amp calculation / low volt power supply choice

30 Apr 2006
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United Kingdom
I need help in working out what 12v power supply I need to purchase if someone would be so kind to help me get my head around calculations.

I have some 12v dc to 5v usb converters that I will have around the house in some media back boxes and am looking at a central 12v dc power supply. the kind of wall mounted metal box used for CCTV/Access control. (I will be using some of the 12v for 12v items not only the 5v converters)

Anyway the converters provide a maximum current output of 2a (I might want to update these in the future to 3-5a should the need arise).
So to provide 2a @ 5v what would the current output for the 12v supply need to be? (maximum distance of cable length around 10m between 12v supply and converter).

What is the calculation I need to then work this out at say 3a @ 5v etc..? Do you require any more information? I know where to source the supplies from I just need to know the 12v can output the required amps (as some will output maximum 1.6a per output and some only 1.1a) from TLC anyway.

Thanks in advanced.
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What's the total potential load?

There's gonna be some volt drop over that distance...

What's the reason for a central supply?
It all depends on how your 12v to 5v converter works.

If they are linear devices (using a simple regulator circuit and a fairly large heat sink) then the current demand at the input terminals is slightly greater than the current capability of the output circuit. The "excess" voltage is dissipated as heat by the regulator - hence the large heat sink. So for a 2 amp output load the input current will probably be about 2.1 amps and the heat dissipated by the heatsink will be somewhere around 14watts.

If however the converter using an inverter circuit to change the 12 volts down to 5 then things get much smaller with less heat produced as these devices are around 90% efficient. The output power is 10 watts (5volts x 2 amps) so the input power demand will be around 10/.9 = 11.1 watts. So your 12 volt supply will only have to provide around 12/11.1 = 1.08 amps. And the heat dissipated by the device is around 10% of the input power = 1.1 watts
The basic formula is W = V * I

watts = voltage times current

watts = volts times amps

So a 5 volt supply at 2 amps is supplying 10 watts

A perfect 10 watt power supply would be taking in 10 watts but there is no such thing as a perfect power supply. All waste some power as heat.

So the 5volt 2amp supply might take in 12 watts.

12 watts at 12 volts is 1 amp.
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Thank you for all the replies.

pPotential load will vary slightly as these are going to be used for USB charging of phones, tablets etc.. I would guess around 10-12w max (currently)

For voltage drop the step downs take between 7v-24v so that's good I guess just means slightly more amps needed from the central supply.

The reason for the central supply is I have a central media closet where all my media outlets from around the house go and wanted to have a usb charging point at each media outlet too. So I chucked an extra cable with each run to provide low voltage to the media plate.

Not sure on the type of converter I have but there is no heatsink. It's a fairly simple circuit with an IC, cap, resistor & (possible) toroid.

Based on the information you guys have provided I think it would be advisable to go for the 12v/1.6a output.
"IC, cap, resistor & (possible) toroid"

If it has a toroid then it's almost certainly an inverter unit - but a photograph would confirm
See image below....

Hi ban, read lots of your posts before so eagerly waiting :cry: :mrgreen: ha-ha

Extra low voltage I shall try remember this and probably based on what im about to say it probably doesn't even class as this :(

Cable size is 24awg, its shielded cat5e so I planned on using 3/all of the pairs (depending on what I decide on IR distribution) but now you got me thinking should I run a dedicated cable for this? I have a roll of 2x0.75mm 2 core. It would be a pain to get back under all the floor boards but hey if going to do it, then lets do it correctly.

Longest run so far is 10m and shouldn't imagine this being any longer. It just depends if I decide to run some cables to the so called sun room.

I see a chip and a coil so could be switch mode or could be simple voltage regulator and smoothing. It would completely change what is required.

A switch mode unit may be able to accept a volt drop of 50% and still work giving a steady 5 volt output but a voltage regulator may only accept a volt drop of 20% and would also load the supply higher than the switch mode. Plus create more heat. So without knowing which type one must assume 2A at 5 volt requires 2A at 12 volt. Assume a simple LM7805 voltage regulator for current calculations.

As for 12 volt clearly there will be a volt drop and much depends how much volt drop can be tolerated. Using 15 volt or more and simple LM7812 voltage regulators could clearly remove the volt drop but in that case one has to ask why not use 230 vac and transform local to where required.

In the main 12 volt supplies are used because they come from a battery in which case rarely 12 volt but 13.2 volt or higher. Again 12 to 12 volt inverters can be used to regulate the voltage but why would you would 230 vac to 12 volt not be better?

The problem with any switch mode supply is they don't go from zero to X amps but Y amps to X amps by sinking the Y amps you can make them do zero to X but clearly that is wasteful. If not switch mode there is a heat and waste problem as well so getting a power supply zero to 12 amps can be more expensive than 12 power supplies zero to 1 amp I really think you need to reconsider the idea of a central supply.
You might consider a 12 volt battery being trickle / float charged as the prome source of 12 volt power. Then you can still charge phones etc during a power cut.

I agree with Eric. that power supply is a switche mode device.

If you are running two or more switch mode power supplies from the same DC source you may find they interact with each other and stop operating properly. The cure is to fit an electrolytic capacitor to the input of each power supply. 100 uF 25 volt should be OK for a 2 amp output supply.

Make sure you get the polarity of the capacitor right. DO NOT use this on an AC supply.

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