What MOFET is good for Arduino to switch low voltage <1.5v

Thread Starter

DanSohan

Joined Jan 6, 2021
18
Hello - I'm new here! Look forward to joining in with the community. Here's to a better 2021.

I've recently built an arduino project of my own design. It charges and discharges batteries and give Ah and works out internal resistance.

I used a high side switching arduino MOSFET switch to connect and disconnect the battery / load. It all seemed to work well.

However (there's always a but!) If I connect a 1.5V battery, the MOSFET fails to switch on / conduct properly.

I only have limited technical knowledge of MOSFETs, but even with 5V PWM applied to the gate, should the FET be able to conduct low voltages below 1.5VDC?

any pro (or non pro) advice would be food for thought. Kind regards

Dan
 

wayneh

Joined Sep 9, 2010
17,153
It's tough to answer without a schematic but in general, yes, a MOSET can conduct if the gate voltage is correct versus the source voltage. The drain voltage can be higher or lower than the source and current can flow in either direction with minimal voltage drop.

Your problem is almost certainly the Gate-Source voltage, or the MOSFET is damaged.
 

Thread Starter

DanSohan

Joined Jan 6, 2021
18
Thank you, yes I understand the difficulties without a schematic, but it's literally a tangle of wires hot glued to a breadboard at this time. Essentially I have a DIY more MOSFET module.

(sorry if links are not permitted) - https://images-na.ssl-images-amazon.com/images/I/41KoYvRT1tL.jpg

This is driven directly from a Uno pwm Pin9

I have a variable rheostat type resistor connected to the output and a battery across the input pins of the mos unit.

All works perfectly, but if I drop the input voltage to ~2 volts or less, no current flows. I also tried taking the pin low or high manually to see if it would switch on.

Confused as I would expect this from a transistor, but expected the mosfet to function as you described. Either open or close at low resistance. Maybe it is a damaged unit. Only have 1 - so something to check

back to the drawing board. (I don't have one hence no diagram ;)
 

Thread Starter

DanSohan

Joined Jan 6, 2021
18
Do you have the specs on that module or a link?
I pasted a link, maybe it was blocked.

It's the F5305S. On reviewing the datasheet I noticed data with: VSD Diode Forward Voltage ––– ––– -1.3 V
does this suggest the MOSFET drops 1.3V across it like a diode? If so I'm surprised it's worked as well as it has so far.
 

Thread Starter

DanSohan

Joined Jan 6, 2021
18
Back in the day on the typewriters. I suspect more is going on with the MOSFET module, as it has an opto-isolator on it, but i don't have the skill to backward engineer it.
Capture.JPG
 

Audioguru again

Joined Oct 21, 2019
3,330
The Fet module has an opto-isolator input with a 3V minimum voltage (it has two series LEDs) and an ordinary P-Channel Mosfet.
The datasheet of the IRF5305 Mosfet shows an ordinary P-channel Mosfet with a maximum Vgs threshold voltage of 4V for it to conduct only 0.25mA and a Vgs of 10V for it to fully turn on.

The entire module can be replaced with an IRLxxx N-Channel Mosfet that turns on fully with a Vgs of 5V. I would use an IRLZ44 Mosfet that has an on resistance of only 0.025 ohms maximum when Vgs is 5V.
 

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Audioguru again

Joined Oct 21, 2019
3,330
If you use an ordinary logic level N-Channel Mosfet then use its drain output in series with a variable resistor and the Mosfet as a switch to discharge the positive wire of a Ni-MH (?) battery cell, then won't the variable resistor smoke then burn out?
Because my Name Brand AA Ni-MH cells can produce a current higher than 10A.
 

sghioto

Joined Dec 31, 2017
2,373
On reviewing the datasheet I noticed data with: VSD Diode Forward Voltage ––– ––– -1.3 V
does this suggest the MOSFET drops 1.3V across it like a diode?
No. That data is the voltage drop across the IR LED inside the EL817 optocoupler. As Audioguru again mentioned the module requires at minimum 3 volts from the Uno to switch but it also specifies a minimum input voltage of 5 volts.
 
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Thread Starter

DanSohan

Joined Jan 6, 2021
18
If you use an ordinary logic level N-Channel Mosfet then use its drain output in series with a variable resistor and the Mosfet as a switch to discharge the positive wire of a Ni-MH (?) battery cell, then won't the variable resistor smoke then burn out?
Because my Name Brand AA Ni-MH cells can produce a current higher than 10A.
Thank you very much - did you create that diagram on the fly!?

the variable resistor is a 50W rheostat so it wont smoke, just warm the room up which I need. Eco friendly testing.

next step is

adding 0.1R 5W current resistor on the ground to measure current flow
the ability to use it in charge mode. So move the 1.5v to the other side and connect a 2v psu to recharge it.

I ran into trouble using low side switching and bi-directional current sensing.
 

sghioto

Joined Dec 31, 2017
2,373
I found that number on the semiconductors datasheet
Correct, that's the voltage drop across the reverse P-N junction diode or body diode as it is also called inside the mosfet. That doesn't mean you are loosing 1.3 volts across the mosfet.
All works perfectly, but if I drop the input voltage to ~2 volts or less, no current flows.
It's because of the component values on the module.
EDIT: What are the max charging and discharging currents?
 
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Audioguru again

Joined Oct 21, 2019
3,330
Thank you very much - did you create that diagram on the fly!?
I asked Google for the module then
the diagram was online along with all the Chinese retailers who sell it..

the variable resistor is a 50W rheostat so it wont smoke, just warm the room up which I need.
I smoked some AA and AAA Ni-MH batteries doing that.

Next step: adding 0.1R 5W current resistor on the ground to measure current flow
the ability to use it in charge mode. So move the 1.5v to the other side and connect a 2v psu to recharge it.
A battery is charged with a certain current, not with a voltage.

I ran into trouble using low side switching and bi-directional current sensing.
If the series current sensing and load resistor connect to the negative terminal of the battery and to ground, then you need to switch the positive terminal of the battery to a positive current for charging and switch the positive terminal of the battery to ground for discharging it.
 

Thread Starter

DanSohan

Joined Jan 6, 2021
18
Correct, that's the voltage drop across the reverse P-N junction diode or body diode as it is also called inside the mosfet. That doesn't mean you are loosing 1.3 volts across the mosfet.

It's because of the component values on the module.
EDIT: What are the max charging and discharging currents?
up to 5 amps, maybe 10 max.

I've essentially got 100s of batteries to test the capacity off, and was hoping to make a charge then discharge circuit. Maybe make 10 of them to increases the speed of testing and working out the health of the cells. I have all sorts of cells, some NIMH AA, some 18650s, and a whole bunch of 10 year old lifepo4 prismatic.

I was attempting to limit the current by adjusting the PWM value on the mosfet using a PID code.

Audioguru

your suggestion last there. I think your suggesting something like a change over relay be used, or physical removal of the cell and switching it around. I'll experiment along those lines.
 
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