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'[PIC] Harddisk motor SOLUTION'
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Here's a circuit that will run a hard disk motor. Shown is a 3 wire
motor. If you use a 4 wire motor just leave one leg unconnected(connect
the center of the star to the 220uF caps).
Inverters are sections of a CD4049. Transistors are one each jellybean
NPN and PNP, I used PN2222A and PN2907A.
Supply voltage to suit your motor, I used 12 volts.
Circuit is NOT SELF STARTING. With pot at greatest resistance, spin the
motor by hand to start. Then advance the pot to desired speed.
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Many thanks Bob! Tomorrow i will try!
Bob Blick wrote:
Nicola, the previous link I posted mentions a variable frequency
speed control. A similar simple speed control is shown going into
a4 in this circuit
Many other alternatives for discrete components (555, transistor
bistable, logic etc)
Again, sorry for colour scheme. Too many drugs. Or too few
What is the purpose of the 74LS06? Shouldn't the PIC have enough power to
drive the FETs directly? Is it the gate capacitance that gets you at high
----- Original Message -----
From: "Jinx" <clear.net.nz> joecolquitt
To: "Microcontroller discussion list - Public." <mit.edu> piclist
Sent: Friday, April 11, 2008 6:06 PM
Subject: Re: [PIC] Harddisk motor SOLUTION
: You could try this - simple, works well
: (sorry about colour scheme. Really sorry. On extensive 'To Do'
: list. It should have been on a 'To Don't' list initially)
Gordon Williams wrote:
> What is the purpose of the 74LS06? Shouldn't the PIC have enough power to
> drive the FETs directly? Is it the gate capacitance that gets you at high
> Gordon Williams
The '06 has open collector outputs for level shifting to the fets which
want to see ground to 12v switching. Note the pullup resistors.
> What is the purpose of the 74LS06? Shouldn't the PIC have
> enough power to
> drive the FETs directly? Is it the gate capacitance that
> gets you at high
In Joe's circuit the 7406 serves two functions that the PIC
- It provides 12 volt drive to the FET gates.
Whether this is needed, compared to the 5V drive that the
PIC can provide, depends on the FETs used, but many FETs are
not "fully enhanced" at 5V drive and benefit greatly in
Rdson (Vds on resistance) with more drive voltage.
While the FETs that Joe shows have a nominal 4V Rth max they
can barely pull the skin off a rice pudding at that drive
level and really start working well at about 8v Vgs and are
best at Vgs of 10V or more. As they have a Vgs of 20V max
using 12V when it is available makes sense. With 5V drive
they typically drop about 0.5V at 1.5A . Not utterly world
ending althought teh 0.75W dissipation approaches the
package 1W at 25C limit.
The quoted Rdson is 0.2 ohm at 1.7A implying a voltage drop
of V=IR = 1.7 8 0.2 = 0.34V which would need 6 to 8V gate
drive to achieve.
- FET gates have substantial capacitance. To get fast
switching this capacitance needs high currents to "sweep
out" the charge stored in the gate capacitance. Currents in
the 100 mA - 1A range for very short periods are typically
appropriate. A PIC is generally not capable of providing
such current levels and the result is substantial slowing of
switching times when a PIC drives a FET gate directly. The
7406 can provide improved gate drive and consequently
improved switching times, especially when turning the FET
off, when used as shown.
Note that Joe has shown the gate resistors as 47k and 4k7
for low and high side FET drivers. I suspect that this is a
typo. A value of 4k7 would be preferred and even lower would
be good. At 4k7 the on current is only 12/4k7 ~= 2.5 mA so
turn on will be 'pedestrian'. In this case the motor
switching rate is slow so this (and the section on gate
drive above) is less critical.
> ending althought the 0.75W dissipation approaches the
> package 1W at 25C limit.
> Note that Joe has shown the gate resistors as 47k and 4k7
> for low and high side FET drivers. I suspect that this is a
The components, their positions and values are actually off
the original Conner PCB
>> ending althought the 0.75W dissipation approaches the
>> package 1W at 25C limit.
>> Note that Joe has shown the gate resistors as 47k and 4k7
>> for low and high side FET drivers. I suspect that this is a
> The components, their positions and values are actually off
> the original Conner PCB
And they worked fine. As you noted on your schematic, the switching
frequency was in the ball park (*) of 1KHz. The drive is feeble but is
more than enough so why hit it harder?
APPTECH failed to note the fact that the bridge was made of
complementary fets: n and p channel. It isn't a question of "whether
this is needed or not". Anything less than 12 volts applied to the pfet
would leave it turned on when it should be off.
What would this do? With both the n and p fets on at once, global
warming would be terrific at least locally.
The n channel fets in the bottom have 12 volt drive through 47k
resistors... about 1/4 mA. Feeble but look at the numbers. The IRFD010
has around 1/4 nF input capacitance. To slew 10 volts will take 10 uS or
about 1% of the switching time. Slower would be sub-optimal but as is
is good enough.
(*) Would it be more appropriate to say "in the cricket pitch"?
> ball park (*) of 1kHz
Pelotta court / free kick / coo-ee / bull's roar
> With both the n and p fets on at once, [phrase deleted with
> extreme prejudice] would be terrific at least locally.
Very true. Do not run that circuit without checking drive
waveforms. The FETs get very hot indeed if switching is
Vey nice and non-PIC sollution !
The propeller's father is still angry on PIClist ?
On 4/11/08, Bob Blick <ftml.net> wrote: bobblick
Maybe there is a solution for that. I think you need to supply one
coil directly with the oscillator signal (with separately NPN-PNP
driver) and after the motor start running, disconnect that coil.
As you see there is one coil on which the current flow is insignifiant
small in your schematic. That's the one. On the other two there is a
full current flowand a phase modified high current (the one with 220uF
non polarized in series)
With pot at greatest resistance, spin the
> motor by hand to start. Then advance the pot to desired speed.
> Cheerful regards,
More... (looser matching)
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