Replacement SMPS repeatedly dying in an LED powerboard with benchlight

Thread Starter

advarp

Joined Jan 12, 2018
57
Hi all,

I have purchased a while ago a powerboard that includes a very handy dual LED light. It worked for a while then started to not turn on in summer (but worked in winder) and eventually blew up. I opened up the case - the two LED lights are on the edges of the powerboard and the power adapter is housed in the left hand side together with the left LED PCB and reflector. I measured the voltage required for the LEDS and the amperage - 9V 0.5A.

So I got one of those cheap SMPS 9V 1A off Ebay (I am using these in all my webcams 24/7 with no problems for years) took the pcb out of the case and soldered the connections to it. All good - I put it back. After only 2 weeks or intermittent use, bang. I opened the case - the high voltage part was at fault, 2 of the 4 rectifier diodes blown and a transistor (swtching transistor? no idea about SMPS unfort) and a resistor - oh yes and the fuse. Before blowing the SMPS was emitting a high pitched noise (for a few days).

SO I thought - must have been a defective power supply. I put in another of the same type. This time it only lasted a few hours! Now granted, it is quite hot and humid here and the case is quite tight - however the whole thing didd nto seem to get excessively hot in operation. This time, the same area was damaged ie diodes, transistor and a resistor - the fuse was Ok.

I wonder - what is going on? There is no short in the LEDs, I mean the fuse woudl blow and that would be it? Could it just be the heat?

I am aiming to get another 9V SMPS and this time put in outside the whole case - it will be messy and ugly, but I'd like to try it outside, see if it blows again.

I have a regulated protected bech supply too, maybe hook it up to the LEDS for extensive time, see what happens?

I can probably repair the blown SMPS-es but have no idea what transistor that may be - and the resistor. The SPMS is cheap - $4AUD - but it takes time to arrive from China (now its NY as well there) and it is very annoying to create so much waste also...

Any idea would be appreciated, thank you. Ill try to put up some photos..

upload_2018-2-17_8-25-39.png
 
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Thread Starter

advarp

Joined Jan 12, 2018
57
Photos attached... the sponge is my idea of insulator :) There was no sign of shorting because of that, sponge is bone dry. Low voltage wires for the two LED clusters are soldered together (so I remember which is which), the free leads are the mains controlled by the green switch.

upload_2018-2-17_9-38-34.jpeg

upload_2018-2-17_9-39-18.jpeg

See damage on the two PCBs, same area - blown transistor and fuse removed on the righthand PCB. NO shorting on the PCB side as far as I can see
 

Thread Starter

advarp

Joined Jan 12, 2018
57
Well, the total power consumption is 0.4A (for both LED clusters together) - how come the '1A' can't do it... and besides they woudl work well in other applications - must be heat... BTW hooked to my bench power supply the LEDs did not misbehave - of course...
 

Thread Starter

advarp

Joined Jan 12, 2018
57
upload_2018-2-18_15-52-6.jpeg

To end the mystery and nailbiting - I have found an old transformer based 9V adapter (non regulated, 9V at 500mA) and put it in. Testing it now. Only a transformer, 4 diodes and a capacitor... no switchmode stuff.

Oh yeah, and I have put the power supply in the powerboard body rather than together with the LED board.
 

ebp

Joined Feb 8, 2018
2,332
What limits the current in the LEDs? The power supply boards in the photo don't look like they have the components required for constant-current operation.
However, blowing up parts at the input of the supply is strange. The top of the electrolytic capacitor in the lower left corner (input filter) in the photo on the right looks bulged. This is typically the result of over-voltage wear-out due to overtemperature. Is there a chance your AC line voltage is unusually high?
 
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Thread Starter

advarp

Joined Jan 12, 2018
57
Nothing limits the current. Measured it and it was around 300-400mA. No idea what to do but no more SMPS. I'll just run it like that, I had it on 30 mins and nothing happened. Can I regulate the current in a very simple way? I.e. 1-2 components. What you see there was inside a 9V 'battery eliminator' from Jaycar or Dicksmith Electronics. Line is 240V - actually lower in the shed most likely.
 

dendad

Joined Feb 20, 2016
3,323
It could well be transients from a power tool you use in the power board popping the supply. Or overheating due to lack of ventilation.
 

Thread Starter

advarp

Joined Jan 12, 2018
57
I have not used any power tool on that. Probably heat... Can i get any advice in relation to how to 'limit the current' for those two LED clusters, or should i just leave it? I'm getting tired of opening that powerboard... Right now I have only a transformer 4 diodes and a capacitor. This is the way it was in the case. It does not seem to burn out the LEDS or to get excessively hot after 30 mins. Maybe the transformer limits the current? What happened before the days of regulated power supplies? End of the world ? :)
 

ebp

Joined Feb 8, 2018
2,332
First you need to understand the nature of LED current versus "forward voltage" - you'll be able to find lots about that on the web.

There are two general ways to control the current in a LED or string of LEDs.
When a switch-mode power supply is used, the usual way is to design the switcher so that it delivers constant current instead of constant voltage. This is the most efficient driver.
If only a constant-voltage power supply (doesn't matter what kind) is available, the usual method is to use a resistor in series with the LED or LED string. This doesn't actually regulate the current as such, but limits it. It wastes power. But it is simple.

Since you have white LEDs and measured the voltage across the string as 9 volts, this means there must be 3 LEDs in series. It looks like each cluster probably is 4 (strings of 3 in series) connected in parallel. This is fairly common, though not without some risk.

With 3 LEDs in series, it is quite common to use a 12 volt regulated power supply and select a resistor to set the current.
The voltage across the resistor (by Kirchoff's voltage law) would then be the supply voltage, minus the LED forward voltage
Vr = Vsupply - VfLEDs = 12V - 3*3V = 3 V
To set select the current, you simply use Ohm's law (in lower case her so i isn't confused with l)
r = v/i = 3/i (r in ohms, i in amperes)

How much current? You said you had measured 0.4 A for both clusters, so let's use that, assuming each cluster gets half, or 0.2 A.
r = 3/0.2 = 15 ohms
The power in the resistor will be 3 volts x 0.2 amperes = 0.6 watts - you would use a 1 W rated resistor
You could use a separate resistor of 4 times the value (60 ohms) for each string of three LEDs.

Now, this seems too low to me. It would mean that each LED runs at 50 mA, which is pretty low for an illumination source (as opposed to an indicator light). Without knowing more about the LEDs used or being highly confident in what was originally intended, we don't know if it is safe to increase the current. Certainly if the light is much less at that current than it was originally it should be safe to increase the current. 50 mA is about the current that would be used in the very small LEDs used in fluorescent tube replacements, where nearly 100 LEDs are used. (100 x 3V x 0.05A = 15 watts, so we're in the ballpark since a 40 W tube replacement is usually around 18 watts). There are lots of small LEDs for which 50 mA is right, but larger LEDs can run at 150 mA (half watt rated) or 300 mA (1 watt rated) or several amps.

Someone else want to take over at this point, SVP?
 

Thread Starter

advarp

Joined Jan 12, 2018
57
Thanks for that - I can certainly test with a 60Ohm 1Watt resistor and see if the light gets dim and if the resistor gets hot... The LEDs are just SMDs, not sure what package, 4x3=12 of them.

EDIT: does anyone else want to pitch in before I open he powerboard again :)

To me, putting a 60Ohm resistor in series will result in a voltage drop of 0.4*60=30Volts which will reduce the voltage to ...zero most likely?

EDIT 2: ahh I sort of understand now, I should use a 12V PSU then instead of the 9V - but all the 12v PSU s I have are SMPS-es of crs - no prob I guess? Then resistor is more for voltage drop than current limitation... Bah Im confused.
 
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Thread Starter

advarp

Joined Jan 12, 2018
57
Ok, I'd like to ask: if the transformer is rated at 500mA only and outputs 9v (9.7-10V with no load) - once connected to the 24 LED the voltage will drop quite a bit, so can the LEDS really draw current to burn them or the transformer out? After 30 mins of operation I have not seen excessive heat but If necessary I will open it up again ( a big pain in the...) and test some resistors with a 12 Volt SMPS. But before spending more time and yet again undo the 1,000 screws (plus other probs I have when opening it) I'd like to know if there is any real danger of burning out the LEDs - do nto much care about the transformer and bridge / capacitor.
 
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