Controlling Relay with mosfet

Thread Starter

norova

Joined Feb 8, 2023
7
Hello,

I'm working on an existing project where there is a relay that is "sinked" to ground to enable/disable it.
The existing solution did this, but now I need to design a similar ciruit that does it accordingly.

Relay is 12VDC OMRON type

I curious, could this be done with a PMOS like ->

1675862962645.png

Im using the output of a 555N timer to set the Gate of the PMOS

Regards,
David
 

Thread Starter

norova

Joined Feb 8, 2023
7
VCC = Battery Voltage : 10.8 - 14.15V

The relay needs to be on until voltage drops below Vtrigger voltage 1/3 5V 1.8V appox, and connect back again at VSense 2/3 approx 3.6V
1675864054400.png
 

ericgibbs

Joined Jan 29, 2010
18,849
hi n,
This roughly simulates the switching of a 12V relay with a P MOS, who's Gate is only 0V or 5v.
To turn Off the PMOS the Gate will have to be at say +12V
E
EG57_ 561.png
 

crutschow

Joined Mar 14, 2008
34,452
Is this what you want?:
The circuit component tolerances will, of course, affect the actual trip point.
(Note that pin CV should not be grounded or the circuit will not work.)

1675870955088.png
 

sghioto

Joined Dec 31, 2017
5,390
The relay needs to be on until voltage drops below Vtrigger voltage 1/3 5V 1.8V appox, and connect back again at VSense 2/3 approx 3.6V
If you must use a PMOS then you need an inverter to switch properly because of the voltage differences.
1675872298672.png
Or just use the inverter to drive the relay like this. Q2 can be a NMOS also.
1675872428255.png
Depending on the relay coil current you might be able to drive it directly from pin 7 of the NE555.
 

crutschow

Joined Mar 14, 2008
34,452

Thread Starter

norova

Joined Feb 8, 2023
7
Hello again,
Im also thinking of using the same output for the 55 timer to power all the comparators in the circuit, these are used to indicate voltage level on a leds matrix.
1675926456454.png
 

DC_Kid

Joined Feb 25, 2008
1,072
The 555 is used as a comparator to detect a low and high voltage from a battery.
So the function is, an active band from low threshold-1 to high threshold-2? Like a batt charger?

Is the relay a SPST? If so then is the mechanical relay even needed?

Do all the components run from this batt, if so will they run stable across the volt range?
Beauty of the 555 is that it can sink and source current on it's output, but you really need a way to run it stable across the batt volt range (assuming it all runs from the batt). Maybe find a small dc-dc IC to run things at 5v, and if not then maybe just a linear reg.



If it's just a batt monitor, I did one a few years ago, but it's not a band type, just a single low threshold. But descriptions this far sounds like a batt charger? If it's a charger, just be sure to monitor batt voltage aft of a resistor to batt, otherwise your sense will just see whatever supply you give the batt, and then it flutters.

The LM358 can provide the active band with hysteresis feedback configuration. I just happend to clamp one input of the 358 for a low threshold, and low enough clamp volts to be well under that threshold.

 
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