# decrease 32V 3A DC voltage to 28V 3A

I am now building a power supply for a 100W LED light. Right now I already got 32V DC from my DIY Power Supply, with 3A current. I try to enable my PS to dim the light by regulate the voltage to the light. I am looking to be able to reduce the voltage to around 28-32V. I tried use a 10K pot in series with the light, but it burned. I can't use PWM because I need the light for studio illumination and PWM will filcker the light.

here is the circuit that i build.

any suggestion to achieve my goal? thanks

IDEA EDIT: here is a stupid idea. lets say, i can create 32V DC and 25V DC, and connect them both to left and right leg of pot respectively, then connect the output from the middle leg of the pot. will I be able to adjust the voltage between 32 and 25 V ?

• PWM at a frequency of a few hundred hertz (possibly less depending on who is looking at it) will not produce visible (to the naked eye) flicker. May 28, 2016 at 13:23
• the reason i avoid PWM is because the light will be used in video and photo production, which use camera May 28, 2016 at 13:25
• A buck converter will do what you want. May 28, 2016 at 13:39
• Perhaps if you put up the circuit of your DIY power supply we could suggest ways of how to modify it to produce a variable output in the 25 -28V range. May 28, 2016 at 13:40
• @RussellMcMahon I can't get my hand on one that able to handle 100w May 28, 2016 at 13:47

IDEA EDIT: here is a stupid idea. lets say, i can create 32V DC and 25V DC, and connect them both to left and right leg of pot respectively, then connect the output from the middle leg of the pot. will I be able to adjust the voltage between 32 and 25 V ?

Your edit idea is bad, because it's not a regulator – the voltage drop will depend on how much current you draw and how your different sources react. Other than that, it wastes a lot of energy, and you will probably break your 25V supply, because now there's not energy flowing out of that supply, but being sunk into it. Also, the resistors that can withstand that load will probably be pretty expensive.

No, really, Aaditya's answer is the way to go. If you can't find cheap electronic suppliers that deliver to Indonesia, you haven't looked hard enough. There's a lot electronics produced in Indonesia, so supply chains can't be that bad.

100W switch mode power supplies are not that hard to build.

A few notes on what you're looking for:

• LEDs don't need a specific voltage, they need a specific current. The voltage is what results automatically when trying to put that much current through the LED. Notice that you still want to limit the maximum voltage (in case some wire etc breaks). So what you're looking for is in fact not really a constant voltage source, but a constant current (3A) source.
• Most (all?) relevant suppliers of Switch Mode Power Supply Controller ICs have design tools that help you.

For example, take the TI LM3409-Q1 IC. It's a buck (=step-down) converter specifically for high-power LED purposes. You can by a single one over some standard distributor for probably around USD 2.00 , but you can also just get a ready to use, tested [evaluation board]((http://www.ti.com/tool/lm3409eval) from TI for USD 75. Notice that the high price is not because they want to take money from you, it's just that they make a couple dozen of these devices, and assembling these simply is expensive. You're usually free to copy these reference designs and build the device yourself. The schematic and recommended component values are in the PDF on the eval board page, and if you follow the design recommendations in the LM3904-Q1 datasheet itself, you can't really do much damage. Generally, use a solidly oversized inductor for things like that. In fact, on OSHPark someone shared a project that does exactly that – a minimal LM3409 based driver. If you reach out for the author, he might just tell you the components or even give you the full design file!

That would be the right thing to do.

Now, if you search the internet for "LM317 constant-current supply", you will find plenty of websites explaining how to make a linear, constant current supply with just a 10ct LM317 and a few resistors. You can put these in parallel, so not to exceed the maximum current per LM317, and have a quick and dirty power supply. Warning: you're going to drop about 7V at a total of 3A in that linear mode; that's 7W, and that means that you'll need to have a big cooler so that these 21W can go somewhere. It's not nearly as elegant or efficient as the switch mode power supply approach mentioned above, but it will work.

• this is actually very helpful. thank you. i think i might build myself a "LM317 constant-current supply" May 28, 2016 at 16:05
• oh and about the problem on finding the parts, its mainly caused by my location. its quite far from the big cities, and having to buy parts online will cost me more to ship them. I might try to find that LM317 since i think its should be available nearby. and I might try to make it adjustable. May 28, 2016 at 16:22
• There are multiple variants of the LM317 that withstand different voltages and more importantly current ratings. You will need the most sturdy variant for this, the TO-220 case one. May 28, 2016 at 16:29

From your question I think you want to step down your DC voltage. Now there are various ways to do so:
1) You could use a linear regulator which supports high current, Wikipedia page on linear regulators
2) You could use a switching regulator which is technically perfect for your job as it handles high currents effectively Switching Regulator.
If I were you I would have gone with a switching regulator. Why? Reasons are clear :

• You won't easily find linear regulator for that sort of voltages
• Switching Mode power supplies are more reliable and very efficient.

Good Luck!

• I can't find buck converter available with more than 10W capacity. yet i need 100W. and I don't have enough money to buy 10 buck converter and put them up in paralell. any idea? May 28, 2016 at 13:46
• @FuadHanif google.co.in/… check this one out May 28, 2016 at 14:05
• Im sorry, what I meant is I can't find above 10W available in my area. the only way for me to get that 100W+ is by buying overseas, which cost more and time. May 28, 2016 at 14:09
• @FuadHanif If you could tell me where you live ? May 28, 2016 at 14:11
• I'm in Indonesia. just found some 100W buck converter, but its quite expensive for me. I got an Idea, pls check on my original post and see if it will work May 28, 2016 at 14:23

You're very vague about how expensive "expensive" is. However, I'd just get rid of your transformer and work with something like a readily integrated, reliable, tested, enclosed AC/DC constant current driver like these ones, which are conveniently available in Indonesia from mouser.

Now, frankly, if you can live with variations between individual lamp's brightnesses, a well-regulated 25VDC power supply will do, too. Those sell for a couple of USD – as printer, scanner, laptop computer power supplies. 25V isn't a very common voltage, but the typical 24V power supply will arguably lead to very little light output reduction.

• well to explain expensive, i've spent about 2.5 USD on the light, and (about) 10 USD on the power supply. and I have almost no more money (yeah, I'm broke). so I try to keep things as cheap as possible. and buying things online will cost about 3USD to get shipped to where I am now. thats why i try to get simplest part to do the job, because there is not much part available around May 28, 2016 at 16:09
• why did you buy a 10 USD power transformer that doesn't match your need? May 28, 2016 at 16:21
• because thats the closest thing available that to my need. besides the voltage is correct, i just need to make it adjustable to control the broghtness May 29, 2016 at 5:22