Sizing DC Power Supply for a Machine

[allscott]

On a multiple output SMPS, the first voltage is the one that's regulated. All others slave off of it. In the case of a PC power supply, the 5 volt (or 3.3 volt) supply needs a load. About 500mA will do, I prefer 1 amp. It won't run at all without a load.
Maybe some of them will, but this is true for the majority.

Guess I got lucky, I didn't know about that, the one I used was an old generic AT style power supply.

 

[rsdoran]

He was refering more to the 3.3v, the 5v (+ or -) I am not sure about, I do know the 3.3v slaves off the 5v so you have to beware of the current limitations involved when using both of them.

That is why I say do not use in that link I gave.

When using 24vdc only, the only limitations are the wattage.

 

[keithkyll]

The Primary voltage. On an old AT, that would be 5 volts. On newer systems, I assume 3.3 would be the primary. Maybe it's still 5?
The transformer has multiple taps. The feedback circuit is on the 5 volt line, so that's the one that's regulated. -5, +12, and -12 all track the 5. On some other supplies, +12 and -12 might be seperate regulators.
In most cases, the supply needs some kind of load or it won't run. Some have small resistors inside. This might be enough to keep it running, but may not be enough to keep it stable.
In all cases, put a 1 amp load or so on the first voltage. If your tearing into a supply trying to find out which one is the primary, look for an optoisolator (6 or 8 pin). That's the feedback device. It will be connected to the primary. That's how they keep the primary and secondary isolated.

 

[bikerdude]

HI,

Trying to follow along here:
What is "SMPS"??????????????
How do you apply an artificial load? Resistor??
BD

 

[rsdoran]

Switched Mode Power supply.

Yes Keith the old AT's were different.

You can use the newer ATX over a wider range, still best to leave 3.3v alone, especially if using 5v.

In simple language 250w (if rated for that) can be obtained at 24vdc.

 

[keithkyll]

bikerdude. Use a resistor or automotive lamp (1157) for a load.
See this link for more info, including instructions on turning a PC supply into a bench supply!

The 250 watt rating is the total of all outputs. On a 250 watt supply, the +12 is 10 amps, but the -12 is only .5 amps. That means you only get 6 watts at 24 volts. You can get away with drawing quite a bit more, but you'll get 50KHz ripple because the cap on the -12 is quite small.

See this link for supply ratings.

 

[Gummit]

Jad5150,

Thought I'd jump in and give my two pennies worth. You mentioned that originaly DIN rail space was prevalent in the design, I have found that many Engineers are not very aware that there are now some very slimline PSU's for DIN rail mounting (some brands have been mentioned).

My company sells some slimline PSU's, if anyone is interested take a look at www.digital-instrumentation.co.uk

Rob

 

[timbo_uk]

Are you planning on building lots of these? Is so then I can see the concern over cost.

If so the best way is to put an ammeter in series with your power supply and actually measure what you are drawing over a period of time while the machine is running. Your power supply needs to be able to handle whatever the peak is.

I have started to use the Omron S8VS-06024B Din rail PSU's. These have an LED display showing Run time in tousand hours, o/p voltage and current, also peak current. I did a rough estimate for my prototype machine, then with the display realised it was oversised, so the next unit had the smaller version.

Data sheet here http://www.rswww.com.hk/1/372020366-SMPS-DIN-RAIL-S8VS-06024B.html

 

[MikeW]

bikerdude. Use a resistor or automotive lamp (1157) for a load...

I would use a resistor, what happens to the "load" when the filament burns out? (Been there, done that many, many years ago)

 

[keithkyll]

Use a resistor for permanent installation. Sorry, I'm always thinking what I use on the bench when servicing. I use a Halogen lamp with reflector. It runs cooler.