I've already assembled a working version of the following circuit, minus the pulldown resistor. It has worked wonderfully so far, but another discussion on this forum made me realize that if I ever had 24V connected, but the Arduino was off, the Darlington base would be floating and strange things might happen.

So I want to add a pulldown resistor to the base, but I don't really know what determines the sizing. I vaguely remember hearing once that 10x the value of the base resistor is a good start. Are there other factors to consider? Obviously if the pulldown resistance is too low, it will act as a voltage divider and significantly reduce the voltage (and therefore current) through the base. What is the risk if it's too high?

Thanks in advance for your insights!

 

An unrelated question that drawing this schematic reminded me of is this:

When abbreviating resistor values with the letter "k" as shown above, is there any agreement on whether upper or lower case letters are preferred? Every time I look around online, I find numerous examples of both. Does it matter?

18K vs 18k
3K9 vs 3k9
etc.

 

R1 and R2 in the TIP120 already provide a pull-down resistor.
Why do you think you need an additional one?

 

Refer to the datasheet for the values of those internal resistors.

 

Doh! Hadn't noticed or thought about that. So it looks like I'm good on this circuit without the extra resistor. Thanks for pointing that out.

For future reference, if this hadn't been a Darlington with built in resistors, but a regular bjt, how would I size an appropriate pull down resistor?

 

There are two things going on:

1. Current through the base resistor will be divided into two parts. Part of it will go to the base and part of it will go to the pulldown. The puuldown needs to be large enough so it doesn't hog the currrent you need to turn on the transistor.
2. It needs to be small enough, so that if there is no driving signal it can discharge the base capacitance and keep the transistor off.

Based on the transistor you pick, a compromise somewhere in the middle is usually a straightforward choice.

 

The collector-base reverse leakage current at the highest junction temperature of interest is one of the factors determining the upper limit for the base-emitter.
The resistance value should be low enough to keep the base-emitter voltage below about 0.5V due to this leakage.

 

The official abbreviation for kilo is a lower case k.

http://en.wikipedia.org/wiki/Metric_prefix

ak