ULN Reminder

[JonnyMac]

As Halloween approaches and everyone is busy trying to finish props, a few of you are experiencing the nauseatingly acrid aroma of a blown ULN chip.  My hope here is to prevent some of that.  If you look at a ULN spec sheet you'll find a graph like this:



Note the yellow dots that I've added on the right side of the chart; those dots intersect the 100% Duty Cycle line (full on) which is how the ULN is used in the Prop-1, Prop-2, and Prop-SX.  The curved lines with numbers in them are the number of simultaneous outputs.  So here's what this boils down to:

1) ~500 mA
2) ~475 mA
3) ~350 mA
4) ~300 mA
5) ~250 mA
6) ~225 mA
7) ~200 mA
8) ~175 mA

What you need to do is add up the total number of simultaneous outputs that will be on at the same time then refer to the chart above -- that will tell you the maximum current you can have per outputs.  For example, a customer just called and wanted to know if he could run a 350 mA LED (that's bright!) and two coils on a stepper motor.  It turns out that the stepper coils only need 170 mA each and so with three simultaneous outputs we are still safe.

For valves and relays you will often see a spec in watts or VA.  To determine the current required by this valve or relay, divide the voltage specification by the watts (or VA).  For example, a 12 volt relay that is rated for 3 watts (or VA) will draw 250 mA from the Prop-1 (3 / 12 = 0.250), and this is safe as long as you're not going to run more than five of these at a time.