thread post test - Electronic Circuits Projects Diagrams Free

Hank Fletcher · 23rd March 2007, 04:59 AM

Argh! I'm having a heck of a time trying to post my question! Anyway maybe this will work:
I searched the net for some ideas on how to use a computer supply as a bench supply, and finally followed this guy's notes:
http://web2.murraystate.edu/andy.bat...OWERSUPPLY.HTM
They're pretty good notes and I tried to contact him with my problem below, but I've yet to hear back.

My problem is with the resistor. I used a 10W, 10ohm resistor as is commonly advised, albeit mine is the green, cylindrical wirewound type. It gets really hot, and although I haven't use any heatsink compound the resistor is well ventilated. The supply works for about ten minutes while the resistor gets hotter and hotter, then shuts down. This hasn't fried the resistor yet, which still indicates 10ohms after it's cooled down. Then you can turn the supply on again and watch the whole thing over... but presumably this is eventually going to cook something permanently.
Thanks!

Hank Fletcher · 23rd March 2007, 05:10 AM

Oops, I forgot to include the question!
So, do I just need some heatsink compound? I would have thought the resistor could easily handle the power/heat. Should I try a different resistance? Has anyone tried this, and would you mind sharing what worked for you? Thanks again!
ps: I have tried searching this forum, but wasn't able to find much more than I know already, other than perhaps using a lamp as the load instead of a resistor.

audioguru · 23rd March 2007, 05:22 AM

With only 5V across it, a 10 ohms resistor dissipates only 2.5W. A 10W resistor gets very hot at 10W but should be fine with only 2.5W.

Did you notice the plastic ties fastening the author's resistor to the metal case for cooling?

A lamp is like a dead short when it is cool. The power supply might not turn on with it as a load.

Hank Fletcher · 23rd March 2007, 05:29 AM

Yeah, only 2.5W, right? So why's it getting so hot? The resistor's from The Source (you know, Canada's RadioShack), I would've thought it at least be up to these specs. I saw his ties and his notes on using compound, but when I was testing my supply before making any serious commitment to modding it, I just shorted the green wire to ground, and a red wire to ground through the resistor. I didn't even open the case, figured I didn't have to until I needed to. So the resistor was just kind of floating around, but wasn't a short risk 'cause I'd taken some liberties with electrical tape. Is there any reason why I shouldn't try using, for example, a 100 Ohm resistor instead to limit the current? And will heatsink compound and more diligence on my part towards keeping the resistor cool make that big a difference (what do I have to do, put an ice-cube on it?)? Thanks,
Hank.

Suraj143 · 23rd March 2007, 05:29 AM

Hi Hank Fletcher.You are speaking the resistor is coming directly through 230v AC through the 400V capacitor? Is that right?

Hank Fletcher · 23rd March 2007, 05:33 AM

Sorry, Suraj143, but that doesn't sound right at all. I'm not too sure what you're asking. In short, I think my answer to both your questions is "no."
Hank.

audioguru · 23rd March 2007, 05:34 AM

The red wire that the resistor is connected to is +5V, not AC.
The power supply won't work if the resistor is 100 ohms. I thought 10 ohms is a high value.

Hank Fletcher · 23rd March 2007, 05:38 AM

Quote:

Originally Posted by audioguru

The power supply won't work if the resistor is 100 ohms. I thought 10 ohms is a high value.

I thought as much. Still not too sure what the problem is, though...

BTW, while the supply is working I get pretty good voltage on the 12V lines: consistently 11.9V, which is good, I'm presuming, based on the remarks on the website I mentioned above.

audioguru · 23rd March 2007, 05:57 AM

I think The Source ripped you off and gave you a 2W resistor instead of a 10W resistor.

Hank Fletcher · 23rd March 2007, 06:02 AM

That's what I thought, too. The packet says 10W, though. Could the type of resistor make that much of a difference to its ability to dissipate heat? I've never used heatsink compound before, but maybe I ought to get some to try it out? Also, do you think I could get by with two of these resistors in series (for a total of 20 Ohms) to share the heatsinking job? Sorry for the newbyness of my questions...

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23rd March 2007, 04:59 AM	#1
Hank Fletcher	thread post test Argh! I'm having a heck of a time trying to post my question! Anyway maybe this will work: I searched the net for some ideas on how to use a computer supply as a bench supply, and finally followed this guy's notes: http://web2.murraystate.edu/andy.bat...OWERSUPPLY.HTM They're pretty good notes and I tried to contact him with my problem below, but I've yet to hear back. My problem is with the resistor. I used a 10W, 10ohm resistor as is commonly advised, albeit mine is the green, cylindrical wirewound type. It gets really hot, and although I haven't use any heatsink compound the resistor is well ventilated. The supply works for about ten minutes while the resistor gets hotter and hotter, then shuts down. This hasn't fried the resistor yet, which still indicates 10ohms after it's cooled down. Then you can turn the supply on again and watch the whole thing over... but presumably this is eventually going to cook something permanently. Thanks! Last edited by Hank Fletcher; 23rd March 2007 at 05:06 AM.

23rd March 2007, 05:10 AM	#2
Hank Fletcher	Oops, I forgot to include the question! So, do I just need some heatsink compound? I would have thought the resistor could easily handle the power/heat. Should I try a different resistance? Has anyone tried this, and would you mind sharing what worked for you? Thanks again! ps: I have tried searching this forum, but wasn't able to find much more than I know already, other than perhaps using a lamp as the load instead of a resistor. Last edited by Hank Fletcher; 23rd March 2007 at 05:12 AM.

23rd March 2007, 05:22 AM	#3
audioguru	With only 5V across it, a 10 ohms resistor dissipates only 2.5W. A 10W resistor gets very hot at 10W but should be fine with only 2.5W. Did you notice the plastic ties fastening the author's resistor to the metal case for cooling? A lamp is like a dead short when it is cool. The power supply might not turn on with it as a load. __________________ Uncle $crooge

23rd March 2007, 05:29 AM	#4
Hank Fletcher	Yeah, only 2.5W, right? So why's it getting so hot? The resistor's from The Source (you know, Canada's RadioShack), I would've thought it at least be up to these specs. I saw his ties and his notes on using compound, but when I was testing my supply before making any serious commitment to modding it, I just shorted the green wire to ground, and a red wire to ground through the resistor. I didn't even open the case, figured I didn't have to until I needed to. So the resistor was just kind of floating around, but wasn't a short risk 'cause I'd taken some liberties with electrical tape. Is there any reason why I shouldn't try using, for example, a 100 Ohm resistor instead to limit the current? And will heatsink compound and more diligence on my part towards keeping the resistor cool make that big a difference (what do I have to do, put an ice-cube on it?)? Thanks, Hank. Last edited by Hank Fletcher; 23rd March 2007 at 05:36 AM.

23rd March 2007, 05:29 AM	#5
Suraj143	Hi Hank Fletcher.You are speaking the resistor is coming directly through 230v AC through the 400V capacitor? Is that right?

23rd March 2007, 05:33 AM	#6
Hank Fletcher	Sorry, Suraj143, but that doesn't sound right at all. I'm not too sure what you're asking. In short, I think my answer to both your questions is "no." Hank.

23rd March 2007, 05:34 AM	#7
audioguru	The red wire that the resistor is connected to is +5V, not AC. The power supply won't work if the resistor is 100 ohms. I thought 10 ohms is a high value. __________________ Uncle $crooge

23rd March 2007, 05:57 AM	#9
audioguru	I think The Source ripped you off and gave you a 2W resistor instead of a 10W resistor. __________________ Uncle $crooge

23rd March 2007, 06:02 AM	#10
Hank Fletcher	That's what I thought, too. The packet says 10W, though. Could the type of resistor make that much of a difference to its ability to dissipate heat? I've never used heatsink compound before, but maybe I ought to get some to try it out? Also, do you think I could get by with two of these resistors in series (for a total of 20 Ohms) to share the heatsinking job? Sorry for the newbyness of my questions...