# How to determine the maximum power required by given devices? [closed]

I need to power 4 devices with these power input specs:

Ethernet Relay Module:                  18-27V DC max. 250mA@24V
Ethernet Dimmer Module:                 9-24V DC max. 80mA@12V
Ethernet Dimmer Module:                 9-24V DC max. 80mA@12V
Ethernet Shading Controller Module:     18-27V DC max. 250mA@24V

1. Is a 24V power supply (15A, 360W) enough to power these devices? (like this one)

2. How do I calculate the minimum necessary to power these devices?

UPDATE:

1. How to determine the maximum power required by given devices?
2. What power supply I should use?
• Question 2 can't be answered because there is not enough info to know what the current consumption is of the dimmers at 24V. – Andy aka Aug 26 '15 at 13:39
• @R2D2, power (watt) = voltage (volt) x current (ampere) – Oka Aug 26 '15 at 13:40
• It makes me sad that this question was asked by R2D2. – ninehundreds Aug 26 '15 at 14:01
• @Andy aka: These are just to power modules which control 220V lines. Oka: yeap know the formula. ninehundredt: I'm better with software :-) – Artur Kędzior Aug 26 '15 at 14:36

Two of the devices are spec'ed at 24V so these are easy.

The others are spec'ed to consume 80mA at 12V. It is likely that they will consume the same or even less at 24V but we can't assume that : a conservative approach would treat them as resistive loads, so their current would double as you double the voltage, for 160mA each.

Summing the currents then gives 500mA + 320mA = 820mA at 24V, so a 24V 1A supply should work, but a 1.5A or 2A supply working further from its maximum load is likely to be more reliable.

And a 15A supply may not even regulate correctly at such low loads, as Andy says.

• Thank you for simple explanation. I got one with these specs: (AC): 100-120V 0.3A 200-240V 0.2A 50 / 60Hz: 200-240V 0.2A 50 / 60Hz; (DC): 24V 2A – Artur Kędzior Aug 27 '15 at 13:51

Applying a lot of guesswork- it's unlikely that the current consumption will be more at higher voltages with modern circuitry. More than likely it will be less (approximately constant input power). If we add up all the currents, we get 660mA, which is much, much less than the 15A supply can deliver. So I would say it has more than adequate current capability.

Unfortunately there is no real data sheet supplied for the power supply and it's rather difficult to see if it's actually suitable. Some supplies demand a 10% minimum load, for example (1.5A). That may not be true in this case, but without a full datasheet (and preferably an application manual describing mounting/cooling methods, recommended input and output filters and so on) it's very much guesswork. Since some of the specs you list indicate that 24V is the maximum power supply voltage it would be prudent to check this power supply thing under no load conditions- or (better) to use a pedigreed power supply from a reputable manufacturer.

• I modified the question. Power supply I have found on amazon do not provide datasheets. So it should be fine taking into consideration that the power supply does not require minimum load. – Artur Kędzior Aug 26 '15 at 14:39
• Suggest you find one that does have a datasheet unless you are a gambling type. Specific product recommendations are off-topic. – Spehro Pefhany Aug 26 '15 at 14:52
• What am I looking for in datasheet that will satisfy requirements for these devices? – Artur Kędzior Aug 26 '15 at 14:54
• Just to give you an idea of what is missing from that spec, here is a proper datasheet. Every item on there is important to one degree or another. – Spehro Pefhany Aug 26 '15 at 15:04

Question changed:

A more correct approach to this problem would be to determine the Maximum power required by all devices then select a power supply capable of powering that load.

• A 12V LDO regulator is a linear device and will waste precisely the same amount of power as a resistor. The two loads at 12V will work also at 24V and likely is that they contain a switching converter to keep power down. This answer is at best misleading. I'd consider deleting it. – Andy aka Aug 26 '15 at 13:37
• As commented above, trying to find appropriate power supply. Don't have a datasheet. I have modified the question. – Artur Kędzior Aug 26 '15 at 14:40