# Power Efficient Circuit

#### j0999

Joined Mar 4, 2019
18
Greetings,
I need to operate 40 independent solenoid devices(12V 250mA each) through a single circuit.
To turn them ON, 12V supply is required.
To keep them ON, 4V supply is the minimum requirement.
Since these solenoid devices remain ON for long duration of time I need to make a circuit that provides 12V for about 0.1ms (to turn on solenoid device) and then provide 4V (to keep them on).
Currently I'm using IRFZ44N MOSFETS (controlled switching through 74HC595) that are continuously providing 12V to the solenoid devices to keep them ON. Every solenoid device is independently operated and there is an instance where maximum 15 are continuously ON for 30 minutes.
Thus a lot of power is being wasted.
Thank You.

#### ericgibbs

Joined Jan 29, 2010
9,768
hi jo,
Welcome to AAC,
Do you use two supplies, one for the 12V and one for 4V.??
E

#### Dodgydave

Joined Jun 22, 2012
9,066
You could put a series resistor in with the coil, this would lower the current drain and pull in voltage across coil, so the mosfet can put full supply to the coil, and when the fet is off the solenoid is on low voltage,,, choose a resistor of twice the coil resistance.

#### ericgibbs

Joined Jan 29, 2010
9,768
hi Dave,
As I understand the TS, he also wants to turn them off individually, perhaps he could confirm.
E

#### ronsimpson

Joined Oct 7, 2019
592
controlled switching through 74HC595
I am assuming you have a micro computer involved.
Turn on the MOSFET for 1mS or maybe 10mS then, have the MOSFET on for 1/2 to 1/3 of the time. 1mS on 2mS off. As long as the frequency of ON/OFF is faster than the response time of the solenoid.
-----edited----
The 74HC595 has a output enable pin. If you disabled the part for 1/2 to 3/2 of the time all the MOSFETs will be off for that time. Many micros have a timer that can drive the OE pin directly with no software overhead. You will need to disable the timer and leave all the MOSFETs on, then change the data, wait for 10mS, then go beck to running the timer.

Last edited:

#### j0999

Joined Mar 4, 2019
18
hi jo,
Welcome to AAC,
Do you use two supplies, one for the 12V and one for 4V.??
E
Hello Eric,
Thank you for your humble welcome.
I am using a single power supply 12V then a 5V regulator to step down the voltage to 5Volts.

#### j0999

Joined Mar 4, 2019
18
You could put a series resistor in with the coil, this would lower the current drain and pull in voltage across coil, so the mosfet can put full supply to the coil, and when the fet is off the solenoid is on low voltage,,, choose a resistor of twice the coil resistance. View attachment 197686
Thanks alot for your time and efforts for explaining me with a diagram, Dave. According to me if we do this then the voltage will be dropped continuously. What I need to do is to first provide a "trigger voltage "of 12V for a shot time say 1ms. Then provide a "maintenance/ minimum voltage" of 4V for a long duration say 30minutes to keep it in 'ON' state. Then turn it 'Off" by providing 0V. Please correct me if I misinterpreted your solution. Thank You.

#### j0999

Joined Mar 4, 2019
18
Hello Eric,
Thank you for your humble welcome.
I am using a single power supply 12V then a 5V regulator to step down the voltage to 5Volts.
currently there is a ferrite bead to isolate the two grounds. But I could use a DC to DC isolation if it is recommended.

#### j0999

Joined Mar 4, 2019
18
hi Dave,
As I understand the TS, he also wants to turn them off individually, perhaps he could confirm.
E
May I know what do you mean by "TS" . Yes, I do want to turn them all on or off individually / separately. Every solenoid device's operation is independent of each other.

#### ericgibbs

Joined Jan 29, 2010
9,768
I am using a single power supply 12V then a 5V regulator to step down the voltage to 5Volts.
hi,
Assuming the solenoids are 12V at 0.25A ie: 48R
At 5V the current draw will be ~100mA, so for 40 solenoids you require ~4Amps
So 12V down to 5V at 4A. means 7V * 4A= 28Watts in the regulator,
The problem with that 12V down to 5V, means you are going to 'waste' power in a linear regulator

Have you consider using a SMPS from say mains to 5V at say 5A or a SMPS module from 12V down to 5V.?

Is it should be possible to design a suitable MOSFET pair to switch the 12V or 5V to a solenoid.

E

BTW: TS is the Thread Starter ie You.

Update:

Please post a circuit showing how you are switching the 12V and 5V to the solenoids.

Last edited:

#### j0999

Joined Mar 4, 2019
18
I am assuming you have a micro computer involved.
Turn on the MOSFET for 1mS or maybe 10mS then, have the MOSFET on for 1/2 to 1/3 of the time. 1mS on 2mS off. As long as the frequency of ON/OFF is faster than the response time of the solenoid.
-----edited----
The 74HC595 has a output enable pin. If you disabled the part for 1/2 to 3/2 of the time all the MOSFETs will be off for that time. Many micros have a timer that can drive the OE pin directly with no software overhead. You will need to disable the timer and leave all the MOSFETs on, then change the data, wait for 10mS, then go beck to running the timer.
Thank You for your reply Ronsimpon. A micro processor STM32F103VCT6 is in use. It is performing many other functions as well. The switching of every solenoid should not be affected by operating any one solenoid. waiting for 10ms will turn ON or OFF all MOSFETS and solenoid devices connected to that 74HC595. Thus hampering the function. Moreover, I have been guided before that using MOSFETS for fast frequency switching(like PWN) is not recommended as it reduced the life of the MOSFET in use. Kindly guide if you think otherwise. Thank you

#### j0999

Joined Mar 4, 2019
18
hi,
Assuming the solenoids are 12V at 0.25A ie: 48R
At 5V the current draw will be ~100mA, so for 40 solenoids you require ~4Amps
So 12V down to 5V at 4A. means 7V * 4A= 28Watts in the regulator,
The problem with that 12V down to 5V, means you are going to 'waste' power in a linear regulator

Have you consider using a SMPS from say mains to 5V at say 5A or a SMPS module from 12V down to 5V.?

Is it should be possible to design a suitable MOSFET pair to switch the 12V or 5V to a solenoid.

E

BTW: TS is the Thread Starter ie You.
Looking at an instance for max load; 15 solenoids are operational.
So if 12V is supplied then amp usage is ~ 4Amp taking into consideration the spike due to solenoids.
If 4V is supplied then amp usage is ~ 2Amp max.
I have considered using 2 SMPS one for 12V and the other for 5V but I was stuck at the point where I have to shift from 12V to 5V. If and any instant the solenoid gets 12+5 = 17V it could burn the coil. If I turn off 12V then provide 5V then would the Solenoid remain on or turn off? Plus then 2 MOSFETS would be required for controlling 1 solenoid. Wandering off to find an alternative solution if any. Do you suggest me to use 2 SMPS with a MOSFET Pair? Or could there be a better solution ?
Best Regards
TS

#### ericgibbs

Joined Jan 29, 2010
9,768
hi jo,
Please post a circuit of your present circuit [ just one solenoid driver].
Are the solenoids in the high or low side of the MOSFET drivers.?
If the high side, can they modified to be in the low side of any driver, ie: all solenoids being grounded/0v at one end.??
E

#### j0999

Joined Mar 4, 2019
18
hi jo,
Please post a circuit of your present circuit [ just one solenoid driver].
Are the solenoids in the high or low side of the MOSFET drivers.?
If the high side, can they modified to be in the low side of any driver, ie: all solenoids being grounded/0v at one end.??
E
Hello Eric,
Currently all solenoids have a common VCC of 12V at the moment.
the switching is done with the ground through MOSFETS.
please find attached a circuit diagram of 1 solenoid driver.

If we go as you suggest then will have to give a common ground to all solenoids and with the use of pair MOSFETS control 12V or 5V.

using a n-channel MOSFET IRFZ44n .
Source is connected to the ground.
Regards.

#### Attachments

• 25.5 KB Views: 8

#### ericgibbs

Joined Jan 29, 2010
9,768
hi,
Got your opto isolator circuit, how are you selecting OFF, and 5V or 12V to the solenoid.?

#### j0999

Joined Mar 4, 2019
18
hi,
Got your opto isolator circuit, how are you selecting OFF, and 5V or 12V to the solenoid.?
Currently the circuit is not power efficient as in ON state 12V from solenoid is connected to GND1. This is done when ON state signal is given to the opto cuppler. Keeping the solenoid ON for as long as required by provideing steady 12V.

To turn off the solenoid , OFF state signal is given to the opto cuppler so the gate is also at OFF state thus the source and drain is not connected. So the colenoids are not connected to the GND1 or any other GND. Thus they remain in OFF state.

So at the moment only 12V is being supplied.

hunting for a way to provide 5V to keep the solenoid in ON state for long duration. This will require to redesign the PCB. So trying to understand the best possible technique.
Thank you
Regards

#### ericgibbs

Joined Jan 29, 2010
9,768
hi,
Looking at a possible circuit design, do you have enough logic pins available to switch 40 drivers, by using 2 pins per driver.?
Ie; pin1 for 5V , pin2 for 12V control.

Do you have a budget target cost figure for the project.?
E