... Maybe nobody makes a Mosfet with low Vgs spec's like you want.

They are made, but the problem for the TS is that the newer designs that have come along specifically to fill the need for "logic level" when it now can easily mean less than 3 V are only produced in surface mount packages for the small devices. Not much of anything new gets put into cheap through-hole packages like the TO-92 anymore.

Have a look at the specs of the parts bertus recommended. The Si2300 is a very impressive part, and one I'd certainly keep in mind if I were still doing design. But it's in SOT-23 only.

 

They are made, but the problem for the TS is that the newer designs that have come along specifically to fill the need for "logic level" when it now can easily mean less than 3 V are only produced in surface mount packages for the small devices. Not much of anything new gets put into cheap through-hole packages like the TO-92 anymore.

Have a look at the specs of the parts bertus recommended. The Si2300 is a very impressive part, and one I'd certainly keep in mind if I were still doing design. But it's in SOT-23 only.

Thank you for this. I will try the parts that bertus recommended, they seem adequate for what I need - if there are any more SMD components anyone would suggest I will try all of them!

 

https://www.ebay.com/itm/KP505G-Transistor-silicon-USSR-Lot-of-4-pcs-/142594884410
KP505G Vth=0.4-0.8 for Id=1mA. Idmax=500mA, Idmaxpulse=2A, Vgs=1.7V Ron<1.2Ohm (id=500mA)

 

Transistor KP505G was used in infrared remote control. This transistor is very sensitive to electrostatic electricity.

 

Hi everyone,

I am looking for a MOSFET transistor to use in my circuit and after much experimentation and failure I thought I would ask here.

I am using an ATTiny85 (with a max output from its signal pins of 40mA) powered with 2xAAA batteries to change the speed of a motor using PWM, so the max power to the MOSFET will be 3V from my 2xAAA batteries. The motors I am powering are rated at 1.5 - 3V with a max of 250 - 460mA @ 3V. There is a 0.1uF capacitor across the motor terminals and also at the power source.

I tried using a 2N7000 - however, it seemed to act quite erratically (it seemed like the ATTiny kept resetting) and when working, the motor was very very weak and kept stalling.
I also tried using a 7030BL which was more stable and gave a slightly higher power to the motor than the 2N7000, but was still not close to the strength the motor can get to when you just apply the 3V from the batteries directly.

So I was wondering if anyone would be able to suggest some more transistors to try. I am looking if possible for a TO-92 package, or if this fails a SMD option. The main thing is that the motor is as efficient and strong as possible, without draining too much current from the ATTiny (I believe that this might have been the issue I describe above but not sure)

I believe I want a MOSFET where the Rds(on) is rated at a Vgs of 3V or less, but beyond this I am really struggling with all of the variables of the MOSFET when I look at data sheets.. any explanation or further links would be really useful, I am trying but still struggling!

Thanks so much

A

You might look at the NDT451AN . I use it in many of my simple circuits, but usually have 3XAA or AAA for power. In any event the typical gate threshold for this MOSFET is 1.6V so it might work for you.

TinkerJim

 

You might look at the NDT451AN . I use it in many of my simple circuits, but usually have 3XAA or AAA for power. In any event the typical gate threshold for this MOSFET is 1.6V so it might work for you.

TinkerJim

The threshold voltage is when the Mosfet is barely conducting, the one you mentioned has a max threshold voltage of 3V so many of them will not work when powered from a 2V to 3V battery. Its is spec'd to turn on well when its gate-source voltage is 10V or 4.5V.

 

The threshold voltage is when the Mosfet is barely conducting, the one you mentioned has a max threshold voltage of 3V so many of them will not work when powered from a 2V to 3V battery. Its is spec'd to turn on well when its gate-source voltage is 10V or 4.5V.

Si2333DS is rated for 59mOhms at 1.8V Vgs so will work fine at 2-3V and will handle >2A current
see vishay.com/product?docid=72023
see attached pdf for an example circuit to drive a relay. Note you need to pull the gate to GND to turn the this p-fet on

 

There are a number of TO-220 MOSFETs that work well at 3.3V. See my lists here:

http://redrok.com/electron.htm#nch
http://redrok.com/electron.htm#IRF3708
30V 62A 39mOhm@2.8Vgs TO-220 package.
And many others if you want to self test them.

In my listings the Black values are from the data sheet.
The Green values were tested by me on about 25 devices.
I also show graphs of the transfer curves Ron vs Vgs.

I have used the IRF3708 for many years.

redrok@redrok.com

 

This thread just showed up in my AAC newsletter today, a month after it was active.

There is a 0.1uF capacitor across the motor terminals and also at the power source.

How about across the uC Vcc and ground? If anything, that's the important one, since your uC is the device most likely to act funky if it resets. Though, a prolonged drive of the motor causing the supply voltage to dip isn't going to be masked by a little decoupling cap.

I tried using a 2N7000 - however, it seemed to act quite erratically (it seemed like the ATTiny kept resetting) and when working, the motor was very very weak and kept stalling.

Like the ATTiny kept resetting because it's supply voltage dropped? That'd be expected if you're pulling nearly half an amp off the AAA cells. You should monitor voltage while you're doing that. Alternately, just for testing, run the uC and the motor from separate supplies and tie the grounds together - if it doesn't reset under those conditions, that points to the supply being inadequate. uC voltage requirements vary with clock speed and various other parameters of the uC. If you configure the uC to operate at a lower frequency, it will be able to run at a lower voltage bracket.

You indicate you're doing PWM and what duty cycles - but not at what FREQUENCY. Gate charge on a capacitor that is being switched very quickly can pose issues, pulling a lot of current. While a current limiting resistor to the gate would protect the uC output, it will also slow the switching, resulting in losses in the transistor (which turn into heat).

I also tried using a 7030BL which was more stable and gave a slightly higher power to the motor than the 2N7000, but was still not close to the strength the motor can get to when you just apply the 3V from the batteries directly.

Not surprisingly, the datasheet for this component shows that its typical Vgs(th) is slightly lower at 1.9V than the 2N7000. Additionally. it's Rds(on) at 4.5V (per the datasheet - not what you're driving it at) is 8.5 milliOhms, which is 100's of times smaller than the 2N7000. This is a power mosfet. It has a much higher gate capacitance than the 2N7000, so can't switch as fast, but really, you shouldn't need to be switching super fast - just make sure your code isn't.

Rds(on) and Vgs(th) are two characteristics you should pay attention to on mosfet datasheets, particularly in relationship to what you're doing. Since a _fresh_ battery situation will be 3V, but will drop with use (and during load), you want something with a lower Vgs(th), otherwise, once you switch and the motor starts to pull current, and the voltage drops, presuming everything doesn't fall in on itself and reset, the gate will never be sufficiently driven and you'll be partially driving it - resulting in more dissipation in the FET and less power for your motor.

Here's the datasheet I pulled up for the 2N7000: https://datasheet.octopart.com/2N7000-ON-Semiconductor-datasheet-5936.pdf

That datasheet shows a typical Vgs(th) of 2.1V, so you should be alright there, though a device with a lower specifieed Vgh(th) would be better. The Rds(on) though is 1.8 ohms if the gate is at 4.5V (which for you, it isn't), so you can expect a higher resistance. Your voltage drop to the motor is going to be V(drop) = I * R. Let's say just 300mA current through your motor, and somehow the 4.5V Rds(on) of 1.8 ohms even at a pristine 3.0V 2xAA battery input, completely discounting battery voltage drop under load: 0.3 * 1.8 = 0.54 volts dropped (so the motor would be getting 3.0 - 0.54 = 2.46V at best). and at 300mA, that's 162mW dissipated in the mosfet (though the 2N7000 is rated for 400mW, so you'd be okay - but fact is, you'll be well above 1.8 ohms, so the dissipation would be higher in reality).

Look at table 1 "On Region Characteristics" in that datasheet - at a full 3.0V Vgs, it looks like you'd be lucky to pass 50mA

When it comes to SMD vs. TH - you'll find more modern options in SMD. Probably a good time to consider investing in some SMD-to-TH adapter PCBs. I put some SMD components on those when I receive the components, just to make it easy to quickly prototype something on breadboard, but in the end, I want to use the SMD because they're more compact, and it's way easier to deal with the SMD stuff on etched-at-home PCBs for super quick turnaround and one-off testing (no holes to drill). If you have some of those adapters on hand for the types of packages you might expect to encounter (SOT23 and SO8 are good ones), then you don't need to start your searches looking for through-hole parts which could limit you.

Now, as to a potential alternative FET to use:

http://www.onsemi.com/pub/Collateral/MCH3484-D.PDF

Vgs(th) is 0.3 - 0.8 (max). If you can give it even just 1.8V to the gate, it's good for 1.0A (which is more than your motor is pulling) and max of 49mOhm Rds(on). Gate capacitance is much higher than the 2N7000, but about a third of your power mosfet - switching speed shouldn't be an issue. It is an SMD component (SOT323), but time to move forward.

Bertus already mentioned Si2300 - it's a good choice too.