Electronics > Beginners
Safest way to test unkown transformers with wire taps
<< < (4/5) > >>
ptricks:

--- Quote from: MarkF on May 12, 2019, 05:59:11 pm ---Remember the series light bulb we talked about before.
If the light stays OFF, you are okay.  If the light comes ON, you have a short.

The bulb wattage will determine the current limit. 
This assumes that the device-under-test can handle the current.
You are still at some risk but not near as much.


--- End quote ---

This !
I did tv repair in the 1980's and this was great for troubleshooting power supplies that would blow the fuse often.  One could also use resistors but the light bulb (incandescent not LED or any others) was so much easier and you can change the limit so easily with a bulb swap.
Jwillis:
Checking the resistance of the winding s won't guaranty the identification of the primary winding s because a step up transformer like a MOT (Micro wave Transformer ),can have over a hundred ohms at the secondary as compared to a fraction of an ohm at the primary.Even Some power transformers with multiple secondary winding s can have one or more taps with higher resistance than the primary.

If you know, for a fact, the identification of the primary using the lamp method WILL NOT GUARANTY SAFETY EVER. Because the secondary may have several thousand volts that a multi meter will not handle. DO NOT USE THE LAMP METHOD to identify an unknown secondary voltage EVER!

Look at the winding s .If any winding is connected to the core DO NOT connect the core to ground.If your not sure if the winding is connected to the core do not connect core to ground.
 
Draw a diagram of of the transformer and identify the coils with continuity and orientation.
Always check for continuity between any winding and the core.

Use LOW voltage AC at the primary and then measure the voltage at the secondary. The use of a variac is safe at low voltage.You only need at most 24 VAC but 10VAC is plenty to produce a magnetic field .

Turn off the variac . Set Variac at the lowest setting and connect to the primary and connect a volt meter across primary.The markings on the variac may not be accurate.monitor the voltage.
Set multi meter to the highest AC setting and connect to secondary you want to identify.
Turn on Variac and VERY SLOWLY increase voltage until you get a reading on the multi meter.Very slowly means just the slightest movement.Reading the secondary voltage of a MOT can be as high as several hundred volts even at very low voltages at the primary. 
Record the voltage on the primary and the voltage on your secondary Volt meter.

Divide Mains voltage (120)  by the reading on the primary . This number will be your multiplier for the next step .
Multiply the reading on the secondary by the multiplier . This is the approximate voltage  of the secondary at 120VAC.
Again these will be approximate because as you increase the voltage at the primary the field gets stronger and the secondary will become closer to  to its turn value.

example 1 :  If the variac is set at 10 VAC and you read 5 volts at the secondary. 120VAC / 10VAC = 12 .  12 X 5 = 60. So at 120VAC the transformer secondary will be approximately 60VAC.

Remember if you  get higher voltages at  the secondary relative to a low voltage at the primary  you may have the secondary identified as a primary.Reverse the transformer and retest.

If you get very high voltages relative to the primary .I would advise to shut down and discontinue testing.We can assume that its a step up transformer and further experience is required.You can test high voltage transformers in reverse .But at this point it's best just avoid them until you have more experience.

This is what happens to a multimeter connected to an "unknown transformer" using the lamp method.I sacrificed this meter to show what can happen because I knew the transformer had 3000VAC at the secondary.Do not assume the secondary winding will be lower than the primary.Always use very low primary voltages to test unknown transformers.
bsdphk:
"Checking the resistance of the winding s won't guaranty the identification of the primary winding"

That is not the point really... you use the highest DC resistance winding to minimize the amount of power while you measure the ratios.

And I fully agree on not using the "lamp method" until one is 100% sure how to connect the primary/ies for local grid voltage (or higher).
stj:
it's actually very simple.

use the output of a 12v transformer to feed the unit you want to test,
then multiply the output by 10 to get the true value.

even if you get the primary and secondary mixed up your still going to only see 3digit values and wont kill yourself or damage the meter.
and it's isolated.
soldar:

--- Quote from: MarkF on May 12, 2019, 05:59:11 pm ---Remember the series light bulb we talked about before.
If the light stays OFF, you are okay.  If the light comes ON, you have a short.
--- End quote ---

You can get a lot of information about a transformer or inductance with very simple tools. And you can learn and practice a lot too which may be more valuable. You do not need variacs or other fancy equipment. Just a multimeter. 

Take a winding of the transformer and feed it through a lightbulb. Measure the voltage and current at the winding. Now calculate the impedance of the winding. Now measure the DC resistance of the winding. Now calculate the inductance of the winding. Do that for all windings.

Now you have a good first order approximation of that transformer and you can calculate currents and voltages for inputs and outputs.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod