planning to drive a Solenoid through N channel MOSFET, through a 595 shift register..

Thread Starter

Nick_G94

Joined Sep 18, 2022
2
Hello everyone, I have designed a circuit with an arduino nano and two shift registers daisy chained, pin QA of the first shift register is connected to an irf520 MOSFET gate, the source connected to gnd, and drain connected to a solenoid which is connected to gnd. the code simply cycles through an array of 8bit binary numbers (1, 2, 4, 8, 16, 32, 64, 128) so that one led is on at a time. I am powering the arduino with a 5v supply, and a seperate power supply for the mosfet and solenoid (benchtop power supply, with adjustable voltage and current).


The issue is that the seperate power supply for the mosfet and solenoid is outputting 0.680A when it needs to be at 1A (when i test the solenoid leads with my multimeter the benchtop supplies the 1A and the solenoid clicks no problem) so I cant seem to get enough current to the solenoid, even though when I do a meter reading on the power rails of that board the power supply gives out 1A and I read one amp.

Also when the pin QA is high the voltage on QA to gate was at 4.8v when the irf520 datasheet says the threshold voltage is between 2-4v so i wanted to reduce this, I started with a 10k resistor, that only brought it down slightly, i ended up on a 47k (the highest I have right now) which got the gate voltage down to 4.5v which is still too high.

I will post a picture of the circuit setup below, does anybody know the best way I could go about reducing the voltage on the MOSFET gate, and increase current to the solenoid through the MOSFET?

one last thing, completely ignore the MOSFET above, it is only the MOSFET underneath, with the green wire that is being used in the circuit.

thanks for your time.
 

Attachments

Last edited:

Ian0

Joined Aug 7, 2020
10,276
and drain connected to a solenoid which is connected to gnd.
The solenoid should be connected between the drain and the +12V supply.
All the grounds should be connected together.
Don't forget the flyback diode across the solenoid.

You should use a logic-level FET if you are driving its gate from logic levels. (IRL520 etc.)
 

LowQCab

Joined Nov 6, 2012
4,296
You are going backwards.
The Threshold-Voltage of the FET is the Voltage at which the FET is just barely starting to turn on.
The maximum Gate-Voltage is probably more than ~15-Volts, and could be as high as ~30-Volts.

You need a "Logic-Level" FET, or a small booster Transistor to switch the Gate..
.
.
.
 

Thread Starter

Nick_G94

Joined Sep 18, 2022
2
You are going backwards.
The Threshold-Voltage of the FET is the Voltage at which the FET is just barely starting to turn on.
The maximum Gate-Voltage is probably more than ~15-Volts, and could be as high as ~30-Volts.

You need a "Logic-Level" FET, or a small booster Transistor to switch the Gate..
.
.
.
You are going backwards.
The Threshold-Voltage of the FET is the Voltage at which the FET is just barely starting to turn on.
The maximum Gate-Voltage is probably more than ~15-Volts, and could be as high as ~30-Volts.

You need a "Logic-Level" FET, or a small booster Transistor to switch the Gate..
.
.
.
I have looked at the datasheet for the IRL520, under absolute maximum ratings, i see "gate to source voltage" at 16v, the IRF520 has 20v, im not sure which parameter tells me what voltage i need to drive the IRL520 (i dont have them yet) but I will replace the IRF520 with a IRL520 asap, I plan on running a bunch of MOSFETs and solenoids in parallel soon using a 36v switching powersupply, so the voltage should stay the same across those MOSFETS, but from what I know you can seperate the output voltage across 3 terminals so it shouldn't burn them out.

To use a booster transister I think I would need one PNP but that would invert the signal, so then I would need either a NPN or an inverter to reinvert the signal back to normal, I rather avoid doing that if I can to save money and space, but if need be ill try that.
 

Ian0

Joined Aug 7, 2020
10,276
Look at the “Typical output characteristics” graph, using the line for a gate voltage of 4V. (You have a drive of 5V, but there is a ±1V variation in gate threshold, so assume the worst case).
Then find the solenoid current on the y axis and read off the voltage drop on the x axis.
Multiply the two together and you will know how much power the MOSFET will dissipate. If it is more than 1W you will need a heatsink.
 

Alec_t

Joined Sep 17, 2013
14,408
I plan on running a bunch of MOSFETs and solenoids in parallel soon using a 36v switching powersupply, so the voltage should stay the same across those MOSFETS,
Unless the power supply is rated to handle the sum of the peak currents of all the solenoids in parallel, the supply voltage will most likely drop when the supply shuts down to protect itself.
 
Top