Controlling 2 DC motors from a stamp with a LM-293/L298
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Hi all,
I am trying to make a simple motor control circuit that should
support 2 DC motors ( 3V each ) using an LM-293 quadruple Half-bridge
thingy and two separate power supplies (one for the stamp and one for
the motors).
The first problem is that the LM-293 doesn't seem to support the puny
amount of voltage i need for my motors. The specs (datasheet) i have
suggest that there should be a minimal voltage difference between the
directional pins and enable pin on one side and the second power
source on the other side ( 2.5V ). In practice i am using 5V for the
logical signals ( directional and enable pins ) and only about 3V for
the motors. The specs on the L298 H-bridge tell me the same story
about the voltage difference so that doesn't help me. Does this mean
that i need different components or is there a solution to this
problem?
The second problem is that i read a schematic using the LM293 and
this
was using the ENable pins to vary the current that comes out of the
output pins. For some reason this doesn't really work as i expected
either, but this might have something to do with the first problem.
I know that i can allways revert to using the motormind or other
pre-built circuits commercially available but i'd really like to
built
this myself if only for educational purposes.
Hope to hear from you soon [noparse]:o[/noparse])
Sandor
I am trying to make a simple motor control circuit that should
support 2 DC motors ( 3V each ) using an LM-293 quadruple Half-bridge
thingy and two separate power supplies (one for the stamp and one for
the motors).
The first problem is that the LM-293 doesn't seem to support the puny
amount of voltage i need for my motors. The specs (datasheet) i have
suggest that there should be a minimal voltage difference between the
directional pins and enable pin on one side and the second power
source on the other side ( 2.5V ). In practice i am using 5V for the
logical signals ( directional and enable pins ) and only about 3V for
the motors. The specs on the L298 H-bridge tell me the same story
about the voltage difference so that doesn't help me. Does this mean
that i need different components or is there a solution to this
problem?
The second problem is that i read a schematic using the LM293 and
this
was using the ENable pins to vary the current that comes out of the
output pins. For some reason this doesn't really work as i expected
either, but this might have something to do with the first problem.
I know that i can allways revert to using the motormind or other
pre-built circuits commercially available but i'd really like to
built
this myself if only for educational purposes.
Hope to hear from you soon [noparse]:o[/noparse])
Sandor
Comments
I assume you meant L293, not LM293. I took a quick look at the TI spec
sheet, and you are correct; this driver is not very useful at 3V. High
side drive is through an emitter follower, so you lose ~0.7V from Vcc.
Worse yet, the low side drive is through a darlington pair, which drops
another 1 to 1.5 volts. This means that you are left with about 1.1 volts
for the motor. Notice that the data sheet lists Vcc minimum as 4.5 volts.
Could you feed the bridge with 5V? This would leave you with ~3V for the
motor.
Ray McArthur
Original Message
From: Sandor Rabe <sandor.rabe@e...>
To: <basicstamps@egroups.com>
Sent: Thursday, April 06, 2000 11:16 AM
Subject: [noparse][[/noparse]basicstamps] Controlling 2 DC motors from a stamp with a
LM-293/L298
> Hi all,
>
> I am trying to make a simple motor control circuit that should
> support 2 DC motors ( 3V each ) using an LM-293 quadruple Half-bridge
> thingy and two separate power supplies (one for the stamp and one for
> the motors).
>
> The first problem is that the LM-293 doesn't seem to support the puny
> amount of voltage i need for my motors. The specs (datasheet) i have
> suggest that there should be a minimal voltage difference between the
> directional pins and enable pin on one side and the second power
> source on the other side ( 2.5V ). In practice i am using 5V for the
> logical signals ( directional and enable pins ) and only about 3V for
> the motors. The specs on the L298 H-bridge tell me the same story
> about the voltage difference so that doesn't help me. Does this mean
> that i need different components or is there a solution to this
> problem?
>
> The second problem is that i read a schematic using the LM293 and
> this
> was using the ENable pins to vary the current that comes out of the
> output pins. For some reason this doesn't really work as i expected
> either, but this might have something to do with the first problem.
>
> I know that i can allways revert to using the motormind or other
> pre-built circuits commercially available but i'd really like to
> built
> this myself if only for educational purposes.
>
> Hope to hear from you soon [noparse]:o[/noparse])
>
> Sandor
>
>
>
>
Thanks for your tips. I have been measuring voltage outputs at several points
in the setup and they comply with what you said. I also found that the current
running through the motors in the setup i'm referring to is quite (too) low.
Now i'm thinking of using 2 op-amps for speed control ( as suggested in the
basic-stamp manual ) and a relay circuit to change direction ( probably using
transistors and a double not-gate ). Is there a foolproof ( and preferably
simple ) way to avoid switching direction ( other than avoid this from
happening by programming the stamp the right way ) while the motors are
running at full speed? I don't even know whether such a 'safety' is really
neccessary with the motors i will use... Since i want to create a setup that i
can use with bigger motors in the future ( without too much of a hassle ) it
might be handy though.
thanks again,
Sandor
P.S. maybe this motor stuff should be added to the FAQ's for future
referencing by newbies like me. I have been working on this for a week now [noparse]:o[/noparse])
[noparse][[/noparse]...] org. message snipped for brevety purposes
control. As a DC motor turns the power consumed varies as the contacts
switch from coil to coil. By monitoring these changes one could predict the
RPMs of the motor and adjust the supply voltage accordingly to maintain a
constant or desired RPM change. All DC motors have counters built into
them, all we have to do is listen to their electrical needs. Bill
Original Message
From: "Sandor Rabe" <sandor.rabe@e...>
To: <basicstamps@egroups.com>
Sent: Thursday, April 06, 2000 11:16 AM
Subject: [noparse][[/noparse]basicstamps] Controlling 2 DC motors from a stamp with a
LM-293/L298
> Hi all,
>
> I am trying to make a simple motor control circuit that should
> support 2 DC motors ( 3V each ) using an LM-293 quadruple Half-bridge
> thingy and two separate power supplies (one for the stamp and one for
> the motors).
>
> The first problem is that the LM-293 doesn't seem to support the puny
> amount of voltage i need for my motors. The specs (datasheet) i have
> suggest that there should be a minimal voltage difference between the
> directional pins and enable pin on one side and the second power
> source on the other side ( 2.5V ). In practice i am using 5V for the
> logical signals ( directional and enable pins ) and only about 3V for
> the motors. The specs on the L298 H-bridge tell me the same story
> about the voltage difference so that doesn't help me. Does this mean
> that i need different components or is there a solution to this
> problem?
>
> The second problem is that i read a schematic using the LM293 and
> this
> was using the ENable pins to vary the current that comes out of the
> output pins. For some reason this doesn't really work as i expected
> either, but this might have something to do with the first problem.
>
> I know that i can allways revert to using the motormind or other
> pre-built circuits commercially available but i'd really like to
> built
> this myself if only for educational purposes.
>
> Hope to hear from you soon [noparse]:o[/noparse])
>
> Sandor
>
>
>
>
>
>I am new to this Group, but I've noticed that a lot of you are into DC motor
>control. As a DC motor turns the power consumed varies as the contacts
>switch from coil to coil. By monitoring these changes one could predict the
>RPMs of the motor and adjust the supply voltage accordingly to maintain a
>constant or desired RPM change. All DC motors have counters built into
>them, all we have to do is listen to their electrical needs. Bill
Bill -
I'd guess that mostly depends on the speed of the motor,
the speed of the processor, and the accessability of the contacts.
Many of the "tin can" motors used in small scale robotics would be a bit
difficult to "hear" except for the pitch and whine they make when we
invariably
connect them up wrong <smile>. Additionally, many that we use are highly
geared
down to produce slow speed and high torque.
On the other hand, if you have some neat inductive coupling collar about
twice
the diameter of a wedding ring - we'd probably all want to "listen" to our
motors
"sing".
Nice thought, in any case.
Regards,
Bruce
>
Original Message
>From: "Sandor Rabe" <sandor.rabe@e...>
>To: <basicstamps@egroups.com>
>Sent: Thursday, April 06, 2000 11:16 AM
>Subject: [noparse][[/noparse]basicstamps] Controlling 2 DC motors from a stamp with a
>LM-293/L298
>
>
>> Hi all,
>>
>> I am trying to make a simple motor control circuit that should
>> support 2 DC motors ( 3V each ) using an LM-293 quadruple Half-bridge
>> thingy and two separate power supplies (one for the stamp and one for
>> the motors).
>>
>> The first problem is that the LM-293 doesn't seem to support the puny
>> amount of voltage i need for my motors. The specs (datasheet) i have
>> suggest that there should be a minimal voltage difference between the
>> directional pins and enable pin on one side and the second power
>> source on the other side ( 2.5V ). In practice i am using 5V for the
>> logical signals ( directional and enable pins ) and only about 3V for
>> the motors. The specs on the L298 H-bridge tell me the same story
>> about the voltage difference so that doesn't help me. Does this mean
>> that i need different components or is there a solution to this
>> problem?
>>
>> The second problem is that i read a schematic using the LM293 and
>> this
>> was using the ENable pins to vary the current that comes out of the
>> output pins. For some reason this doesn't really work as i expected
>> either, but this might have something to do with the first problem.
>>
>> I know that i can allways revert to using the motormind or other
>> pre-built circuits commercially available but i'd really like to
>> built
>> this myself if only for educational purposes.
>>
>> Hope to hear from you soon [noparse]:o[/noparse])
>>
>> Sandor
>>
>>
>>
>>
>>
>
>
>
>
>
I am surprised that the motor or stamp ran at all with the stamp regulator
connected to the motor/bridge.· Both the BS1 and BS2 regulatorsare only
rated for ~50 ma!· You need a separate power supply to run thebridge, like
maybe 1 amp or so at 5 volts.
That's what i did: the h-bridge needs 2 power supplies and has 6 inpoutpins right? I hooked up the 1st (external power supply, rated 4.5 to 30Von the bridge i have ) to the secondary power unit, which is not connectedin any way to the stamp. The 2nd power supply ( for the logic ) and thedirectional and enable pins were connected to the stamp, since they won'tneed that much current. since the ground pins on the bridge are all interconnectedi just wired these to the grounds of both power supplies, as you suggest.In short i hooked it all up just like the datasheet on the L293 suggested.
The main problem we seem to have here is that we can only find h-bridgesrated for output voltages at or in excess of 4.5V. Since we are tryingto run puny 3V dc motors the L293 wasn't quite up to the task. Since lastweek i'm working on a totally different approach, which consists of a doubleop-amp rated for the voltages we need and a max-output of 1A, togetherwith four transistors per motor for directional control. The op-amp shouldgive us a nice voltage range between 0 and 3 volts per motor on a steady.3 Ohm current. We are also looking at using a circuit with mosfets togetherwith transistors to make the voltage drop coming from the op-amp even less.
> from flowing in "the wrong direction"?
I think a diode would work.· You would only need 1 diode betweenthe +5
supply and the bridge.· The downside is that you lose ~0.7 voltsacross the
diode.
Thanks for the tip.
What's a 404 error?
404 - page not found
Never seen that error when you were 'surfing' ?
Ray McArthur
Thanks Ray