Dear readers,

Very good morning to all

I have two SMPS blocks lying with me, each 400watt, used in my old PCs. I want to use the 5 volt supply for the LEDs, not touching the 12 volt lines. will this destroy the whole SMPS, as my power requirement is only a few watts, may be up to 10 watts and the SMPS is of 400 Watt type.

when partly used, will it destroy the whole SMPS? is my question.
and
as my requirement is only 10 to 15 watts, will the smps puts the total 400 watts load on my power lines?

Please help me to go ahead.

regards to every one at All About Circuits Forum

V. Hari Hara Prasad, aged 58, from India.

 

There should be no problem in loading only the 5 V supply. Just put a 1K 1Watt resistor as a bleeder across the 12V and -12V lines.

Ramesh

 

thank Mr.Ramesh for the advise. I will follow accordingly.

prasad

 

Precaution should be taken.

Which type is ur LED.?
One Big one LED or just a series of small leds?
U should manually switch on the supply if it is not used with a CPU.
The stby 5V supply cannot handle heavy loads.

 

Thank you R!f@@ for your suggestion. The LEDs are of 0.2 watt, BIG ones, totaling 12 numbers. I have tried using single 1 watt LEDs -O- type, and attaching the heat-sink is becoming a problem for me. (I am new to electronics).

The SMPS I am using, is a stand alone one, which was removed from a working PC of the olden days. I have grounded it, and isolated it from the LEDs being used. I have a magnifying glass fitted with a round CFL beneath it. so, I want to replace that burnt CFL with LEDs.
During my studies (1974), our lecturer used to tell us that the power transistors of the audio stage should not be switched on without proper loads (speaker), and hence I am asking these questions regarding leaving un-used- some of the SMPS power sources.
I liked this forum of all good people, who are answering almost all the questions posed by dummies like me.
Thank you one and all, for keeping knowledge open to the world.
No restrictions, no apartheid, no gender in-equality... I am happpppy to see a generation like this.
Regards to all the masters of the universe!
prasad

 

Another word of caution: The PC SMPS supplies will tend to turn off unless there is a minimum load on the 5 Volt line.

Ramesh

 

Thank you for the kind words Prasad.
As Ramesh said newer ones tend to switch off with light loads but not old ones as far as I have seen.

During my studies (1974), our lecturer used to tell us that the power transistors of the audio stage should not be switched on without proper loads (speaker)

I repair amps everyday and I test it without speaker initially.
I connect speakers when I am sure tht output has no DC to test for thermal and overload and distortion. I dunno why u were taught tht.

U have to find the proper series resistor by calculation and trial and error method used with the LED.

I believe for magnifying glass series leds around the edge will be far better than a single big one. U need to finalize how many leds u will be needing.
As for driving a series string that nominal voltage of one led will add up.

Eg: If ur 0.2W led needs around 2V for normal lighting levels u can use maximum of six leds. 2 volts X 6 leds will give you 12V.
12V is the maximum u can get from a PC supply.

So now u have to find the nominal voltage of ur LED.
Post a picture of the LEDs if u can. Place the LED next to a ruler so we can be sure of the size.

My one cent.
Driving series LED strings with a PC supply is over kill.
U can save power if u find a small transformer. I can tell u what secondary voltage u will need with current capacity if you provide me with the LED details and how many LEDs u will be using.

 

There should be no problem in loading only the 5 V supply. Just put a 1K 1Watt resistor as a bleeder across the 12V and -12V lines.

Ramesh

Some of the PC PSU boxes I've tried using that way wouldn't behave unless I also loaded the 3.3V rail.

On one very cheap PSU, the 3.3V sense line had a pin to itself instead of sharing a power pin, I didn't notice this and didn't link all the orange wire pins together (as would happen if plugged into a motherboard) - it went bang when powered up!

 

Some of the PC PSU boxes I've tried using that way wouldn't behave unless I also loaded the 3.3V rail.

On one very cheap PSU, the 3.3V sense line had a pin to itself instead of sharing a power pin, I didn't notice this and didn't link all the orange wire pins together (as would happen if plugged into a motherboard) - it went bang when powered up!

I have'nt faced such an issue on any output except the main 5 V bus. Maybe, it's better to put bleeders on all unused power outlets.

Ramesh

 

I have'nt faced such an issue on any output except the main 5 V bus. Maybe, it's better to put bleeders on all unused power outlets.

Ramesh

The best way is to hacksaw/tinsnip the power connector from a scrap motherboard and solder whatever wires you need to the reverse side of that - whenever I scrap a motherboard, I usually salvage the power connector along with any other useful bits.