mosfet not right?

[MrDEB]

Trying to control a mosfet with a pic but it does not turn on.
Any reason why not?
Having very little experience with Mosfets but did some research and I think this should work but it doesn't.


https://www.electro-tech-online.com/custompdfs/2012/02/FDP55N06.pdf

 

[Ubergeek63]

you need 10V to turn that on hard... cant be going very fast if you are driving a relay...

 

[audioguru]

Your Mosfet has a threshold voltage from 2.0V to 4.0V for its gate-source voltage when it is barely turned on with a current of only 0.25mA. Who knows how much or how little your Mosfet will turn on with a gate-source voltage of only 5V from your PIC?

Use a "logic-level" Mosfet that is rated to turn on well when its gate-source voltage is 5V.
Usually a Mosfet has a low value resistor (use 47 ohms in your slow circuit) in series with the gate at the gate pin to prevent the Mosfet from oscillating at a very high frequency.

 

[Reloadron]

Per AG what you want is a logic level MOSFET, this link leads to some examples.

Something else you can try to get the gate voltage up is maybe add a ULN2003 between the PIC and MOSFET.

Ron

 

[MrDEB]

I contemplated a NPN transistor to turn it on. Seeing how I have the Mosfet and transistor already.
A logic level Mosfet is perhaps the best choice
THANKS

 

[MrDEB]

Well this should work?

Thought about just using a TIP31c transistor but a Mosfet seems better?
130ma is what the relay draws at 86 ohms.

 

[audioguru]

Why are you using a huge and powerful 55A Mosfet to turn on a little 12V 130mA relay?
Even a big TIP31C power transistor with a 100V at 3A rating is a waste.
I would use an inexpensive little 2N4401 transistor that has ratings of 40V at 600mA.

 

[ronv]

Or, as Ron suggested a logic level FET like the 2n7000.

 

[audioguru]

Or, as Ron suggested a logic level FET like the 2n7000.

No, because it is not logic level unless its load current is extremely low.
With Vgs at 4.5V, it is guaranteed to conduct only 75mA with a max voltage loss of 0.4V.

 

[ronv]

The way I read the graph is that Rds on is 8 ohms max at 200ma with 4.5v gate voltage. Gives him at least 11 volts for the relay.

 

[MrDEB]

I guess because I have it on hand.
The mosfet is already on the perf board.I could unsolder and use a 4401 as I have some where around here.

 

[ronsimpson]

I have used the 2n7000 in this application. But; 4.5V on voltage is close to the 5.0 supply which night be +/-5%.
There are many parts with on voltages in the 1 to 2.5 volt range.

 

[audioguru]

The way I read the graph is that Rds on is 8 ohms max at 200ma with 4.5v gate voltage. Gives him at least 11 volts for the relay.

The graphs on datasheets are for "typical" devices. Some are better but some are worse. If you design a circuit for a "typical" device and make a few then many WILL NOT WORK because their characteristics are less than "typical" (but they are still sold).

Semiconductor manufacturers have a production run every few months. Some of their devices have excellent spec's but others have poor spec's (made late on a Friday?). The datasheet lists the excellent and poor spec's together with the "typical" spec's.

What happens when your favourite manufacturer has no remaining stock on what you want then recently had a production run with poor but passable spec's?"What happens when a huge manufacturer buys all the "best" ones before you come to buy a few?
So if you don't design for "worst case" spec's then many of your products WILL NOT WORK!
Then you will spend the rest of your life testing thousands of circuits and replacing the weak parts with stronger parts. BORING.

I have had tens of thousands of my (worst case) designs manufactured and sold. They were all tested and NONE failed (except a couple that had a part installed backwards). It was easy to swap two ICs.
But if you design a circuit to use "typical" spec's then you might need to swap millions of parts. Lose your shirt.

 

[ronv]

No it is not typical. It is the multiplyer on worst case on resistance vs gate voltage vs drain current.
You can always use somethig like the ZVN2106 with a lower threshold voltage -- it only costs a few times more.
I came from an industry that built 30 million disk drives a year and cost and reliability were king.

 

[audioguru]

ALL my products are reliable because I designed them using WORST CASE SPECS and used major manufacturers.
Every single product was tested and passed with "flying colors".
Dumbo product manufacturers bought the cheap poor failed parts that I rejected and now are bankrupt.

 

[MrDEB]

Used a 2n2222 and it appears to work just fine.
The base is drawing 3ma as it should.
Ran simulation in TINA just to see how close it is and it is 3.4ma

 

[audioguru]

Maybe you are lucky that your Mosfet has a low gate switching voltage.

 

[audioguru]

Used a 2n2222 and it appears to work just fine.
The base is drawing 3ma as it should.
Ran simulation in TINA just to see how close it is and it is 3.4ma

Where is your new schematic/
What is the 2N2222 doing?

If the 2N2222 is saturated and is turning on your 130mA relay then the base current for the 2n2222 is supposed to be 13mA, not just 3.4mA.

 

[ronv]

But the hfe is 100 @ Ic=150ma.

 

[MrDEB]

Revised schematic

With unit ON I measure 210mv at the collector and 4.10 on the base.
Shouldn't make any difference but I still have an LED with a 330 ohm resistor on the RC6 port for testing purposes(coming off upon completion)
At 210mv I assume that the 2n2222 is in saturation?
The transformer marked 12v cube is dc output.