[34S] battery warning indicator / question

[matthiaspaul]

(03-12-2015 12:21 PM)matthiaspaul Wrote:  For those who can modify the calculator hardware, there are several options to at least improve the situation:

- A low-leakage low-ESR capacitor (perhaps 10-47uF) should be added in parallel to C17/C16/C15 as close to the CPU as possible. This is particularly important for those who have added the USB extension, because then VBAT is routed through circuitry on the USB board and via long and thin wires, which will increase the voltage drops under load. Low leakage and clean soldering is important here because otherwise this will have the reverse effect.

As you know, the HP 15c Limited Edition is based on a hardware very similar to that of the HP 20b and 30b. They use the same Atmel AT91SAM7L128 controller (but in a different package), and some of the external parts differ slightly. Using the original firmware current peaks up to 40-44mA have been observed in the HP 15c LE, which could cause "Pr Error" messages in the calculator. While the situation in the WP 34S may differ as it is uses a completely different firmware, the scenario is nevertheless comparable. I just found an article suggesting a solution similar to the one proposed by me above:

http://www.finetune.co.jp/~lyuka/interes...c/hp15cle/

By adding a multi-layer ceramic capacitor (MLCC) TDK C3225X5R0J107M (100uF/6.3V/X5R in 3225 package) they could reduce these spikes down to less than the half (18mA), which is in the normal peak current range for this calculator. Using a cheaper Murata GRM31CF10J107ZE01 capacitor, the current was reduced down to at least 21mA - still good.

Also recommended by them are:

- Murata GRM32ER60J107ME20L, 100uF/6.3V/X5R/3225
- TDK C3216X5R0J107M, 100uF/6.3V/X5R/3216
- Taiyo Yuden JMK316BJ107ML-T, 100uF/6.3V/X5R/3216

In my post above I did not specifically recommend 100uF capacitors because they typically have higher leak currents than 10-47uF capacitors, but you will have to study the datasheets yourself as the exact capacitor models frequently change with advancements in technology.

While a low ESR (combined with relatively high capacitance) is the important parameter to reduce such current spikes, the low leak current is important to not reduce the battery life through leak currents when the calculator is not used. So, if you find multiple capacitors with about equal low ESR and low leak current, go for the higher capacitance, for as long as the nominal voltage is 5V or higher. If the other parameters are equally good, choose X7R over X5R.

Greetings,

Matthias