tapping voltage off batteries in series

Thread Starter

lowrise4

Joined Sep 6, 2010
34
I'm building a 12VDC circuit powered by eight 1.5V cells in series. It's composed of two main sections; one part runs on 12V, the other part runs on 6V.

Rather than the usual way of using a series resistor to reduce the voltage for the 6V section (which would wastefully consume power), could I instead 'tap' voltage off after the first 4 batteries? Is this a standard way of providing lower-than-full voltage, or is there some reason this wouldn't work?
 

dl324

Joined Mar 30, 2015
9,568
Tapping the first 4 would cause them to discharge faster than the other batteries. Normally battery operated devices want all batteries replaced at the same time; so if the first 4 run down, standard procedure would be to replace all of them. What type of batteries?
 

Thread Starter

lowrise4

Joined Sep 6, 2010
34
I don't like the idea of the batteries being consumed at a different rate. So then the only other options I can think of are:

-use an inline resistor for the 6V section and live with the fact that the resistor will be wasting some of the energy
-I could use two different battery packs (one eight-cell, the other four cell) ...as awkward as that is...and knowing that (arrrgh again) one set of batteries might be consumed faster

If there are no other options, I guess I will just have to use an inline resistor to provide the 6V. BTW, my circuit is an 8-step adjustable tempo and tone sound sequencer using a 4017 and 555s. That part works fine on 6V but I'm using a TIP31 transistor to power the speaker, and this is the part that I want to give 12V (or maybe even more) to give it half-decent volume.
 

MikeML

Joined Oct 2, 2009
5,444
I dealt with this issue on my golf cart, but at a larger scale... Motor runs on 36V; everything else, lights, horn, radio runs on 12V. Idiots that built it (Teledyne) were so cheap that they tapped the battery string. Bad design. I am now using a SMPS buck-converter to transform 36Vdc to 12Vdc at an efficiency of ~85%.
 

ian field

Joined Oct 27, 2012
6,539
Thanks. I want to use eight AA cells...either alkaline, or Ni-MH if it uses batteries quickly.
Nickel batteries like being discharged all the way down occasionally.

I'd advise marking the cells in 2 equal groups with coloured tape so you can rotate the sets of cells between the high and low discharge positions.

Nickel batteries are 1.2V instead of 1.5V, so most applications for 12V would use 10 cells instead of 8.
 

Thread Starter

lowrise4

Joined Sep 6, 2010
34
What is your 6V current requirement?
I don't know exactly, but I think it will be less than the transistor ('amplifier') section at typical volume levels. It's one 4017 and nine 555s, and only two of the 555s are running at any given time (one constantly providing clock pulses to the 4017 and one of the other eight at any given time generating a tone).

The TIP31 transistor powers a 4-ohm 4" full range speaker with a small resistor inline (right now 100 ohms but I'm sure it can handle the current if I use a smaller resistor).

So I guess the point is that if the 6V sequencer section consumes a small amount of power relative to the 12V 'amplifer'/transistor section, I shouldn't sweat the modest power consumed by the resistor that steps down the voltage to 6V?
 

Thread Starter

lowrise4

Joined Sep 6, 2010
34
Yes - the 555s and 4017 should easily handle 12V, but if I run everything at 12V, I will have to change many resistor and potentiometer values to get the range of tones I want. But it's an option.
 
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