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# How do i determine mA for any given schematic in a book or websitewith parts and voltage only.?

AS a home hobbiest only I have a problem determineing appropiate amps for any given schematic.

I dont have a problem with construction just determining propper amps so to be sure of success.

tanks kbyrne6925@gmail.com

## Discussions

Don't add up the resistances - That would give you a meaningless value.

What you need to do is identify the outputs of the circuit. This is where the major power usage is going to occur. For example, :-

An amplifier will drive a speaker with a certain number of watts.

A high power LED driver will be designed for LEDs with a certain wattage.

A motor drive circuit will drive a current into the motor windings. The power (in watts) will be the square of the current times the resistance of the coils.

A small LED driver will be driving a typical LED at 20mA. If there are 10 LEDs, that's 200 mA.

The power use of the rest of the circuitry will usually be much less than the output stage. (Microprocessors are an exception to this.)

Buy a variable power supply with both volts and amp gauges. Set the voltage at the proper amount then read the amp gauge. Now buy a wall wart higher than the amp reading.

Ohm's Law. Amps = Volts / Ohms. Ohms = Volts / Amps. Volts = Amps X Ohms. Also Watts = Volts X Amps.

5 mosfet transistors into darlington pairs (5) into 1 jfet transistor into amplifier

LM386 circuit. Where to measure from.?

The output - whatever's connected to the LM386 - will be where the power is. Everything else will be negligible compared to this. LM386 is 1/2 watt max output so work out 2/(supply voltage), add on 5% for the other bits of circuit and you'll have your maximum current draw.

For any given schematic do I add all resistance to get ohms in formula.

NO.