Author Topic: ucurrent with mosfet  (Read 1025 times)

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Offline fedimakniTopic starter

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ucurrent with mosfet
« on: June 21, 2020, 08:18:34 am »
Hello,

I am planning to make a ucurrent but the ranges will not be controlled manually but using mosfet (schematic attached below). I choose to use the DMG3406L datasheet: https://www.diodes.com/assets/Datasheets/DMG3406L.pdf
as an n-channel mosfet and i plan to turn it on and off from the raspberry pi. I will power this device from the raspberry pi 3.3V pin.
But also the DUT (device under test) will also be powered from the same pin. and as i planned to use a low side measurement the GND and -IN pin could be the same net.
I would like to have an idea does this could really work?
Do the mosfet will turn on and off normally as the voltage reference of the gate may not be the same with the voltage in the source?
Is there any enhancement i could do?

Thanks

 

Offline SilverSolder

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Re: ucurrent with mosfet
« Reply #1 on: June 22, 2020, 02:09:39 am »
Hello,

I am planning to make a ucurrent but the ranges will not be controlled manually but using mosfet (schematic attached below). I choose to use the DMG3406L datasheet: https://www.diodes.com/assets/Datasheets/DMG3406L.pdf
as an n-channel mosfet and i plan to turn it on and off from the raspberry pi. I will power this device from the raspberry pi 3.3V pin.
But also the DUT (device under test) will also be powered from the same pin. and as i planned to use a low side measurement the GND and -IN pin could be the same net.
I would like to have an idea does this could really work?
Do the mosfet will turn on and off normally as the voltage reference of the gate may not be the same with the voltage in the source?
Is there any enhancement i could do?

Thanks

The MOSfet in series with R1 may have higher Rds(on) than R1 itself -  and will therefore introduce significant measurement error.    The MOSfet probably needs to be placed somewhere else, where it is not being "measured" by the uCurrent...

 

Offline fedimakniTopic starter

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Re: ucurrent with mosfet
« Reply #2 on: June 22, 2020, 02:14:21 am »
Could you please suggest any enhancement? Where I need to place it?

Another thing, driving the mosfet from a 3.3V source with GND reference but the mosfet has the source connected to -IN which has a different voltage level than GNd, Do that could cause some problem during powering on and off the device?
Thanks
 

Offline SilverSolder

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Re: ucurrent with mosfet
« Reply #3 on: June 22, 2020, 02:22:52 am »

It might make sense to think in terms of replacing the uCurrent's sliding switch range selector with a relay or two.
 

Offline fedimakniTopic starter

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Re: ucurrent with mosfet
« Reply #4 on: June 22, 2020, 02:27:38 am »
A relay is very slow to make the switch also i think here we are dealing with max of 1A and most of the case with few uA so do you think it's a good idea?
 

Offline apoorv3in

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Re: ucurrent with mosfet
« Reply #5 on: June 23, 2020, 11:12:43 am »
Have a look at this.
 

Offline apoorv3in

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Re: ucurrent with mosfet
« Reply #6 on: June 23, 2020, 11:19:55 am »
I am working on something similar it turns out not to be that simple :)
https://www.eevblog.com/forum/projects/current-wave/msg2840098/#msg2840098
 
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Offline SilverSolder

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Re: ucurrent with mosfet
« Reply #7 on: June 23, 2020, 03:20:27 pm »

Using a couple of relays will simplify the project enormously and you won't have to deal with the resistance and temperature dependence of the MOSFETs introducing problems.

How often and how fast do you envision changing the range?  -  a relay will be fine, for most imaginable purposes.

Many (most) professional instruments use relays for range switching...  for good reasons!
 
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