How to measure current and computer PSU's

[MikePL]

I've seen the answer here but cannot find it now. I would like to know how to measure current without using the LED's. What else can I use instead of the LED? Can I use a bulb? I am simply experimenting with different power supplies and I don't want to burn any LED's.

This is connected with my other question whis is: can I use computer power supplies to drive LEDs? Do they have a constant current? I am simply looking for a source to drive 30-40 LEDs and I have a few PSU's which have a rating of 30A on the 5V line.

Any comments welcome.

 

[mds82]

what kind of LED's are you looking to power first off?

 

[MikePL]

I am planning to power 3W white LEDs at a current of 600-750mA

 

[Gomer]

AFAIK, all computer power supplies are voltage regulated, not current.

 

[mds82]

You may want to look into the Xitamium drivers.
http://ledsupply.com/xi-led120a0024v18f.php

This one will power a lot of LED's, 6 in series, 5 strings, 30 led's total at 350ma

 

[MikePL]

Gomer,
Does it mean that the current changes significantly? So in general they are useless?

mds82,
I am aware that there are drivers for multiple LEDs but I have 20 or more computer power supplies and simply don't want to throw them away. And my current project is to make lighting around my house.

So is there even a remote possibility to use the computer power units?
How to measure current? What should I put in the place of the LED?

 

[Gomer]

You may want to look into the Xitamium drivers.
http://ledsupply.com/xi-led120a0024v18f.php

This one will power a lot of LED's, 6 in series, 5 strings, 30 led's total at 350ma

Doesn't that driver supply regulated 24v and can supply up to 1750mA but nor regulated ?

I thought the only regulated Xitanium drivers were the explicit 350 and 700mA ones.

 

[Mash]

regarding PC PSUs:
I have played around a bit with them. The best way to use them is to supply one or multiple low voltage drivers for LEDs. Or use one of the LM317 type circuits.
The only problem with PC psus is that a lot of them dont operate at a constant level. They increasae their output according to the load placed on them, this is so they cut down on the fan noise, ie they dont generate excess heat if the power is not needed. I have played around with 12V led srips, and if they are the only things connected to the PSU, they are either very dim or flash repeatedly, meaning there is not enough current. However if you put an additional load ie a fan or something, suddenly the current output is higher and the strip functions correctly.
SO you might need a constant load (maybe a resistor) on the output circuit to ensure the PSU is outputting enough.

 

[mds82]

Doesn't that driver supply regulated 24v and can supply up to 1750mA but nor regulated ?

I thought the only regulated Xitanium drivers were the explicit 350 and 700mA ones.

The datasheet shows that they can be run in a few different configurations, depending on th model. I know that i have the 750ma one, which can be use to power 6 led's at 750ma, or 2 rows of 6 at 350ma.

I dont first hand experience with this higher output one either though

 

[MikePL]

I hate being a pain in the back but this time I will have to. I am really grateful for the replies but unfortunately the two questions are still unanswered.

Mash, you said that the PSUs need some kind of load. Will 30 or 40 3W LEDs be enough load? I guess so, it's 90-120W after all.

And here comes my second question... How to meaure current without using LEDs. I do not want to connect 10 3W LEDs in parallel and hope that they will receive 700mA each. They migh as well get 1800mA and burn out (all 10, which is a lot of $$$). Should I connect a bulb as a load? Or a fan?

Oh and here comes an unexpected third question. Will I get significant current drops if I have to use 200 feet of cable? I simply need those LEDs around my fence and this is why I am talking about a circuit of 200 or more feet in length.