Driving a DC motor

P

Pedro Duarte

Guest
Hi,

I pretend to control a small dc motor (about 5 - 6V) using a
microcontroller (PIC) and I think I need some precautions since the
motor may be an inductive load...

What I want to do is this : the PIC connects to the base of a
transistor working in saturation zone, so that is behaves like a
switch turning on and off the motor's power supply (you know, the
motor is wired to the power source and connected to the transistor
colector, the usual stuff.)

By doing PWM on the signal outputed by the PIC to the transistor's
base, it's possible to control the motor speed (the motor is "quickly"
beeing turned on and off).

Since the motor is an inductive load (all dc motor are? correct me if
i'm worng) each time I power it and "unpower" it, a spike of current
is generated so the life of this circuit would be seriously shortened
....

What is the easiest yet functional way to protect it agains these
spikes ? Some diodes ? Where and how ? I'm know very little about this
matter ... A small schematic or a detailed description of an example
circuit is really welcome.

Thanks a lot

Pedro Duarte
 
assuming the transistor is an NPN device, put a diode (capable of taking the
shunt) across the collector and emitter with the anode on the collector...
see the diagram below, if this doesn't look right, copy the lines into and
editor (notepad) and use a fixed pitch font.

-------------------------+---+---------------- +power
| |
| |
/ |
|/ -
|\ ^ 1N4004 or similar
|
| |
Click to expand...
| |
-------------------------+---+---------------- -power


"Pedro Duarte" <paraasnews@yahoo.com> wrote in message
news:95002a38.0401040504.1a3c9683@posting.google.com...
Hi,

I pretend to control a small dc motor (about 5 - 6V) using a
microcontroller (PIC) and I think I need some precautions since the
motor may be an inductive load...

What I want to do is this : the PIC connects to the base of a
transistor working in saturation zone, so that is behaves like a
switch turning on and off the motor's power supply (you know, the
motor is wired to the power source and connected to the transistor
colector, the usual stuff.)

By doing PWM on the signal outputed by the PIC to the transistor's
base, it's possible to control the motor speed (the motor is "quickly"
beeing turned on and off).

Since the motor is an inductive load (all dc motor are? correct me if
i'm worng) each time I power it and "unpower" it, a spike of current
is generated so the life of this circuit would be seriously shortened
...

What is the easiest yet functional way to protect it agains these
spikes ? Some diodes ? Where and how ? I'm know very little about this
matter ... A small schematic or a detailed description of an example
circuit is really welcome.

Thanks a lot

Pedro Duarte
Click to expand...
 
On 4 Jan 2004 05:04:37 -0800, paraasnews@yahoo.com (Pedro Duarte)
wrote:

Hi,

I pretend to control a small dc motor (about 5 - 6V) using a
microcontroller (PIC) and I think I need some precautions since the
motor may be an inductive load...

What I want to do is this : the PIC connects to the base of a
transistor working in saturation zone, so that is behaves like a
switch turning on and off the motor's power supply (you know, the
motor is wired to the power source and connected to the transistor
colector, the usual stuff.)

By doing PWM on the signal outputed by the PIC to the transistor's
base, it's possible to control the motor speed (the motor is "quickly"
beeing turned on and off).

Since the motor is an inductive load (all dc motor are? correct me if
i'm worng) each time I power it and "unpower" it, a spike of current
is generated so the life of this circuit would be seriously shortened
...

What is the easiest yet functional way to protect it agains these
spikes ? Some diodes ? Where and how ? I'm know very little about this
matter ... A small schematic or a detailed description of an example
circuit is really welcome.
Click to expand...
Yeah, diodes. But a TTL level power FET would be a better bet than a
transistor. They're better suited to this application. What's the
power requirement of the motor? John Crighton might make your day if
he's reading this and post the link to a clever little circuit that
would probably do the job for you just fine...
--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill
 
Pedro Duarte wrote:
Hi,

I pretend to control a small dc motor (about 5 - 6V) using a
microcontroller (PIC) and I think I need some precautions since the
motor may be an inductive load...

What I want to do is this : the PIC connects to the base of a
transistor working in saturation zone, so that is behaves like a
switch turning on and off the motor's power supply (you know, the
motor is wired to the power source and connected to the transistor
colector, the usual stuff.)

By doing PWM on the signal outputed by the PIC to the transistor's
base, it's possible to control the motor speed (the motor is "quickly"
beeing turned on and off).

Since the motor is an inductive load (all dc motor are? correct me if
i'm worng) each time I power it and "unpower" it, a spike of current
is generated so the life of this circuit would be seriously shortened
...

What is the easiest yet functional way to protect it agains these
spikes ? Some diodes ? Where and how ? I'm know very little about this
matter ... A small schematic or a detailed description of an example
circuit is really welcome.

Thanks a lot

Pedro Duarte
Click to expand...
The inductance tries to reduce the rate of change of current by
producing voltage. When you first switch the supply in to the motor,
it produces a voltage equal to the supply to slow the rise of
current. This makes it very easy on the switch, since it does not
have to deal with current rising while it is in the process of turning
on. But when you try to turn the switch off, the motor inductance
generates a large and sudden voltage than adds to the supply voltage,
to continue to pull current through the switch as it is turning off.
It is this high voltage spike (much like the high voltage from an
ignition coil primary) that damages the switch. So you need to give
the motor current somewhere else to go (other than through the switch)
with the motor voltage rising a modest amount. The most common
approach for single direction motors is to just put a diode across the
motor, so that when it adds a single diode drop to the supply voltage,
the motor current detours through the diode while the switch turns
off. That way, the PWM circuit alternates the motor voltage between
the supply and a diode drop in the other direction, with the motor
inductance and generated EMF averaging these two and acting as if the
average voltage were applied.

--
John Popelish
 
UncleWobbly wrote:

assuming the transistor is an NPN device, put a diode (capable of taking the
shunt) across the collector and emitter with the anode on the collector...
see the diagram below, if this doesn't look right, copy the lines into and
editor (notepad) and use a fixed pitch font.

-------------------------+---+---------------- +power
| |
| |
/ |
|/ -
|\ ^ 1N4004 or similar
|
| |
| |
-------------------------+---+---------------- -power
Click to expand...
I think it would work better with the diode across the
inductor instead of the switch, like this:

+---+--- +power
| |
| |
_| |
(_ -
Motor (_ ^ Diode
(_ |
| |
| |
+---+
|
|
/
__|/ NPN
|\
|
Click to expand...
--+--
---
-

(John Popelish explained the theory in a related post in this
thread.)
 
Pedro Duarte wrote:

Hi,

I pretend to control a small dc motor (about 5 - 6V) using a
microcontroller (PIC) and I think I need some precautions since the
motor may be an inductive load...

What I want to do is this : the PIC connects to the base of a
transistor working in saturation zone, so that is behaves like a
switch turning on and off the motor's power supply (you know, the
motor is wired to the power source and connected to the transistor
colector, the usual stuff.)

By doing PWM on the signal outputed by the PIC to the transistor's
base, it's possible to control the motor speed (the motor is "quickly"
beeing turned on and off).

Since the motor is an inductive load (all dc motor are? correct me if
i'm worng) each time I power it and "unpower" it, a spike of current
is generated so the life of this circuit would be seriously shortened
...

What is the easiest yet functional way to protect it agains these
spikes ? Some diodes ? Where and how ? I'm know very little about this
matter ... A small schematic or a detailed description of an example
circuit is really welcome.

Thanks a lot

Pedro Duarte
Click to expand...
Try these, you can drive them with a PIC and they take care of
everything....

FAN8200 or TC4426

http://members.cox.net/berniekm/Hot_Parts.html

--
Luhan Monat, "LuhanKnows" At 'Yahoo' dot 'Com'
http://members.cox.net/berniekm
"The future is not what it used to be."
 
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