[stdly]

I have a DB25 breakout board (PMDX-122) that calls for 7-12 VDC @ 200 milliamperes maximum. Is there a way I can reduce the amperage of a small switching supply that has a 1 amp or higher rating?
Steve

[Al_The_Man]

You don't need to reduce the current, the current is supplied according to the demand, in other words, your board will only demand 200ma and thats all the supply will provide.
Al.

[gar]

060221-1657 EST USA

stdly:

Your question is not clear.

If you have an ideal voltage source, then the output current is determined by the voltage of the source and the resistance of the load.

If you have an ideal current source, then the output current is defined by the source, and the output voltage is a function of the source current and the load resistance.

You have not defined what your switching supply is, except I will assume it is approximately a voltage source up to some maximum current limit point. Also switching supplies may require a minimum load to function correctly.

So if your switching supply is nominally a voltage source, then your only concern is if there is a minimum load current required that you exceed that value, and that the maximum current capability of the supply is greater than the maximum load you will put on the supply.

.

[stdly]

You don't need to reduce the current, the current is supplied according to the demand, in other words, your board will only demand 200ma and thats all the supply will provide.
Al.

Thanks Al,
Boy do I feel dumb!
Steve
:withstupi I don't usually use these but this to fit.

[stdly]

060221-1657 EST USA

stdly:

Your question is not clear.

If you have an ideal voltage source, then the output current is determined by the voltage of the source and the resistance of the load.

If you have an ideal current source, then the output current is defined by the source, and the output voltage is a function of the source current and the load resistance.

You have not defined what your switching supply is, except I will assume it is approximately a voltage source up to some maximum current limit point. Also switching supplies may require a minimum load to function correctly.

So if your switching supply is nominally a voltage source, then your only concern is if there is a minimum load current required that you exceed that value, and that the maximum current capability of the supply is greater than the maximum load you will put on the supply.

.

I just looked at supplies today I have not bought it yet. It was a new supply on an alluminuim plate that said it was 120vac in and 1-3.3@10a 1-5@8a and 2x12@1a VDC Output. I will have to check on that minimum load requirement but it is at a surplus store and the documentation was not very good.
Steve

[Al_The_Man]

Looking at the voltages, it almost conforms to one of the PC formfactors, maybe ATX or later, although the currents are a little low for PC use.
In some cases, just putting a fan for load will allow it to run, if this is the design criteria.
Al.

[paulC]

There are devices available that will detect excessive current draw and shut down the curcuit in the event of a fault condition.
They are called fuses.
Paul

[gar]

060221-2134 EST USA

There are many voltage regulated supplies that have built-in current limiting.

These supplies will be an approximately constant voltage source up to some predefined current maximum. This may be adjustable with a knob in some lab supplies. At this predefined current the supply becomes a constant current source.

In some cases these supplies are designed with a fold-back characteristic. In this case when you exceed the maximum current the supply current drops way below maximum. The intent is that if you short this supply that power dissipation in the series pass regulator will be reduced in comparison to a constant current limiting.

.

[stdly]

Thanks All!
I bought the supply I was looking at it is made by Power-One an MAP110-4300 there is a link to the specs.
HERE
I can't find anything about the minimum load in the specs. Will do some testing latter and I thought for $24 CDN I will use this supply for something.
Steve

[stdly]

There are devices available that will detect excessive current draw and shut down the curcuit in the event of a fault condition.
They are called fuses.
Paul

I have a 1 amp breaker I use as this device you talk about.
Steve

[stdly]

060221-2134 EST USA

There are many voltage regulated supplies that have built-in current limiting.

These supplies will be an approximately constant voltage source up to some predefined current maximum. This may be adjustable with a knob in some lab supplies. At this predefined current the supply becomes a constant current source.

In some cases these supplies are designed with a fold-back characteristic. In this case when you exceed the maximum current the supply current drops way below maximum. The intent is that if you short this supply that power dissipation in the series pass regulator will be reduced in comparison to a constant current limiting.

.

Yes I was looking around last night and saw this option on a few voltage controls.
Steve

[Al_The_Man]

There is a note on minimum load only if you are using V3 V4 as per the app notes. These supplies usually output with no load. Nice supplies.
Al.

[stdly]

There is a note on minimum load only if you are using V3 V4 as per the app notes. These supplies usually output with no load. Nice supplies.
Al.

Al,
Yes I saw that just now and V4 is +12 I will be useing it.
Will the 3 fans for the Geckos and a case fan and indicator light be enough load, 1 amp seems like allot?
Steve

[Al_The_Man]

They refer in a couple of places to the 'main output' on yours I assume this is the 3.3v, This is what you may have to load up in order to obtain the regulation on the V3 V4.
Also it does not specifically say if the sense input has to be connected, this is usually strapped accross the the 'main' output when not used, when you get it, I would just fire it up and see what the results are on the outputs, you can place moderate loads on to experiment, you should not do any damage doing this.
Al.

[stdly]

They refer in a couple of places to the 'main output' on yours I assume this is the 3.3v, This is what you may have to load up in order to obtain the regulation on the V3 V4.
Also it does not specifically say if the sense input has to be connected, this is usually strapped accross the the 'main' output when not used, when you get it, I would just fire it up and see what the results are on the outputs, you can place moderate loads on to experiment, you should not do any damage doing this.
Al.

Al,
I like the way you said FIRE it up and that is what I will do.
If the 3.3 @ 1 amp load is right (which makes more sence because the 12 is rated at 1 amp and 1.5 max so having a constant 1 amp load would not be right) and needs a 1 amp load could a resistor or something of that type be used to create load or am I thinking like a toolmaker again?
Steve

[Al_The_Man]

Yes, for experimentation you could use a resistor between 3 & 1 Ohm 5 to 10 watts.
Al.

[stdly]

Yes, for experimentation you could use a resistor between 3 & 1 Ohm 5 to 10 watts.
Al.

Al Thanks,
I will not be needing the 3.3 vdc in my encloser so do you have any sugestions for a permenent load? Would that resistor be good or is there a better way to permently laod the 3.3 with 1 amp.
Steve

[Al_The_Man]

I would suspect the load for regulation also applies to the 5v output, so a 5v load of some sort would also probabally work.
Al.