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ATX PSU keeps shutting down with heavy 12v load

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mauserman48

New Member
Joined
May 5, 2017
My ATX psu only puts out 11.6v when idling. when I hook a 12v load at 6 amps start up load it shuts off. I have 2 ATXs and they both do this.? what gives..
 

Lochekey

Senior Pink Member
Joined
Sep 13, 2015
What are you using to load them and have you checked the outlet voltage. Can you provide model number and age of the power supplies.
 
OP
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mauserman48

New Member
Joined
May 5, 2017
i'm using a 12v weed wacker motor w/o the gearing etc so it's free wheeling. my atxs are from 1999 d3ell 250watts,and a hp compaq presario 250watt . I have an old 16volt ac transformer that has been rectified to 14.50volt and the motor runs just fine. with the atxs they shut off when on the 12v rail which only puts out 11.65v. however when the motor is attached to the 5v rail, which has [email protected],it runs fine.
 

Lochekey

Senior Pink Member
Joined
Sep 13, 2015
Well let's start by saying those psus are old and junkish.

How are you determining that you only have a start up load of 6 amps. My guts would be that you have a bigger current draw on startup that is causing the psu to trip.
 

caddi daddi

Godzilla to ant hills
Joined
Jan 10, 2012
I have an old xp rig from that time.
if any of my newer stuff lasts as long I'll dance a jig on my own grave for you.
 
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mauserman48

New Member
Joined
May 5, 2017
The one psu is from a 2011 hp compaq. ..why when I run the motors on the 5v rail they run just fine..From both psus they run on the 5v rail. Is it cause the 12v rail doesn't have volt sensing? How can I bring the 12v rail back up to 12.2-12.5v? Brand new the hp was never higher than 11.75v
 
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Alaric

New Member
Joined
Dec 4, 2011
Location
Satan's Colon, US
The one psu is from a 2011 hp compaq. ..why when I run the motors on the 5v rail they run just fine..From both psus they run on the 5v rail. Is it cause the 12v rail doesn't have volt sensing? How can I bring the 12v rail back up to 12.2-12.5v? Brand new the hp was never higher than 11.75v

That's because the HP PSU was junk when it rolled out of the factory.
 

EarthDog

Gulper Nozzle Co-Owner
Joined
Dec 15, 2008
Location
Buckeyes!
Actually, ATX spec is 5%. Which is 11.4V on the low side (12.6 on high side) so it is in fact in spec. Outside of splitting that hair for clarity, I agree its trash. :)
 

Mr.Scott

Beamed Me Up!
Joined
Jun 9, 2013
Actually, ATX spec is 5%. Which is 11.4V on the low side (12.6 on high side) so it is in fact in spec. Outside of splitting that hair for clarity, I agree its trash. :)

My bad, but the bottom line is the 12v rail is not putting out enough amperage to fire the motor. Still trash.
 

Dolk

I once overclocked an Intel
Joined
Mar 3, 2008
Mauserman, are you using the PSUs in a computer or are you attaching them to the motor? I am a bit confused by your original post.
 
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mauserman48

New Member
Joined
May 5, 2017
The dell was given to me and the hp came from my desktop built in 2011.I wore out the HDD in the HP, and received a new desktop for christmas in 2015.. The psus were to be used as benchtop pwr supplies. and was wanting one of them to power the motor for my mini wood lathe... guess I'll just tear the dell down and scrap for parts and build a new power supply from scratch..the weedwacker motor turns out 13,000 rpms free wheel. using an ammeter it uses about 6amps to startup, then tapers down to 1.85a very quickly.I have a 16.5volt tranformer rectified to 14.25v with a 2.5a rating at 9.6v and it runs just fine.
 

Dolk

I once overclocked an Intel
Joined
Mar 3, 2008
That DC motor will consume a lot more than 6A at startup due to massive inrush. This is why you see the PSU die during those times. The CPU/PCI-E connectors will give you 12V @ ~20A, but you may need to load share with a bunch of these ports just to mitigate the in-rush current.

Now typically I do not suggest anything of the like with these PSUs. Most of the lower priced ones are not built to sustain constant max current draw. They will blow faster than usual, but no clue when that may happen.

If you want cheap 12V DC power, try to find old Server Power Supplies. These do not follow standard ATX design and are built to handle big in-rush currents on their 12V loads. They are easy to find in scrap/pawn stores, but you may not find any details on the power supply (unless you can read Chinese).

Transformer would always work (even though it can't sustain the 12V) since it is designed to handle large in-rush currents. You can go with your transformer but it sounds like you can't run the motor at full speed.
 

JrClocker

AKA: JrMiyagi
Joined
Sep 25, 2015
If you want to limit inrush, then you can put an inrush limiter between the PSU and the motor.

The motor only draws (or supplies) an energy spike during voltage and/or speed change...until the magnetic field is stabilized.

The simplest would be an RC circuit. A slow increase on the input voltage will allow for a more controlled motor inrush. But you would have to choose a small R and a big C to avoid voltage drop across the resistor. And, you would have to make sure the capacitor is drained before plugging and unplugged for safety reasons.



Another approach would be to build yourself a speed control circuit. The speed control will also control the inrush. Something with this sort of topology:

Linear_amplifier_schematic.jpg

This circuit allows for the motor to go in forward/reverse. As you will have far less -12 V current than +12 V current, substitute the -12 V with ground. You can flip the PNP and NPN transistors around (it's just me...I prefer to have the NPN transistor on the bottom - hehe).

You can setup a potentiometer to go off the 12 V into the + input on the opamp for speed control. You can put an RC circuit in the feedback loop of the opamp to make a low pass filter, and limit the surge current this way.

If you go this route, get transistor pairs that are 15 A and 45 V or more as there may be much larger current/voltage spikes when you are changing the motor speed. Look for a good quality audio amplifier pair, and they should work nicely.

Finally, these is the better option as it will keep your motor at a constant speed as it is put under load (and as the load is removed.)
 

RJ88

Disabled
Joined
Mar 24, 2017
11.4v a few that still didn't trip down to 11.1v...just some won't boot in or freeze once you really load it...Glacier 1000w still killing it hasn't budged at all yet on the rails in like what 6-8months.