Help me "load" the power supply?

[petreza]

Hi,

I need to put a load across the 12V leads of my power supply to simulate a hard drive so that the power supply never shuts down. I know I have to put a resistor but i do not know how to figure the exact specifications.

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My hunch is this:

- let's say I want to load 7Watts (a regular HD just spinning)

- 7Watts = (12V)*(Ampers)

- Ampers = ~0.5A

- 0.5A = (12V) / (Resistance)

- Resistance = 24 Ohms

--------------------------

Am I right in saying that I need a 24Ohm resistor capable of dispensing 7Watts? ( I guess they make only 25Ohm ones but that is fine)


Any coments and suggestions are welcome?

 

[bigben2k]

You're correct, but you should make sure that the resistor is rated for at least the same power.

However...

It still won't be meant to stay on for extended periods of time. Maybe 1 minute: these things get toasty, fast!

If you want to go that route, I'd thermal eopxy some heatsinks on those power resistors, and run a fan over them.

 

[petreza]

While I waited for a responce I checked the catalog at digikey.com. Here is what I found:

- download Section F of the catalog - 1.9MB

- turn to page 760

- on top it says - "YAGEO 5 and 10 Watt Wirewound Rectangular Resistors 5%"

Should I get the 10 Watt 24 Ohm resistor - will it do the job? (It is square - good, and already have the heatsinks lined up.)

 

[hitechjb1]

Resistors with higher power rating than what it would be dissipated should be fine.

In your example,
R = 24 ohm
V = 12 V
P_dissipation = V^2/R = 12 x 12 / 24 = 6W

So a 10 W resistor should be fine, with enough margins.

 

[hitechjb1]

Is there is better way to prevent the PSU from shuting down? Since the way you try to do consumes power all the time, in theory is not desirable for a design to consume and waste power and put out heat all the time, ...

What exactly you are trying to do? Is the PSU connected to a motherboard, if so, can you use the mb to control the PSU? What do you mean by simulating a hard drive? ....

 

[petreza]

I am buying one of those Mini-Itx fanless motherboards - ME6000 - for home file server. There will be 3-4 hard drives conected to it. I want the whole system to be fanless and the HD to suspend after only 3min of idile.

The problem was that there are no cheap fanless power supplies that can support that many HDs. The one (PW60-K2) that I am buying for the ME6000 can NOT spin-up even one regular HD.

I am solving this problem by using a fanless power supply from an old computer just for the HDs in addition to the PW60-K2. But this creates a new problem - when the HDs go into suspend mode the PSU shuts down. When I try to wake them up Windows tells me that I have unpluged the HDs and that I shouldn't do that. My hunch was that if there was a small constant load on the 12V wire the PSU will not shut down. I was right: I disconnected one of the HDs from the computer so that it does not suspend with the others. It continued to spin while the others suspended. When I tried to look for something on the suspended HDs they came back to life.

So the conclusion was - I need a resistor that can simulate the load of a HD in order to keep the PSU on.

Of course I would apreciate it greatly if you have more elegant solution.

Thank you!

 

[emericanchaos]

you've made your solution to complicated. if i were you i'd get a 300W fortron for $30 and leave that go. since it's the only fan you have in the box (which imo is a bad idea cause those disks are going to get hot) it shouldn't make much noise. i'd just get some 120L1A's and 7V them if you want silence. that way you'd have some air cooling the disks and silence and a PSU that won't shutdown.

 

[petreza]

for the HDs I have prepared big heatsinks for passive cooling plus they are 5400rpm and will not be ON for more than a few minutes at a time - just transfer the needed file and suspend - so I am not worried about the HDs overheating. I want a silent system - it is going to be a few feet away from my head when I seep - so even a 7volt-modded quiet fan will botter me.

I do not think my setup is TOO COMPLICATED. An extra PSU, which I already have sitting around, and a 50cent resistor, for a completely silent system is worth it, in my opinion.

 

[hitechjb1]

Now I understand what you are trying to do.

1. It is hard to get cheaper than using your existing PSU. If you don't have an existing PSU for the HD's, then there are very quiet PSU which may be a solution. And you can use the motherboard and Windows for power management to turn on/put sleep the HD and PSU.

2. For your existing solution, you may not need to sink 0.5 A current to prevent the PSU from shuting down. A smaller current and higher resistance may be sufficient, and so generates less heat. How small the current is needed, you may have to try it and find out or from the PSU spec.

E.g. if 0.1 A is enough, then the resistance
R = 12 / 0.1 = 120 Ohm

You can test with a 100 Ohm resistor.
The heat generated would be much less and the resistor is much smaller.
P = 12 x 12/ 100 = 1.44 W (25% heat as previous one)

You can also try
1. 200 Ohm
current = 12 / 200 = 0.06 A
power = 12 x 12 / 200 = 0.72 W

2. 50 Ohm
current = 12 / 50 = 0.24 A
power 12 x 12 / 50 = 2.88 W
etc etc

So you just have to try it with various resistors:
24 Ohm 0.5A 6W
50 Ohm 0.24A 2.88W
100 Ohm 0.12A 1.44W
200 Ohm 0.06A 0.72W
500 Ohm 0.024A 0.288W

Starting with highest resistance (smaller watt, less heat generated) until it works.

If resistor is under 1W, probably you can get it from Radio Shack.


PS: Are the two PSU set up in dual PSU mode? If so, can the motherboard and Windows power manage the PSU for the HD's. Then no need to force the PSU on all the time???

 

[petreza]

Wow - you did a lot of math for me - THANKS!

Yes, I was also thinking that I might NOT need the full 7Watts or so to keep the PSU alive. When I order the resistor I will order the whole set and will test them by the method prescribed above. Thanks again!

Are the two PSU set up in dual PSU mode?

I do not understand what you mean there. How do you setup two PSUs in dual PSU mode? (other than those special server mobos that have this feature)

 

[hitechjb1]

Dual PSU means connecting two regular ATX PSU's so that both can be controlled by one power button and both can be detected by the mb and hence both can be power managed by the mb and windows. Since you have a special PSU PW60-K2, I don't know whether it can be done.

In your case, one PSU powers the mb + some stuff, and the other PSU powers all the HD's. But both will be controlled by the mb and Windows.

Only the GND and the GREEN wire of both PSU have to be connected together. And one more wire to GND for the slave PSU. It does not mean shorting the 3.3, 5, 12 V together. There is a sticky on Dual PSU, refer to it if you don't know for details.

 

[bigben2k]

In this case, the resistor would be a bad option, because of the heat it will dump in the case.

You definitely want to ground the PWR_ON line of the mobo header of that PSU, to leave it on all the time.

Alternatively, as has been suggested, you can wire that line to the other mobo header, and you should be OK.

Unless of course this is an AT PSU, and not an ATX. In that case, I would highly recommend getting a cheap ATX PSU, and doing the above. You can find them for less than $10, if you look hard enough.

 

[petreza]

it is an old PSU; its motherboard conection has only 6 cables - red, red, yellow, blue, black, black.

I am much more comfortable to add a resistor than to cut wires and put two PSUs on one mobo.

Ventilation would not be a problem - It will be an open case.

 

[bigben2k]

If that's the way you want to go, that's your choice!

In that case, I would opt for a resistor that has a much higher power rating: you can actually find 75 Watt resistors, if you know where to look. It'll allow the resistor to maybe run a bit cooler, in density, so that you don't accidentally burn yourself if you stick your hands in there (yes, it gets that hot!).

Couldn't you turn off the sleep function of the HDDs?

 

[petreza]

....you can actually find 75 Watt resistors....

.....Couldn't you turn off the sleep function of the HDDs?

- Of course if I can find such a thing and it does not cost an arm and a leg, I will buy it. I will email DigiKey.com with a question like that - their catalog is too big to search manualy ( for inexperianced searcher). Hopefully they have the "beast".

- No. The 3-4 HDs will be 3-4 Feet away from my ears when I sleep and the noise will most likely botter me. When I go to sleep they will have to go to sleep too.

 

[Punkmasterflick]

so you would turn off the suspend mode on your hdd's if you knew how? what o/s are you running?

 

[pik4chu]

Im pretty sure he WANTS the suspend mode active because he(she?) needs it to be silent shortly after it finishes doing whatever it needs. just that the two PSUs are not connected in any way so when windows puts the hard drives to sleep, the PSU controlling them shuts off.

 

[larrymoencurly]

I've found that resistors have to be rated for at least twice the power going through them to keep them from getting scorchin' hot, but for larger resistors, like over 1W, they should be rated for at least 3-4W the actual power.

 

[petreza]

DigiKey were not very helpful with their email reply. I guess I am on my own. I have ordered their paper catalog but can not wait until it comes.

Can someone in the know give me a link to someone who sells the resistors as explained by hitechjb1 above but are rated for 3-4 times the wattage they are going to be carrying?

Thank you!

 

[star882]

You may also need to load the 5v line.
I find that 5 ohm, 10W resistors work good for loading the 5v line.