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Unread 11-14-2002, 10:47 AM   #1
jtroutma
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Default 12V mod for the experts

Ok guys here is the deal:

I have a dinky PSU that can put out 12V @ 2.5Amax and 5V @ 5.5Amax. I want to be able to drive (2) Seagate 15K SCSI drives off of this powersuppy. I have already looked up the power requirements for these drives and the idle power consumption is only about 0.5 - 0.8 Amps on the 12V line per device.

The PROBLEM is that each drive requires 1.8 - 1.5 Amps at startup/spinup. I will be using the Staggared spinup option BUT as you can see, I will be exceeding the max amperage that the PSU can handle at one time.

The 5V rail is not a problem because both drives spinning up at the same time only draw 0.8 Amps each and only 0.5 at idle.

WHAT I want to do is create some sort of buffer for the 12V rail so that durring spinup I am not overloading the PSU. Is it possible to create a series of large capacitors to create an electrical "buffer" of sort so that the PSU will not be overloaded as much?

Comments? Suggestions? Theories?

Thanks
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Unread 11-14-2002, 10:52 AM   #2
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Hum...

Should I ask why you want to power your drives seperately?

The PSU will also have a peak output rating. I'm going to assume that you don't know this value.

You should be able to use it "as is", if that cheap PSU can handle the load, as a peak. Do you have a time frame for the HDD startup/spinup sequence? (Usually in milliseconds).
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Unread 11-14-2002, 11:32 AM   #3
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Correction: both drives will be powered by the same PSU.

According to the specs that I was able to get off the PSU (without taking the whole damn box apart ) was that the units PEAK power rating is 2.5Amps on the 12V line. I am assuming that those numbers are the peak ratings; if they are only normal ratings then I wouldnt be so concerned. BUT better safe tahn sorry, especially since we are talking about $600+ worth of computer equipment Keep in mind, I am talking about a small PSU used to power (2) OLD Toshiba 1X CDROM drives in a CD-ROM tower.

The time frame for the drives spinning up is about 6 seconds............DAMN I was wrong --> 20Seconds!!!!!!! (just checked the specs sheets)

Links to HDD specs:
Cheetah X15 36LP 15K Drive

Cheetah 36ES 10K Drive

I put the specs to the 10K drive just for comparison.

Strange though... why would the 80pin version of the Cheetah consume LESS power than the 68pin version????
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Unread 11-14-2002, 12:23 PM   #4
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a large capacitor will be a buffer, but I would just try it without. Most PSU's have conservative ratings. You will NOT heat up the PSU's heat sinks very much in 20 seconds.
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Unread 11-15-2002, 11:58 AM   #5
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Well guys I found out more info about the unit.

I was able to find a model number on the board and a link the the manufacturer. According to them this unit does have a max current draw and a typical current draw as well. For those who are interested here is the link to the specs of the PSU in question:

Minature PSU specs

Thanks for all your help.
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Unread 12-09-2002, 03:03 AM   #6
iroc409
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put a lightning audio 1 farad 12v cap on that thing and disconnect the power supply! wheeee!

lol... my 500+watt car stereo ran for several minutes on mine, until i got the bitch disconnected...
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Unread 12-09-2002, 03:04 AM   #7
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but yeah, as meltman said, it would work. if you're worried, i'd certainly do it. the only thing i'd be concerned about is the hard drives staying on for a little bit... you'd have to make some sort of loop or something to keep the caps charged, and turn the drives off when the machine goes off.
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Unread 12-09-2002, 06:03 AM   #8
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Quote:
Originally posted by iroc409
but yeah, as meltman said, it would work. if you're worried, i'd certainly do it. the only thing i'd be concerned about is the hard drives staying on for a little bit... you'd have to make some sort of loop or something to keep the caps charged, and turn the drives off when the machine goes off.
When you say having the hard drives on for a little bit, you mean after shutdown, right?.... If so... this might actually be a *good thing*... many of you may be familiar with the issue that several of the MS OS' have on particularily fast machines - That is that the computer shutsdown and powers off before the harddrives get the chance to finish writing their cache back to the harddrive. A Common symptom of this is a corrupt registry (particularily the sofware hive) since it is something that is constantly being read from/written to and hence is typically kept in the harddrive's cache...

O/T: Iroc409... <to-be-taken-semi-seriously> are you going for the highest post per second record or something?... or just trying to inflate the post-count? your name is all over the main-page... in one instance you replied to the same thread multiple times within minutes of your previous post... what's up with that?... from what I've noticed the people here dont seem to care so much about post count but rather care more about the quality and manner in which people present their ideas - something I dont want to see change.

PS: Strikethough BB code should be added to the forum.

Last edited by ymboc; 12-09-2002 at 06:17 AM.
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Unread 12-09-2002, 08:54 AM   #9
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jtroutma:
Looking at the specs for the hard drives and the power supply you should be just fine running it normally. As long as your not going to be spinning the hard drives up and letting them spin down every five minutes you should be just fine. The power supply's ratings should allow you to do this without adding anything else.

The answer for the question why does the 68 pin draw more power than the 80 pin is that the 80 pin uses a different signaling voltage than the 68 pin.

Just one other thing remember to take the ground of this power supply and connect it to the ground of the pc's power supply. You wouldn't want to have a lot of noise (electrical) being generated between the supplies.
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Unread 12-09-2002, 07:10 PM   #10
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no, i'm not trying to inflate my post count, sometimes i forget to write things, and wish to elaborate. forgive me for posting erraticly. i'll try to remember the edit button next time.

back on subject however:

if that would be desirable for the drives to have some sort of way to re-charge the caps while the setup was off, like right after shut-down, otherwise the caps would not be charged for the next startup. if the caps were empty at startup, then they wouldn't really serve the purpose. not exactly sure how this would be done, but i'll ponder it. probably have to be done with some sort of timer that shuts everything down, and then comes on to recycle the caps only, then off again.
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Unread 12-09-2002, 07:22 PM   #11
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Isn't there a line from the PSU that's on, when the comp is off?
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Unread 12-09-2002, 07:58 PM   #12
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This PSU was part of an external CDROM drive tower that I was using to power (2) Seagate Cheetah 15K 36GB HDDs. It used its own power plug and everything.

Fortunately I did some more searching and I found the PSUs spec sheet and discovered that it was rated for a MAX of 4A on the 12V (2.5A average) and 10A MAX on the 5V (5.5A average).

So everything worked out. I have the two cheetahs in the box in RAID 1 config and I will be attaching it to the servers hopefully within the next few days.

I can get Pics if you really want them......It was a very interesting mod job.
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Unread 12-10-2002, 12:12 AM   #13
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I'd be interested in a couple of pics, it sounds very interesting
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Unread 12-10-2002, 12:15 AM   #14
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I will see about geting some for yea guys... unfortuatly I wont have any before picutres. Didnt think about it at the time
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Unread 12-10-2002, 12:27 AM   #15
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Quote:
Originally posted by iroc409
put a lightning audio 1 farad 12v cap on that thing and disconnect the power supply! wheeee!

lol... my 500+watt car stereo ran for several minutes on mine, until i got the bitch disconnected...
You forgot to mention that those 1 ferad caps are about the size of a grown man's forarm :lol:

Glad you got it figured it out!
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