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PICList Thread
'[EE](noob) Power transistor & PIC'
2006\03\14@101108 by Metis Adrastea

picon face
Hi!

I know that this question is pretty obvious for many of the list
members, but I have some trouble connecting a power BJT to a pic
because I don't know much about transistors.

I have a RC helicopter which I want to switch on (max 20 minutes) and
off with a PIC, and I would want to keep the circuit smallish. I have
a ST BD243C NPN power transistor
(http://www.st.com/stonline/products/literature/ds/5053/bd243c.pdf)
lying around here, and I've thought about using it.

The heli works with a 8.4V 700mAh NiMH battery that goes up to 10V on
full charge, and I suppose that it doesn't sink much more than 2A (I
may measure it) :-)

So, I've thought using a bc557 (PNP general purpose) in a darlington
configuration to make it work, but i'm pretty lost about what
resistors I would have to use, and if there is any better solution in
terms of space/component saving (maybe a power fet directly to the
PIC?).

When I manage to make this prototype work I would want to make a final
version using smd components, so it also could be taken into account
:-)

Any ideas?

Thanks in advance, and regards

Metis Adrastea
--
He comentado ya que tengo un blog? O:-)
http://metisadrastea.blogspot.com/

2006\03\14@172814 by Metis Adrastea

picon face
Hi!

I know that this question is pretty obvious for many of the list
members, but I have some trouble connecting a power BJT to a pic
because I don't know much about transistors.

I have a RC helicopter which I want to switch on (max 20 minutes) and
off with a PIC, and I would want to keep the circuit smallish. I have
a ST BD243C NPN power transistor
(http://www.st.com/stonline/products/literature/ds/5053/bd243c.pdf)
lying around here, and I've thought about using it.

The heli works with a 8.4V 700mAh NiMH battery that goes up to 10V on
full charge, and I suppose that it doesn't sink much more than 2A (I
may measure it) :-)

So, I've thought using a bc557 (PNP general purpose) in a darlington
configuration to make it work, but i'm pretty lost about what
resistors I would have to use, and if there is any better solution in
terms of space/component saving (maybe a power fet directly to the
PIC?).

When I manage to make this prototype work I would want to make a final
version using smd components, so it also could be taken into account
:-)

Any ideas?

Thanks in advance, and regards

Metis Adrastea
--
He comentado ya que tengo un blog? O:-)
http://metisadrastea.blogspot.com/

2006\03\14@204838 by kravnus wolf

picon face
   From my understanding you can use the BD243C NPN
power transistor through the PIC. Just pull the pin
high from the PIC to the base of the NPN and it would
start powering the heli. I assume that you are
attaching the heli in series to the NPN?


*BEFORE attempting this please check value on the base
resistor*
  John


   
--- Metis Adrastea <spam_OUTmetisadrasteaTakeThisOuTspamgmail.com> wrote:

{Quote hidden}

(http://www.st.com/stonline/products/literature/ds/5053/bd243c.pdf)
{Quote hidden}

> --

2006\03\15@061140 by Michael Rigby-Jones

picon face


{Quote hidden}

I think you may have underestimated the current.  Even the small Dragonfly/Hummingbird type Heli's can take over 10Amps peak with stock motors

>So, I've thought using a bc557 (PNP general purpose) in a
>darlington configuration to make it work, but i'm pretty lost
>about what resistors I would have to use, and if there is any
>better solution in terms of space/component saving (maybe a
>power fet directly to the PIC?).
>
>When I manage to make this prototype work I would want to make
>a final version using smd components, so it also could be
>taken into account

By far the simplest method would be to use a power MOSFET. The problem with bipolar power transistors is that their gain is typical quite low, so you need to pump significant current into the base.  A darlington helps the base current problem, but introduces it's own problem of high Vce(sat) which is likely to be problematic in this application.  A MOSEFT takes zero gate current, apart from when actualy turning it on or off, so it can be comfortably driven by the PIC.  If you have to switch the positive line you will need a P-channel device, and another transistor (can be bipolar) to translate the voltages required on the gate.  Easiest would be to use an N-channel device and switch the ground connection of the heli if possible.

Regards

Mike

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2006\03\15@102348 by kravnus wolf

picon face
Nice tip there Michael.

Thanks,
John

--- Michael Rigby-Jones
<.....Michael.Rigby-JonesKILLspamspam.....bookham.com> wrote:

>
>
> >{Original Message removed}

2006\03\15@135937 by Metis Adrastea

picon face
Ok, so an N-channel power MOSFET could be the best option to suit my
needs (no problemo switching ground line). I've understood that I
should use another transistor to drive the gate, maybe because of the
current drain on switching?

I don't mind switching speed very much, so could I put a resistor (1K
or so) in series with the gate? (pic pin --> resistor --> gate) In
this manner the current would not be very high on switching, and it
could fall as long as the gate capacitor gets charged, I'm ok? Could
the pin on the PIC float when the transistor is off?

I've looked for something suitable and I've seen the STB16NF06L (smt
is ok), could this critter suit my needs? (ID max=16A) Any clue?
Another component (easier to buy or something)?

(As you can see, I know almost nothing about transistors, sorry if
this topic is too noob)

Regards

Metis Adrastea
--
He comentado ya que tengo un blog? O:-)
http://metisadrastea.blogspot.com/

2006\03\15@204309 by kravnus wolf

picon face


--- Metis Adrastea <EraseMEmetisadrasteaspam_OUTspamTakeThisOuTgmail.com> wrote:

> Ok, so an N-channel power MOSFET could be the best
> option to suit my
> needs (no problemo switching ground line). I've
> understood that I
> should use another transistor to drive the gate,
> maybe because of the
> current drain on switching?
 the transistor is used to translate the voltage to a
suitable level of the MOSFET gate.

>
> I don't mind switching speed very much, so could I
> put a resistor (1K
> or so) in series with the gate? (pic pin -->
> resistor --> gate) In
> this manner the current would not be very high on
> switching, and it
> could fall as long as the gate capacitor gets
> charged, I'm ok? Could
> the pin on the PIC float when the transistor is off?
 a PIC pin must never float. For it consume power
unnecessarily and could ON certain component. The
forward bias for a pn junction is 0.7v.

 try playing around with a npn BJT. build simple
circuits for it. measure it's Vce,Vbe.

http://www.allaboutcircuits.com/vol_3/index.html

john

{Quote hidden}

> --

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