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LED Driver unit with no output
- Thread starter partyanimallighting
- Start date
I put in the new LM2575 and it promptly blew out so I decided to do some further probing around. From the datasheet I see that pin 1 is voltage in, pin 2 is output and pin 3 is the Ground rail so I removed the blown out regulator and tested the circuit board for continuity from Pin 1 to 2, 3, 4 and 5 and got nothing which seems correct to me as the regulator would receive voltage in then output to the other pins. I then checked continuity from pin to pin and, keeping in mind that I am not an electronics expert, I found it strange that I'm getting continuity from pin 2 (output) to the Ground rail and also to pins 4 and 5. I rechecked resistance at Pin 3 as follows, Pin 3 to Pin 2 ~ 0.3Ω / Pin 3 to Pin 4 ~ 0.6Ω. I bought 2 of the 2575's so I still have one available but I really do not want to blow this one prematurely as I paid about $7.00 for one, which is cheap for parts like this in my little island where I live.
This is a sign of a short circuit somewhere:
- Pin 2 to GND should have a resistance significantly > 0Ω.
- Pin 2 to pin 4 has a very low resistance, namely the resistance of the output inductor. This could be mistaken for a short and is not per se a point of concern.
- Pin 2 to pin 5 should have the same resistance as pin 2 to GND as pin 5 is probably directly connected to GND to turn on the regulator permanently.
All the measurements I listed in my last post were with the blown regulator removed. You mentioned that Pin 2 to Ground should have a higher resistance and I'm checking resistance from Pin 2 to the Pin 3 / Pin 5 sockets, which are on the Ground rail. Am I doing this correctly or should I check to Ground somewhere else on the circuit board? Pin 3 and Pin 5 are on the same solder trace on the back side of the board which seems to be a Ground trace and I'm getting a zero reading across from Pin 2 to Pins 3, 4 and 5 when I tested for continuity. As far as signs of overcurrent or damaged components, I'm not seeing any sort of damage with my untrained eyes. The only thing I see that looks a little suspicious is the pin sockets on the rear side of the board. Pin 1 seems to be OK, coming in from the rectifier, Pin 2 seems to go to the largest capacitor in the group of three capacitors next to the regulator, Pins 3 and 5 to Ground Rail and Pin 4 goes to the other leg of the biggest capacitor. As far as removing components to check, I will definitely need your guidance to know parts what to remove and what to test for as I'm not seeing anything like transistors, only IC's, capacitors and disc capacitors. Once again, thanks for all the assistance so far!!
I would track down the diode connected between the output and ground and remove it. See if the short disappears.
If not, it is likely that the failure is in some component powered from the 5v rail. Have a close look at the board for any toasted components, especially ic's.
Post some pictures if you're unsure.
If not, it is likely that the failure is in some component powered from the 5v rail. Have a close look at the board for any toasted components, especially ic's.
Post some pictures if you're unsure.
No signs of burnouts. The previous page has the pictures i originally posted. If you need closeups of specific areas, let me know because I don't have a clue where to start to look for faulty parts.
Here are the closeups. Let me know if they're good enough. As far as connectors go, it's just the 120VAC/24VAC transformer going into the board then 8 LED outputs being fed out. The only thing connected is the transformer to supply power for testing.
Great. I'll post my findings in a bit.
OK. I lifted one end of the diode and took resistance readings as follows: Negative tester lead from Pin 3 to Pin 2 = 10.7kΩ. Pin 3 to Pin 4 = 14.5KΩ. Positive tester lead from Pin 3 to 2 and 4 = similar values as listed above. Does this signify that this diode is faulty?
I removed the diode and the number is SR160. I did a search on Google for information on diodes and these act as a one way switch for current flow, and I should get an OL reading when metered and low resistance when the leads are reversed. Did not know this so another day gone, another thing learnt. I did measure continuity across the diode and I'm getting a reading of 0.3Ω across read both ways and with the Diode Test on the meter (I had to Google this also to make sure I was doing the correct thing) I got a 0 reading.
What value would a "meatier" diode be?
Something like this may be suitable. It's just the first my phone found for me 
http://www.digikey.com/product-detail/en/SB3100E-G/641-1418-1-ND/2075782
http://www.digikey.com/product-detail/en/SB3100E-G/641-1418-1-ND/2075782
Thanks! I'll pick up whichever one's available, install it with the regulator, power up and update you accordingly.
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