Base resistor and MOSFET + Rectifier Diode Selection for Solenoid actuation?

protoengineer

Oct 7, 2018
2
Hi all,

Problem:
I have to calculate the base resistance required between a voltage supply and the base leg of a MOSFET transistor, given the following parameters:

1) voltage supply to the base leg is 5V and the emitter leg is connected to ground with the voltage supply.
2) the collector leg is connected to a rectifier diode and solenoid (inductor) which are in parallel with each other.
3) the solenoid and diode are connected to a separate 12V voltage supply which is connected to ground with the emitter leg and base voltage supply.
4) the solenoid operates at 24W with an equivalent 6ohm resistance.

I also have to figure out what transistor and what diode I can use for the system.

My attempt:

So, I assumed that you could use KVL on the left side of the circuit, with Ohm's Law V=R*i -> 5=R1*i1.
I also assumed that using the equivalent resistance of the solenoid, the current across it can be calculated with P=V*i -> 24=12*i -> i_L1=2A.
So, using KCL, I know that if I choose a diode with a 1A rating, the current into the parallel connection should be 2A+1A=3A.
If I then choose a MOSFET transistor with a gain of 1000, the current out of the resistor should be i1=3mA.
Thus, using Ohm's Law again, with V=R*i -> 5=R1*0.003 -> R1=1667 Ohm.
I know that a 1N4004 has a 1A rating and a TIP120 has a 1000 gain, so if the system uses two standard resistors in series in place of R1 with resistances of 1600 Ohm and 68 Ohm, with a 1N4004 and a TIP120, the solenoid should operate at 23.97W and 12V. The power on the resistors is less that 15 mW combined so a 250 mW rating on the resistors would not be overloaded.

Can someone please tell me if I did this correctly? Thank you very much.

duke37

Jan 9, 2011
5,364
You have shown a NPN transistor, not a MOSFET.
Which one do you intend to use?

The current through the solenoid can be calculated I=V/R. You seem to have this right.
The base current will be less than this by the current gain of the transistor which could be as low as 10. It certainly will not have a gain of 1000. The TIP120 is a Darlington which is two transistors in one package and could have a gain of 1000.
You will need a cosiderable base current, R1 can be calculated from R=V/I where V= 5V minus base/emitter volts.

The 1N400x diode is rated at 1A but it will need to pass the full solenoid current for a short time. It may do this but why not get one well on top of the job.

If you chose a MOSFET then the gain is essentially infinity and any value of R1 will do, a value of 1k may be used to prevent oscillation. The MOSFET will need to turn on fully with 5V drive so a sensitive gate would help.

Where did you get the 2A+1A = 3A from? The 1A rating of the diode is the current which should not be exeeded, it does not control the current, just allows current to pass in one direction and not the other.

protoengineer

Oct 7, 2018
2
You have shown a NPN transistor, not a MOSFET.
Which one do you intend to use?

The current through the solenoid can be calculated I=V/R. You seem to have this right.
The base current will be less than this by the current gain of the transistor which could be as low as 10. It certainly will not have a gain of 1000. The TIP120 is a Darlington which is two transistors in one package and could have a gain of 1000.
You will need a cosiderable base current, R1 can be calculated from R=V/I where V= 5V minus base/emitter volts.

The 1N400x diode is rated at 1A but it will need to pass the full solenoid current for a short time. It may do this but why not get one well on top of the job.

If you chose a MOSFET then the gain is essentially infinity and any value of R1 will do, a value of 1k may be used to prevent oscillation. The MOSFET will need to turn on fully with 5V drive so a sensitive gate would help.

Where did you get the 2A+1A = 3A from? The 1A rating of the diode is the current which should not be exeeded, it does not control the current, just allows current to pass in one direction and not the other.

Thanks! I am teaching myself and I was completely clueless, so thanks for the clarifications. I intend to use the n-type enhancement MOSFET. I understand now that the value of the resistor at the "gate" leg does not really matter too much.

Thanks for the info about the diode. Since there are 2A for the solenoid, do you recommend a specific diode for this that meets a 2A rated current?

I just learned that the MOSFET "gate threshold voltage" should be 5V for a 5V source when the "sink" leg is connected to ground, as in this problem. And also that the power across the drain-sink interface (P=I^2*R) should be less than (T_junction-T_ambient)/R_thermal, where I is the load current (2A) and R is the Rds(on) resistance across the drain-sink interface when the MOSFET is turned on by the 5V source, to prevent need of a heat sink.

So really I just need to find a MOSFET transistor that is "logic level" and doesn't heat up over the no-heat-sink condition; plus a diode that is rated at 2A. Which MOSFET and diode are good for this application? Thanks again!

also, I'm assuming the diode has to stay under a certain heat value as well

I have a question however:

If I did use a darlington TIP120, then would my analysis of needing 2500 ohm be correct? I would need the tip120, a 2500 ohm, a 2A rated diode?

Last edited:

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,507
In the resources section the are a couple of resources dealing with switching loads using transistors which will help you in the case you've drawn.

For a MOSFET, the threshold voltage (Vgs(th)) should be significantly less than the gate voltage you intend to apply.

duke37

Jan 9, 2011
5,364
I am not going through your TIP120 calculation, I have a scope to repair.
I have not used logic level MOSFETs, you can look up the data.
A common family of 3A diodes is 1N540x

Audioguru

Sep 24, 2016
3,650
The datasheet for most Mosfets show that when the gate-source voltage is at the "threshold voltage" then the Mosfet is almost turned off. You want the Mosfet to turn on so look at the gate-source voltage when it is turned on.

BobK

Jan 5, 2010
7,682
And please learn the correct names of the pins. There is no sink pin on a MOSFET. Perhaps you meant source, which is, of course, the opposite of sink!

Gate threshold voltage is typically the voltage that will cause 250 microamps to flow from drain to source.
Yes, current flows from the drain to the source in N-channel MOSFETS, and more descriptively from the source to the drain in P-channel MOSFETS.

Bob

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