Troubleshooting 12v lighting transformer

[sw_ross]

When troubleshooting a 12v transformer, like the ones used for undercabinet puck lighting, why can you only check voltage when a load is applied?

I have a Greenlee DMM and when I test off the secondary leads of the transformer I get no reading until a load is applied. Additionally, if there's much of a run between between the transformer and the lights, and I'm checking for voltage drop at the end light (where I know there's a decent VD - to the degree that the end light is noticably dim) I can't get a reading with my Greenlee, even with the load on.

In the last scenario I was able to borrow a fluke and get a reading of <4v. We changed out the transformer to a higher quality landscape transformer that solved the problem.

My questions:
1- The smaller transformers (60-100 watt) that are used for under cabinet lighting don't used windings? How do they reduce voltage? and why can't I test voltage off of the secondary leads without a load attached?
2- Is my Greenlee meter in need of an upgrade? Would a higher quality fluke style meter solve some of my problem?

 

[Dennis Alwon]

All the tranies I use for pucks are electronic and not magnetic- no windings. The voltage is transformed electronically- ask the pros how it's done

Not sure why you cannot read voltage without a load but some of the smaller electronic tranies need a minimum wattage to work correctly however I wouldn't think that they would have no voltage with no load. Again we need the geeks.:grin:

 

[dbuckley]

Not uncommon in electronic power supplies.

If you put a scope on the output you'll see an investigatory "pulse" of a few cycles whilst the electronics sizes up the load; if the load is too big or too little then the PSU goes asleep for a bit, then tries again. Its part of th protection system that makes the power supply more reliable. If the investigatory pulse measures well then the PSU ramps up to the required output.

The other thing is that the output is a high frequency waveform, and not necessarily a sine wave, so make sure your testmeter is a modern electronic RMS reading meter with the AC voltage measuring capability operable to beyond 20KHz.

 

[Sierrasparky]

Great info.

 

[jeremysterling]

I heard somewhere that the 12V ballast will shut itself off if the lamp burns out. By doing this, the life of the ballast is preserved. I do not know if this is true.

I check the ballast by briefly striking the output leads together. If a spark jumps, the ballasts is usually good.

 

[Dennis Alwon]

If you put a scope on the output you'll see an investigatory "pulse" of a few cycles whilst the electronics sizes up the load; if the load is too big or too little then the PSU goes asleep for a bit, then tries again.

I have seen this effect when the wattage was too low. Two lights would light dim while the third wouldn't come on then a different two may come on, etc. It was odd but that was many, many years ago. I guess I thought things would have changed.

 

[sw_ross]

Thanks for the info. Is it possible to use some sort of solenoid style tester, like a wiggy, that can put a load on the transformer to test it's output voltage?

Thanks again,
Sky

 

[K8MHZ]

Thanks for the info. Is it possible to use some sort of solenoid style tester, like a wiggy, that can put a load on the transformer to test it's output voltage?

Thanks again,
Sky

12 volt test light from a car parts store. Spend 9 bucks and get the good one. After a while you won't need a meter, you will get pretty good at guessing the voltage by how bright the light is.

 

[MikeyinLA]

12 volt test light from a car parts store. Spend 9 bucks and get the good one. After a while you won't need a meter, you will get pretty good at guessing the voltage by how bright the light is.

Great idea

Nice call sign too!

 

[K8MHZ]

Great idea

Nice call sign too!

Thanks. It's a vanity call, but another op just a few miles south of me got N8MHZ as a sequential I have talked to him a couple times on 2 meters.

I spent a couple decades doing automotive, marine and heavy equipment electrical troubleshooting and still dabble in it once in a while. Right now there is a stump grinder in my yard waiting for me to mount an aftermarket voltage regulator on it and fabricate a harness to connect same.

I don't know where I got my last light (I have killed many) but its got a great, thick and very flexible cable and a real nice clamp. The body is made of clear acrylic and is very tough. The tip came so sharp it had a cover. It's still pretty sharp, enough to pierce wires if need be. I think I paid 12 bucks for it at Advance Auto. Don't buy the cheap lights. They are garbage.