# Multiple resistors or one trimmer pot?

Discussion in 'The Projects Forum' started by jerseyguy1996, Apr 6, 2011.

1. ### jerseyguy1996 Thread Starter Active Member

Feb 2, 2008
206
9
I am building the circuit listed on page 5 here:

http://focus.ti.com/lit/ug/sluu023b/sluu023b.pdf

Resistors R12, R14, and R19 are part of a divider that specifies the Bulk charge and Float charge characteristics. I am shooting for a bulk charge voltage of 14.7 volts and a float charge voltage of 13.5 volts. The resistor values are based on equation 1 and 2 on page 6 here:

http://focus.ti.com/lit/an/slua017/slua017.pdf

My question is this. I am using 1% resistors. If I consider all of the different permutations of the three resistors being +/- 1% of their stated values I end up with Vblk having a maximum and minimum possible value of 15.2V and 14.7V respectively and Vflt having a maximum and minimum possible value of 13.94V and 13.48V respectively.

First off is this amount of variation acceptable for charging a 12V SLA battery? Second, if this amount of variation is not acceptable does it make sense to stick a trimmer pot in series with either R12 or R14 to sort of tune the circuit?

2. ### wayneh Expert

Sep 9, 2010
12,382
3,241
The trimmer pot makes sense, unless you want to try several values and hope you get lucky. On the other hand, your odds of getting lucky are better than you might think, since most of the 1% resistors will be better than the max tolerance. Personally, I'd aim for voltages within ~0.1v of target.

3. ### jerseyguy1996 Thread Starter Active Member

Feb 2, 2008
206
9
Considering the cost of a trimmer pot versus the cost of just buying a bunch of resistors, I think the multiple resistors wins.

4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
You can get a 15 turn trimmer for \$1.27. And very precise voltage settings.

5. ### wayneh Expert

Sep 9, 2010
12,382
3,241
Right, and it will work first time, every time. Definitely the best approach if you just want to assemble quickly without horsing around. This approach also allows you to compensate for changing temperatures, which will affect resistors and the optimum charging voltages.

6. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
For each resistor, use a fixed to get you around 90% to 95% of the way there, and then use a 15-turn pot that will give you +/-10% or +/-5% adjustment.

7. ### jerseyguy1996 Thread Starter Active Member

Feb 2, 2008
206
9
Sounds like a pretty good consensus for the trimmer pot. Thanks everyone!