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 ->



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

Regards,
David

 

hi norova,
Welcome to AAC,
You cannot power a 555N with +33V, it is 18V max.
Why not use an N MOS FET?
E

 

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

 

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

 

hi n,
Check this simulation circuit for accuracy.
E

 

Why is the CV pin grounded?

 

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.)

 

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.

Or just use the inverter to drive the relay like this. Q2 can be a NMOS also.

Depending on the relay coil current you might be able to drive it directly from pin 7 of the NE555.

 

A simple Opto-isolator could have done it also.

 

If you must use a PMOS then you need an inverter to switch properly because of the voltage differences.

A simple Opto-isolator could have done it also.

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

To get those trigger points the 555 is needed (or a window comparator).

 

To get those trigger points the 555 is needed (or a window comparator).

Correct. That's the circuit in post #3, although it has an error showing CV pin5 at ground.

 

Why would not a Opto with output trigg3er point work?
The base resistor can be adjusted for the trigger point.

 

Why would not a Opto with output trigg3er point work?
The base resistor can be adjusted for the trigger point

How definitive is the the switching? Needs to switch at 10.8 volts and reset at 14.14 volts according to the TS

 

Why would not a Opto with output trigg3er point work?
The base resistor can be adjusted for the trigger point.

The trigger point is temperature sensitive
It also needs hysteresis to meet the TS's requirements.

 

All nice examples '

what about this?

 

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.

 

What's the function of 555 in the ckt? I assume it's not frequency.

 

What's the function of 555 in the ckt? I assume it's not frequency.

The 555 is used as a comparator to detect a low and high voltage from a battery.

 

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.