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'[PIC]:interfacing power transitor'
2002\02\12@184436 by Dave King

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Hi

Just wondering if anyone has any information or know of any on-line for
interfacing a
power transistor. I'm trying to drive about a non-inductive 2 amp load.
This circuit for
lack of a better description would be the blinking led on steroids. The
signal is 2 pulses
(1/10th duration) at 1hz. Board voltage is 3.3vdc.

I've tried a few various fets and such but haven't found one I can get to
turn on cleanly.
I also tried driving a mosfet with a 2n2222 but that didn't work out either
(even though it
should).

I'm missing something here but I think I've been staring at it too long to
figure out what so
HEEEELLLLPPPPP!!!!


Cheers

Dave

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2002\02\12@185531 by David VanHorn

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>
>I'm missing something here but I think I've been staring at it too long to
>figure out what so
>HEEEELLLLPPPPP!!!!

What is the Vth of the fet?

You have to get the gate above Vth, to turn it fully on.
For very low voltage designs, you end up needing either some mechanism to
create a gate voltage supply, or just use bipolars.

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2002\02\12@205421 by Dwayne Reid

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At 03:31 PM 2/12/02 -0800, Dave King wrote:
>Hi
>
>Just wondering if anyone has any information or know of any on-line for
>interfacing a
>power transistor. I'm trying to drive about a non-inductive 2 amp load.
>This circuit for
>lack of a better description would be the blinking led on steroids. The
>signal is 2 pulses
>(1/10th duration) at 1hz. Board voltage is 3.3vdc.

Do you have a higher supply voltage available?  Most FETs - even logic
level FETs - will not fully enhance below 4V.

You can use a Darlington bipolar transistor: one NPN part I use is
TIP121.  Feed the base with a 220R resistor (Ib ~ 10 mA if Vdd=3.3V).  The
collector will handle 2 Amps easily - but with a fairly nasty saturation
voltage of about 1V.

If you DO have a higher supply available, use it to turn on a MOSFET.  For
example, where is your 3.3V derived from?  Can you use that?

dwayne


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2002\02\12@213820 by Gordon Varney

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Dave,
       For lack of information, I will explain a few ideas. First use an N-Channel
FET and drive the circuit from the ground side. Use a logic level FET if
using a microcontroller directly. Example: controller output pin to a 100
ohm resister to the gate of a logic level FET.

       If trying to high side switch the FET, you will require a gate driver that
can produce about 15Vdc above the input voltage source. You pulse period is
to long to boot strap the gate.

       Use a PNP transistor on the high side and drive it like a series pass
regulator.

Good Luck

Gordon Varney
http://www.iamnee.com



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2002\02\12@225211 by Dave King

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>Do you have a higher supply voltage available?  Most FETs - even logic
>level FETs - will not fully enhance below 4V.
I actually compromised and went with 3.3V on the board. I thought that
was a good thing as the per board cost dropped from about $47 to about
$7. I didn't think I was going to give myself a stroke getting these #$%
transistors to behave.

>You can use a Darlington bipolar transistor: one NPN part I use is
>TIP121.  Feed the base with a 220R resistor (Ib ~ 10 mA if Vdd=3.3V).  The
>collector will handle 2 Amps easily - but with a fairly nasty saturation
>voltage of about 1V.

I'll see if that would work with this. I'm sure there is some part that
will but
I've not stumbled across it yet.

>If you DO have a higher supply available, use it to turn on a MOSFET.  For
>example, where is your 3.3V derived from?  Can you use that?

I have a smart smps as I don't know exactly what my power is going to be
out in the field. So all I can count on is the 3.3.  I've been debating if
I need
to scrap the 3.3 and go back to 5vdc but then I need to start adding back in
parts.

TNX

Dave

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2002\02\12@233157 by Bob Ammerman

picon face
PIC pin via resistor to base of 2N2222

Emitter of 2N2222 to ground

Collector of 2N2222 via resistor to base of PNP power transistor

Emitter of PNP power transistor to +V

Collector of PNP power transistor is your output

Driving the PNP into full saturation your output voltage will be +V - Vcesat

Select R's to ensure full saturation of both transistors

You may also want a resistor between PNP base and +V to ensure it remains in
cutoff when 2N2222 isn't driven.

Bob Ammerman
RAm Systems


{Original Message removed}

2002\02\13@014251 by Andrei B.

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--- David VanHorn <.....dvanhornKILLspamspam@spam@CEDAR.NET> wrote:
> >
>
> What is the Vth of the fet?
>
> You have to get the gate above Vth, to turn it fully on.
> For very low voltage designs, you end up needing either some
> mechanism to
> create a gate voltage supply, or just use bipolars.

Another way to do it is to use a dedicated MOSFET driver which contains
a charge pump to create the necessary high gate drive voltage. One such
circuit is the MAX622 (see http://www.maxim-ic.com).

I seen recommendations for it on some pages, but never used it as it's
not available from any retailer here in Romania.



=====
ing. Andrei Boros
Centrul pt. Tehnologia Informatiei
Societatea Romana de Radiodifuziune

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2002\02\13@025458 by Vasile Surducan

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On Tue, 12 Feb 2002, Andrei B. wrote:

> Another way to do it is to use a dedicated MOSFET driver which contains
> a charge pump to create the necessary high gate drive voltage. One such
> circuit is the MAX622 (see http://www.maxim-ic.com).
>
> I seen recommendations for it on some pages, but never used it as it's
> not available from any retailer here in Romania.
>
 Andrei, maybe that's because romanians know to compute and design easily
a step-up driving transformer for any MOSFET or better to use bipolar
transistors, cheapest and easyest to drive ( but with problems at really
high currents... )

regards, Vasile

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2002\02\13@033843 by Mike Blakey

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Hi Dave,

Is your 2A load using the 3.3V supply or a higher voltage?



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2002\02\13@043023 by Dave King

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At 08:25 AM 2/13/02 +0000, you wrote:
>Hi Dave,
>
>Is your 2A load using the 3.3V supply or a higher voltage?

Everything is running off of the single 3.3v supply.

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2002\02\13@071010 by Andrei B.

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--- Vasile Surducan <vasilespamKILLspamS3.ITIM-CJ.RO> wrote:
> On Tue, 12 Feb 2002, Andrei B. wrote:
>
>   Andrei, maybe that's because romanians know to compute and design
> easily
> a step-up driving transformer for any MOSFET or better to use bipolar
> transistors, cheapest and easyest to drive ( but with problems at
> really high currents... )

Eh, in cautarile mele despre punti H am dat si peste solutia asta.
Oricum, pentru un montaj cat mai compact posibil, dar care tine la
curent mare, parca l-as prefera.
Iar la facut rost/confectionat traf-uri, intru in alta belea si mai
tare... Cel putin cu materialele pe care le gasesc pe aici.

=====
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Centrul pt. Tehnologia Informatiei
Societatea Romana de Radiodifuziune

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2002\02\13@170732 by Dave King

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>Select R's to ensure full saturation of both transistors
>
>You may also want a resistor between PNP base and +V to ensure it remains in
>cutoff when 2N2222 isn't driven.
>
>Bob Ammerman
>RAm Systems

Thanks Bob!

This was close to what I had tried with a few differences. I used a ZTX851
as one of the
other fellows suggested as well as your suggestions and it worked (sort of).

It's now functional but I need to clean up the signal a bit more to make it
perfect.
On the scope it would be best described as somewhat ugly and grungy. I've also
lost my clean square wave and have a saw tooth now.

Cheers

Dave

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2002\02\13@231607 by Bob Ammerman

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----- Original Message -----
From: "Dave King" <.....kingdwsKILLspamspam.....SHAW.CA>
To: <EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU>
Sent: Wednesday, February 13, 2002 2:04 PM
Subject: Re: [PIC]:interfacing power transitor


> >Select R's to ensure full saturation of both transistors
> >
> >You may also want a resistor between PNP base and +V to ensure it remains
in
> >cutoff when 2N2222 isn't driven.
> >
> >Bob Ammerman
> >RAm Systems
>
> Thanks Bob!
>
> This was close to what I had tried with a few differences. I used a ZTX851
> as one of the
> other fellows suggested as well as your suggestions and it worked (sort
of).
>
> It's now functional but I need to clean up the signal a bit more to make
it
> perfect.
> On the scope it would be best described as somewhat ugly and grungy. I've
also
> lost my clean square wave and have a saw tooth now.
>
> Cheers
>
> Dave

Sounds like one or more of the following:

The source impedence on your signal is a bit high.

You have some capacitance floating around that is slowing transitions.

Your R's are too high and you are not driving the transistors fully into
saturation.

Q: Refresh my memory:

1: source of signal

2: frequency of signal

3: duty cycle of signal


Bob Ammerman
RAm Systems

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2002\02\14@145057 by Dave King

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>The source impedence on your signal is a bit high.

K

>You have some capacitance floating around that is slowing transitions.


>Your R's are too high and you are not driving the transistors fully into
>saturation.

I have a feeling this is most of it. If you looked at the scope the signal
would be one or two pulses fairly clean then it behaves like it never
cleanly shuts off or over drives

>Q: Refresh my memory:
>1: source of signal

25ma source from a 74hc04 which is in turn driven by the pic. Everything
is perfect going in and coming out of the 04. I'm actually pulling two
identical
signals from the 04. Each separately goes to a 2n2222 then to the ztx851
and then to the load. Each 851 has a separate line to the power source.

>2: frequency of signal

2 pulses @ 1hz

>3: duty cycle of signal

total 20% - 2 x 100ms duration pulses with a 100ms low space between the pulses
then 700ms of low.

looks like (bad ascii art following....)    1010000000   or  -_-_______

I know there is a stoooopid simple solution to this but I've been staring
at it too long. ;-]

Dave

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