Trying to bump up voltage on a switching power supply

[wirednuts]

I'm not sure if this is possible or not on this model, but I would like to bump up the voltage output to 12.5v or so...

Right now, its 12v, and that works OK but to account for line loss it would be nice to get another half volt out of it. I'm running LED's off them, so I need all the voltage that they can take to get the full brightness out of them.

the datasheet is here- http://www.jameco.com/Jameco/Products/ProdDS/2105666.pdf

I just can't find anywhere obvious to tweak the voltage. Most of these little supplies have a pot on them to do it, but these don't. Sucks, because they are perfect for what I need otherwise. The datasheet does say its overvoltage protection starts at around 13.5v, so there should be enough headroom to do what I want.

And no, I cant reduce the resistance circuitry on the LED's, they are built in ropelights, so the resistors are preassembled and fixed. If nobody has any idea, then ill start tweaking the resistance on a couple paths that I think would correlate to voltage output... but for now I want to see if anyone knows.

this is where I bought them from- http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_2105666_-1

 

[PottedMeat]

If nobody has any idea, then ill start tweaking the resistance on a couple paths that I think would correlate to voltage output... but for now I want to see if anyone knows.

sounds like you already got the idea. did you look up the switching regulator chip datasheet? the supply manufacturer usually doesn't deviate too far from the regulators' reference design and they specify where in the loop to set the voltage. it shouldn't take more than shifting the value of some resistor tens or hundreds of ohms to get what you want.

 

[bryanl]

Look for a variable resistor, and try tweaking it slightly. Mark its original position in case it's not the right resistor and you need to restore its original setting.

Many power supplies have no variable resistor but instead use a pair of fixed resistors to set the output voltage. Typically they're not the usual 5% tolerance resistor with a brown to light tan body and 4 color bands to indicate value but precision resistors with 5
bands and a body that's grey, blue, or green. Here's an example, where those resistors are labeled R1 and R2. The easiest way to adjust this is by replacing either of those resistors with a variable one, or bridge across one of those resistors with a fixed or variable resistor. Bridging across R2 will increase the voltage; bridging across R1 will decrease it.

However LEDs are controlled mostly by current, not voltage, and simply applying voltage to them can burn them out. You need to limit the current flow, either with a resistor in series or an electronic current regulator. Increasing the voltage will increase the current when only a series resistor is used. Each LED in a series string needs at least 2-3V, or 3.5V for blue or white LEDs.

 

[wirednuts]

thanks bryan!! this is exactly the details i was looking for. i do understand the circuitry, but i just wasnt quite sure and i dont want to guess, as these are going into production models i am selling as a side project.

the led's are on a rope, with the resistors built in. the power supply outputs 12v, but after 30ft of extension cord, there is a bit of voltage drop... im just trying to compensate for it.

im too exhausted right now, but tomorrow i will hunt down those special resistors. it makes perfect sense to me now.

and i think i will also do what potted meat suggested. i had already looked up the transistor, but again i just wasnt sure how they normally use them in basic power supplies like this.

ill take pics too. i had originally used 24w chinese power supplies, and they work, plus have a voltage adj pot on them... thing is, these 15w meanwell supplies i found seem MILES better in design and build. they have twice the components, and larger caps, then the chinese supply rated at a higher output. plus, these meanwell supplies have 2 year warranties.

 

[wirednuts]

600ohms more to get it to 12.4v output at the power supply!

One is 2A@12v rated, the other is 1.2A@12v. Guess which one is which at first glance?

Even the solder job on the back is better on the meanwell.

I suspect these are the resistors bryan is talking about! So i started (and finished ) there.

There is the regulator

And the last pic shows my pot hack! i was able to put a random potentiometer i had in series with the one resistor. it was just a 8k log pot but it did the trick! as the ohms increased, the voltage went up. 12.4v tested at 600ohm on the pot.

i guess i need to order some precision resistors though, because i do see now how much just a little difference makes. granted, it was about 500ohms to get a half volt more, but i want to run these led's at their peak capacity without risk of shortening their life... so ill just get the proper resistors and sleep well at night.

thanks again! this is going to work great.