Cheap 100 amp 12 volt power supply

[ellisco]

Hi,
I need to power a DC motor for short and infrequent periods. Motor draws ~80 amps under full load. I thought that I might use 2 ATX power supplies in parallel to provide the needed current. I foresee a problem with small differences in output voltage causing feedback and or other issues.
So the question: Is the solution to this as simple as putting a diode in series with the output of each power supply to prevent current flow from one supply to another? In other words:

+12 DC PS1----------->|--------|
| |
Ground---------------Load-------|
| |
+12.1 DC PS2----------->|------|

 

[KMoffett]

If there is a difference in voltage and you wire the diodes that way, only the highest voltage power supply will provide current to the load. It's a bad idea to parallel power supplies in an attempt provide a higher current.
What are the times of the "short and infrequent periods"? I think a 12V auto battery with a charger would be your best bet.

Ken

 

[WTP Pepper]

Have you tried looking around computer scrap merchants. I had a 100amp 12V PSU made from 2 x 5V PSUs tweaked up from an old PDP mainframe I used to run CB radio kit from.

As previously said it's a bad idea to parallel power supplies for reasons stated.

 

[Nigel Goodwin]

Use a car battery and a trickle charger - cheap, reliable and effective.

 

[ellisco]

So maybe I'm asking the wrong question

First thanks to all for their input so far.
Second regarding the use of a battery and charger, that was my first thought as well but there are some practical safety concerns with having a battery in this location as it is a power source that cannot be de-energized.
Some background:
I need an 80 amp 12 volt DC source to run a motor for about 1 minute ever few days. Yes a battery would be perfect but I can't have one. I am trying to prototype a device that could go into production so price is a real concern but complete field ready design is not required at this point. There are 1+ Kw DC power supplies for servers readily available but all the ones I've seen in this power range are for 220 VAC input which is no good for me. I have been looking around for a 110 to 12 volt transformer in the 1+ KW range but have had no luck finding one. I know they exist I just don' know where. With that component the rest of the problem becomes pretty straight forward.
Here is the question(s) I probably should have asked:
1) Any thoughts on a cheap way to get 80 amp 12 volt DC out of 110 AC? (We're after power here not precision)
2) Does anybody know where to find a 110 VAC-12 VAC transformer in the 1+ KW range?
3) Given how cheap ATX power supplies are is there a reasonable circuit that I could use to allow two to be used in parallel?
Thanks to all in advance.

 

[ellisco]

KMoffett,
Do you suppose that applying a current limiting circuit after each PS could be a reasonable solution? Say limit each supply to 45 amps or so. I'm thinking that if the motor is operating at less than 45 amps I don't really care if it all comes from one supply as they are rated at ~55 amps.

 

[unclejed613]

the method that works the best for running power supplies in parallel is to use a small resistance (0.01-0.1 ohms usually) to isolate each power supply, yet allow both to feed current into the load. a 0.01 ohm resistor would give you a 0.4V drop from each supply to the load. the resistor will dissipate 16 watts, so best to make them 25W resistors. digikey and newark (farnell) both carry them for about $3.00 each. since the motor will require a large starting current, put a very large capacitance (a few hundred thousand microfarads at least) at the junction between the resistors, so you don't bog down the supplies starting the motor. ATX psu's go into shutdown mode if you draw too much current, so they would just sit there and chirp if the starting current is too much for them.

 

[WTP Pepper]

First thanks to all for their input so far.
Second regarding the use of a battery and charger, that was my first thought as well but there are some practical safety concerns with having a battery in this location as it is a power source that cannot be de-energized.
Some background:
I need an 80 amp 12 volt DC source to run a motor for about 1 minute ever few days. Yes a battery would be perfect but I can't have one. I am trying to prototype a device that could go into production so price is a real concern but complete field ready design is not required at this point. There are 1+ Kw DC power supplies for servers readily available but all the ones I've seen in this power range are for 220 VAC input which is no good for me. I have been looking around for a 110 to 12 volt transformer in the 1+ KW range but have had no luck finding one. I know they exist I just don' know where. With that component the rest of the problem becomes pretty straight forward.
Here is the question(s) I probably should have asked:
1) Any thoughts on a cheap way to get 80 amp 12 volt DC out of 110 AC? (We're after power here not precision)
2) Does anybody know where to find a 110 VAC-12 VAC transformer in the 1+ KW range?
3) Given how cheap ATX power supplies are is there a reasonable circuit that I could use to allow two to be used in parallel?
Thanks to all in advance.

They exist but aren't cheap
https://uk.farnell.com/murata-power-solutions/d1u-w-1200-12-ha2c/psu-1200w-12v-5v/dp/1574398
https://uk.farnell.com/xp-power/mhp1000ps12/psu-medical-1000w-12v-83a/dp/1821479

Most modern PSUs will be universal voltage (90VAC to 270VAC)

 

[WTP Pepper]

After reading a moderated post, how do I insert a PDF instead of a link?.

 

[ericgibbs]

After reading a moderated post, how do I insert a PDF instead of a link?.

hi WP.
I dont see a Mod in this Thread.?

E.

To post a 'file', use 'Go Advanced', then 'Manage Attachments', upload the file from your PC.

When the window confirms its uploaded Close the window.

Eric

 

[harold777]

Hi there, new member here, I think OP needs to provide further details, the notion that the load is 100A at 12v is a bit simplistic, as motors draw many times the running current any PSU design needs to take this into account.

Someone suggetted a 12v car battery and this is an excellent 1st choice, if only to determine the power demands. Obviously fit a decent fuse, but at 100A that may not be so easy.

OP needs to define that power demand and then get a suitable switch-mode PSU designed to meet it AND the spread of motors (Plus the mech. load attached).
Most SMPS will cover 110v-240v ac at normal power frequencies.

The real issue is the 100A @ 12v - - - Yikes!

 

[bountyhunter]

I need an 80 amp 12 volt DC source to run a motor for about 1 minute ever few days. Yes a battery would be perfect but I can't have one. //// There are 1+ Kw DC power supplies for servers readily available but all the ones I've seen in this power range are for 220 VAC input which is no good for me. I have been looking around for a 110 to 12 volt transformer in the 1+ KW range but have had no luck finding one.

Ahhh... been retired so long I forgot what it was like to be a designer: simple engineering problem with an easy solution but "you can't use that".....

The reason you don't see 1kW+ power converters off 115 single phase is because with power factor and efficiency, they would draw about 25 Amps off the AC line.

 

[bountyhunter]

I need an 80 amp 12 volt DC source to run a motor /////
3) Given how cheap ATX power supplies are is there a reasonable circuit that I could use to allow two to be used in parallel?
Thanks to all in advance.

Check the magnitude of current that motor is going to need to start up..... then thank the stars for lead-acid batteries. An ATX or even multiple in parallel won't deliver that kind of current.

 

[tcmtech]

You guys are way over analyzing this.

100 amps at 12 volts is well within the range of a good and common transformer based battery charger and they obviously don't have issues with running high power motor loads for short bursts.

As far a paralleling smaller ones if needed they are all the same brand and model they will have identical internal parts which means stacking the guts two or more smaller units side by side will work just fine.
Strip the guts out of the charger, put a big audio amplifier booster capacitor or two on it for DC side filtering to and you are good to go.

As far as 1 KW/KVA on common 120 VAC power thats not a problem either. A basic 15 amp circuit is still good for 1800 VA peak which means there is plenty of headroom for a ~1000 VA system driving a 750 watt motor.

 

[JimB]

TCM hits the nail on the head!

I am not sure that a motor will benefit all that much from the smoothing capacitor, but it cannot hurt.

Another blindingly obvious simple solution...

... would it be possible to change the motor for a 120volt type?

JimB

 

[bountyhunter]

You guys are way over analyzing this.

100 amps at 12 volts is well within the range of a good and common transformer based battery charger

Here's one that runs of 220V, still doubt you will find one for 110V.

https://www.akkutolto.hu/english/12V100ACharger.html

 

[tcmtech]

Here's one that runs of 220V, still doubt you will find one for 110V.

Standard issue 200 amp battery charger booster unit. No American farmer would be caught dead without one.
https://www.centurytool.net/87422_Century_40_15_2_200_130_6_12_Battery_Charger_p/87422c.htm $159.95!

Parts break down for the Sears 200 amp units made by century. https://parts.sears.com/partsdirect...8/0705000/00056312/00001?blt=06&prst=&shdMod=

If you are searching for large low voltage secondary transformers with high surge capacities they are most often referred to as Buck/Boost transformers and are relatively cheap as well. 8/16, 9/18, 10/20, 12/24, 16/32 volt AC secondaries are fairly common in sizes from 25 VA well into the 10's of KVA ranges.

The 10/20 volt secondary ones make excellent high power unregulated 12 and 24 volt DC power supply transformers.