12V Transformers

[Dennis Alwon]

I have had a bunch of low voltage 120v- 12V ac transformers that I cannot measure the voltage. In fact, one specifically states not to measure the secondary side of the transformer but I cannot remember if that was a driver or a trany. I have used 4 different brand meters with the same result

What's the story with this? Some say soft start, whatever that means on a trany.

 

[infinity]

Could it have something to do with the inherent class 2, current limiting design?

 

[Dennis Alwon]

Cold it have something to do with the inherent class 2, current limiting design?

I have no idea... These are small 60 watt transformers that work fine but I cannot measure the voltage. Why would it even say not to test the secondary side? :?

They are class 2 tranies

 

[jumper]

Google says if there is not a minimum load the tranny “sleeps” until it sees a proper load, your DMM is prolly high impedance, low load.

Maybe similar to this?

http://forums.mikeholt.com/showthread.php?t=130713

http://www.ecmweb.com/lighting-amp-control/low-voltage-lighting-primer

 

[kwired]

Is it simple two coil device or is there more to it?

 

[Dennis Alwon]

Is it simple two coil device or is there more to it?

Simple electronic trany

 

[Dennis Alwon]

Google says if there is not a minimum load the tranny “sleeps” until it sees a proper load, your DMM is prolly high impedance, low load.

Maybe similar to this?

http://forums.mikeholt.com/showthread.php?t=130713

http://www.ecmweb.com/lighting-amp-control/low-voltage-lighting-primer

I hooked up a 12v 50 watt bulb and still did not read voltage.

 

[texie]

I have had a bunch of low voltage 120v- 12V ac transformers that I cannot measure the voltage. In fact, one specifically states not to measure the secondary side of the transformer but I cannot remember if that was a driver or a trany. I have used 4 different brand meters with the same result

What's the story with this? Some say soft start, whatever that means on a trany.

My experience as well. These modern class 2 "transformers" are not really traditional transformers as in core and coil but rather electronic devices. Notice how light they are, a sure sign. With load on them you can read the voltage as I recall.

 

[jumper]

I hooked up a 12v 50 watt bulb and still did not read voltage.

You did space out and read across the load by chance? That would be like trying to read across a switch.

 

[kwired]

Simple electronic trany

simple two coil and simple electronic sound like they can be very different to me.

 

[Dennis Alwon]

You did space out and read across the load by chance? That would be like trying to read across a switch.

How is reading across the load like reading across a switch? :?

Kwired- I don't know what a 2 coil trany is . One is a wall wart

 

[texie]

I hooked up a 12v 50 watt bulb and still did not read voltage.

My guess is that this will depend on the meter. I'm sure these things do no out put anything close to a normal sine wave and frequency.

 

[kwired]

How is reading across the load like reading across a switch? :?

Kwired- I don't know what a 2 coil trany is . One is a wall wart

One primary coil, one secondary coil, both wrapped around same ferrous core - what we were taught is a transformer in basic electricity 101.

Your electronic device may perform similar function but is not a basic transformer.

 

[Dennis Alwon]

My guess is that this will depend on the meter. I'm sure these things do no out put anything close to a normal sine wave and frequency.

I guess so because I just was able to read one with a cheap meter but the reading was not 12v or anything close

 

[kwired]

Try setting to DCV and see if you read anything. Might be all there is to read until it is loaded.

 

[Dennis Alwon]

Try setting to DCV and see if you read anything. Might be all there is to read until it is loaded.

DC gets zero v

 

[jumper]

How is reading across the load like reading across a switch? :?

It is not.:slaphead:

I spaced and brain wondered, just woke up, stayed up for that moon last night, need more coffee. Sorry.:ashamed1:

Edit: that should be wandered! stupid auto correct.

 

[Jraef]

It’s probably a particular type of wall-wart transformer called an “NFG” *.

The cheapest way to make one and get a UL Class 2 listing is to use a sacrificial wire, basically a fuse wire, in the design. Yes, they are designed to be current limiting, but not forever. People plug them in to something with a short, then don’t know it’s getting hot until it eventually melts that fuse wire inside and dies. But because they discover later that the device it was powering was shorted too, they assume that was the only problem and toss that, but keep the wall-wart. The reading you got with the cheap meter was likely reading something across the carbon trace left behind when the wire melted.

* Look it up...

 

[Dennis Alwon]

It’s probably a particular type of wall-wart transformer called an “NFG” *.

The cheapest way to make one and get a UL Class 2 listing is to use a sacrificial wire, basically a fuse wire, in the design. Yes, they are designed to be current limiting, but not forever. People plug them in to something with a short, then don’t know it’s getting hot until it eventually melts that fuse wire inside and dies. But because they discover later that the device it was powering was shorted too, they assume that was the only problem and toss that, but keep the wall-wart. The reading you got with the cheap meter was likely reading something across the carbon trace left behind when the wire melted.

* Look it up...

This is a brand new wall wart-- never used before I tested it and as I stated it works

 

[gar]

180131-2033 EST

Inadquate information and testing so far.

What is all the label information on the power supply, and in any manual associated with the supply?

What kind of a load is this power supply to work with?

With no load on the power supply what is the measured output voltage? Measured with a Simpson 260 set to the 50 V range. Do this set to DC, AC, and using the Output input. If no Simpson use a Fluke 27 or equivalent. Here you only have the choice of DC or AC. Based on post #1 information the result is near 0.

Post #3 says the power supply is rated at 60 W. So maximum I is about 5 A. This is equivalent to a 2.4 ohm resistive load. A 250 W tungsten incandescent draws about 0.7 A at 12 V, or 17.1 ohms. At full voltage a 120 V 250 W incandescent is about 57.6 ohms. At room temperature and no current flow the resistance is about 4.4 ohms. A 1500 W space heater is about 10 ohms. 12 V DC might not hurt the motor. But I would not power a space heater with a fan in it more than a few seconds on DC. Just long enough to get a reading. Another possible test load is a 12 V car headlight using the low beam filament.

If you have a suitable load on the power supply, and you do not overload the supp;y, then across that load you will read voltage, and current thru the load.

.