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I’m trying to replicate the circuit in the below diagram with a solenoid drawing 1.85A at 36V.

enter image description here

First question:

I think the TIP120 transistor should be fine, as it can handle switching up to 60V and 5A - is that correct?

Second question:

How do I determine the specs of the diode in parallel with the solenoid? It needs to be able to withstand any reverse voltage generated when power to the solenoid is cut off, right? How do I go about determining that?

Third question:

Let’s say I want to expand this circuit with an additional seven sets of solenoids, transistors, and diodes for a total of eight. Each is being controlled by a single digital pin on the Arduino. The power supply would need to be capable of supplying ~15A at 36V, correct? Would I need to make any changes to the components I’m using in the single-solenoid test?

agaricaceae
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2 Answers2

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The TIP120 is a Darlington transistor and might lose about a volt when switched on. This means it might dissipate 1.8 watts when powering the solenoid. Try choosing a smaller package size N channel MOSFET. You can get tiny ones that will drop a few tens of milli volts when taking a load of 1.8 amps.

The diode is only reverse biased when the transistor is on hence it needs to be rated at about 1.5 times the supply voltage. When conducting it needs to be rated to take a peak current of 1.8 amps. More than likely a 1N400x will be fine.

I would rate the power supply possibly 20% higher in current maximum. If you are wanting 8 channels then there is an octal MOSFET that might fit the bill.

Andy aka
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  1. The diode will turn on in the forward direction (with the inductive energy) when the switch turns off, so it needs to handle the voltage applied across the solenoid and transistor plus margin (1.5-2x). (You show 12V in the schematic, but reference 36V in the text, not sure which is is.) It should also be able to handle the full solenoid current.

  2. The transistor you show is OK, but a logic-level FET may be a better solution. Faster, easier drive, lower voltage drop.

  3. If you want to run a bunch of (say N) solenoids you will need a power supply that can supply N times the solenoid current plus margin at whatever voltage they are rated for.

John D
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