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Electronics => Beginners => Topic started by: shai on January 19, 2019, 08:50:15 am

Title: How to calculate total amps required for various voltages?
Post by: shai on January 19, 2019, 08:50:15 am

Say I have 12V for power, but I have a PCB with loads that use 3v, 5v and 12v. How do I figure out the total amps all the loads consume? Do I do 3V x amps = watts and then add up all the watts and divide by 12v to figure out the total amps? Is watts used as a common denominator? Thanks!

Title: Re: How to calculate total amps required for various voltages?
Post by: Rerouter on January 19, 2019, 09:03:46 am

in this case it would be (3V x amps) / efficiency = input watts, then input watts / 12 = input amps.

your probably using switchmode regulators, but they are not 100% efficient so they will pull a bit extra than just the load watts would say.

Title: Re: How to calculate total amps required for various voltages?
Post by: Ian.M on January 19, 2019, 02:05:38 pm

If there are any linear regulators, their input current is their output current + their quiescent current (usually negligible).  The difference between their input and output powers (calculated from V*I) is thrown away as heat.

Title: Re: How to calculate total amps required for various voltages?
Post by: shai on January 20, 2019, 12:02:12 am

Hi, thank you for the help, but I am confused what you mean by efficiency? I'm using a linear regulator, specifically the LM1117. Can you please post an example calculation on how I can figure out how much 12V amps I need to feed the PCB if I have a 12V 2A component and a 3V 0.5A component?

Title: Re: How to calculate total amps required for various voltages?
Post by: sleemanj on January 20, 2019, 12:05:57 am

Since you are only using linear regulators, just sum all three (load) currents.  Input current = output current for a linear regulator, ignoring quiescent.

Title: Re: How to calculate total amps required for various voltages?
Post by: Rerouter on January 20, 2019, 12:07:12 am

Linear regulators essentially burn off the voltage difference as heat, so a 0.5A load on 3V draws 0.5A at 12V, and burns 4.5W of heat in the process.

Be aware this will put your regulator into thermal shutdown if your not careful, that is a lot of heat.

Title: Re: How to calculate total amps required for various voltages?
Post by: james_s on January 20, 2019, 04:17:16 am

Think of linear regulators as automatic series resistors that adjust their resistance as needed to maintain the specified output voltage into whatever load that is on it. The current through any point in a series circuit is the same, therefore the current at the input of a linear regulator must equal the current drawn from the output.

Title: Re: How to calculate total amps required for various voltages?
Post by: bson on January 20, 2019, 07:19:25 am

Quote from: shai on January 20, 2019, 12:02:12 am

Hi, thank you for the help, but I am confused what you mean by efficiency? I'm using a linear regulator, specifically the LM1117. Can you please post an example calculation on how I can figure out how much 12V amps I need to feed the PCB if I have a 12V 2A component and a 3V 0.5A component?

Currents add regardless of voltage, so you need 2+0.5 = 2.5A.