Query regarding capacitor based transformers

Thread Starter

mahanteshec28

Joined Jul 22, 2013
8
Dear All,

Recently I burnt a capacitor based transformer which I used to power LEDs. It was 12V 1Amp. I bought new one with same ratings but what is observe is -
1. After turning ON, after 1-2 mins, transformer turns off by itself
2. After being OFF for 10-20 seconds it'll automatically turn ON
3. Again after being ON for 10-20 seconds it'll automatically turn OFF
4. This process repeats

Can anybody please tell me what I'm doing wrong?

Thanks in advance!

Regards,
Mahantesh
 

gerty

Joined Aug 30, 2007
1,287
Show us how you have it wired up, a schematic would be nice.
What do you mean by a "capacitor based transformer"?
What is all of this connected to, what is your load?
 

Thread Starter

mahanteshec28

Joined Jul 22, 2013
8
Thanks for quick response, Gerty :)

"capacitor based transformer" - light weight power transformers

LEDs are connected in series and positive of 1st LED is connected to positive of power supply and negative of power supply to negative of last LED. Like this I have connected 10 LEDs in series. Hope you got the picture. I'm not able to draw pictures here and I cannot upload any file from my office computer.

Regards,
Mahantesh
 

mcgyvr

Joined Oct 15, 2009
5,394
What is enforcing current regulation for the LED's? (I'd guess nothing as thats what the problem is)
They CANNOT just be attached to a regular constant voltage power supply. They MUST be powered by a constant current power supply or include series resistors to limit current.
 

MaxHeadRoom

Joined Jul 18, 2013
20,889
Why not pick up a bell transformer and a small bridge and maybe a 10μF capacitor and you should be good to go. Maybe have to add another LED or 2?
Max.
 

mcgyvr

Joined Oct 15, 2009
5,394
Buy a proper constant current power supply or include some form of current limiting or it will happen again and again.
 

Thread Starter

mahanteshec28

Joined Jul 22, 2013
8
@mcgyvr - I'm tired searching for the supply. I already returned 2 power supplied. Luckily I got full refund. Both of them behaved the same.

@Max - Can I just add couple of LEDs and try? Coz to get all those stuffs I need to drive for 2 Hr in full traffic :(
 

mcgyvr

Joined Oct 15, 2009
5,394
Why don't you post a schematic of the LED circuit you are attaching to this power supply..
Including the specifications of the LED's (forward voltage/current rating)

You are ONLY having problems because you are NOT doing it right.
 

Thread Starter

mahanteshec28

Joined Jul 22, 2013
8
@mcgyvr - It is a LED strip where LEDs are in series. I bought it on ebay. So other than LEDs I cannot see what is in there. I have break it open to see what else is present in there.

Let me try connecting some other load and check. I'll update you soon. Meanwhile if you have any other solution, please let me know.
 

WBahn

Joined Mar 31, 2012
25,914
If you have 10 LEDs in series, then you probably need 18V to 24V to actually get them to conduct. Many power suppllies like the one you describe will put out considerably more than their rated voltage when there is no load. If it will put out enough to overcome the turnon voltage, then current will start to flow. But as soon as current starts to flow it will start regulating, so the voltage will start to drop. But as soon as it drops very much, the LEDs will turn off and the current flow will stop and so the voltage will go back up. What you end up doing is operating at the point where the output voltage that is supplied exceeds the threshold voltage of the LED string by just enough to drive the corresponding current. That current is probably not going to be very much, but it is going to vary considerably from one power supply to the next. Your old supply reach equilibrium at a current that it could handle. The new one reaches equilibrium at a current that it can't, possibly because it is being forced to operate so far outside its designed range, and so it goes into thermal shutdown. As soon as it cools down a bit, it turns back on. Since it is already starting at a higher temperature than when first turned on, it doesn't take it as long to reach the shutdown temperature again.

You need a supply that produces the correct amount of voltage for your LED string, AND you need to properly limit the current in the string.
 
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