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I'm having an issue with a TIP120 I'm using limiting current flowing through it.

I'm using the TIP120 to switch on/off a 12 V electromagnet. The circuit is working, but it appears the TIP120 is limiting the amount of current that is able to flow through the electromagnet. If I hook the electromagnet directly up to my bench supply I can get 290 mA of current (it has a 39 ohm internal resistance).

When I use the TIP120 to switch the current on/off the most current I can get through the electromagnet is around 240 mA, a 50 mA drop. I'm pretty sure the transistor is getting switched on all the way as I've experimented with everything from 220 ohm to 22 kilohm resistors on the base to the controlling voltage and the resistance to the base of the transistor isn't having any significant effect on the amount of current flowing through the magnet (collector to emitter).

Is this current drop expected? I don't see anything in the specification sheet of the TIP120 that lists an internal resistance that would be limiting the maximum current.

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    \$\begingroup\$ everything has a resistance... \$\endgroup\$
    – Solar Mike
    Commented Jul 30, 2018 at 14:29
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    \$\begingroup\$ A mosfet would probably be a much better choice than a darlington for this application. \$\endgroup\$
    – dim
    Commented Jul 30, 2018 at 14:37
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    \$\begingroup\$ A transistor doesn't introduce a current drop, so you won't find that parameter in the datasheet. But it will introduce a voltage drop, which means that your electromagnet will draw less current. The parameter that you're looking for the is the collector-emitter saturation voltage, Vce(sat) in the datasheet. This is the voltage dropped across the transistor when it is turned on fully. In your application this is probably around 0.7V which doesn't quite account for the reduction in current that you measure. You need to add a schematic to your question for us to work out why. \$\endgroup\$
    – Steve G
    Commented Jul 30, 2018 at 14:41
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    \$\begingroup\$ The Voltage drop only accounts for 18mA of Current drop. I think the rest of the current drop is coming from the resistance of the multiple jumper wires in use. I've found the alligator clip jumpers (2 ft) can add 2 or more Ohms each and the smaller jumpers wires add about 0.1 ohms each. \$\endgroup\$
    – Jeff Wahaus
    Commented Jul 30, 2018 at 15:22
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    \$\begingroup\$ While it's true that every real wire has resistance, the "resistance" of this transistor has a very small effect in comparison to its saturated \$V_{CE}\$. If you think of the transistor as having a resistance from collector to emitter then you will miss the big picture and be confused by the true behavior. \$\endgroup\$
    – Elliot Alderson
    Commented Jul 30, 2018 at 15:38

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I'm pretty sure the transistor is getting switched on all the way

The TIP120 is a Darlington transistor and NEVER can get switched "all the way". It will always "drop" at least 0.5 volts across it and reduce the voltage across your solenoid.

enter image description here

If you think about the picture above, for the right hand transistor to be "on" there must be about 0.7 volts at its base with respect to its emitter. That 0.7 volts is sourced via the left hand transistor from the collector pin hence, the collector can never be lower than about 0.7 volts and the right hand transistor can never be fully turned "on".

Here's an extract from the data sheet that shows what I mean: -

enter image description here

At 240 mA collector current \$V_{CE(SAT)}\$ is about 0.75 volts. And this assumes that your base driving circuit is delivering a current of about 1 mA.

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