Watt the ??? What do you mean W=VA?

It’s always good to perform some basic power calculations before specifying or purchasing a suitable power supply (or power supplies) for your project.

Seems simple enough – Once you’ve decided on the operational voltage, just take the current draw of each device that needs to be powered and add them together.

An Example:

8 x sets of Vanderbilt PE Beams 13.5Vdc @ 50mA ea = 400mA
22 x Aritech PIR’s 13.5Vdc @ 10mA ea = 220mA

Total current required is 400mA + 220mA = 620mA – therefore a 13.5Vdc 1A Power Supply should be sufficient.

Now add a couple of IR (Infrared) Illuminators for your CCTV system:

2 x IR Illuminators 12-32Vdc @ 13Watts

WATT ????  13W – what do you mean Watts, what happened to Amps (or Milliamps / mA) ? ? ?

Hmmmm . . .  that’s concerning – why don’t manufacturers just refer to Amps (A or mA) for current consumption and leave Watts to the car stereo guys  . . . Well, that’s a long story – but without delving too deeply into why, here’s a simple way to estimate current consumption in A or mA instead of W (Watts):

Basically Watts = Volts x Amps  (or W=VA) so, if we know the wattage and the operating voltage – in the case of this example 13W ea and an operating voltage of 13.5Vdc – we can estimate as follows:

13(W) / 13.5(V) = 0.963(A) or 963mA ea

Referring to our example:

8 x sets of Vanderbilt PE Beams 13.5Vdc @ 50mA ea = 400mA
22 x Aritech PIR’s 13.5Vdc @ 10mA ea = 220mA
2 x IR Illuminators 13.5Vdc @ 963mA ea = 1926mA (1.926A)

Total current required is 400mA + 220mA + 1926mA = 2.546mA (or a little over 2.5A)
So, we’ll need to upgrade our 13.5Vdc 1A Power Supply to (say) a 13.5Vdc 5A supply.

You’re probably thinking that if we use the industry standard practice of only loading a power supply to approx. 70-80% of it’s rated output, we should be able to get away with (say) a 3A supply ? And at first glance, that would appear to be right – BUT (there’s always a BUT !) for devices like IR Illuminators (and maglocks and drop bolts / strikes and PTZ CCTV cameras – the list goes on), we have to allow for in-rush (start-up) current.

In-rush current can be considerably higher than equipment manufacturers’ stated maximum running current (particularly for inductive loads like locks and PTZ cameras), so erring on the cautious side, a 5A supply would be a good starting point (a 10A supply would be even better). There will be a post regarding In-Rush Current coming shortly, so please check back regularly.

So next time someone says “watt the?”, you’ll know how to work it out for them !

This is an extract from the Tactical Technologies website

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