Help with MOSFET circuit to control Muscle Wire

[gophert]

Invalid Request

 

[Kian]

It still doesn't make sense for me. If i look at the graph Fig 2, for VGS 3.25V (which is what I am using), ID saturates at about 2.3A. I measured VDS to be 1V across the FET, then shouldn't I be getting up to 2.5A?

The threshold voltage is when the mosfet is barely tuned on, that is what the graphs are showing. The graph, Fig.2, shows how much each gate voltage level alows the mosfet to conduct current wise. Look for what is called a "logic level" mosfet. But it seems what you have is one, you just need one with a higher pass through current rating.

 

[gophert]

First try to activate your muscle wire from 3v power (2 D-Cell or 2-AA batteries in series) to see if the wire activates slowly as voltage is decreased.

Then check that your pin assignments are correct.

Then check that all solder joints are good.

And make sure your MOSFET is still ok (no damage from static or over current).

 

[Kian]

Thanks everyone who replied and provided suggestions. I managed to get it to work with the DMN2075 MOSFET. However I need to have the lipo battery full charged at 4.2V for it to work. So now I have 4.2V connected to one end of the muscle wire and the other end is connected to the Drain of the MOSFET. However, I think the circuit can be improved with a better MOSFET because they battery will lost its charge and the voltage will drop over time. I want to be able to this working even if the battery voltage drops to 3.5V.

I am still hoping someone can advise me how to choose a MOSFET with high drain current.

 

[alec_t]

Breadboards are not designed to handle high currents. Poor contact and inherent resistance in the breadboard connectors may be a contributing factor to your less-than-hoped-for current through the FET.

 

[Nigel Goodwin]

It still doesn't make sense for me. If i look at the graph Fig 2, for VGS 3.25V (which is what I am using), ID saturates at about 2.3A. I measured VDS to be 1V across the FET, then shouldn't I be getting up to 2.5A?

Current is dependent on ohms law, the graph has no bearing on it - until the current (from ohms law) tries to exceed the figure on the graph.

Your problem is that VDS is FAR too high - as I said above, simply short out drain and source on the FET (with a screwdriver, or piece of wire) and ensure it works then, also try it with the battery when it's more discharged to ensure it still works. Assuming all is well in those cases?, then you can worry about getting an FET that switches correctly.

Assuming all does work OK?, then you 'might' consider a dual FET - these are often the same 8 pin SM encapsulation as you already have, and are an N-Channel and P-channel together. With a few resistors you can configure it as a high side switch, and there's no issues with only 3.3V because you have the extra gain of two FET's.

I use them to switch various supplies in commercial products, including switching Li-Ion 4.2V to SIM800L modules.

 

[Kian]

I am thinking, instead of making this so troublesome with a MOSFET, can't I just use a 3.3V LDO with an enable pin and use it to drive the muscle wire instead? Whenever I need to actuate the muscle wire, just enable the LDO?

 

[gophert]

I am thinking, instead of making this so troublesome with a MOSFET, can't I just use a 3.3V LDO with an enable pin and use it to drive the muscle wire instead? Whenever I need to actuate the muscle wire, just enable the LDO?

Yes.