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'PIC and power MOSFET'
1998\04\09@234025 by FScalini

picon face
I'm trying to use a PIC to control several amps (PWM).  From what I read in
catalogs, etc. MOSFETs are the way to go so I bought a 4A MOSFET (IFR510) at
RadioShack.  The ratings and specifications that came with the mosfet says
"low drive current".  The PIC manual says "high source current (~20mA).  Am I
safe to attach a PIC output pin directly to the gate of the mosfet or is there
a safer way to do this.

Looking throught the MOSFET list in the DigiKey catalog I only see one column
with the mA units (Id) so I'm guessing that's the drive current.  But those
values are ranging from about 100 to over 1000mA so I'm reluctant to
experiment with my one remaining functioning PIC.

One more question:  The MOSFET lowers it's resistance somewhat when I apply 5V
to the gate but doesn't seem to saturate until i hit around 10V.  Whats the
typical (or most efficient/lowest cost/lowest hassle) way to use the PIC and
the MOSFET to their full potential.

Thanks.

1998\04\10@021142 by Mike Keitz

picon face
On Thu, 9 Apr 1998 23:39:05 EDT FScalini <spam_OUTFScaliniTakeThisOuTspamAOL.COM> writes:
>  Am I
>safe to attach a PIC output pin directly to the gate of the mosfet or
>is there
>a safer way to do this.
[...]
>with the mA units (Id) so I'm guessing that's the drive current.  But
>those
>values are ranging from about 100 to over 1000mA so I'm reluctant to
>experiment with my one remaining functioning PIC.

Id is the *drain* current, the current that the FET can control on the
output side.  The drive current at DC is very close to zero.  But, there
is a large capacitance from the gate to the source and drain which mean
that rather heavy transient drive currents are required to turn a MOSFET
on or off quickly.  Usually a small resistor (22 ohms) is connected in
series with the gate to help reduce instability from feedback through the
internal capacitances.  The series resitor will also reduce the current
surge from the PIC pin when the logic level changes.

>One more question:  The MOSFET lowers it's resistance somewhat when I
>apply 5V
>to the gate but doesn't seem to saturate until i hit around 10V.

The older type MOSFETs (such as the IRF510) require about 10V to turn on
fully.  "Logic Level" MOSFETs have been developed which require only 5V.
For example, the IRL510 is similar to the IRF510 but has a lower turn-on
voltage.  It will work with 5V drive supplied directly from the PIC.  The
logic level devices tend to be a little more expensive and aren't
available in the highest voltage / power ratings.

>Whats the
>typical (or most efficient/lowest cost/lowest hassle) way to use the
>PIC and
>the MOSFET to their full potential.

Set the IRF510 aside and order some logic level FETs (as well as more PIC
chips).  Connect a resistor of about 22 ohms in series from the PIC to
the gate.  Also connect a resistor of about 10K from the gate to source
(ground).  When the PIC is first reset, the pins are inputs so the drive
voltage to the gate is indeterminate (it is floating).  The resistor to
ground ensures the MOSFET will stay off until the PIC starts to drive it.


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1998\04\10@073850 by John Sanderson

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face
Hello PIC.ers,
..
{Quote hidden}

..
You'll need >10 volts to gate the mosfet properly on, try it with
less and the thing will likely cook.
..
I've done this very satisfactorily from the open-collector output RA4 pin
of a 16x84 or '71 PIC, using a 10k pullup to 12-14 volts.
This again only works well 'till you want to switch the PWM at a high
frequency, then you need to drive the gate a hunk harder.
The 10k pullup won't allow the gate rise time to be short enough to
keep the dissipation acceptable.
..
Solution, cheap & effective:
Use RA4 as before but as input to a 555 set up as a Schmitt-trigger
invertor. Drive the mosfet gate straight from the 555 output.
This is a stiff buffer, switches fast and will handle a continuous 200 mA.
Just remember the PWM is thereafter inverted in sense.
..
Best regards,   John.
..
..
email from John Sanderson at
JS Controls, PO Box 1887, Boksburg 1460, Rep. South Africa
Manufacturer & purveyor of laboratory force testing apparatus
and related products and services.
Tel/fax: Johannesburg 893 4154    Cellphone 082 453 4815

1998\04\10@105518 by Wynn Rostek

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face
At 11:39 PM 4/9/98 EDT, you wrote:
>I'm trying to use a PIC to control several amps (PWM).  From what I read in
>catalogs, etc. MOSFETs are the way to go so I bought a 4A MOSFET (IFR510) at
>RadioShack.  The ratings and specifications that came with the mosfet says
>"low drive current".  The PIC manual says "high source current (~20mA).  Am I
>safe to attach a PIC output pin directly to the gate of the mosfet or is
there
>a safer way to do this.

Yes, you can connect direct in this case.  When in doubt, use a 1k ohm
resistor between the PIC output pin and the device you are driving. MOSFETs
have a very high input impedance, well, well under a microamp drive.  To
the driving device it looks more like a cap than anything else.

Wynn Rostek
WB4ZUY

1998\04\10@145751 by H.P. de Vries

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face
At 10:53 AM 4/10/98 -0400, you wrote:
>At 11:39 PM 4/9/98 EDT, you wrote:
>>I'm trying to use a PIC to control several amps (PWM).  From what I read in
>>catalogs, etc. MOSFETs are the way to go so I bought a 4A MOSFET (IFR510) at
>>RadioShack.  The ratings and specifications that came with the mosfet says
>>"low drive current".  The PIC manual says "high source current (~20mA).
Am I
>>safe to attach a PIC output pin directly to the gate of the mosfet or is
>there
>>a safer way to do this.
>
>Yes, you can connect direct in this case.  When in doubt, use a 1k ohm
>resistor between the PIC output pin and the device you are driving. MOSFETs
>have a very high input impedance, well, well under a microamp drive.  To
>the driving device it looks more like a cap than anything else.
>
It is a capacity actually. ;)

Hans
Hans de Vries - H.P.d.VriesspamKILLspamstud.tue.nl
+-------------------------------------------------+
|         the Official MeadoW Homepage            |
|         http://www.dse.nl/~meadow               |
| email:  .....meadowKILLspamspam.....dse.nl                           |
+-------------------------------------------------+

1998\04\10@204841 by Andy Kunz

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face
WHen I really need to use a 10V FET from a PIC, I always put a FET driver
in front of it.  They cost a little, but it sure help to get RDSon way down
there.

Andy



At 11:39 PM 4/9/98 EDT, you wrote:
>I'm trying to use a PIC to control several amps (PWM).  From what I read in
>catalogs, etc. MOSFETs are the way to go so I bought a 4A MOSFET (IFR510) at
>RadioShack.  The ratings and specifications that came with the mosfet says
>"low drive current".  The PIC manual says "high source current (~20mA).  Am I
>safe to attach a PIC output pin directly to the gate of the mosfet or is
there
>a safer way to do this.
>
>Looking throught the MOSFET list in the DigiKey catalog I only see one column
>with the mA units (Id) so I'm guessing that's the drive current.  But those
>values are ranging from about 100 to over 1000mA so I'm reluctant to
>experiment with my one remaining functioning PIC.
>
>One more question:  The MOSFET lowers it's resistance somewhat when I
apply 5V
>to the gate but doesn't seem to saturate until i hit around 10V.  Whats the
>typical (or most efficient/lowest cost/lowest hassle) way to use the PIC and
>the MOSFET to their full potential.
>
>Thanks.
>
>
==================================================================
                    Andy Kunz - Montana Design
         Go fast, turn right, and keep the wet side down!
==================================================================

1998\04\10@214237 by Robert McAtee

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face
>I'm trying to use a PIC to control several amps (PWM).  From what I read in
>catalogs, etc. MOSFETs are the way to go so I bought a 4A MOSFET (IFR510) at
>RadioShack.  The ratings and specifications that came with the mosfet says
---------- SNIP-------------
I used the IFR510 to control a small RS motor from the basic stamp pin.
Output was 5v and I put a 500K resistor in series with the gate. A "micro"
ampmeter did not show any gate current at all ???. I used a 6v lantern
battery for motor power..... ==Mac==

1998\04\11@050522 by Alex Torres

picon face
> From: Robert McAtee <EraseMEw5tnjspam_OUTspamTakeThisOuTCAMALOTT.COM>
>
> >I'm trying to use a PIC to control several amps (PWM).  From what I read
in
> >catalogs, etc. MOSFETs are the way to go so I bought a 4A MOSFET
(IFR510) at
> >RadioShack.  The ratings and specifications that came with the mosfet
says
> ---------- SNIP-------------
> I used the IFR510 to control a small RS motor from the basic stamp pin.
> Output was 5v and I put a 500K resistor in series with the gate. A
"micro"
> ampmeter did not show any gate current at all ???. I used a 6v lantern
> battery for motor power..... ==Mac==

Why you need _SERIES_ resistor in MOSFET ???  You may shortly connect PIC's
output to gate.
One thing - during the reset (when power on) the pic;s pins are in
Z-condition, so it is good practice to use resistor from the gate to ground
(to prevent floating gate).

Alex Torres, Kharkov, Ukraine (exUSSR)
altorspamspam_OUTgeocities.com
2:461/28 FidoNet
http://www.geocities.com/SiliconValley/Lab/6311

1998\04\11@182028 by Andy Kunz

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face
>Why you need _SERIES_ resistor in MOSFET ???  You may shortly connect PIC's
>output to gate.

This series R is going between the Gate and the PIC.  If you turn it on
(and off) without the resistor, the rise time is very short and generates
many harmonics which can keep your device from FCC approval.  The PIC will
provide 20mA of current if required, so it will turn on the device VERY
quickly.

Andy

==================================================================
                    Andy Kunz - Montana Design
         Go fast, turn right, and keep the wet side down!
==================================================================

1998\04\12@114909 by J Nagy

flavicon
face
       Just my two bits worth...I think the high current capability of the
PIC is a big part of the problem in this case. Output impedance is
relatively low so that when combined with the wiring inductance and the
gate capacitance, a tuned circuit is formed. By adding a small (<100 ohm)
resistance, oscillations are damped without degrading gate voltage rise
time too badly. If the wiring between PIC and MOSFET is relatively short
(<12"), you likely won't need a resistor, as the inductance will be small
(so the damping factor will be large).

       Jim

{Quote hidden}

1998\04\12@171211 by Robert McAtee
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face
Yes Alex you are right. I use resistor just to see what it do. Understand,
good idea for resistor from gate to ground. I found MOSFET do strange
things when gate float..... ==Mac==

{Quote hidden}

1998\04\13@062559 by Andy Kunz

flavicon
face
>time too badly. If the wiring between PIC and MOSFET is relatively short
>(<12"), you likely won't need a resistor, as the inductance will be small
>(so the damping factor will be large).

I drive FETs with a PIC-to-gate length of two inches or less at 3 KHz.  I
use a 22 ohm R to keep down noise.

It isn't the inductance _here_ that's the problem - the wiring to the
battery pack is often several inches.  Turning on a low-ohm load (an
electric motor) at that speed with a fast rise time makes the wires and
motor to "sing."

Because this is in an RF environment (RC model) it is necessary to keep the
noise down.  The fast rise time generates lots of harmonics.

Andy

==================================================================
                    Andy Kunz - Montana Design
         Go fast, turn right, and keep the wet side down!
==================================================================

1998\04\13@234851 by Dwayne Reid

flavicon
face
>Why you need _SERIES_ resistor in MOSFET ???  You may shortly connect PIC's
>output to gate.
>One thing - during the reset (when power on) the pic;s pins are in
>Z-condition, so it is good practice to use resistor from the gate to ground
>(to prevent floating gate).

A small series resistor is useful in preventing parasitic oscillations.  I
generally use a 100 Ohm resistor in series with the gate, as close to the
FET as possible.  As Alex says, you may also need a resistor to ensure the
FET stays off while the PIC is in reset.

dwayne

Dwayne Reid   <KILLspamdwaynerKILLspamspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, Alberta, CANADA
(403) 489-3199 voice     (403) 487-6397 fax

1998\04\14@013524 by tjaart

flavicon
face
Alex Torres wrote:

> > From: Robert McAtee <RemoveMEw5tnjTakeThisOuTspamCAMALOTT.COM>
> >
> > >I'm trying to use a PIC to control several amps (PWM).  From what I read
> in
> > >catalogs, etc. MOSFETs are the way to go so I bought a 4A MOSFET
> (IFR510) at
> > >RadioShack.  The ratings and specifications that came with the mosfet
> says
> > ---------- SNIP-------------
> > I used the IFR510 to control a small RS motor from the basic stamp pin.
> > Output was 5v and I put a 500K resistor in series with the gate. A
> "micro"
> > ampmeter did not show any gate current at all ???. I used a 6v lantern
> > battery for motor power..... ==Mac==
>
> Why you need _SERIES_ resistor in MOSFET ???  You may shortly connect PIC's
> output to gate.

Nope, you can't. The maximum capacitance _directly_ connected to an output pin
is something like 50pF (or was is 25pF - I can't remember). The inrush current
into the MOSFET gate could cause a few nasties. A small series resistor keeps this
down to an acceptable level. 5V/20mA = 250ohm minimum, so use a 270 Ohm.

--
Friendly Regards

Tjaart van der Walt
spamBeGonetjaartspamBeGonespamwasp.co.za

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1998\04\14@120541 by Jason Sachs

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(new to this list, so I hope I'm not answering a Q that
someone has already answered)

The IRF510 requires 10V to guarantee full turn-on.
It may turn on somewhat at 5V... you will be better off with
an IRL510 (100V, .54 ohm) or an IRLZ14 (60V, .24 ohm), which
are "logic-level" FETs that turn on fully at 5V. (And IR even guarantees
on-resistance for some of them at 4V.) These are jellybean
MOSFETs available from DigiKey for 60-70 cents in single quantities.

Technically the "max" capacitance in the specs refers to
the max. capacitance you can hook up to the pin that Microchip
will still guarantee normal operation. A small MOSFET running
from 6V w/ gate tied directly to a PIC output shouldn't hurt a PIC
(although it's outside the mfr-guaranteed operation of the PIC), but
it's a larger-than-normal capacitive load; the PIC will most likely
provide as much output current as it can to pull the gate up,
and it'll just mean a longer than normal rise time.

(this is true for pretty much any digital chips w/ output tied directly
to a FET gate)

Strictly speaking, though, if you want the the behavior of the part
to be guaranteed correct, you should use a resistor or a MOSFET driver
to ease the load on a digital output. (i.e., what Tjaart said.) The
larger the resistor the slower the turn-on time; it should be
in the 100's of ohms.

Check out "The Art of Electronics" by Horowitz & Hill:
they are a great source for how to use FETs (and just about any other
electronic component) & will tell you stuff like putting a freewheeling
diode to prevent inductive kickbacks from the motor from damaging
your circuit.

Jason Sachs
Electrical Engineer

Deka Research & Development
340 Commercial Street
Manchester, NH 03101
(603) 669 5139 x327

> {Original Message removed}


'PIC and power MOSFET'
1998\06\14@215525 by Harold Hallikainen
picon face
       I've driven a few fes directly from PICs without any problem.
These are typically low frequency applications (60Hz or so).  While
inrush current due to gate source capacitance and the Miller capacitance
between the gate and drain can result in a high inrush current, I've just
let the PIC current limit.
       However, another reason to put in a resistor is source lead
inductance.  I had an application where I was switching 240 VAC at 50 KHz
using FETs.  When the FET switched off, the source would swing negative
with respect to ground (and the gate).  This would cause the gate-source
voltage limit to be exceeded.  Adding a series resistor between the pulse
transformer driving the gate and the gate allowed the gate to follow the
source during the transiient, resulting in far fewer blown fets.


Harold


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