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Relay or MosFet to switch a 12V signal?

PhilN

Jul 13, 2021
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Hi all,
I am looking to provide a 12V signal when a control circuit switches a ground connection. Total current is a few mA (less than 200mA for sure).

The simplest thing to do would be to use the switched ground to ground a relay coil but I am concerned about the longevity of a relay in what will be a vibration prone environment so am thinking of using a Mosfet as the switch.

Does this look like it would do what I want and are the resistor values in the right ballpark?
Any suggestions or recommendations would be greatly appreciated.

upload_2021-7-28_11-16-18.png

Many thanks
Phil
 

PETERDECO

Dec 19, 2019
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You are going to have 12V on your output regardless of the position of your switch. In about 4 hours I will be at work with several schematics for what might work for you - unless someone else answers you first.
 

PhilN

Jul 13, 2021
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Hi Peter,
Wasn't sure if that would be the case or not so thank you very much.
Any suitable schematics would be greatly appreciated.
Regards
Phil
 

WHONOES

May 20, 2017
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See the attached circuit and plot.
If have understood you requirements correctly, the attached circuit should satisfy them.
Referring to the schematic, Q1 provides the switching function for you 12V signal.
R3 connected between gate and source keeps it switched off.
When S1, shown on the schematic as a voltage controlled switch, closes, the gate of Q1 is pulled towards ground thereby turning Q1 on and providing 12V to your load. When the switch is opened again R3 pulls the gate back up to 12V and turns Q1 off again.
See the switch graph for functionality.

Hope this helps.
 

Attachments

  • 12V switch Schematic.pdf
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  • 12V switch graph.pdf
    11.6 KB · Views: 7

PhilN

Jul 13, 2021
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Jul 13, 2021
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See the attached circuit and plot.
If have understood you requirements correctly, the attached circuit should satisfy them.
Referring to the schematic, Q1 provides the switching function for you 12V signal.
R3 connected between gate and source keeps it switched off.
When S1, shown on the schematic as a voltage controlled switch, closes, the gate of Q1 is pulled towards ground thereby turning Q1 on and providing 12V to your load. When the switch is opened again R3 pulls the gate back up to 12V and turns Q1 off again.
See the switch graph for functionality.

Hope this helps.
Many thanks for this.
However I am not sure I am following you exactly here. S1 in my drawing is an external ground connection and is external to this device, this ground connections needs to switch the the Mosfet to allow the 12V output.
Also what is V2 on your schematic?
 

PhilN

Jul 13, 2021
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Many thanks for this.
However I am not sure I am following you exactly here. S1 in my drawing is an external ground connection and is external to this device, this ground connections needs to switch the the Mosfet to allow the 12V output.
Also what is V2 on your schematic?

This is what I was meaning, does this replicate your schematic?
upload_2021-7-28_14-3-14.png
 

WHONOES

May 20, 2017
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Almost. R1 is your load and should be connected directly to ground and not go through the switch. The value depicted is fictional and is only there to show where your load goes.
Does that make sense. Other than that you are correct. Note, both the grounds should be connected together, if one is floating then it will not work.
V2 on my schematic does not really exist it is there just to operate the voltage controlled switch in my simulation as there is no provision in it for a manually operated switch. You will be using a conventional switch.
Hope that's not too confusing.
 

PhilN

Jul 13, 2021
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Almost. R1 is your load and should be connected directly to ground and not go through the switch. The value depicted is fictional and is only there to show where your load goes.
Does that make sense. Other than that you are correct. Note, both the grounds should be connected together, if one is floating then it will not work.
V2 on my schematic does not really exist it is there just to operate the voltage controlled switch in my simulation as there is no provision in it for a manually operated switch. You will be using a conventional switch.
Hope that's not too confusing.

OK thanks, so in reality the bottom of R1 and the ground connection go to the same point but will only be be grounded and the output switch the 12V output when the switch is closed and completed the ground connection for the whole circuit like this:

upload_2021-7-28_18-6-23.png

Thanks for all your help with this

Phil
 

WHONOES

May 20, 2017
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No, R1 which is your load does not go through the switch, it goes directly to ground.
 

PhilN

Jul 13, 2021
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I don't have a load (R1) as such, I am merely looking to provide a 12V signal on the output when S1 has a ground connection.
S1 is external to this circuit but physically exists and all it does is to make a ground connection, I am looking to use this ground connection to ground the Mosfet and provide the 12V output.
Apologies if I wasn't clear.
 

WHONOES

May 20, 2017
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The output of your circuit is going to experience a load of some sort. You inferred that it could be up to 200mA though most likely less. That is all R1 represents and is not meant to be absolute, it is just an indication of where whatever is attached to your circuit should go.
The ground end of the 12V supply needs to be attached to directly to ground and not go through the switch otherwise the circuit will be permanently enabled.
If the ground connection on the bottom of your switch is totally independent to the rest of your circuitry as in part of another system then all bets are off as there could be a huge potential difference between the two which could be catastrophic.

In order for your circuit to function correctly, it needs to be organised exactly as shown in my schematic ignoring V2 which is just a signal generator used to drive the virtual switch in my circuit. R1 represents whatever load you choose to put on the output.
 
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