safe to use a voltage divider continuously?

Hi,

The actual value of the resistor will make a difference. For example is you
use a 1K resistor on the high side of the divider you'll be able to deliver
max 6ma at 6volts. If you have a 10k you'll only have 0.6ma.

What ever values you use you'll be draining your battery for no reason. Use
a voltage regulator or 2 battery packs
 one of the motor, the other for the controller.

Regards
Ian Dobson

Home of the Atmel based UDP mobile web cam http://www.planet-ian.com
All mails scanned with av-filter.pl (F-Prot / perl)


 > I have a motor controller that requires a logic voltage of no more than
 > 5.5V (but is content with as little as 2.5V).  But my motors are happy
 > with 6V, and I was planning to run them off of 4 AA alkaline cells,
 > which total about 6.5V when fresh.  Unfortunately this means I can't
 > safely connect the same battery pack directly to the motor controller.
 >
 > However, I could pretty easily set up a voltage divider (just using a
 > couple of resistors) to cut the voltage to the the controller by, say,
 > 20%.  That should put me safely within the range the controller wants.
 > But is it safe, reasonable, and kosher to set things up this way?  Or is
 > it going to cause problems, for example, the resistors growing hot or
 > wasting power?
 > Also, if this is a reasonable thing to do, does it matter what size
 > resistors I use?  The division ratio depends only on the ratio of their
 > resistance -- but does their absolute value also matter, or do I just
 > pick whatever's convenient?
 >
 > I've got some 7085 voltage regulators on order, but since the motor
 > controller accepts such a wide range of input voltage, I'm wondering why
 > I shouldn't just use a couple of resistors instead.
 > Many thanks,
 > - Joe

Thanks, that's exactly the sort of insight I was looking for.  The
educational resources I've found are great at explaining how to make a
voltage divider, but utterly failed to point out these realities of
actually using them.


Will do, thanks again!
- Joe

that's slightly off.  if you have a 1k resistor on the high side it
will only deliver 6ma at _0v_, i.e. a dead short.  suppose your
microcontroller needs between 0ma 50ma at 5v and a 500mv output voltage
drop is acceptable under that load.  (and that is a pretty huge margin
for a power supply)  you need a supply with an impedance of 500mv/50ma
= 10 ohms.  if you have a voltage divider with r1 to 6v and r2 to gnd,
you need 1/(1/r1+1/r2) < 10.  r1, r2` gets you that but burns 83
milliamps continuously.  and like i said, 50ma is a tiny power supply
and 10% is awful.  that's why nobody uses voltage dividers for power
supplies unless the current in question is really, really tiny.

you want a linear regulator.  be careful with 7805s though -- they have
pretty awful dropout, on the order of 2 volts or something.  you can
get LDO regulators with dropout of like 0.5.  that is important because
your battery voltage isn't much higher than your operating voltage.

using two batteries is sort of silly and i would advise against it.

-chris.


reason.Thanks, that's exactly the sort of insight I was looking for.  The

Thanks, that's a good explanation -- I need to let it sink in again and
work through the details, but in broad strokes I think I get it.


True.  With the voltage regulator, I was planning on using a separate 9V
battery.  That's a bit of a nuisance, but not too terrible since I would
expect that battery (unlike the motor batts) to last for months even
under heavy use.

What does a voltage regulator do when the input voltage drops too low?  
Does the output voltage simply drop in proportion?

Maybe I could find a 3.3V LDO -- that would give me a fair amount of
wiggle room.


Well, I do like the idea of a single set of batteries, mainly because it
enables me to have a single on/off switch.  But I've already gotten used
to having separate batteries on my servo-based robots, where the servos
draw so much current (and are so noisy) that they frequently cause the
servo controller to reset if driven from shared batts.

I haven't heard of that being a problem with this motor controller, but
it might be one of those things I learn about when my robot suddenly
freezes.  :)

But I have heard that many builders try to keep two separate power
busses, a high-current, noisy one for motors (and certain active
sensors) and a lower-current, clean one for electronics and control
signals.  That seems sensible to me... but how would you achieve it with
a single set of batteries?  Would a voltage regulator on the "clean"
line be all that's needed?

Thanks,
- Joe

[snip resistor voltage divider]


A linear regulator is a better choice than a voltage divider.

A LDO regulator is a better choice than a non LDO regulator
in this application.  (LDO regulators cost more, but not enough
to matter.)

There is no free lunch on linear regulators.  The key equation is:

     Power = Voltage * Current  or P = IV

If you have a device that wants to eat 20mA at 3.3V,
then the device is chewing up .020A * 3.3V = .066 Watt or 66mWatt.
However if the battery is is 9V volts, the power that the
voltage regulators is (9V - 3.3V) * .020A = .114W = 114mWatt.
Linear regulators are nice and easy, but they can eat up some
power.

A switching supply, can be used to drop deliver more battery
power to your robot electronics, at additional cost and
complexity.


Separate battery packs are reasonable for hobbyist grade robots.
A single battery pack can be used with careful engineering,
but most of us would rather spend our valuable enginnering
time on other aspects of the robots.


Separate battery packs can introduce their own set of problems
such as ground loops.  I typically design my electronics to
be opto isolated these days so that the "power" battery is
completely electronically isolated from the "logic" battery.
Spurious resets are a thing of the past for me.

Your mileage may vary,

-Wayne

not proportionally per se; more like "output will never be more than
(input - x) volts".


sure.  of course, you'll regret it later when you want to attach
something that needs all 5.

-chris.

You don't need a voltage divider with two resistors, you only need one
resistor.  Your motor controller acts as the other resistor and you get the
same voltage dividing effect.

The problem with using resistors is that the amount of voltage drop changes
based on how much current is being drawn (E=I*R).  If your controller has a
stable enough current demand it might actually work ok.  You would have to
determine the maximum and minimum current draw and see if you can find a
resistor value (using the simple E=IR formula) that would keep the voltage
in the range needed.

If the current dropped to zero at any point however, the voltage drop
across the resistor would be zero which means the full 6.5 V would be
applied to the microcontroller and could damage it.  Dangerous stuff.

This of course is why you would add a second resistor in parallel with the
controller - to guarantee a minimal current flow which guarantees enough
voltage drop across the first series resistor so the voltage never got over
the maximum.  Any current flowing through that second resistor, would just
be wasted energy draining your battery.  But if the current is low enough
compared to say what the motors are draining, it might be a reasonable
solution.

Now, once you have two loads in your circuit (both the controller and the
extra load resistor in parallel with the controller), then the current
through the series resistor is the sum of the currents through each load.
This means the voltage drop across the series resistor won't change as much
the current draw from the controller changes - which means the voltage will
be more stable.

So that's two reasons to use the two resistors in a divider circuit as you
first thought about. It guarantees the voltage to the controller will
always be less than what it is when the controller is not connected (or is
drawing 0 ma or current) - and it means the voltage to the controller will
change less as the current draw on the controller changes.

You also asked about the resistors heating up.

All resistors heat up when they are working.  That's what resistors are -
electrical heaters.  At normal current levels the heat is so low we don't
even notice it.  Resistors are rated based on how much heat they can take
without damage.  It's their wattage rating.  As long as the resistor is
operated below it's rating, everything is fine.  To calculate how much
power the resistor is consuming, just multiply the voltage across the
resistor, by the current going through it. W=EI. (or W=I^2 * R)  If the max
current through your resistor is 500 ma and the max voltage drop is 5
volts, then that makes for 5 * .5 or 2.5 watts.  A typical 1/4 watt
resistor would not be big enough in this case.  You would need a special
power resistor rated for something greater than 2.5 watts.  And it would
get warm if it had to deal with that high of a current and voltage.  But if
the max load was 2 volts, and 50 ma, then the watts would be .1 and a small
1/4 watt (.25 watts) would work just fine).

Another option other than a real regulator is to use a zener diode in place
of the second resistor in the voltage divider.  This is how you make a
cheap regulator.  A zener diode conducts when the voltage across it goes
over it's voltage rating.  So if you use a 5 volt zener diode, the voltage
across it will never go over 5 volts (approximately).  This means that if
the controller is not drawing enough current to make the voltage drop to
the needed 5V, the zener diode will draw the current instead.  But unlike
the second resistor, when the controller was drawing enough current and the
voltage dropped below 5V, the zener diode would stop conducting and stop
wasting your battery power.

If the currents are low enough, the zener diode solution might be cheaper
and easier than a real regulator - but it's only used in low power
applications because it does regulate by wasting power.

When you use a real regulator, it works the same way as the series resistor
does - it drops the voltage by acting as a resistor.  The only difference,
is that it automatically changes it's resistance as required to keep the
voltage output constant.  So, your regulator is going to be producing just
as much heat, and wasting just as much battery power, as the series
resistor was wasting.  So it's no better or worse in that regard.  It only
saves energy by not needing that second resistor (or zener diode) - that's
where those other two simpler systems would waste extra power.


I've not had enough experience with these circuits to know the practical
issues with that, but you should be able to solve that with the correct
application of capacitors - but maybe the needed capacitors to solve the
problem are so large that it's easier to just use separate batteries?


Yeah, it's also nice that the brains of the bot keep functioning even when
the power to the servos/motors starts to die because they tend to behave
better that way (and sensors and indicator lights can keep working etc).


Yeah.  You can just have one buss which is the raw battery buss which is
connected to the input of your regulator as well as the servos and/or
motors and the output of the regulator is the clean buss for the
electronics.  You need capacitors on both sides of most regulators (across
the power) and you can add additional capacitors at different locations on
the "clean" buss to keep it clean (high speed digital electronics can
generate a lot of high frequency noise - and the capacitors act to filter
that out).

It's typical to use a large electrolytic cap on the input and output of a
regulator and to put a small non-electrolytic in parallel on the output.
Electrolytic don't act like capacitors at high frequencies so that's why
the small cap is also needed.  Without the small cap, the output could
oscillate at a high frequency.

If you experiment with the voltage divider idea, you should use similar
caps as well in the circuit to reduce noise effects.


--
Curt Welch                                            http://CurtWelch.Com/
curt@kcwc.com                                        http://NewsReader.Com/

Many thanks to all for your insightful (and amazingly detailed) answers
to my questions.  I've learned a lot here in the last two days, and I'm
deeply grateful.

Best,
- Joe