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Testing mains transformers (wall warts) etc.

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leviticus49

New Member
Could anyone give a way to test transformers (for computer equipment , modems etc.) My Plustek scanner gave up the ghost recently and so I checked as best I could and after trying the tip polarity test, getting no reading assummed the trannie was dead. Just to check, I tried my radio scanner trannie - gave 15v tip positive. Tried two other similar trannies with no readings so I was very supprised when I plugged them into a modem and the lights came on. What do I do to test if a trannie is good other than plugging them into tackle they were not designed for? Any ideas?
 

Scubasteve

New Member
Let me get this straight :

You have a device that stop working, then you decided to troubleshoot it. Found that you were not getting a 'reading' off of the transformer with your equipment. I am not sure about that particular peice of equipment, is it a multimeter? If it is, it should be no problem to use its AC voltage setting and to measure the output off of the seconary of the transformer. You should be getting a reading of some sort when it is plugged it. Make sure that there is actually power going into the primary coil with the AC meter also. You decided to plug in random transformers to see if it worked? This is a bad idea without taking into consideration the requirements of the device.

The 'blown' transformer should indicate several important specs., more importantly the 'voltage output'. You need to take into account the configuration of the transformer (one secondary, center-tapped, three secondaries, etc.). You also need to consider the power capability, this is expressed as a current or a rating in Volt-Amperes.

Once you have these specs., you can find a similar replacement. You must be able to supply the same voltages as the damaged transformer. You need to make sure that the transformer has a power rating which is equal to or higher then the original. Also, the configuration must match.

If you have too much voltage, your circuit may not be able to handle the power losses associated or it may exceed voltage ratings. Also, if you don't have enough power, it may not be able to supply current during current demanding tasks (ie. highspeed upload/downloading, rebooting modem, etc.)

Hope this helps your understanding,

Steve
 

leviticus49

New Member
Re your reply

Scubasteve said:
Let me get this straight :

You have a device that stop working, then you decided to troubleshoot it. Found that you were not getting a 'reading' off of the transformer with your equipment. I am not sure about that particular peice of equipment, is it a multimeter? If it is, it should be no problem to use its AC voltage setting and to measure the output off of the seconary of the transformer. You should be getting a reading of some sort when it is plugged it. Make sure that there is actually power going into the primary coil with the AC meter also. You decided to plug in random transformers to see if it worked? This is a bad idea without taking into consideration the requirements of the device.

The 'blown' transformer should indicate several important specs., more importantly the 'voltage output'. You need to take into account the configuration of the transformer (one secondary, center-tapped, three secondaries, etc.). You also need to consider the power capability, this is expressed as a current or a rating in Volt-Amperes.

Once you have these specs., you can find a similar replacement. You must be able to supply the same voltages as the damaged transformer. You need to make sure that the transformer has a power rating which is equal to or higher then the original. Also, the configuration must match.

If you have too much voltage, your circuit may not be able to handle the power losses associated or it may exceed voltage ratings. Also, if you don't have enough power, it may not be able to supply current during current demanding tasks (ie. highspeed upload/downloading, rebooting modem, etc.)

Hope this helps your understanding,

Steve
May I first apologise for not making my query a little clearer. My Plustek scanner stopped working after moving its position. Checking that all switches etc. were on I tried to find the problem in the scanner. Nothing! Using my digital multimeter, I tried to see what if anything the mains tranformer that fed the scanner was giving. I am no electrician so carried out the only test I knew, how to acertain the tip polarity of the transformer, thinking if it is giving anything out it should register on the LCD readout, meaning that the trannie was O.K. and the repair/purchase of a new scanner. Setting the meter to DC Volts 20V range and putting the test prods in the correct terminals (red to VmA, black to COM) I inserted the tip of the red prod in the centre of the co-axial plug and touched the black prod to the outer. Nothing at all, so I assumed that the trannie was dead. I tried the exact same procedure on another trannie that I knew was O.K. and obtained the reading I expected, 15V tip positive. On a whim I tried another two trannies that had the correct size plug (and correct Voltage output and rating) getting no reading. Plugging these into the modem they were compatible with, the LED's on the front of the modem came on as if nothing was wrong with the trannies. The suspect trannie is a sealed unit so rather than destroy it just to find out if it was duff after all, I will buy another. This still leaves me with the problem of how to test a trannie. Any Ideas will be greatfully accepted.
 

Gene

New Member
1. Some voltmeters will read 0 if they are switched to read AC and attached to a DC source. The opposite is also true. These little transformers come in both AC and DC output - depending on the device they are designed to operate with. I know it is a long shot but . . . if your device is designed to operate from an AC transformer and your meter was switched to read DC, you might have seen a 0 reading.

2. Another long shot. If your transformer is designed to deliver DC, it probably has a transformer, a bridge, and a cap. A dying cap could cause screwy readings.

3. Some of these things are designed to deliver pulsed AC (high wattage in a small package) and simply will not read anything on some voltmeters. A test that will work is a small light bulb connected to the transformer (block).

Personally, if the problem is not resolved quickly, drop the $10 and replace the thing- your time is valuable too.
 
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