Recommend mosfet for muscle wire

Hi all,

I have a muscle wire that the manufacturer has rate for 3-3.6V operation and requires a current draw of about 700mA. The manufacturer also recommended 0.5s on time to actuate the wire.

I am trying to actuate this muscle wire with a NMOS. One end of the muscle wire is connected to a lipo battery (3.7-4.2V) while the other end is connected to the Drain. The Source is connected to Ground. And the Gate is connected to Arduino output pin @3.3V. There is also a 10k pull down resistor at the Gate.

I am having a hard time trying to get this to work as I can't seem to find the right MOSFET. I have tried with a few part numbers eg MGSF1N02 and DMN2075. Using the DMN2075 seems to work, but it only actuates the muscle wire after I turn on the MOSFET for 5-10 seconds. The MGSF1N02 takes about 20 seconds to actuate the muscle wire. If I connect the muscle wire directly across the lipo battery, it actuates almost immediately (within 1 seconds).

Can someone please help and recommend me a suitable part number for the MOSFET? I need something in SOT23 footprint if possible.

Thanks!

The DMN2075 should be fine in that application. Connections sound correct, it could be a software issue, like failing to set the Arduino pin to output mode. Also, it would be good to know which Arduino.

I’d recommend posting your code and a photo of your wiring as there isn’t enough info to help.

Hi,
Welcome to the forum.

Please read the post at the start of any forum , entitled "How to use this Forum".
OR
http://forum.arduino.cc/index.php/topic,148850.0.html.

Have you got the gnd of the muscle wire and the source of the MOSFET and the gnd of the Arduino connected together?

What model Arduino are you using?

Thanks.. Tom... :slight_smile:

Thanks @WattsThat and @TomGeorge. I am using an Arduino Pro Mini at 3.3V 8MHz.

I can confirm that the muscle wire is working as it actuates when connected directly between the lipo battery terminals. I also checked that the connection is working and the MOSFET is turning on because I replaced the muscle wire with a piezo buzzer and its generating a sound. My program is simple, just set the pin as digital output and set it to high for 1 second then set it to low.

I somewhat figured out what is the problem. If the lipo battery is fully charged at 4.2 or 4.1V, then my circuit works with the DMN2075 MOSFET. However, I was testing it yesterday with a slightly drained battery at 3.8V and that didn't work. This is still a problem for me as I want to ensure that my circuit can work over a large range of battery battery voltage. I think it wouldn't make sense for the muscle wire to stop working when the battery is 3.8V and still has a fair bit of capacity left.

Should I choose a better MOSFET? If so, how do I choose one? Or can someone recommend me a specific part number? Thanks in advance!

Edit: Just to feedback one more thing. I was prototyping this on breadboard and someone told me that breadboards are bad with conducting high current.

Changing the mosfet will not change anything, it is the correct part for the job with about 35 milliohms resistance with 2.5 volts on the gate.

You’ve identified the source of your problem, the breadboard. The power connections must not run through the breadboard - which can add several ohms of resistance to your circuit. That doesn’t matter when you’re dealing with logic level signals, which it was designed for, but pass an amp through a breadboard and you’ll very quickly discover contact resistance and the impact of said resistance. Ohms law is all you need to see it will be a problem just as you’ve discovered.

Do you have a digital multi-meter? That will identify the problem connection(s) in seconds. The solution is to hard wire/solder every connection in the loop that contains the muscle wire current.

kian_y79:
Edit: Just to feedback one more thing. I was prototyping this on breadboard and someone told me that breadboards are bad with conducting high current.

Absolutely, this is why you are losing power to the wire. 700mA is far too much for a breadboard, you will be losing significant voltage on the contacts (and may overheat them).

Directly solder or use screw terminals for high current circuits, not a breadboard.

Thanks everyone for the help. I am thinking, can I just use a 3.3V LDO voltage regulator (with an enable pin) for this instead? I just have to choose one that can provide up to 700mA output @3.3V. Then use the Arduino to enable or disable the LDO whenever I want to actuate the muscle wire.

Any pro and cons for using this method?

Bad solution. A regulator is not a switch. It needs capacitors to even work, and a minimum drop-out that is very likely more than what you have available when the battery drains (and voltage drops to about 3.2V when drained).

A MOSFET is the appropriate solution for this. The specs of the DMN2075 are good - it’s fully on at just 2.5V on the gate and can handle 3.1A, so indeed it may be a software related issue: is the pin set to OUTPUT?

For these low gate voltages you’re pretty much limited to SOT-23 parts… hard to find suitable MOSFETs in bigger footprints actually! The only minor concern related to the part you chose is that it’s end of life, not recommended for new designs. The replacement is DMN2056U - which is I think the first ever I see with a RDS, ON given for just 1.5V! Gotta remember this one. Another difference is the higher current carrying ability.