Logic Level MOSFET

I'm working on some electronics for part of my personal winter project and I could use some advice.

I would like to control an incandescent bulb (yes, can't be an LED.) with the output from a 3.3 volt logic device (

Reply to
James Fraser
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It's spelled "hobbyist". /pet peeve

Reply to
a7yvm109gf5d1

Well it depends what kind of MOSFET you are using, whether it is hard on at 3.3v. Also the pull up resistor and the impdeance of your drive output from your uC(?).

Look at the spec for your device. Is it hard on at 3.3v.

Regards, Rob.

Reply to
Robert Wilson

I haven't chosen a MOSFET. That's my question. How do I find the right piece? Browse through lots of data sheets? Or does anyone have a recommendation?

Hobbyist with less than perfect spelling... James Fraser

Reply to
James Fraser

You can use a FET with a low threshold but that threshold should be guaranteed. For example:

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Digikey P/N: SI2312BDS-T1-E3CT-ND

Around 50 cents a pop if you buy 25 and it's in stock. It is SMT, pretty much like everything else these days. If needed it can switch a whole lot more than 150mA so you might want to get some spares for other projects. Happy soldering ;-)

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

On a sunny day (Wed, 14 Nov 2007 13:51:46 -0800) it happened James Fraser wrote in :

A lightbulb, when cold, has about 1/10 the resistance from a hot one. So your MOSFET will have to be able to switch at least 1.5A with minimal losses.

A IRLZ34N MOSFET may do, it has a maximum gate threshold of 2V (minimum is 1), a TO220 case, and does 30A at 55V. It also has a build in zener to protect against avalance breakdown.

Reply to
Jan Panteltje

See my other post for a part number.

Other than that, Digikey helps: Key in FET, click "in stock", then scoot to the right and pick some that have the right current and Rdson @ Vgs. You can hold the CTRL key and left click to select all that could fit.

One more hint: Place a 1K resistor or so between gate and uC, in case your uC is expensive or hard to unsolder. It protects the uC in case you shorted a lamp an the FET "nukes out".

None of us is perfect. Some might think they are but they aren't :-)

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Vgs(th) doesn't tell you much; it tells you how much gate voltage you need to get a tiny drain current.

You need a part with seriously low Rds(on) with 3.3 volts on the gate... a couple of ohms max. There are lots of "logic level fets" around, so it shouldn't be a problem. Take a look at the Fairchild or IR or Zetex web sites.

The blinking/turnon transient may warm up the fet a bit, so don't use a really tiny (like SOT-23) package.

One trick is to add a resistor in parallel with the fet, to keep the filament warm all the time, to reduce the turnon surge. Cute, but you really don't need that here.

John

Reply to
John Larkin

Why not a NPN common emitter with open collector to the lamp as the load?

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Reply to
Jamie

Mmmmmmmmmmnah...not done that way:

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(fig 6) or for a very simplified approach on light loads (pun intended) US6518713....

Reply to
Fred Bloggs

Oh, quit being hobbier-than-thou.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

If I correctly interpret your "not done that way" as "nobody does this", you're just flat wrong. It's been done for decades, in thousands of applications.

If one must use incandescants, and inrush is a problem, PWM would be a simpler solution than all that stuff in the ancient Motorola appnote.

The patent is absurd. Why not just taper the PWM drive and be done with it?

John

Reply to
John Larkin

The hardest and hobbiest hobs are rumoured to be located in H-E-double-hockeysticks.

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

The first non-mechanical-rotary-switch turn signals for the T-bird used honking Ge transistors as the switches and had a "pre-warm" current active as soon as the ignition switch came on.

Later I used SCR's...

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...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Middle name "Elbert"? Is there some German blood flowing in your veins?

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Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

On a sunny day (Thu, 15 Nov 2007 09:47:15 -0700) it happened Jim Thompson wrote in :

That they allow that patent... seems trivial to me. You did not need the thermal strip. you can switch of a SCR in several ways, one to short it for say a few ms with a transistor (makes hold current zero), the other to reverse-voltage it by firing a second SCR connected via a capacitor. me 1968.

Reply to
Jan Panteltje

Thanks all for the tips and advice. Right now I'm leaning towards IRF3706 series that specs max Rds at 22 mOhm at Vgs 2.8V.

But your comment got me thinking more. Yes, my uC is expensive and hard to change. I hadn't really thought about failure modes much. If I put in a resistor between uC and gate, then I have to up my pull-down resistors (I suppose I don't need those and the FET gate can discharge through the uC. I need to look into that.)

Should I look at more isolation between the FET and controller? Not hard to do, I think, since I'm not worried about frequency response.

Again, thanks to all for the help and tips... James Fraser

Reply to
James Fraser

Yep. My grandmother Thompson's maiden name was Rhoda Warner and Riverton, WV, is in an area of the Appalachians called Germany Valley.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Wow, those are huge.

Nah. Just make sure the FET turns on and off fast enough so it spends the least amount of time in the linear region. Your FETs are a bit largish, about 2500pF gate capacitance. 1K in series from uC pin to gate would mean an RC constant of 2.5usec, still pretty fast. This roughly will be your turn on and turn off time. In the event that the gate isolation melts down or pops for some reason it might worst case get connected to +5V, meaning above the VCC of your uC. However, since there is that 1K series resistor the substrate diode will only be hit with a few mA. Look in the absolute maximum table. Most can tolerate 10mA, except for example the MSP430 which is spec'd at 2mA although it should certainly be able to take more.

What is more important is that the 3.3V rail has enough load that a broken gate isolation layer doesn't drive this 3.3V rail up through the

1K resistor. This can happen regardless of whether you have a buffer driver or not.

I only do that if I need to preserve very fast transition times. Then I use 74HC buffers and the like. Since you are just turning lamps on and off at slow rates I don't see the need for that here.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Me too: Wussner, Munsch, and Wycks.

John

Reply to
John Larkin

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