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Running a DC motor from BS2px

Q

QWERTY

Jan 1, 1970
0
Hey group

I have bought a small hobby motor with the following characteristics.

Torque: Low
Operating Volts: 1.5- 4.5V
Nominal Volts: 3V
Speed (no load): 12000rpm
Current (no load): 0.28A
Speed (max eff): 8540rpm
Current (max eff): 0.69A
Torque (max eff): 7.94gcm
Size dia: 20mm
Size length: 25mm
Shaft length: 8.1mm
Shaft dia: 2mm.

I would like to run this of a Basic Stamp, BS2px in particular. The
datasheet for the BS says that the I/O pins each can source and sink
30 mA. Therefore I believe there is not enough current to run this
motor direct from the ic.
I only want to be able to turn the motor on and turn it off. It will
be used to create vibrations. ie like a mobile phone.

Is a mosfet the best way of isolating the motor from a chip or is
there another simple solution?

Thanks for any help
 
A

Anthony Fremont

Jan 1, 1970
0
QWERTY said:
Hey group

I have bought a small hobby motor with the following characteristics.

Torque: Low
Operating Volts: 1.5- 4.5V
Nominal Volts: 3V
Speed (no load): 12000rpm
Current (no load): 0.28A
Speed (max eff): 8540rpm
Current (max eff): 0.69A
Torque (max eff): 7.94gcm
Size dia: 20mm
Size length: 25mm
Shaft length: 8.1mm
Shaft dia: 2mm.

I would like to run this of a Basic Stamp, BS2px in particular. The
datasheet for the BS says that the I/O pins each can source and sink
30 mA. Therefore I believe there is not enough current to run this
motor direct from the ic.
I only want to be able to turn the motor on and turn it off. It will
be used to create vibrations. ie like a mobile phone.

Is a mosfet the best way of isolating the motor from a chip or is
there another simple solution?

Here is a document that might interest you:
http://lgjohn.okstate.edu/4213/lectures/IO.pdf

My suggestion is to start with a very simple circuit using a 2N2222 or
2N3904 and a 5K base resistor. For maximum learning potential be prepared
to cook a transistor or two, they're cheap. If you have access to an
oscilloscope, that would be great since you could see the actual voltages
showing up on your motor terminals. You will want to experiment with
clamping diodes as well. Motors are great for learning the trials of
connecting your "fragile" PIC to a real world inductive load.

MOSFETs are great too. 2n7000 is a nice logic level input low-power MOSFET.
For higher power, use one of the IRLxxx types as these have so-called "logic
level" inputs. This means that 5V is usually enough to turn the MOSFET on
hard.
 
C

Christopher Ott

Jan 1, 1970
0
Anthony Fremont said:
Here is a document that might interest you:
http://lgjohn.okstate.edu/4213/lectures/IO.pdf

My suggestion is to start with a very simple circuit using a 2N2222 or
2N3904 and a 5K base resistor. For maximum learning potential be prepared
to cook a transistor or two, they're cheap. If you have access to an
oscilloscope, that would be great since you could see the actual voltages
showing up on your motor terminals. You will want to experiment with
clamping diodes as well. Motors are great for learning the trials of
connecting your "fragile" PIC to a real world inductive load.

MOSFETs are great too. 2n7000 is a nice logic level input low-power
MOSFET. For higher power, use one of the IRLxxx types as these have
so-called "logic level" inputs. This means that 5V is usually enough to
turn the MOSFET on hard.

A little more costly, but easier approach would be an Internationar
Rectifier PVN012 Solid State Relay. Just put a 200ish ohm resistor in series
with the LED, and you can switch AC or DC. Digikey has them for $5.85 in
single qty. Considering the BS2PX runs about $79 each, I'd certainly prefer
the electrical isolation of an SSR.

Chris
 
E

ehsjr

Jan 1, 1970
0
Anthony said:
Here is a document that might interest you:
http://lgjohn.okstate.edu/4213/lectures/IO.pdf

My suggestion is to start with a very simple circuit using a 2N2222 or
2N3904 and a 5K base resistor. For maximum learning potential be prepared
to cook a transistor or two, they're cheap. If you have access to an
oscilloscope, that would be great since you could see the actual voltages
showing up on your motor terminals. You will want to experiment with
clamping diodes as well. Motors are great for learning the trials of
connecting your "fragile" PIC to a real world inductive load.

MOSFETs are great too. 2n7000 is a nice logic level input low-power MOSFET.
For higher power, use one of the IRLxxx types as these have so-called "logic
level" inputs. This means that 5V is usually enough to turn the MOSFET on
hard.

The suggestion to start simple is darn good, but needs
a little more said. Those transistors are great for
experimenting, cheap, and so what if you fry them.

The transistors mentioned are not the best choices for
connection to the stamp or PIC. So the experiemnts should
initially be done without it. When he goes to the stamp/PIC/uC
the 2N3904 & 2N7000 are way too small in terms of continuous
collector/drain current, with absolute maximum ratings of 200 mA.
He might squeek by with the 2N2222 for a short time before it
overheats and dies, if the motor is loaded. If the transistor
is connected to the stamp and shorts, the stamp potentially dies.
A TIP33 would work well. It has sufficient gain, continuous
Ic and power dissipation ratings to switch the motor current
on/off.

Ed
 
A

Anthony Fremont

Jan 1, 1970
0
ehsjr said:
The suggestion to start simple is darn good, but needs
a little more said. Those transistors are great for
experimenting, cheap, and so what if you fry them.

The transistors mentioned are not the best choices for
connection to the stamp or PIC. So the experiemnts should
initially be done without it. When he goes to the stamp/PIC/uC
the 2N3904 & 2N7000 are way too small in terms of continuous
collector/drain current, with absolute maximum ratings of 200 mA.

You're right, I should have checked first. I thought the 2N3904 would
handle 500mA or so, oops.

As for the 2N7000, I didn't intend for that to be interpreted as a
recommendation for the motor. I only said it was a handy part for low power
switching. ;-)
He might squeek by with the 2N2222 for a short time before it
overheats and dies, if the motor is loaded. If the transistor
is connected to the stamp and shorts, the stamp potentially dies.

On that I must disagree. Even if the transistor shorts to ground, the base
resistor will limit the current protecting the STAMP.
A TIP33 would work well. It has sufficient gain, continuous
Ic and power dissipation ratings to switch the motor current
on/off.

My personal favorite is a TIP120 for heavier loads, probably because radio
shack used to carry them. They will handle the OP's current requirements
easily and the off leakage is very small. Being darlingtons, they don't
need much drive either (Hfe is above 1000).
 
E

ehsjr

Jan 1, 1970
0
Anthony said:
You're right, I should have checked first. I thought the 2N3904 would
handle 500mA or so, oops.

Yabut - it's really a very minor oops - it could be a
valuable learning experience, which you stated at the outset.
Frying a few transistors - which you warned him of - can really
make the lesson sink in.
As for the 2N7000, I didn't intend for that to be interpreted as a
recommendation for the motor. I only said it was a handy part for low power
switching. ;-)

Don't get me started.
I am drooling for the day when they make jellybean Mosfets
that work well as switches with 1 volt on the gate - or
(grudgingly) 3 v on the gate - and can carry heavy current
with low Rds on. Give me some 10 amp, 6 cent, low Vg, <.1 ohm
50 Vds etc Mosfets! Yeah, yeah, I know - you can get them at
Unobtanium Incorporated.
On that I must disagree. Even if the transistor shorts to ground, the base
resistor will limit the current protecting the STAMP.

Yup, you're right. I forgot the base resistor.
My personal favorite is a TIP120 for heavier loads, probably because radio
shack used to carry them. They will handle the OP's current requirements
easily and the off leakage is very small. Being darlingtons, they don't
need much drive either (Hfe is above 1000).

Good point. I like them, too, but you pay a Vcesat penalty with
darlingtons. It (the penalty) might work real well for him in
this case. He could use a 4.5 or 5 v supply, and the Vcesat would
drop it near nominal.

Ed
 
A

Anthony Fremont

Jan 1, 1970
0
ehsjr said:
Anthony Fremont wrote:

Good point. I like them, too, but you pay a Vcesat penalty with
darlingtons. It (the penalty) might work real well for him in
this case. He could use a 4.5 or 5 v supply, and the Vcesat would
drop it near nominal.

It's "only" about .6V at 1A. Logic-level MOSFETs are much nicer though IMO.
 
E

ehsjr

Jan 1, 1970
0
Anthony said:
It's "only" about .6V at 1A. Logic-level MOSFETs are much nicer though IMO.

The datasheet shows Vcesat as .75, minimum. That
moves his supply near enough nominal, even with
a 5V supply, so the Vcesat "penalty" would work in
his favor. His maximum is 4.5, and the darlington
would lower the 5V to 4.25. The TIP120 would be a
good choice. If he wants to go closer to nominal he
can add a diode between the transistor and motor.

The logic level mosfet is another good option with
that series diode and a 5V supply.

Ed
 
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