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Topic: Transistor Issue Plagues Me (Read 3 times) previous topic - next topic

dc42

16v is the absolute maximum rating for Vgs, i.e. you must never exceed 16v between gate and source. The critical figure is the gate-source voltage at which Rds(on) is quoted. The datasheet for that mosfet quotes it at both 10v and 5v, both for Id = 8.5A, which is well above what you need.
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boingaon

So, those transistors will fully open up when the Arduino's 3.3V is applied at the gate?

dc42

#17
May 06, 2012, 09:45 pm Last Edit: May 06, 2012, 09:52 pm by dc42 Reason: 1

So, those transistors will fully open up when the Arduino's 3.3V is applied at the gate?


No, I assumed you were using a 5v Arduino because the photo you posted looks like the edge of an Arduino Mega. Are you intending to use a 3.3V Arduino instead?

[EDIT: you can get mosfets which will switch with 3.3v gate drive but they are mostly in SMD packages, e.g. http://uk.farnell.com/nxp/pmv16un/mosfet-n-ch-20v-5-8a-sot23/dp/1894627RL]
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boingaon

I'm using a Mega. A previous poster said that the digital I/O pins were 3.3V so I made an assumption... Now that I've check it with a multimeter, I'm reading 4.86V off the pins.

So, now that we've clarified things, you think this will work? The voltage is close to but but quite at 5V.

dc42

I think that part is suitable; but if you want to play really safe, go for a part that has Rds(on) quoted at 4.5v. For example, the SMD part I mentioned in my last post, or this through-hole part: http://uk.farnell.com/international-rectifier/irlu8726pbf/mosfet-n-ch-30v-86a-ipak/dp/1698317.
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boingaon

The through-hole one looks perfect. Thank you

boingaon

Update:

I got some new transistors and they seem to be working. However, I believe that when current is sent through the solenoids, there is not enough resistance, so it results in a short. For example, when I activate two or more solenoids at any given time, the DC adapter shuts off to protect itself. What can I do here? The resistors I see at Radio Shack can handle a max of 1Watt power. I'm running 1.8 Amps and 16.6 Volts through the solenoids, which have impedance of 12 Ohms.

Grumpy_Mike

Simply get a bigger power supply, one that can give more current.

MarkT

There are 3 voltages commonly quoted in the specs.   One is in the "Absolute Maximum Ratings" section, that's the absolute maximum Vgs (gate-source voltage) - this is usually something like +/-20V.   If you take the gate source voltage that high then the gate oxide may breakdown (device dies).

There is the VT - the FET threshold gate voltage.  This varies a lot between devices, so is often quoted as loosely as "2--4V".  This is the point at which the device starts to conduct (usually a few microamps is the criterion).  Below this the device is OFF.

Then there is the Rds(on) specification, something like  "Rds(on) = 0.004 ohms at Vgs=4.5V"  - the Vgs specified here is the gate voltage for the FET swiched fully on.  Typically the Vgs quoted is 10V or 4.5V (for logic devices).  Some devices quote both (the on resistance is less at 10V gate drive of course, but only a little less since the device is basically fully ON at 4.5V).  4.5V is quoted rather than 5.0V to allow for the device that drives the gate not being able to pull fully up to the rail.  If the spec is for 10V this is usually taken to mean "use 12V of drive".

This "Rds(on)" is the important section, its saying what voltage you need to drive the gate to get the device turned on (and how low a Rds resistance there will then be guaranteed).  You should probably check the absolute maximum as well of course, but you can ignore VT

If you take Vgs to somewhere between VT and the quoted Vgs you'll half-switch-on the device - since VT is highly variable you cannot be sure how well the device is turned on - you are in the linear region rather than the switching regime.
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