I have a mosfet question..

[Ian Rogers]

I don't use these things much, but!!! How would you use a mosfet in low voltage systems....

I am going wireless and I need to switch in / out devices to save power... If I use mosfet's surely I can't fully turn it on??
If I use bipolar I'll lose a tad voltage!!

 

[MaxHeadRoom78]

The lowest level devices are Logic level FETs AFAIK.
Look at the Vgs(th) on threshhold.
e.g. the logic level 2n7000 has a Vgs of .8v - 3v.
What level do you have available?
It may be a case of individual device test & selection.
Max.

 

[Ian Rogers]

I'm running on a 3.7 lithium battery pack... I'll have to switch in a loadcell and amplifier and two proximity switches

I was under the impression that if a fet has a Vgs of 3v will the device be fully on at 3.7V then as the circuit loses voltage over the space of time, the voltage I have to use will become less and less... In short! Will I be better off with bipolar... Good old 2n2222a??

 

[MaxHeadRoom78]

This may help understanding the performance curves etc.
https://www.microsemi.com/document-portal/doc_view/14692-mosfet-tutorial
Max.

 

[ericgibbs]

hi,
Check the data sheets for these logic level Gate types.
https://www.ebay.co.uk/sch/i.html?p...crdt=0&treatment_id=7&clk_rvr_id=904795158277

 

[Ian Rogers]

Okay.. Lets assume I go for a BSS138 with a Vgs of (max) 1.5v... If I switch on a 1k ohm bridge loadcell .. According to the datasheet I'll get an RDon of about between 1 and 2 ohms... I could live with that!!

 

[crutschow]

Okay.. Lets assume I go for a BSS138 with a Vgs of (max) 1.5v... If I switch on a 1k ohm bridge loadcell .. According to the datasheet I'll get an RDon of about between 1 and 2 ohms... I could live with that!!

That's typical, not worst-case. That device is only rated for minimum ON resistance at a Vgs of no lower than 4.5V.
I would look for a logic-level MOSFET that has an ON-resistance specified at a Vgs of 3V or 3.3V.

 

[jpanhalt]

Hi Ian,

As for mosfets that are fully on at the voltage of a lithium cell, use the parametric search of any supplier. Vgs(on) <<3.0 V are quite common today. Also, if you need a high side switch, the mosfet and driver are also available in a single package, such as the VN340SP.

Regards, John

 

[audioguru]

Do not look at the threshold voltage range because that is when the Mosfet barely turns on and conducts only 0.25mA for most Mosfets or 1mA for a 2n7000 Mosfet.
your "3.7V" lithium battery will be 4.2V when fully charged and its voltage drops as it is used until it is 3.2V when its load should be disconnected.

 

[large_ghostman]

I am not sure I understand what your switching on or what power it needs, but recently I used a mems isolator (silicon labs isolators) to switch on a couple of fans. My problem was the control board was powered by a coin cell and I needed 24V for the fans, also the control board uses a tiny solar cell when there is enough light. The fans run from an ATX supply.
Could you maybe use something like an isolator as they use tiny amounts of currents to switch on and off? Sorry if i have totally misunderstood

 

[Mikebits]

Have a look at this part http://www.irf.com/product-info/datasheets/data/irlml6302gpbf.pdf
The RDS (on) = .9 ohm @ VGS 2.7v Id = .31A

 

[Willen]

Does a MOSFET turns ON partially too? (like BJT) Hm.... MOSFETS can give sine wave in some audio amplifier mean....I think yes. I used to think that if a mosfet has 3V V(th)/Vgs then it turns ON fully at 3V and turn OFF fully at 2.9V. Clear me please!

 

[MaxHeadRoom78]

Here is another explanation of the transconductance for a 2n7000.
http://www.physics.csbsju.edu/trace/nMOSFET.CC.html
Max.

 

[ronsimpson]

Does a MOSFET turns ON partially too?

Yes.

it turns ON fully at 3V and turn OFF fully at 2.9V.

No.
1) every different type of part has a different turn on voltage. The "logic level" parts turn on at a lower voltage.
2) the on/off is over a much bigger range than 0.1 volts.
3) The voltage is temperature dependent.
4) A BJT is a current amplifier. Current in and current out. (in the linear area) While a FET is more of a voltage in resistance out.[/QUOTE]

 

[Willen]

The "logic level" parts turn on at a lower voltage.

logic level means?

4) A BJT is a current amplifier. Current in and current out. (in the linear area) While a FET is more of a voltage in resistance out.

Then the 'Rds' also depends to gate voltage? Eg. if Vgs/Vth is 3V then Rds= 0.3R, if we applied 2.5Vgs then the Rds= around 0.5R and if we applied 1.5Vgs then the Rds= around 1 or 2 ohms, like this? (And the increment of Rds makes output voltage drop too.)

 

[ericgibbs]

hi Willen,
Its the Rds resistance [resistance between Drain and Source] that as to be considered with MOSFETs.
Look at this image of Rds, you can see if the Vgs [ voltage Gate to Source] is 'low' the Rds is high, so you will have a bigger voltage drop across the Rds and more heat being generated in the FET body.

Driving the Vgs higher will reduce the effective Rds and so improve the overall efficiency of the FET.
ie: lower voltage drop across Rds and less heating.

E

 

[Ian Rogers]

I'm going to be switching on a 2.5v amp and cell circuit... I have to maintain some degree of accuracy..

Current isn't really the issue.. A loadcell with a 1k bridge will take 2.5mA and the amp about .5mA.. but the transmission circuit will need 12mA ~ 20mA whilst transmitting... Luckily the radio has an OFF pin so I wont need to worry on that score..

So how to keep a ratiometric voltage level on my amp circiut... I could use an ADC pin and read the amp voltage so I could read the loadcell amp at the same level ( dynamically so to speak ) but the proximity switches are power hungry little devices... I have made my own pickups so the mA is as low as I dare go, but at 3v the signal isn't very high so I darn't go any lower.... Switching with a bipolar will use 0.6v so its very tight... I was hoping the mosfet would give me nearly 3.7v.. but the RDon has to be non exsistant.... When I use the mosfets in my motor driver, I get virtually 0 RDon as I have mosfet drivers.... I'm going to buy several of these ( suggested by you all Thanks) and do some experimenting....

 

[MikeMl]

So you need to buy a FET with a very low Rds at a Vgs of 3.5V (they exist) , or you need a charge pump to drive the gate of a less-capable FET.

Is the FET switch high-side, or low-side?

 

[Ian Rogers]

I was thinking low side but I'm open to suggestion!!

 

[jpanhalt]

For lowest RDS(on). N-channel, low-side is till the best. If you need high side, your choice will be a P-channel, a high-side driver, or an integrated device as I mentioned above (post#8).

John