Why voltage drop?(74HC595 & MOSFET)

I want to control many electromagnets.
‘74HC595’ and ‘FQP50N06 (N_mosfet)’ were used.
Power source is used, adapter (5v 3a) and SMPS (12v 30a).
current of each electromagnet is 1.3a(9ohm)

However, when the electromagnets are operated at the same time, a slight voltage drop occurs.

Is it a problem with ‘74HC595’?

Is it a problem with '74HC595'?

No it is a problem with your power supply, and perhaps inadequate or no bulk decoupling.

I don't think you have your diodes wired correctly.

Also, R1 to R8 could be much smaller, like 1KOhm, vs 2MegOhm, to ensure the MOSFETs turn on & off quicker.

Where are you measuring the voltage?

30A is quite a lot, and the voltage drop down wires is significant...

I bet if you measured right at the psu it would be OK



That means, capacity of the capacitor?


I tried 10k ohm. But the result was the same. (The same was true without resistance.)
Connect the diode as you say (noise is reduced. Thanks)

This circuit adjusts the magnetic force from 0 to 255 steps with the PWM signal.
Therefore, when operating with a low resistance value, noise is generated. (Vibration of electromagnet)

Is the wire thickness problem?
I’m testing it with a breadboard wire. (Connected to the 12v SMPS by thick lines)

Change the gate resistors to 1k (not 10k, not 2Megohm), as CrossRoads already said.
That reduces switching losses, but not "ON" losses.

The diodes go across the solenoids. Not in series with the drain.

The indicator LEDs draw >12mA.
Not good.
That reduces the HIGH voltage level on the output of the 74HC595.

That FQP50N06 fet is NOT a logic level mosfet.
Not suitable for 5volt logic.
5volt on the gate could be still ok for the 1.3Amp solenoids.
But adding a 12mA load to the outputs of the 595 will drop gate drive voltage well below 5volt.
Change the 220ohm LED resistors to 1k (minimum). I would use 4k7 or 10k.

Fix those resistors to be 1k or lower - otherwise you risk burning up the MOSFETs on PWM
by operating in the linear area.

The diodes must be across the load coils, by not doing so you may have already have damaged the MOSFETs

If the solenoids are acoustically noisy you need a higher frequency PWM.

When you use 2M resistors and PWM you are operating in the linear range so that power is wasted in
the MOSFETs and might fry them if not using heatsinks.

Of course it could be that your electromagnets are not designed for PWM in which the way you are
doing things is superior, but has a number of issues - firstly 2M is too high
a gate resistor, it allows lots of noise pickup. An RC low pass filter with more sensible values can be used.

Your way of doing things exposes you to device variations in threshold voltage which have a wide

Normally you'd either use PWM and switchmode, or you'd use a programmable current sink using a sense
resistor and feedback for analog control.

OPs revised schematic.

Tom… :slight_smile:

:slight_smile: :slight_smile: :slight_smile: :slight_smile:
Right now, this problem has been solved!!!
Just connect the 12v connection to one breadboard.
So I connected the other terminals of 12v SMPS.
It seems to be related to the thickness of the wire.
Thank you so much for your kindly advice! :slight_smile: