Electronics - Running HID lights from PC power supply

Submitted: Wednesday, Aug 15, 2012 at 16:38
ThreadID: 97430 Views:2738 Replies:5 FollowUps:7
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G'day , this is probably not the forum for it but I've received a lot of advice here from people who are fairly knowledgeable in electronics etc so giving it a shot.

I've got two HID 55W globes with Ballasts that I'm trying to power with a computer switchmode power supply. When I test the HID lights off a battery they work well & I measure 9.5 amps drawn for the two of them. When I connect to a PC power supply on the +12v rail it lights up for about 30 seconds then shuts down & needs to be unplugged to try again. The PC power supply is 400 Watts with 17amps on the +12v ..

I tried 2 power supplies (300W & 400W) & both doing the same thing.. One thing I did notice is when I turn it on it starts at 12v then slowly drops to 8V then dies..

I'm going to try get my hands on a higher rated PS but thought I'd ask the question here incase someone knows

Thxs Don
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Reply By: MrBitchi (QLD) - Wednesday, Aug 15, 2012 at 16:55

Wednesday, Aug 15, 2012 at 16:55
Most PC power supplies rely on recieving a "Power Good" signal back from the processor or they shut down. A PC powersupply does not make a good bench supply. If you want 12v on the bench you should get a proper power supply. Maybe a battery charger will do what you want.
AnswerID: 492928

Reply By: Member - Boobook - Wednesday, Aug 15, 2012 at 17:12

Wednesday, Aug 15, 2012 at 17:12
The issue is that HID's draw a lot of current on start up. Each 55w globe will draw up to about 20 Amps for up to about 20 seconds. 2 of them is about 40 Amps or 480 Watts.

What you are seeing is exactly what will happen if they don't get enough start up current when they change from start up to constant mode.

First try just one at a time, that *may* work. Otherwise you could put say a 7AH battery in parallel to act as a large current source at start up.
AnswerID: 492932

Follow Up By: Fab72 - Wednesday, Aug 15, 2012 at 18:20

Wednesday, Aug 15, 2012 at 18:20
Hello Boobook,
You seem pretty switched on (no pun intended) regarding HID lights. At the risk of hi-jacking the thread, could I ask for some advice please.

I have a pair of Light Force 240 Blitz that I have converted to HID with 55W globes. Both lights run through a single relay with two "out" terminals. I used 20 Amp wire through out.

The problem I have is that when I go to turn them on, sometimes only the LH one works, other times the RH one, sometimes both and sometimes none.

Any ideas? Should I run them each through their own relay? Go for higher Amp wiring? Go for a battery with a higher CCA rating? Swap globes to 35W? Or give up and go back to the Halogen globes?

Any help appreciated.
FollowupID: 768539

Follow Up By: Member - Boobook - Wednesday, Aug 15, 2012 at 19:32

Wednesday, Aug 15, 2012 at 19:32
Well i am not sure how Swtiched on I am Fab but I think heavier wiring will help. Wire is usually underrated.
FollowupID: 768541

Follow Up By: Fab72 - Thursday, Aug 16, 2012 at 04:43

Thursday, Aug 16, 2012 at 04:43
I'll give that a shot for sure Boobook. I'll run two relays (one per light) also. Costs next to nothing and if it works, I'll be happier than a pig in stuff.

FollowupID: 768564

Follow Up By: AdrianLR - Thursday, Aug 16, 2012 at 06:55

Thursday, Aug 16, 2012 at 06:55
Also check the earth connections. My HID spotties played up when I first installed them until I found a better earth point (for the relay in my case but same applies for individual lights).

FollowupID: 768565

Follow Up By: Fab72 - Thursday, Aug 16, 2012 at 16:50

Thursday, Aug 16, 2012 at 16:50
Thanks Adrian,
I must admit that I used a factory earth point and have just assumed it would be OK. One thing I didn't do was to put a mutimetre on that point then back to the negative of the battery and check the resistance value.

I'll do that for sure also.

FollowupID: 768597

Follow Up By: AdrianLR - Thursday, Aug 16, 2012 at 17:29

Thursday, Aug 16, 2012 at 17:29

The multimeter may not show up a problem. It's only when you have a high current draw (eg when the lights start up) or you get some vibration whilst driving that the connection becomes sufficiently bad to cause a problem.

The factory earth points are usually fine but even these get corroded. In my case, the attachment point for the relay as suggested by Nissan (I was using a factory driving light loom) was painted and although the bolt made some contact it wasn't good enough after a couple of weeks.

Also check any crimped connections. Don't be afraid to give the wires a good tug - if they come out then it was a poor connection which would need to have been fixed anyway at some point.

FollowupID: 768601

Follow Up By: Fab72 - Friday, Aug 17, 2012 at 04:59

Friday, Aug 17, 2012 at 04:59
Thanks again Adrian.
The weather looks pretty ordinary for this weekend so I might make it a shed weekend and sort these things out.

FollowupID: 768630

Reply By: ss--ss - Wednesday, Aug 15, 2012 at 17:24

Wednesday, Aug 15, 2012 at 17:24
Thanks for the replies !
I just checked it with one 55W on & it's doing the same thing. there is a surge at startup but not as high as 20amp (only 7amp). These are new so technology may have improved in the surge part. But it dies out after about 20 -30 seconds.

I then got out my CTEK charger & it doing the job just fine without any cutout, however I can only run one light as it's a 7amp charger.. Think your right John it's something to do with these PC power supplies.

thxs Don
AnswerID: 492933

Reply By: The Bantam - Wednesday, Aug 15, 2012 at 18:02

Wednesday, Aug 15, 2012 at 18:02
As has been said for a number of reasons computer power supplies unmodified do not make good bench supplies, particularly the later ones.

PC supplies have very little surge capacity and they are designed to run lots of little loads.

Additionally many switchmode power supplies do not like running with outputs unloaded

Also I would have thaught to have any hope of sucess you would be better off looking at AT supplies with a mechanical power switch rather than ATX supplies with a fly by wire power switching system.
BUT....then ya looking as uncommon high power supples, some of the early At supplies where only 150 watts all up.

If ya trying to run HIDs of 240v mains, what ya doing is a round about way of getting the result...reinventing the wheel so to speak.

ya probably beter off doing some homework and getting an archtrual or projection balast to do the job.

what are you trying to achieve?

AnswerID: 492935

Reply By: Cravenhaven - Thursday, Aug 16, 2012 at 09:48

Thursday, Aug 16, 2012 at 09:48
I'm not sure if it still holds true with PC power supplies, but they originally required a 5v load and a 'power good' signal back from the motherboard before the other outputs would even light up. They are primarily designed to provide 5v power as that is what most of the load runs on, the 12v is normally an ancillary supply.

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