How to build a 24V to 17V Simple Voltage Regulator?

[Fluence]

Hi,

I have a power supply with 24Vdc 6A and i need to step down the voltage to 17Vdc 6A.

I'm thinking in a simple solution, like a voltage regulator and 2 capacitors, but i have no idea how to build it.

I've seen some products already built to do this, but need the voltage regulator -yesterday- so i can't buy it and wait 2 weeks to arrive, i have to build it myself.

I'm making a LED panel with 7 Red LED strings (2.2V each LED), and i've used a 82Ohm resistor (yeah i did it bad, i should've used 10 Red LED strings and a 100Ohm, but anyways...), so in order to not burn all the LED and maintain the same 82Ohm resistors, i need to step down the voltage from 24Vdc 6A to 17Vdc 6A.

 

[ericgibbs]

Hi,

I have a power supply with 24Vdc 6A and i need to step down the voltage to 17Vdc 6A.

I'm thinking in a simple solution, like a voltage regulator and 2 capacitors, but i have no idea how to build it.

I've seen some products already built to do this, but need the voltage regulator -yesterday- so i can't buy it and wait 2 weeks to arrive, i have to build it myself.

I'm making a LED panel with 7 Red LED strings (2.2V each LED), and i've used a 82Ohm resistor (yeah i did it bad, i should've used 10 Red LED strings and a 100Ohm, but anyways...), so in order to not burn all the LED and maintain the same 82Ohm resistors, i need to step down the voltage from 24Vdc 6A to 17Vdc 6A.

How much current is flowing in each LED string, 6amps seems to be very high..??

 

[Mosaic]

Have a look see here...

https://www.electro-tech-online.com/threads/need-help-about-a-7805-regulator-circuit.15890/

U can use an LM338K instead of the 7805 for a total of about 9Amps Transistor augmented capacity , variable voltage. Just use the formula in the thread to calc your resistors based on the target loading.

 

[Fluence]

Each LED string is consuming 20mA, but i need 6A, because the power supply will power many LED strings, i didn't make an exact count but i think it will need to power about 1750 LED, that's 250 strings at 20mA each, consuming 5A.

 

[Fluence]

I've read that topic and the LM338K really seems a nice solution.

The problem is that i'm not good with maths and i don't know how to build it in order to step down the voltage from 24v to 17v.

I've seen on the LM338K datasheet many "regulators" but i just need the exact 17V, i don't need to be able to regulate it from 0-24V.

I know it's troublesome, but can you make me a simple schematic on how to build this? Or tell me what's the formula i need to use, because on that topic you gave me, there's many people saying different opinions and i don't know what's right or wrong. D:

 

[MikeMl]

Your biggest problem is what to do with the waste heat. Reducing 24V to 17V at 6A means the power lost = I*E = 6*(24-17) = 42W, which is about 1/2 the heat produced by a 100W light bulb. Unless you go to a Switch Mode Regulator, you will need a very large (several hundred square inch) heat sink.

 

[Fluence]

The other solution i have is to replace all the 82Ohm 1/4W resistors for 470Ohm 1/2W resistors, i would lose about 188mW in each resistor, i wonder if 188mW is very hot...

Do you think it's "viable" to change the resistors instead?

 

[MikeMl]

The other solution i have is to replace all the 82Ohm 1/4W resistors for 470Ohm 1/2W resistors, i would lose about 188mW in each resistor, i wonder if 188mW is very hot...

Do you think it's "viable" to change the resistors instead?

Yes, much better solution. Even better, reconfigure the LED strings to have more LEDs. That way, almost all the power you are paying for goes into light, not heat.

 

[Fluence]

Eh, i will change all the resistors instead, i just want to know if 188mW it's very hot since i will use plastic near the resistors and they may burn it lol.

I know that using 11 LED strings would be better, but i've already soldered all the LED in 7 strings, and desolder then and reorganize in 11 LED strings... just the thought of it lol..., at least the resistors are fast to replace and are just a few (about 250).

 

[BrownOut]

188mW should barely produce enough heat in, say a 1/4 watt resistor, to even "feel" warm.

 

[crutschow]

188mW should barely produce enough heat in, say a 1/4 watt resistor, to even "feel" warm.

188mW will cause a 1/4 watt resistor to get quite warm (it's 75% of its maximum rating). 188mW in a 1/2 watt resistor will give a much cooler resistor and would be the preferred resistor wattage to use for that amount of dissipated power.

 

[Fluence]

Eh, i will use 1/2W resistors.

Thank you people! ;D

 

[BrownOut]

I wouldn't worry about it. 75% is rated power is very safe, and the power is too low to make much of a heater.

 

[carmusic]

can you lower your 24v supply, they often have a ajustement on the power supply, you may be able to lower a t 22v

 

[crutschow]

I wouldn't worry about it. 75% is rated power is very safe, and the power is too low to make much of a heater.

In general that's true, although I prefer to use a 50% derating if I can. According to the derating curves, a 1/4 watt resistor will heat up about 40C above the ambient when dissipating 75% of it's rated power, which would only be a problem if the resistor ambient temperature became abnormally high due to lack of good air flow.