I was reading through some pin outs in the back of my Commodore 64 manual when I saw this:


Reading online, I found a couple pages that also said (of the +5V pin) "maximum load for extensions is 100 mA."

What does that mean? That sounds to me like you can attach any device (or multiple devices) to the +5V pin, and each one will take the current it needs, as long as it adds up to less than 100 mA. But that doesn't make sense, I thought that the line puts out a constant amount of current, and if that amount is 100 mA, a device that only takes 3 mA would blow if connected!

 

That sounds to me like you can attach any device (or multiple devices) to the +5V pin, and each one will take the current it needs, as long as it adds up to less than 100 mA

That is correct. Lines don't "put out a constant amount of current".
Think of your domestic mains supply. It is capable of supplying 100A or so maximum, but a 5W lamp bulb, or your electric toothbrush, doesn't self destruct.

 

If it's a device that needs 3mA then 3mA is all that will be taken on the 5V supply. It's not constant current so you can put as many devices on the 5V pin a long as you don't exceed a total load of 100mA.

 

If it's a device that needs 3mA then 3mA is all that will be taken on the 5V supply. It's not constant current so you can put as many devices on the 5V pin a long as you don't exceed a total load of 100mA.

Ok, but now I have another question. On the datasheet for the 74ls04 (just for example) it gives typical (1.2 mA) and maximum (2.4 mA) supply current (when all inputs are high). So if the power supply offers 100 mA, how much will the chip take?

 

Ok, but now I have another question. On the datasheet for the 74ls04 (just for example) it gives typical (1.2 mA) and maximum (2.4 mA) supply current (when all inputs are high). So if the power supply offers 100 mA, how much will the chip take?

A power supply doesn't "offer" current. That 100 mA figure you are worried about is a rating; that is, it says how much current may be safely drawn from the power source. Power sources DO NOT force current through things; the current is determined by the power source's voltage and how much current the load draws at that voltage.

For example, a "12V 5A" power supply will output 12 volts at up to 5 amps. How much current flows from the supply depends entirely on what the load draws, NOT on any characteristic of the supply.

 

The datasheet for the 74LS04 is telling you that under the specified conditions the chip will typically draw 1.2mA but, because of manufacturing tolerances, may draw up to 2.4mA. No two chips are identical and the one you buy may not be 'typical'.
When designing your circuit you should allow for this current variation.

 

A power supply doesn't "offer" current. That 100 mA figure you are worried about is a rating; that is, it says how much current may be safely drawn from the power source. Power sources DO NOT force current through things; the current is determined by the power source's voltage and how much current the load draws at that voltage.

For example, a "12V 5A" power supply will output 12 volts at up to 5 amps. How much current flows from the supply depends entirely on what the load draws, NOT on any characteristic of the supply.

The datasheet for the 74LS04 is telling you that under the specified conditions the chip will typically draw 1.2mA but, because of manufacturing tolerances, may draw up to 2.4mA. No two chips are identical and the one you buy may not be 'typical'.
When designing your circuit you should allow for this current variation.

Ohhh, now I see. Thank you!