I have been given a transformer that has no markings to indicate primary or secondary voltages. One one side of the transformer is one heavy red lead, one smaller red lead and a black lead of the same size. On the other side are three leads. I know only that it came out of a UPS battery backup so I feel sure it is 120 volt primary. I ohmed out the side with the heavy wire and it measured 8 ohms between any two of the three leads. I ohmed out the other side and it measured 1.2 ohms between any two of the three leads. Is there any safe way to test this thing out and determine which two of the three leads is input voltage? I am not at all experienced with transformers and I don’t want to make a mistake and smoke it. It looks to be in excellent condition so if I can figure it out, I can possibly use it in the future.
You can either use another transformer to feed low voltage into the unknown windings (a model railroad transformer is an excellent choice) or put an incandescent lamp in series to limit the current to a safe level and hook it to the mains.
An isolation transformer is always a good investment, by the way
Hi, your measured values 8 ohms and 1.2 ohms sound a little too low if you ask me. It would have helped a lot if you had the exact specs of that UPS gear. a classic style transformer that has a primary designed to be (directly fed with mains voltage IE: 120V ac) should /would usually have a much higher measured resistance than 1.2 or 8 Ohms.
Now…. having said that the transformer you have could also be part of a SWITCH MODE Power supply system and it that’s the case then it would explain why such a low measured value. However such a transformer is only to be (ever) used in a SMPS because of its design. If fed with mains voltage (directly) chances are it might either blow in your face or burn out in flames or the protection box in your house will trip. So…beware.
PS: are you certain that your measurements are correct and accurate ? Alternatively you might actually have a transformer with an internal short (meaning is faulty)
the larger gauge wires are the secondary (12-volt/1.2=10-amps) the smaller is the primary (120/8=15-amps) the other lead may be a 5-volt circuit board power. That is what I think. (IMHO)
Hello Follks:
to determine the values of the transformer you need to determine the turns ratio of the transformer. To do that you are best to use a Variable transformer (variac). First group all the leads that have a connection to each other by using your ohm meter. The primary and secondary leads do not always come off the same side of the transformer. As noted the larger diameter wire is likely the secondary. but check to make sure you do not have a circuit with the other leads. Draw a sketch and indicate the leads. The resistance between them usually indicates where they are on the winding. IE Red-Bl is greatest therefore either end of the winding etc. Do this for all groups of wires. Now you have a visual of what you are doing.
Second feed a very small AC voltage into the group with two wires that have the largest resistance. Measure the resultant voltage from the other group of two wires with the largest resistance ratio. This will give you the voltage ratio. IE Vin removed link
This is a really rough way of figuring it out. I recommend you research “turns ratio”. That will be the best way to make sense of it.
Good luck. Be Careful, One hand only when playing with Voltage.
Latest Post: looking for tech data emc 502 Our newest member: billjoye Recent Posts Unread Posts
