let's say I want to do H-bridge to run motor with 2x 1.2V AA rechargeables. Pretend there is no way in getting higher voltage. MOtor drwas ~25mA at that voltage. I'm looking for some mosfets to do the H-bridge. Every mosfet I see has Vgs od 4V and not lower. Does that mean it won't work with lower voltage? Or does that mean it won't supply that much current with lower voltage (which would be fine, I don't need much current).
For example, I'm looking at IRF4905 P-mosfet (datasheet here) that should do like 1-2A with only -4Vgs. Would it run at -2Vgs and supply current of ~0.05A? These are not the cheapest, that's why I ask. Well, 2$, but multiply it and it's not that cheap for "fun" project - I would buy them, but I would like to hear some opinions.
There are plenty of mosfets which will work with low Vgs if you look at mosfets in SMD packages. For example, this one http://www.farnell.com/datasheets/1685598.pdf has quoted Rds(on) = 100millohms max @ Vgs = 1.8V, Id = 2.9A.
Threshold voltage is not the right parameter - you need the Vgs value at which Rds(on) is
measured. For that device the lowest Vgs quoted is 2.5V which is borderline for this 2.4V
supply (probably fine, but device variability might be an issue).
The threshold value is where the device transistions between completely off and starting
to carry a tiny little current (measured in microamps usually).
I wanted to know if there are any of these mosfets and I know it now.
I would like to use non-SMD mosfets, because they are more - you know - breadboards friendly
I will experiment with my L293D a little more and see if it could be of some use if I supply logic with 5V and connect 2.4V to motor voltage pin, but I don't know if 2.4V could go through the circuitry inside and to output pins.
In case of mosfets - They would be supplied (on gate) from Arduino. GND will be the same (negative from batteries and Arduino GND pin). This is ok in case of N-mosfets because Vgs would be 5V, but would it be ok in case of P-mosfets, where Vgs would be -2.4V only? There is the problem, I couldn't go lower.
Pavouk106:
I will experiment with my L293D a little more and see if it could be of some use if I supply logic with 5V and connect 2.4V to motor voltage pin, but I don't know if 2.4V could go through the circuitry inside and to output pins.
No way, because of the voltage drop in the L293D. But if you supply 5V to the motor voltage pin of the L293D, you should get 2 to 3V out of it under load.
Pavouk106:
In case of mosfets - They would be supplied (on gate) from Arduino. GND will be the same (negative from batteries and Arduino GND pin). This is ok in case of N-mosfets because Vgs would be 5V, but would it be ok in case of P-mosfets, where Vgs would be -2.4V only? There is the problem, I couldn't go lower.
dc42: Thanks for your patience and for your ideas. And especially for that SMD escape adapter. I will keep that one in my mind, I will certainly use some in future.
I kept on searching and found this IRF4905 P-mosfet measurement, why they can't have this as standard in datasheets... It looks like I would have 1 ohm resistance with -3.5 Vgs, this is suitable for my application. I don't have problem with buying these, if I know they would work for me. Plus it won't hurt having some high power mosfets around ready to use
Pavouk106:
dc42: Thanks for your patience and for your ideas. And especially for that SMD escape adapter. I will keep that one in my mind, I will certainly use some in future.
I kept on searching and found this IRF4905 P-mosfet measurement, why they can't have this as standard in datasheets... It looks like I would have 1 ohm resistance with -3.5 Vgs, this is suitable for my application. I don't have problem with buying these, if I know they would work for me. Plus it won't hurt having some high power mosfets around ready to use
Not a good choice. That mosfet isn't even logic level and it has gate threshold voltage anywhere between 2 and 4V. So it might need 4V gate-to-source before it even conducts a few microamps.
PS - you could consider a device in a SOT223 package such as http://www.farnell.com/datasheets/11864.pdf, since they have lead spacing just under 0.1", making them usable with stripboard and other prototyping boards.
