Building a MAX712 Battery charger -- A few questions

[Jack.Straw]

Hello. I'm plan to build several NiMH battery chargers using the MAX712 IC and have some questions for you gurus. The datasheet for the MAX712 is located here.

My first question concerns the Rsense resistor. Per the datasheet, Rsense is calculated as:

RSENSE = 0.25V/(IFAST), where IFAST is the fast-charge current.

For the sake of simplicity, lets assume I want a 1A charge current. That means Rsense would be a .25 ohm resistor. A .25 ohm resistor isn't something I can pick up locally, and the only such things i can find online aren't really suited for my perfboard application. My thought is to simply put 4x 1 ohm resistors in parallel and call it a day. However, I've done a lot of internet searching on the MAX712, and in every tutorial I find the author doesn't do that. They mostly make their own resistors out of wire or use large power resistors. Using 4x 1 ohm resistors in parallel seems to me like the most obvious and easiest solution, yet noone seems to do that. I'm thinking there must be a good reason. Can anyone see a problem with me just using parallel resistors?

Thanks for your time,
-Scott

 

[kam1787]

no problem, just mke sure they can handle the power

 

[Jack.Straw]

no problem, just mke sure they can handle the power

Thanks Kam. All the resistors I have on-hand are 1/4 watt, but I know there are bigger ones available. How would I calculate the wattage requirement of those resistors? My "biggest" charger will be for 10x AA batteries (12 vots). I will be using a 24v - 1A power supply, and plan to charge the batteries at 750mA.



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My 2nd question concerns temperature control. I want to use absolute values for temperature cut-off. I want to disable cold temperature cutt-off, and set a high temperature cutt-off at 40°C.

The 10k thermistors I have are Epcos part number B57863S0103F040 (**broken link removed**).

Using the Epcos NTC R/T Calculation tool I see that my thermistor's values are:

25°C (room temp) - 10k ohm
40°C (hi cut-off) - 5.3k ohm

Using those values I came up with this:

(TLO would be connected to BAT- to disable low temp cut-off, per the datasheet)

I feel like that is correct, but page 13 in the datasheet says "All resistance values in Figures 9a and 9b should be chosen in the 10kΩ to 500kΩ range."

Looking further, the author of this tutorial article says:

First choose a thermistor whose R0 is about 10kOhm, not to exceed 100kOhm (this is required by the inputs of the ‘712). It makes logical sense to pick a value of R2 that is the same resistance as the thermistor’s nominal resistance to make the voltage on the TEMP pin 1V. For illustration I’ll use the muRata NTSA0XV103, a resin coated, twin lead NTC thermistor with nominal resistance of 10k and B=3900. We’ll choose R2 to also be 10k. With these values and desired temperature limits we can determine what voltages to apply to THI and TLO.

Let’s start by finding the voltage THI equivalent to 40 degrees C (313 K). Taking room temperature to be 300 Kelvin, plugging everything in to the equation for R(T) we find that the warm thermistor resistance will be about 5.8k. A simple solution is to match R3 to T1 (when warm) and R4 to R2. Since the warm value of T1 is likely to be something obscure, you can always just match the two divider networks instead by choosing R3/R4 = T1/R2 , so long as the values of the R3 through R6 stay in the 10k to100k range. The low temperature cutoff is calculated in the similar fashion.

It seems to me that his value for R3 is also going to be below 10k (5.8k). Furthermore, as the batteries get hot, T1 will also fall below 10k. Am i misunderstanding both this author and the datasheet? Is it the combined values of R3 & R4 that must be at least 10k??

Thanks for your help!
-Scott

 

[kam1787]

P = VI or P = I**2 * R gives the power the resistor must handle. Convention is to double that number for the resistor's power rating.

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I feel like that is correct, but page 13 in the datasheet says "All resistance values in Figures 9a and 9b should be chosen in the

I am NO expert on the Max IC, but it appears to me that the restriction applies if ".... battery packs come with a temperature-detecting thermistor ..."