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Transistor as switch with very low voltage drop

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neazoi

neazoi

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Hello I have a device that draws 20mA at 1.2V.
I need to switch it on/off electronically but the switch must introduce only a tiny voltage drop.
I think that a mosfet could be used.
Any ideas or schematics would be useful.
 

Just use a low power N-channel Mosfet placed in series with your circuit (the Mosfet source connected to GND and the drain tied to your circuit ground terminal).

You didn't mention anything about the control signal parameters (voltage, current) but there's a wide range of Mosfets with a minimum Vgs starting as low as 0.8-1 V.

So any small signal greater than 1 V could switch on/off that Mosfet.
 
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    neazoi

    neazoi

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You could always use a conventional low rdson N channel power mosfet, but it will need a voltage pump to provide sufficient gate voltage.
As the gate consumes no power, I don't see that as a problem.
 
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    neazoi

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kathmandu said:
Just use a low power N-channel Mosfet placed in series with your circuit (the Mosfet source connected to GND and the drain tied to your circuit ground terminal).

You didn't mention anything about the control signal parameters (voltage, current) but there's a wide range of Mosfets with a minimum Vgs starting as low as 0.8-1 V.

So any small signal greater than 1 V could switch on/off that Mosfet.
Click to expand...

I would avoid connecting the mosfet that way, since the circuit is RF and needs a good grounding. I would prefer to connect it right in series with the battery.

It only needs to switch 20mA at 1.2v. Obviously a 0.8V voltage drop is too much! I would expect something in the range of 50-100mV max to be satisfied. Can this be done or else I would need to look for a low voltage coil relay better.

- - - Updated - - -

Warpspeed said:
You could always use a conventional low rdson N channel power mosfet, but it will need a voltage pump to provide sufficient gate voltage.
As the gate consumes no power, I don't see that as a problem.
Click to expand...

Can you please give me some examples/schematics?
 


neazoi said:
I would avoid connecting the mosfet that way, since the circuit is RF and needs a good grounding. I would prefer to connect it right in series with the battery.
Click to expand...

You could connect it this way too but in this case the gate control signal should be greater than Vgs + Vcc (Vgs = minimum Vgs threshold, Vcc = your circuit power supply voltage (1.2V))

What's your control signal anyway? (voltage level)

neazoi said:
It only needs to switch 20mA at 1.2v. Obviously a 0.8V voltage drop is too much! I would expect something in the range of 50-100mV max to be satisfied.
Click to expand...

By 0.8-1V I was reffering to Mosfet Vgs threshold parameter (the actual voltage level you need to control that Mosfet).

The voltage drop in on state is very-very low for almost any Mosfet you may use (as your load current it's small).

- - - Updated - - -

Warpspeed, we still don't know the parameters of his control voltage. Maybe he actually have a 3.3-5-12V signal (a MCU or something) to control the Mosfet (after all, I believe he wants to remote control the switch turn-on/off).
 

kathmandu said:
Warpspeed, we still don't know the parameters of his control voltage. Maybe he actually have a 3.3-5-12V signal (a MCU or something) to control the Mosfet (after all, I believe he wants to remote control the switch turn-on/off).
Click to expand...
True,

I was assuming all he had was a single cell battery to work from.
There is now another thread where he is asking about 1.2 volt latching relays.
And that is suggestive...
 

kathmandu said:
You could connect it this way too but in this case the gate control signal should be greater than Vgs + Vcc (Vgs = minimum Vgs threshold, Vcc = your circuit power supply voltage (1.2V))

What's your control signal anyway? (voltage level)



By 0.8-1V I was reffering to Mosfet Vgs threshold parameter (the actual voltage level you need to control that Mosfet).

The voltage drop in on state is very-very low for almost any Mosfet you may use (as your load current it's small).

- - - Updated - - -

Warpspeed, we still don't know the parameters of his control voltage. Maybe he actually have a 3.3-5-12V signal (a MCU or something) to control the Mosfet (after all, I believe he wants to remote control the switch turn-on/off).
Click to expand...

An idea for the control voltage will come from the RF signal itself. A rectified (germanium diode?) voltage using an envelope detector on an RF of no less than 50mW and not higher than 5W at 50R.
Another idea for the control voltage is to come from a low voltage MCU that takes it's power from the 1.2v battery. I am not sure if there are such a low voltage MCUs around, but I would nit like to use a charge pump or other inverter since they will interfere with RF circuit I am using this switch (2000m-6m active antenna)
 

You want to control the on/off switch (Mosfet) locally, using some sort of timers or using a strong RF signal as a remote control?

Warpspeed's idea still work but you have to isolate that boost converter using a Faraday cage or by moving it away from your RF circuit (if that's possible).

You may also use a voltage doubler/quadrupler circuit (or any other multiplier rate) using a simple (not switched) circuit topology.

Search for RF-DC converters, too.
 

kathmandu said:
You want to control the on/off switch (Mosfet) locally, using some sort of timers or using a strong RF signal as a remote control?

Search for RF-DC converters, too.
Click to expand...

The control will be better to be done remotely using the RF signal.
 

A boost converter running with open circuit output will pump a couple of times then essentially turn itself off.
Most are not PWM but gated on and off, so they work in burst mode.
 

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