DC Power Supply Sizing

[fifty60]

I have 4 instruments that I need to size a power supply for. The data sheet for the instruments say they are rated for 11-26VAC/VDC, 15VA, 7W maximum.

The "15VA" would only apply if I were powering the instruments from an 11-26VAC power supply, right? If I am using a 24VDC power supply, I can just use the 7W in my sizing calculation?

 

[Smart $]

That's how I see it.

 

[GoldDigger]

Seeing a difference between VA and W for the OUTPUT of a DC power supply seems rather strange unless the intended use has really high peak currents or ripple.
To me the separate VA specification would only be meaningful if it was intended to tell you what level of peak current (spikes or pulses) the supply for the instruments has to be able to provide. And that current level would vary depending on the input voltage used.

On the AC input to a DC supply having greater VA than W makes perfect sense, representing a low distortion PF on the supply input.

One example would be if the instrument has its own DC to DC converter (switching type) to provide a selected stable internal voltage. In that case the input current could be pulsed if there is not a large series inductance designed into the converter input. And the pulse duty cycle would get worse as the input voltage increased.

 

[Besoeker]

I have 4 instruments that I need to size a power supply for. The data sheet for the instruments say they are rated for 11-26VAC/VDC, 15VA, 7W maximum.

The "15VA" would only apply if I were powering the instruments from an 11-26VAC power supply, right? If I am using a 24VDC power supply, I can just use the 7W in my sizing calculation?

You'd need at least four times that if each instrument is 7W. But we are looking at very inexpensive supplies. Be generous with the sizing.

 

[Smart $]

Seeing a difference between VA and W for the OUTPUT of a DC power supply seems rather strange...

He furnished power input spec's for four instrumentation units... not the spec's for the power supply unit. A 24VDC power supply will only see the W maximum times 4. If he used an AC power supply, the unit should be capable of furnishing up to 15VA times 4.

 

[Smart $]

...But we are looking at very inexpensive supplies. Be generous with the sizing.

I agree.

 

[GoldDigger]

He furnished power input spec's for four instrumentation units... not the spec's for the power supply unit. A 24VDC power supply will only see the W maximum times 4. If he used an AC power supply, the unit should be capable of furnishing up to 15VA times 4.

I assumed that the four instruments were identical units and so the spec for each one had both VA and W and they were different.
That would correspond exactly to requiring both that VA rating and that watt rating times four for the power supply output. Hence my discussion of non-constant-current loads.

Depending on the design, the power supply might be able to supply a peak VA far higher than its continuous wattage rating, or if it included foldback current limiting it might be able to produce a peak VA which is identical in value to the continuous watts.

And there was no indication that the instruments would accept an AC input.

 

[wwhitney]

And there was no indication that the instruments would accept an AC input.

The data sheet for the instruments say they are rated for 11-26VAC/VDC

Cheers, Wayne

 

[GoldDigger]

Cheers, Wayne

OOPS, missed that. :slaphead:
Looks like when fed AC it will have a low power factor. Probably an input full wave bridge followed by a capacitor before getting down to business.
The higher VA would be attributable to distortion power factor rather than phase power factor.