12v power supply overheating on a 2w LED?

[howardb]

Hi all,

I had to re-wire our ornamental garden lamp this weekend - it used to run off 240v and had a day/night CFL globe in it. I decided to go the 12v low voltage route and used an on-hand small 20w 12v G4 halogen fitting that was already pre-wired and had it's own 12v power supply brick that came with it. BTW everything in/on the ornamental lamp is watertight/waterproof.

I also decided to replace the G4 20w 12v halogen globe with a 2w waterproof LED globe instead, but still run it off the same power supply brick. Anyway, switched it all on last night around 7pm and all looked good and working; however after about 30 minutes the power supply was VERY hot, to the point of smelling electrical burning, so I switched it off - below's a photo of the power supply and globe used - wondering what the issue could be that is causing it to "overheat" so quickly. The 2w LED uses 10x less wattage and the current draw certainly cannot be more than a few hundred mA; I'd think its much less than a 20w halogen draws?

Any ideas before I go and buy a different power supply to try?

 

[savage]

Hi all,

I had to re-wire our ornamental garden lamp this weekend - it used to run off 240v and had a day/night CFL globe in it. I decided to go the 12v low voltage route and used an on-hand small 20w 12v G4 halogen fitting that was already pre-wired and had it's own 12v power supply brick that came with it. BTW everything in/on the ornamental lamp is watertight/waterproof.

Did you actually *test* and *confirmed* this, or just assumed? Any joints in the wires?

Use a amp meter, and measure the actual current through the wire. If there's a short (could be due to water / dampness, crappy electrical connections, etc.), current will sky rocket, causing your transformer to overload and burn out.

That's the whole problem with 12V... No earth leakage behind the transformer, and putting a earth leakage in front of the transformer is pointless. I would believe that you have a leak to earth on the 12V side... Fix it, ASAP.

 

[desiganp]

Looks like it might be a step down AC transformer rather than a AC-DC adaptor. Presuming the G4 is DC rated. Try it with a 12V DC supply.

 

[savage]

Looks like it might be a step down AC transformer rather than a AC-DC adaptor. Presuming the G4 is DC rated. Try it was 12V DC supply.

12V LEDs has built in rectifiers, they operate on AC (see the picture of the globe, it can be inserted any way around, i.e. it's not polarity bound).

That being said, it could be because it's a step down transformer and not a LED driver (i.e. proper LED transformer), but for that low wattage I doubt it.

 

[agentrfr]

Use a switch mode DC power supply, and not a transformer plox. Here's a decent one for you (600mA or enough for 7 of your 2W LEDs)

http://za.rs-online.com/web/p/plug-in-power-supply/7350737/

 

[agentrfr]

Oh and the reason you see it is overheating is that the load on the transformer is very low. Such little current is being pulled from the secondary coil (therefore almost no back EMF) that the primary coil essentially becomes a resistor instead.

Hence you should use a proper switch mode power supply

 

[howardb]

Did you actually *test* and *confirmed* this, or just assumed? Any joints in the wires?

Use a amp meter, and measure the actual current through the wire. If there's a short (could be due to water / dampness, crappy electrical connections, etc.), current will sky rocket, causing your transformer to overload and burn out.

That's the whole problem with 12V... No earth leakage behind the transformer, and putting a earth leakage in front of the transformer is pointless. I would believe that you have a leak to earth on the 12V side... Fix it, ASAP.

Thanks Savage, yes tested and confirmed before installing everything, then again after installation. There are 2 joints either end, both soldered properly and staggered joints, heat-shrink on each individual wire and heat-shrink again over both wires. I don't have a current tester, only a volt meter - showing 12v at the power pack and 12v on the LED side.
I cannot see where there could be any shorts, there's no dampness in the conduit nor the joints.

Looks like it might be a step down AC transformer rather than a AC-DC adaptor. Presuming the G4 is DC rated. Try it with a 12V DC supply.

Thanks desiganp - just been checking the power adapter model specs - does seem to be a 12v AC, not DC; not very clearly marked though - I also understand that the LED globe can take either, so probably why it lights up...I guess the power brick was made for the halogen globe...
Will try with a spare 12v DC adapter I know works with similar LED's.

12V LEDs has built in rectifiers, they operate on AC (see the picture of the globe, it can be inserted any way around, i.e. it's not polarity bound).

That being said, it could be because it's a step down transformer and not a LED driver (i.e. proper LED transformer), but for that low wattage I doubt it.

Thanks Savage, noted. I have a spare DC LED driver from another LED fitting where the LED itself was DOA - the driver was fine when checked - was rated for a 7w globe/fitting so should be able to power this 2w LED.

Use a switch mode DC power supply, and not a transformer plox. Here's a decent one for you (600mA or enough for 7 of your 2W LEDs)

http://za.rs-online.com/web/p/plug-in-power-supply/7350737/

Thanks agentrfr, will check it out.

Oh and the reason you see it is overheating is that the load on the transformer is very low. Such little current is being pulled from the secondary coil (therefore almost no back EMF) that the primary coil essentially becomes a resistor instead.

Hence you should use a proper switch mode power supply

Thanks agentrfr, noted and will try a switch mode DC supply

 

[howardb]

Quick update: the power pack was AC230v-AC12v and the globes rectifiers fried... replaced the globe and used a standard AC240v-DC12v adapter (2A rated) and all working fine now.