I am trying to determine the necessary power supply specs/requirements for a set up I've been working on. Doing some background research on the subject of course brings one to the Power Equation (P=I*V).

I already have the rated voltage and amperage requirements for most of my components. An example list goes as follows:

  • Motor 1 - 30V, 4 Amps
  • Motor 2 - 30V, 4 Amps
  • Peripheral 1 - 30V, 2 Amps
  • Peripheral 2 - 30V, 2 Amps
  • Peripheral 3 - 30V, 2 Amps
  • Controller Board - 5V, .5 Amps

Based on the examples I have seen, one performs the power equation on each item and then these can be individually summed up, which gives me a total of 422.5 Watts.

My first question, is this the correct procedure ?

Second, in addition to the watt rating of the power supply I also see power supplies have a 'max current out' rating... How does this figure relate to the power calculation above ? Or I can understand I need a power supply that is rated at least 422.5, but is the required max amperage out dependent on the highest single consumer in the system (i.e. max amperage out of at least 4 amps)? Or is it somehow determined by some other formula ?


You are correct in the power calculation, however that's not directly useful in specifying a power supply (except that you can remove all supplies with less than that supply rating from consideration).

Each output of the supply must be capable of supplying the maximum possible current draw. In this case you need a supply with at least two outputs, one 30V at 14A or better rating (in a real situation you'd have to look at the stall current of the motors and other possible conditions that could cause the motor to draw more than the stated current). In a real situation you might not want to share the same supply between the electrically noisy motors and whatever those peripherals are, so you might want two outputs for 30V or even two separate supplies.

Generally speaking, under no conditions should you exceed the maximum current rating of the supply on each output.

So, in theory, from the numbers you give, a supply with two outputs, 30V rated at 14A or better and 5V rated at 0.5A or better would fill the need. In some cases there may be a constraint on the total power, in which case you would check that the total power of 422.5W does not exceed the rated total power.

In real situations you must look more deeply at the datasheet and applications data because the top-line specifications often must be derated for elevated temperature or if you want long lifetime. You might find you need a 600W 30V supply, for example.

  • \$\begingroup\$ Thanks very much for the feedback. Unfortunately, among the conditions I have been looking at, 30V AC/DC supplies at such high amperage can become easily really expensive. So I have look been looking either higher 48V, or lower, 24V and thinking of adding the needed buck/boost circuits. Still not sure if one 'is preferable' to another-- For example, you can buy a 5V supply with 'massive' amperage (24+) for not that much... I wonder am I losing out then in this case going for 'low voltage' and then just 'boosting' up where I need it ? \$\endgroup\$ – Anthony Balducci Feb 8 '15 at 7:31
  • \$\begingroup\$ How critical is the 30V? A 36V open-frame supply or two trimmed to 32V is not at all expensive. \$\endgroup\$ – Spehro Pefhany Feb 8 '15 at 8:02
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    \$\begingroup\$ I have a +/- .5 V acceptable range on the peripherals from there (30V), but a much wider range on the motors. Also, cool to see you are in Toronto-- I am a U of T grad, though admittedly not in EE. ;) \$\endgroup\$ – Anthony Balducci Feb 8 '15 at 9:00

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