# Using MOSFET to turn on/off auxillary electronic circuit

1. ## Using MOSFET to turn on/off auxillary electronic circuit

I have custom-build e-bike which is basically a motor, battery, controller and an LCD screen with 3 buttons. When I push ignition button, the LCD sets voltage on one of it's signal wires (blue) to 51 - 67.2V (BAT+, depending on battery capacity) which is then wired to the controller.

I would like to sense voltage on this ignition wire and switch on/off an external dc-dc converter (8-72V to 5V5A) in order to charge my phone while riding a bike. One downside is that I can't pull too much current on this signal wire - dc converter need to be wired directly to the battery by mosfet or something else.

After 3 minutes of idle, LCD automaticly turns e-bike off (0V on ignition wire) which is okay, i want to charge my phone only while riding.

I need a solution that uses no power when switched off in order to not drain the battery. I'm thinking of using a MOSFET as a switch where this ignition wire would be connected to the gate of MOSFET. As far as I understand I need a MOSFET with Vgs of at least 67.2V? Could anyone point me at a right mosfet? Wiring schematic and any tips on this would be great too. Thanks!

2. ## Re: Using MOSFET to turn on/off auxillary electronic circuit

Hi,

I need a MOSFET with Vgs of at least 67.2V?
Unlikely.
V_GS is the control voltage for the Mosfet. I can't see why the control voltage needs to be that high.
And you will hardly find a Mosfet with that high V_gs.

There are logic level Mosfets which can be controlled with V_GS if 0V/5V.
Standard Mosfets usually are controlled with 0V/12V.

Klaus

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3. ## Re: Using MOSFET to turn on/off auxillary electronic circuit

I suppose a simple rocker switch would be just too 20th
century?

There are many load switch products although most are
lower voltage. There are high side PMOS and NMOS
controllers for the higher voltage applications.

1 members found this post helpful.

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4. ## Re: Using MOSFET to turn on/off auxillary electronic circuit

Hi,

Whatever you do, that battery voltage would need to be reduced for most devices from what I know.

Typical one is voltage reference and resistive divider coming from ignition line into a comparator controlling MOSFET gate. Comparator, reference and MOSFET would need a regulated voltage as the highest you'll get is a 30V comparator.

I would look at discrete IC solutions, they should be better and far smaller than homebrew circuits. Power management is one term that comes to mind. I'm surprised there are no DCDC converters with an enable pin, that would make it a "simple" case of ensuring the ignition voltage were divided down with a resistive divider into whatever the enable pin can accept as min/max for ON.

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Hi,

Originally Posted by dick_freebird
I suppose a simple rocker switch would be just too 20th century?
You truly deserve a prize.

5. ## Re: Using MOSFET to turn on/off auxillary electronic circuit

Thanks for your answers. What if I use a relay with a reverse voltage diode for safety? I see some of them have coils that operate within my voltage range. I would like not to use any additional switches like rocker switches.

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6. ## Re: Using MOSFET to turn on/off auxillary electronic circuit

A relay would work.

7. ## Re: Using MOSFET to turn on/off auxillary electronic circuit

OK, so I used a HF41F/048-ZS relay with reverse bias diode (1n4007) in parallel. Now the problem is when I apply 67V to the coil, relay switches ON, but after I turn off coil it stays in ON state (i hear clicking of the relay when connecting and disconnecting voltage to the coil). I have to knock the relay with a screwdriver a few times in order to switch OFF. What may be the cause of this? Is this poor quality relay?

8. ## Re: Using MOSFET to turn on/off auxillary electronic circuit

Hi,

Could you draw a schematic of some sort of how you have all this set up, please?

Another thing, I don't think you should be applying 67V to a 48V coil.

Chatter would be when there is a fluctuating voltage still applied, I'd guess. Chatter at turn on is when there isn't quite enough voltage to turn it on; at turn off - I'm unfamiliar with that.

9. ## Re: Using MOSFET to turn on/off auxillary electronic circuit

Originally Posted by lypse
OK, so I used a HF41F/048-ZS relay with reverse bias diode (1n4007) in parallel. Now the problem is when I apply 67V to the coil, relay switches ON, but after I turn off coil it stays in ON state (i hear clicking of the relay when connecting and disconnecting voltage to the coil). I have to knock the relay with a screwdriver a few times in order to switch OFF. What may be the cause of this? Is this poor quality relay?
It sounds like the relay contacts are welding together by what you describe.

10. ## Re: Using MOSFET to turn on/off auxillary electronic circuit

If the load has a large filter capacitor, the high inrush current may be welding the contacts.
You may need to add a small resistor in series to limit the surge current.

And you need a resistor in series with the coil to reduce the coil voltage to 48V.
For that relay with a 10.6k ohm coil, you should use about a 4.2k ohm, 1/4W resistor.

11. ## Re: Using MOSFET to turn on/off auxillary electronic circuit

Originally Posted by crutschow
If the load has a large filter capacitor, the high inrush current may be welding the contacts.
You may need to add a small resistor in series to limit the surge current.

And you need a resistor in series with the coil to reduce the coil voltage to 48V.
For that relay with a 10.6k ohm coil, you should use about a 4.2k ohm, 1/4W resistor.
I will add 10kOhm resistor to the coil. What resistor value should I use on the load side?

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12. ## Re: Using MOSFET to turn on/off auxillary electronic circuit

Originally Posted by lypse
I will add 10kOhm resistor to the coil. What resistor value should I use on the load side?
i would try about 10 ohms.

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