I have a 3.3V arduino pro mini. The pins can handle something like 40mA or whatever... but I need to switch a load of about 2A (actually half that, but I want to be safe).

At first I was thinking relay, but then I ran into two problems: 1) all the cheap relays are 5v; 2) big. I need it to fit in a small space.

So then I get reading about using a mosfet, but I'm getting lost in the specs. I've used mosfets as switches before, but in situations where the specifications really didn't matter. With 50,000+ to choose from on Digikey and specs that don't mean a whole lot to me... where to start?

The load is going to be active briefly (max 500ms, probably much less), so I'm not too terribly concerned with heat. I'm hoping for small and as close to dirt cheap as possible.

ideas?

 

hey ,
match this one with your specification MOSFET 3055L.
https://www.onsemi.com/pub/Collateral/NTD3055L170-D.PDF

 

You look for a "logic level" type MOSFET.
Those have the Rds(on) rated at a low Vgs voltage.
You want one where the Rds(on) is rated at a Vgs of 3.3V or less.
For that the maximum Vgs(th) will likely be less than 2V

 

as we seen in electrical characteristic table -
https://www.onsemi.com/pub/Collateral/NTD3055L170-D.PDF

ON CHARACTERISTICS

VGS(th) 1.0 − 1.7 Vdc mV/°C range so ,
this may easily on Rds at 3.3vdc

 

Here's an example transistor from Mouser that specifies the RDSon down to a Vgs of 1.8V (Table 7).


That's what you want to look for.
One that specifies RDSon at a Vgs of 3.3V or less.

 

You look for a "logic level" type MOSFET.
Those have the Rds(on) rated at a low Vgs voltage.
You want one where the Rds(on) is rated at a Vgs of 3.3V or less.
For that the maximum Vgs(th) will likely be less than 2V

Its still something that needs to be checked before placing the order.

Many TO92 MOSFETS are 1V at the lower end of their spread - ordering enough for a quantity price break can sometimes be cheaper than how many you wanted, then you can select the ones that work from a heap of them.

 

Its still something that needs to be checked before placing the order.

Did I imply otherwise?
I don't understand that statement.

 

Thank you for your help. The 3055L will do nicely.

 

Did I imply otherwise?
I don't understand that statement.

Do I really need to explain that some logic level MOSFETs are better than others?!!!

 

Do I really need to explain that some logic level MOSFETs are better than others?!!!

No.
And condescending remarks are not appreciated.
I just needed clarification of this cryptic statement:

Its still something that needs to be checked before placing the order.

 

Thank you for your help. The 3055L will do nicely.

Just be aware that its operation as a switch is not guaranteed at a Vgs of 3.3V.

 

Wait, what? Why?

Its because they don't specifically state an RDS on in that range? But the Gate Threshold Voltage is 1 to 3V. What am I missing?

 

Is this a better choice?

http://www.vishay.com/docs/67072/sqa410ej.pdf

Edit: except... grr... package isn't suitable for hand soldering.

 

Wait, what? Why?

Its because they don't specifically state an RDS on in that range? But the Gate Threshold Voltage is 1 to 3V. What am I missing?

The Threshold is the voltage where the transistor just begins to draw current, typically 250 uAmp or 1 mAmp.
Find the Drain-Source On-State Resistance section in the datasheet.

 

Wait, what? Why?

Its because they don't specifically state an RDS on in that range? But the Gate Threshold Voltage is 1 to 3V. What am I missing?

You are missing the fact that the gate threshold voltage is where the MOSFET just barely starts to turn on, not when it's fully on.

Note that it's specified for a drain current of 250μA.
That's why you need a transistor the specifies the on resistance at the Vgs you want to use.

Did you look at the one I referenced in post #5?

The one in your post #13 should also work.

 

I didn't notice your post #5 contained a link (ie. didn't realize it was a suggestion).

For the same reasoning that the 3055L can't be guaranteed to switch... isn't the PMF63 not guaranteed to handle the current? It only has current values and power dissipation for V=4.5? Do you *think* it would work?

 

Maybe I'm going about this wrong? Instead of trying to find a MOSFET that apparently doesn't exist (available + lowish cost + suitable for 3.3V, 2A, 4W) is there a better way to switch a load that I should be looking at? Should I just level convert the 3.3V to 5V and then (hopefully) have more options on the FET? Or???

I honestly didn't expect to get stuck on this part of the design lol

 

isn't the PMF63 not guaranteed to handle the current? It only has current values and power dissipation for V=4.5? Do you *think* it would work?

You are correct.
It's ON resistance is rated at 1.8A at a Vgs of 2.5V.

Here's one http://www.mouser.com/ds/2/302/PMV28UNEA-1022845.pdf that is specified for 3.9A @ a Vgs of 2.5V.
You can buy it here http://www.mouser.com/ProductDetail...hyDBzk1/WiwHIj//1rPuithT8BcfHemDMRyBN1Eyafw==.

 

I'll try it. It wasn't coming up on my searches because the power dissipation is too low. However, its probably "close enough" for this application.

Thanks.

 

Wait, what? Why?

Its because they don't specifically state an RDS on in that range? But the Gate Threshold Voltage is 1 to 3V. What am I missing?

Wait, what? Why?

Its because they don't specifically state an RDS on in that range? But the Gate Threshold Voltage is 1 to 3V. What am I missing?

Most FETs have a wide parameter spread - some samples will work down to 1V, but they only guarantee success at 3V.

Either order a device that guarantees the voltage your Arduino can feed it, or buy bulk and select a device from the heap that works.

Some suppliers offer price breaks for quantity orders - this is neatly illustrated by an order I placed for 50x JFETs because of the price break. My order was mis heard over the phone so 50 was heard as 15 - not only did I get less parts than I expected, they cost me more than the 50 would have. They sent me the outstanding balance of components, but refused to refund what they'd overcharged me.

A pile of MOSFETs on a quantity price break can cost less than the few you were going to order, and you can select parts for one that works reliably.

On a contract to completely rework a pile of PCBs, I was handed a big box full of long bandoliers of 1% resistors. When I pointed out that precision components weren't necessary for the job in hand, I was told these parts worked out 1/5 the price of 5% resistors when bought at the 1000 qty price break. You have to look carefully to find the best value for money.