[SOLVED] how to reduce current(amperage)

hi
i am pretty sure this question has been asked before but since my english is not so good i dont know what should i search, sorry

i have a 3V 1000mA source, i need 3V 20mA, what should i do? something very simple please!
(the source has 1.5V 3V 4.5V 6V 7.5V 9V 12V all 1000mA)

by the way i dont know wattage of my resistors, i can only read the colors of them

What is the load? Normally a load will ony take what it needs so if you connect something that takes 3V at 20mA to a 3V 1A power supply it sill simply take 20mA. No extra circuitry is required.

Keith.

i dont think it will, it's an atmega32 micro-controller, (hey dont ban me! it's not microcontroller related, it's basic electronic, reducing current)
the source is a very cheap 'unregulated' 220>4.5V adapter, i'm not gonna regulate it, i'll take my chances... but the current is to high,
1000mA is gonna fry it, how do i reduce it?

When you have a source specified as 3v 1000mA it means that the output is 3v and can provide a current UP TO 1000mA (1 Ampere).
The power supply can't force the current to the load , if your load is 3v 100mA then it will just pull 100mA from your power supply, it is like a car that can run up to 100mpH, it doesn't mean that anyone who drives it has to run with that speed!!!!

Your load will only get more current if you apply more voltage to it that its specification but as long as you keep the voltage to the specified level no damage can be done.

That being said there are two types of variable power supply like the one you describe, the regulated ones in which the voltage is maintained at the set level (for example exactly 3v) no matter how much current you pull and there are the unregulated ones which are actually designed to have a voltage close to the set one (for example 3v) when the current it at its max (1000mA in your case) but at lower current the output voltage can be a couple of volts higher (4v or 5volts in this example).
Your psu probably mentions if it is regulated or not, if it isn't then there may be some devices that don't like this higher voltage output.
The is no need for a resistor unless you have a load (like a led) that requires a resistor to limit the current.

Alex

The same thing I said applies. The fact that it CAN deliver 1000mA doesn't mean it will draw that current if it doesn't need it. I would be more concerned about the voltage when the power supply is lightly loaded. Is it 4.5V or 3V rated? I think I would use a voltage regulator!

Keith.

thanks to both of you keith and alexan
no it's clear

While I was writing my answer you have posted more info on your load.

According to the datasheet the voltage range for the mcu is

–2.7 - 5.5V for ATmega32L
–4.5 - 5.5V for ATmega32

so give it a try , no resistor is needed and the mcu will not be damaged unless you go over the 5.5v , in the worse case it might not work because your voltage may be too low but at some point you have to buy a 7805 regulator to operate the mcu as it is intended to be used.

Alex

how is the regulator used? i guess i have one in my stuff somewhere around

---------- Post added at 13:54 ---------- Previous post was at 13:42 ----------

yes i found it:L7805CV
how do i use it?

For 7805

**broken link removed**

It is also shown in the datasheet
**broken link removed**

Alex

---------- Post added at 16:56 ---------- Previous post was at 16:55 ----------

Note that the regulator needs at least 7v input (2v more than the output)

can those 2 capacitors be replaced with anything else? dont have 33uf right now

actually these are 0.33u (330n) and 0.1u (100n) but you can use different values or even not use them at all although it is usually good idea to place a small cap in the output of the regulator (usually polyester MKT etc)

alexan_e said:

actually these are 0.33u (330n) and 0.1u (100n) but you can use different values or even not use them at all although it is usually good idea to place a small cap in the output of the regulator (usually polyester MKT etc)

Click to expand...

tanx again alex