555 timer and 3V DC motor

[John Popelish]

Hello,

I'm building a little 555 timer circuit that turns a small geared 3VDC
motor on for an as yet undetermined time (estimated max 15seconds).

When calculating the RC for the timing is it best to keep the
capacitor at or under 1uF with larger resistor values so as to avoid
using electrolytics? I thought they were leaky with wide tolerances
and not good for timing applications.

Not necessarily. You also have to consider the bias current
requirements of the 555. If the capacitor charging current approaches
the bias current, the timing accuracy will also be degraded. If you
adjust the time with a variable resistor, electrolytic caps can be
pretty good for a long time. What timing accuracy are you needing?
There is also a timing trick for electrolytics that improves the
leakage situation for long times. Stack to wo similar capacitors in
series across the supply, and use the center point as your timing
node. This method passes the average leakage straight through and
only the difference in leakage current shows up at the timer.

Most of the suitable monostable circuits I have seen so far have an
NPN transistor with resistor on the 555 output controlling a relay
with its protection diode. Is their a particular reason why a power
transistor, eg BD139, can't be used to switch the motor and eliminate
the relay?

No. The relay just has very low DC drop, compared to a transistor.
If you can spare a few tenths of a volt, the transistor is fine. Keep
in mind the current capability of the 555 output and the saturation
current gain of the power transistor.

Also are there any special circuit requirements when using a motor
apart from perhaps a 0.01 - 0.1uF suppression cap across the motors
terminals? A reverse voltage diode placed somewhere?

The capacitor may help with radio frequency interference, but for good
circuit operation, a bypass capacitor across the supply right at the
points where the motor current leaves the area of the power switch is
more important. That would be from the emitter of the power switch to
the positive supply point where the motor connects. You also need a
diode from collector to positive supply rail to carry the motor's
inductive current when the switch first turns off, cathode band to the
positive rail.

At the moment the V+ on my proto-board is regulated 5V and I'm using
three diodes to drop the voltage for the 3V motor but eventually it'll
be battery powered with the motor being activated once or twice a day
for the max 15secs. The battery will possibly be a PP3 9V type. What
is the simplest and hopefully most efficient way of reducing a much
higher voltage for the motor? Dropping resistor, LM317 or... mmmm... a
zener regulator of some sort?

Simplest and most efficient are two different solutions. A series
resistor that sets the speed to about what you need is probably the
simplest. The most efficient would be a something like a second 555
timer set to mid ot high kilohertz frequency , gated on by the slow
555 described, above, to act as a pulse duty modulator to lower the
average voltage applied to the motor. If it puts out a pulse width
that is about 1/3 of the total cycle time, the average voltage to the
motor will be about 1/3 of the supply voltage, with no intentional
losses. And the battery current will average a little more than 1/3
of the motor current, extending the battery life, considerably. But
you will have ot pay more attention to transistor turn on and turn off
times and use a fast diode (Schottky) across the motor to keep the
switching losses low.

 

[Jamie]

using the range of caps that you have suggested may not work so well
with the 555 timer considering the 15 sec's your looking for.
you must look at the bias current the 555 timer needs to properly
calculate the highest value resister that you can use.
the Cmos type 555 may allow for much higher value resisters.
--
as far as the motor being switched on via a transistor? the answer is
obviously yes. you should use place both a Electrolytic and .o1 Ceramic
type Disc Cap across the terminals of the motor for reduce both the
RF Skin effects ( disc cap) and brush pulse noise and possible
inductive regen ( Electrolytic)..
adjusting the voltage is not a problem if you bias a NPN transistor
from the output of the 555 with a voltage divider or a resister with
a zener to regulate the voltage for constant speed.

 

[Rubicon]

Hello,

I'm building a little 555 timer circuit that turns a small geared 3VDC
motor on for an as yet undetermined time (estimated max 15seconds).

When calculating the RC for the timing is it best to keep the
capacitor at or under 1uF with larger resistor values so as to avoid
using electrolytics? I thought they were leaky with wide tolerances
and not good for timing applications.

Most of the suitable monostable circuits I have seen so far have an
NPN transistor with resistor on the 555 output controlling a relay
with its protection diode. Is their a particular reason why a power
transistor, eg BD139, can't be used to switch the motor and eliminate
the relay?

Also are there any special circuit requirements when using a motor
apart from perhaps a 0.01 - 0.1uF suppression cap across the motors
terminals? A reverse voltage diode placed somewhere?

At the moment the V+ on my proto-board is regulated 5V and I'm using
three diodes to drop the voltage for the 3V motor but eventually it'll
be battery powered with the motor being activated once or twice a day
for the max 15secs. The battery will possibly be a PP3 9V type. What
is the simplest and hopefully most efficient way of reducing a much
higher voltage for the motor? Dropping resistor, LM317 or... mmmm... a
zener regulator of some sort?

Thankyou,

Andrew.


Remove the ZZ from E-Mail address to contact me.

 

[John Popelish]

Rubicon wrote:
(snip)

Using a dropping resistor.
Never very good at maths but with a wouldn't it go like this?
9V supply, 3V motor
0.5A startup current drawn
9V - 3V - 0.6V (transistor) = 5.4V
5.4V / 0.5A = 10.8 ohms 2.7Watts

But...

0.23A constant current drawn.
5.4V / 0.23A = 24 ohms 1.24Watts

Which means that if I used the 10.8 ohm resistor the voltage would be
O.K for the initial startup current but rise to almost 6V when the
current drawn falls. At 3V the gearing provides 0.5rps, under 3V and
the rpm falls and the torque drops sharply while much over 3V and I'm
afraid of damaging the motor.

I did say that it was simple. A series resistor does a poor job of
regulating the motor resistance. Better than a series zener might be
a linear adjustable voltage regulator like an LM317 on the positive
side of the motor. You can tie the reference pin divider to the
switch, so that the regulator compensates for the switch drop, too.

Not sure but with a zener regulator using a 3V3 zener the series
resistor would be.
R = (Vin - Vzener) / I
R = (9 - 3.3V) / 0.5A Max
R = 11.4 ohms

P = E * I
P = (9V - 3.3V) * 0.5A Max
P = 2.85 Watts

and

Pzener = Vz * ILoad
Pzener = 3.3V * 0.5A Max
Pzener = 1.65 Watts

As the 3V3 zener doesn't come in 5W I'd have to go higher to 5V1 or
something and lose some voltage via diodes?

If the Zener goes in reverse bias mode from the V- to before the
switching transistors base resistor and the zeners resistor goes from
that junction to V+ is the transistors 0.6V and motor diode included?
ie 3V motor + 0.6 transistor + 0.6 diode = 4.2V or 4V3 closest zener?

I shall read up on pulse duty modulators before asking about them.

3.3 volt zeners have no sharper knee than a series string of silicon
diodes, and it is easier to find diodes rated for your motor current.
But they all will provide regulation that is poor, compared to an
active regulator.

 

[Rubicon]

Thankyou both for replying to my post.

John,

In answer to your question on timing no it's not critical and a 100uF
electro with a 120K resistor and 50K trimmer is giving me a good range
to work with. One thing I've found is when the circuit is first turned
on the timing is more accurate to my calculations than if the
temporary 10K resistor & switch for negative triggering is pushed
again immediately afterwards. I suspect without any understanding that
the electro isn't fully discharged. It's not a problem really, just an
observation.

I had tired the transistor replacing the relay before asking here and
now I know why it didn't work the first time. Not easy for it to with
a 100K base resistor! 150 Ohms works fine at the proto-boards 5V+

Using a dropping resistor.
Never very good at maths but with a wouldn't it go like this?
9V supply, 3V motor
0.5A startup current drawn
9V - 3V - 0.6V (transistor) = 5.4V
5.4V / 0.5A = 10.8 ohms 2.7Watts

But...

0.23A constant current drawn.
5.4V / 0.23A = 24 ohms 1.24Watts

Which means that if I used the 10.8 ohm resistor the voltage would be
O.K for the initial startup current but rise to almost 6V when the
current drawn falls. At 3V the gearing provides 0.5rps, under 3V and
the rpm falls and the torque drops sharply while much over 3V and I'm
afraid of damaging the motor.

Not sure but with a zener regulator using a 3V3 zener the series
resistor would be.
R = (Vin - Vzener) / I
R = (9 - 3.3V) / 0.5A Max
R = 11.4 ohms

P = E * I
P = (9V - 3.3V) * 0.5A Max
P = 2.85 Watts

and

Pzener = Vz * ILoad
Pzener = 3.3V * 0.5A Max
Pzener = 1.65 Watts

As the 3V3 zener doesn't come in 5W I'd have to go higher to 5V1 or
something and lose some voltage via diodes?

If the Zener goes in reverse bias mode from the V- to before the
switching transistors base resistor and the zeners resistor goes from
that junction to V+ is the transistors 0.6V and motor diode included?
ie 3V motor + 0.6 transistor + 0.6 diode = 4.2V or 4V3 closest zener?

I shall read up on pulse duty modulators before asking about them.

Cheers,

Andrew.

Not necessarily. You also have to consider the bias current
requirements of the 555. If the capacitor charging current approaches
the bias current, the timing accuracy will also be degraded. If you
adjust the time with a variable resistor, electrolytic caps can be
pretty good for a long time. What timing accuracy are you needing?
There is also a timing trick for electrolytics that improves the
leakage situation for long times. Stack to wo similar capacitors in
series across the supply, and use the center point as your timing
node. This method passes the average leakage straight through and
only the difference in leakage current shows up at the timer.



No. The relay just has very low DC drop, compared to a transistor.
If you can spare a few tenths of a volt, the transistor is fine. Keep
in mind the current capability of the 555 output and the saturation
current gain of the power transistor.


The capacitor may help with radio frequency interference, but for good
circuit operation, a bypass capacitor across the supply right at the
points where the motor current leaves the area of the power switch is
more important. That would be from the emitter of the power switch to
the positive supply point where the motor connects. You also need a
diode from collector to positive supply rail to carry the motor's
inductive current when the switch first turns off, cathode band to the
positive rail.


Simplest and most efficient are two different solutions. A series
resistor that sets the speed to about what you need is probably the
simplest. The most efficient would be a something like a second 555
timer set to mid ot high kilohertz frequency , gated on by the slow
555 described, above, to act as a pulse duty modulator to lower the
average voltage applied to the motor. If it puts out a pulse width
that is about 1/3 of the total cycle time, the average voltage to the
motor will be about 1/3 of the supply voltage, with no intentional
losses. And the battery current will average a little more than 1/3
of the motor current, extending the battery life, considerably. But
you will have ot pay more attention to transistor turn on and turn off
times and use a fast diode (Schottky) across the motor to keep the
switching losses low.

Remove the ZZ from E-Mail address to contact me.