need help in purchasing adjustable power supply

[mattstolte]

Hello I am looking to buy adjustable power supply to test things like main board led strips and such. What voltage and amps do i need when looking for such power supply thanks

[ReeceyBurger123]

Re: need help in purchasing adjustable power supply

If your buying an "adjustable power supply" you can adjust the output voltage and amps lol. For Led you can use 2* AA batteries in series to power an Led which normally tv leds use 3v 100ma to power them.

[mattstolte]

Re: need help in purchasing adjustable power supply

If your buying an "adjustable power supply" you can adjust the output voltage and amps lol. For Led you can use 2* AA batteries in series to power an Led which normally tv leds use 3v 100ma to power them.

I'm.asking like any range of amps and volts do i need my power supply to handle

[stj]

Re: need help in purchasing adjustable power supply

forget it.

most psu's go upto 15 or 30v
for led strips you may need as much as 100v

better to test the led's individually with a battery.

[mattstolte]

Re: need help in purchasing adjustable power supply

What about testing one whole strip

[budm]

Re: need help in purchasing adjustable power supply

You count how many LEDs are in the strip, multiply that by 3V (Close enough Vf for the White LED), that will be the power supply Voltage you need, then set the limit current to may be 20~50mA range for testing.
So you are not going to easily fine the power supply that can go up to 100V or more with current limiter function if the strip has >30 LEDs in series.

[Ltank]

Re: need help in purchasing adjustable power supply

I have a BK tripple PS. its fine for testing low voltage circuits. For testing LED strips i have a Sharp PS from a LED set that had a bad Pannel. What PS you use is up to you. You can simply jumper it on. Keep in mind that there is no enclosure. Different LED strips use different voltages. The PS you use need to have close to the same strips that you are testing. Got more than one LED PS available?

[mattstolte]

Re: need help in purchasing adjustable power supply

So say for example I have a led strip I wanna test that has 5 led lights on it. So just set the power supply to 15v? I just get mixed up with the whole amp and volt things. I seen a guy testing a main board with external power supply. But at low voltage because he was touching the components looking for hot ones.... Now I was getting mixed up cause he was saying something about 1amp. But then I'm doing understand dont it need 3.v to power that main? Where does the amps come in. To you have to set the power supply.to volts and amps.I just wanna buy a external power supply But I wanna do stuff like this but not sure what outputs I need a power supply to push out.

[mariushm]

Re: need help in purchasing adjustable power supply

So say for example I have a led strip I wanna test that has 5 led lights on it. So just set the power supply to 15v?

No, the voltage has to be bigger than the sum of the forward voltages of each led in the series for the leds to start working. But note that the forward voltage varies with manufacturing, so each led will have a slightly different value. In addition, as the LEDs warm up (when they're working), the forward voltage will also change a bit. So basically, if you have 5 leds and you think the average forward voltage is 3v, then your input voltage should be a bit higher than 3x5 = 15v.
BUT, if you just connect this strip of leds to 15v, you'll blow up the LEDs because a huge amount of current would flow through the leds and burn them up. You need to limit the current flowing through the series, and the most basic way to limit the current would be using a resistor added at the beginning or end of the series of leds.
The value of the resistor must be chosen depending on the maximum current you want to go through the leds and the input voltage :

Resistor = [Input Voltage - (number of leds x forward voltage) ] / Current in Amps

So for example, with 5 leds with assumed forward voltage of 3v and input voltage of 19v (let's say it's a laptop power adapter) and we want to limit current to 100mA (0.1A) then we have R = (19v-5x3v ) / 0.1a = 40 ohm (so you pick the closest standard resistor to this value, more than 40 would mean less current, less than 40 would mean more current)

But if your leds are really capable of MAXIMUM 100mA, then it would make sense to configure your circuit to limit the current to a bit less than that, let's say 90-95mA, for the reasons I mentioned above. As the leds warm up, the forward voltage will change slightly and with a fixed resistor, the current would change (increase a bit)
For this and other reasons, led driver chips were invented - they monitor the current and dynamically adjust things to keep the current fixed, regardless of forward voltages and other things.

--

LED strips can be of various types. On most tvs and monitors, there are two or more groups of leds connected in series.

TV manufacturers usually connect enough leds in series in such a group that the sum of the forward voltages of those leds goes to around 60-80v, so to even get that segment of strip to light up, your power supply will need to be able to output that much voltage.
Naturally, you can connect one wire from the power supply to the first led and then just touch the solder pads of a led in the series to light up the leds up to that particular led you touch with your second wire.

Adjustable linear power supplies or even switching power supplies capable of such high output voltages would be expensive.

My advice would be to build yourself a small led driver circuit, which you'd be able to power up from whatever voltage you want, and output a voltage high enough to power up a segment of that strip.

For example, a relatively cheap and simple IC that would do this would be Sanken LC5205D : http://www.digikey.com/product-detai...05D-ND/4454288

The IC can run with up to 450v so since you're in US you can grab a small 110v to 110v 100-500mA transformer to have the isolation from the mains, and then use a bridge rectifier and a capacitor to have up to about 160v available for the strip of leds. For example this 20VA transformer ( http://www.digikey.com/product-detai...797-ND/4915211 ) will give you up to 120v x 1.414 = ~ 170v at about 100mA, which should be enough to test any led strip.

You can then make the circuit as shown in the datasheet and just replace R2 with a potentiometer, to be able to adjust the output current going to that strip of leds (the values in datasheet set the strip to about 0.35A and you may blow the strip with so much current). The datasheet explains how to calculate the peak current so you can figure out the range of a potential potentiometer (do the math for let's say 5mA and for 100-200 mA and then put a resistor and a potentiometer in series so that when potentiometer is 0, the resistor in series with the potentiometer will set the peak current to that 5mA value)

As a DIP package chip, it can easily be soldered onto a prototyping board and all the other parts can also be found with leads so it should be quite simple to make it.

[budm]

Re: need help in purchasing adjustable power supply

So say for example I have a led strip I wanna test that has 5 led lights on it. So just set the power supply to 15v? I just get mixed up with the whole amp and volt things. I seen a guy testing a main board with external power supply. But at low voltage because he was touching the components looking for hot ones.... Now I was getting mixed up cause he was saying something about 1amp. But then I'm doing understand dont it need 3.v to power that main? Where does the amps come in. To you have to set the power supply.to volts and amps.I just wanna buy a external power supply But I wanna do stuff like this but not sure what outputs I need a power supply to push out.

The problem I see you are having right now is that you are skipping the learning of the basic, you need to learn the basic about the Volt, the AMP, the resistance and how it relate to each other for example.
If you have to ask 'Where does the amps come in. ' then you need to study up the basic first.
In other thread you also ask about how to test the capacitors, transistors, etc.
What have you studied so far? for example: Did you try searching how the get the simple LED to light up and able to figure out the Voltage and the current, the needed resistor, etc? If you really want to learn you need to study and do the experiment by making the circuits and study the result by taking the measurement, you have no idea how lucky you are to have internet to find so many information out there, I wish I had that available for me 40 years ago, so do some works.
Good tools in the wrong hands can cause a lot of damages.

https://www.badcaps.net/forum/showthread.php?t=49654
https://www.badcaps.net/forum/showthread.php?t=50018
https://www.badcaps.net/forum/showthread.php?t=49166

[mattstolte]

Re: need help in purchasing adjustable power supply

Yea i learned tv repair all in my own. I guess backward not knowing anything about electronics. I can easily troubleshoot a tv to figure out what is causing the problem and swap the board but now inwanna learn electronics theory and component level and really not sure where to start there seems to be so much

[budm]

Re: need help in purchasing adjustable power supply

Some example of free online learning:
http://www.digikey.com/techxchange/message/7678
http://www.electronicstheory.com/
http://www.learnelectronicsonline.com/
http://www.electronics-tutorials.ws/