Resistor Current?

Newbie electronics question

I'm sure this is very simple, but we've all to start somewhere.


When selecting resistors for the voltage divider from a 9V AC/AC adaptor, does the resistors current rating need to be higher than the current from the adaptor?

The adaptor I plan to use is the - IDEAL POWER Plug In Adaptor Power Supply, UK, 240 V, 10 W, 9 VAC, 670 mA


Harescti's picture

Re: Resistor Current?

I'll try and answer my own question with ohms law

If the mains adaptor output is 9v AC at 10W and the voltage divider uses these two resistors

100k (Should the resistor current rating be higher than 0.09mA)

10k (Should the resistor current rating be higher than 0.9mA)

As a newbie there are thousands of resistor choices and want to ensure I'm on the right track.



Robert Wall's picture

Re: Resistor Current?

Resistors are limited by their power rating, in other words the amount of heat that they can lose to their surroundings before they become damaged. The amount of current available from the supply has nothing to do with it. Therefore, you need to calculate the power being dissipated by the resistor given its resistance and the voltage across it.

Doing the sums very roughly - there's about 10 V across the 100 kΩ resistor  and about 1 V across the 10 kΩ resistor. Therefore:

The 100 kΩ resistor will dissipate V2/R Watts = 10 × 10 / 100,000 = 1 mW, the 10 kΩ resistor will likewise dissipate 1 × 1 / 10,000 = 0.1 mW.

The smallest wire-ended resistor you'll be able to buy will be rated at 0.1 W, which will be absolutely fine for both. You'll more likely end up using a 0.25 or 0.3 W 1% metal film, which will be fine too (and what was supplied with the emonTx V2 kit) and is what I'd recommend you use.

Harescti's picture

Re: Resistor Current?

Robert, Thanks taking the time to reply. Trevor

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.