, so I was using a DMM on V AC for which the values should represent 34V and 17V RMS and the peak values would then be 49Vp and 24Vp which in my analysis is double of the rating of the transformer or am I mixing things up here?
 

ZCochran98

Joined Jul 24, 2018
88
Ah. In that case, I'm not entirely sure myself. From what I know, DMM VAC measurements (attempt to) return the RMS value, so if you were reading 34 and 17 VRMS, then the true peak value would, indeed, be about 48 and 24V. In that case, the transformer could possibly be marked wrong. It's just really odd that the ratio of "wrongness" is almost exactly 1.414. It's always possible there's some detail we're both missing here.

One thing I may suggest is to see what the voltage is on the secondary with a large load - connect something akin to a 1MOhm (or similar) resistor across the output of the secondary and try again. Apparently, from what I read, over-voltage is somewhat common for transformers.

You may also want to check the voltage on the primary - if it actually reads 230V or 115V, then something's up with the transformer (especially if providing a large load on the secondary doesn't change the reading). If it reads 325 and 163, then your DMM is reading high (which is possible if it's not a "true RMS"-type meter or if it's been awhile since calibration).

Unfortunately, that's all I can determine or suggest. Sorry I couldn't be more helpful!
 

AlbertHall

Joined Jun 4, 2014
10,258
The secondary voltage of transformers is usually specified at the rated secondary current so ideally you would connect two 72(ish) ohm, 2W (or higher) resistors, one across each secondary and measure the secondary voltages again. But this transformer does seem to be a long way off.

With the primaries in series what voltage do you read across each primary - they should be equal.
 

Thread Starter

JochenV

Joined Jun 7, 2020
6
Thanks for the responses, I put a 1/8th load on it yesterday (I didn't have the proper high wattage resistors to put in) and measured the voltages again. Primary is 230V over the series and as expected I measure 115V over the individual windings. Secondary side I still get 34V RMS when in series and this doesn't drop a bit with a small load. My conclusion now would be that this device is wrongly marked as there is no way to get 12V out of it.

What I also did not expect is the hum it generates when connected to mains without current flowing. I have another 4VA transfo which doesn't generated any noise. As the winding resistances and voltages match I did not expect internal failure but it is not normal behavior I would say.

Any further thoughts are still welcome.
 

crutschow

Joined Mar 14, 2008
25,273
Measure the primary and secondary winding resistances with no other connections.
Short the ohmmeter leads together before the measurements to account for that offset error.
 

Thread Starter

JochenV

Joined Jun 7, 2020
6
Primary side I measure 662 and 660Ohm, secondary side it comes down to 12.4 and 12.5Ohm (offset of 0.2 in the 0-200ohm range of DMM leads subtracted)
 

Thread Starter

JochenV

Joined Jun 7, 2020
6
Thanks for pointing that out agian and I know it was mentioned before but I didn't really grasp the total amount it adds up to. 35% is quite substantial. I didn't see a drop with a 1/8 load. I will test again as soon as I have some 0,5W resistors. Thanks for the feedback, still feel a bit stupid that I missed that from the datasheet.
 

djsfantasi

Joined Apr 11, 2010
6,724
Ah. In that case, I'm not entirely sure myself. From what I know, DMM VAC measurements (attempt to) return the RMS value, so if you were reading 34 and 17 VRMS, then the true peak value would, indeed, be about 48 and 24V. In that case, the transformer could possibly be marked wrong. It's just really odd that the ratio of "wrongness" is almost exactly 1.414. It's always possible there's some detail we're both missing here.
The TS is connecting 230VAC across the primary. The transformer is marked 12V for 115VAC. Hence, 24VAC and 48VAC are the expected values.
 

Thread Starter

JochenV

Joined Jun 7, 2020
6
The transformer would not be happy with 230VAC on one winding of the primary so as per intended use the primary windings are put in series with 230VAC across.

I got my 0,5W resistors and put 275Ohm across with 8 resistors in parallel which resulted in an output voltage of 28,7VAC with a current of around 0,100A which is almost 3VA so a 3/4 load of the transformer. Voltage clearly dropped but it is still on the high side in my opinion, though I guess this will be within expected tolerance margins and you have to design around this variation.

Thanks for the feedback.
 
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