electronics Q -

-Laser- · 22 Mar 2006 at 11:46

i don't have the exact numbers handy, but can anyone help me with this situation - if i have a 5v power supply running an item that is meant to run on 12v, at say 12amps, what amperage would it be at 5v? is it as simple as 5amps? im guessing not but its too long since i did this at school?

daz · 22 Mar 2006 at 11:52

If the power is kept constant, i.e. it needs say 144 Watts (as you have 12V, 12Amps and then P = IV) then if you have a voltage of 5V you'd need a current of just under 29A.

However, with higher current you're increasing the heating effect on wires etc used - thus you experience wastage.

Adam_151 · 22 Mar 2006 at 12:35

-Laser- said:
i don't have the exact numbers handy, but can anyone help me with this situation - if i have a 5v power supply running an item that is meant to run on 12v, at say 12amps, what amperage would it be at 5v? is it as simple as 5amps? im guessing not but its too long since i did this at school?

Its not quite as simple as that, it all hinges on how much power the item will try and take on 5v , if its a resistive load then power will be equal to voltage squared over resistance, 12v at 12A would be a power of 144w, and rearranging the formula to give R=V²/P can tell you that the resistance would be 1 ohm, if you then plug that into P=V²/R at five volts you get a power of 25w which at 5 volts is 5A. But even if its a resistive load such a lamp, its likely to be off because the normal operating resistance of the filament won't be reached.

If its not a resistive load, then practically anything goes because the nature of the load will determine the way it reacts when presented with undervoltage

Sputnik II · 22 Mar 2006 at 13:17

Adam_151 said:
Its not quite as simple as that, it all hinges on how much power the item will try and take on 5v , if its a resistive load then power will be equal to voltage squared over resistance, 12v at 12A would be a power of 144w, and rearranging the formula to give R=V²/P can tell you that the resistance would be 1 ohm, if you then plug that into P=V²/R at five volts you get a power of 25w which at 5 volts is 5A. But even if its a resistive load such a lamp, its likely to be off because the normal operating resistance of the filament won't be reached.

If its not a resistive load, then practically anything goes because the nature of the load will determine the way it reacts when presented with undervoltage

That's a very long winded way to say that the voltage would be the same as the current for a load of 1 Ohm.

But, your spot on.
If the OP could tell us what this electrical item was it would help us to give a more accurate answer based on his question wouldn't it.

-Laser- · 22 Mar 2006 at 13:26

Sputnik II said:
That's a very long winded way to say that the voltage would be the same as the current for a load of 1 Ohm. But, your spot on.
If the OP could tell us what this electrical item was it would help us to give a more accurate answer based on his question wouldn't it.

sorry, yes it's a lamp. trying to get my head round it still lol, but these posts have really helped thx. i can work out how to wire the circuit, but i need amperage etc for fuses etc.

Sputnik II · 22 Mar 2006 at 13:40

-Laser- said:
sorry, yes it's a lamp. trying to get my head round it still lol, but these posts have really helped thx. i can work out how to wire the circuit, but i need amperage etc for fuses etc.

Assuming that it uses a 'normal filament' bulb it would be a resistive load so just work it out using the info that adam gave.

-Laser- · 22 Mar 2006 at 14:41

Sputnik II said:
Assuming that it uses a 'normal filament' bulb it would be a resistive load so just work it out using the info that adam gave.

cool cool, thx