Voltage divider question

dev/null

Hi

I am using a 7.2V battery pack to power a PSC (Parallax Servo Controller).

For some reason, when I measure the voltage on the pack, it shows up to 8.5V.
The max rating for the PSC is 7.5 volts, so I want to make sure I stay well below this (two of my PSC's are damaged).

Is it appropriate to use a voltage divider to accomplish this?

What I don't understand with voltage dividers, is the magnitude of the resistor values. How do I choose which of the following circuits to use?



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will a black hole evaporate if you send it to /dev/null ?

stamptrol

The voltage from your battery pack sounds as if its got an extra cell in it or your're checking the voltage immediately after taking it off the charger. A 6-cell nicad pack will have a voltage of 6 x 1.2 volts = 7.2 volts; a 7 cell pack will show 7 x 1.2= 8.4 volts. Any thing above 7.5 volts is going to damage things.

Nonetheless, the voltage dividers you show will not work satisfactorily for you. Too much resistance in series plus the fact that the current drawn by the controller and servos will be changing all the time. Just basic Ohm's law.

If you must use the battery pack with the non-standard voltage, you could put 1 or two diodes ( 1N5408) in series with the +ve lead of the battery to drop the voltage a bit ( about 0.6 v per diode). Check the voltage going into the PSC with a meter.

Cheers,

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Tom Sisk

http://www.siskconsult.com
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Alan Bradford

Hi,
I do not know much about the PSC but looking at your drawing either one will have the same basic results.
Using Kerchovs current and voltage laws:
1) The current flow around the loop will equal the total current from the power source.
2) The voltage drops across the loop will equal the total voltage from the power source. (Wow wont Prof Sindelar be proud of me after 35 years!!!!)

I have to assume the current draw will vary depending on activity in rthe PSC.
This will always be changing the current through the series resistor.
Using Ohms Law this changing current will·alter the voltage drop across the resistor.
In turn this varing current will change the voltage drop across the other 2 resistors in paralel with the PSC.

This is why voltage dividers and dropping resistors only work when the current draw is constant.
It also wastes power from the batteries.

I am not a battery expert, but if you measuere the batt voltage no load it will be higher than with a load.
What is the loaded voltage when you power up the circuit.

You can add 2 diodes in series with the + lead and they will drop .7 volts each, and test the voltage drop.
If loaded the voltage drops to 7.2 or lower then you should be ok, and remove the diodes.
If you leave the diodes they will lower your voltage 1.4 volts through the battery life, and shorten the run time.
(Sorry there is no 'Free Lunch' in this world.)

Someone who knows the PSC more than I may also want to comment.





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Alan Bradford ·N1YMQ

Plasma Technologies
Canaan NH 03741
www.plasmatechnologies.com

dev/null

Thanks for tips. I will try the diode solution. I didn't know that voltage dividers work for constant currents only, so now I know!!

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will a black hole evaporate if you send it to /dev/null ?

dev/null

BTW the diode solution works perfectly. When applying two 1N4001 in serial, the voltage drops 1V. PERFECT!

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will a black hole evaporate if you send it to /dev/null ?

randyaz

Why not keep it simple and put a volatage regulator on your battery pack leads...
http://www.parallax.com/Store/Components/AllIntegratedCircuits/tabid/154/CategoryID/26/List/0/SortField/0/Level/a/ProductID/242/Default.aspx