Nuts and Volts - January 2009

still a bit confused about the ANSEL and CMCON command. Can you shed a little light on this? Thanks.

— Bob

MAILBAG

AI use PICBASIC PRO from microEngineering Labs, Inc. You can download a demo version plus MicroCode Studio, plus the PICBASIC PRO manual from: www.mecanique.co. uk/products/compiler/ pbp.html. You can download just the manual which will answer all your questions. I’m not the one to ask because I am a newbie myself.

TELEPHONE HEADSET ADAPTER

QI am hard of hearing and have a difficult time hearing on the telephone. Headsets help but most cover only one ear. I have a set of headphones for my computer that cover both ears and have a boom microphone. I would love to be able to plug these into my phone.

Can you recommend a circuit that would allow some amplification for incoming audio and provide phantom power, as well as interfacing for the condenser mic which is what I think these computer mics are? Many thanks.

Dear Russell,

Good job continuing the Q&A column! I enjoy reading it.

One comment: one reader asked you about a power supply for a battery charger, and you guessed the oddball transformer in question was a ferro-resonant regulating transformer. Seems like a good guess to me. But at the end of your answer you said "batteries don't care about being pulsed; in fact, it may be better than smooth DC."

This seems not to be true of laptop batteries. If you look at the Maxim 8731A charger chip, the Intersil 88731, the TI bq24702, or any of the other modern chips, they all use buck converters and the battery charge current is regulated via the duty cycle of a power FET that controls the current into the coil of the buck converter.

The output of that buck converter is, of course, a non-smooth DC waveform. The charger chip makers all warn you to use the proper ripple-absorbing capacitors in parallel with the battery and go to great lengths to show you what types of caps work, which don't, what values the caps should have, and where they should be placed. Some of them specifically warn you not to let the batteries absorb the ripple themselves because it heats them up substantially.

— Bob Colwell

Thanks for the feedback, Bob; I was thinking of automotive type batteries which absorb pulses easily. A capacitor in parallel with the battery will only work if the battery has high impedance which may be the case in those types. Certainly a temperature rise due to internal losses would not be good in a fast charger that determines full charge by a temperature rise.

— Denny Sioux City, IA

ANot knowing anything about your phone or the microphone, I cannot come up with a circuit, but I do have a solution to your problem. I have a Panasonic model KX-TG5672 portable telephone. The handset has a speaker phone button which increases the volume so it is audible across the room. My wife loves it; she is hard of hearing, also. The handset has a phone jack on the side that accepts a 2.5 mm plug, in case you want some privacy. You no doubt will have to change the 3. 5 mm plug that is on your headphones, look for Mouser part number 17PP053-EX. NV

30 January 2009

Dear Russell,

Regarding the "Battery Charger" inquiry by Michael Craft in the October 08 N&V issue, page 24...my take on the question is this: The charger is used to charge and to maintain a fully charged state of a string of lead-acid cells while simultaneously supporting an external load. The reader describes a float voltage of 36 volts. The "float" or recharge voltage under which a lead acid cell will experience long life is 2.25 volts; this suggests a battery composed of 16 cells, having a float voltage of 36.0 volts, a fully charged open

circuit voltage of 35.2 volts, and an end-of-discharge voltage of 32.0 volts.

The load characteristic of a lead acid battery — that is, the impedance presented to the charger — is non-linear with voltage. As the battery charges (from a discharged state), the voltage impressed across the battery terminals rises so that the charging current — if delivered from a constant voltage source — decreases. As the float voltage point is approached, the charging current drops to a very small value, and it is important not to force charging at this point, else electrolysis will occur and water will be lost from the electrolyte solution.

The voltage and current charging characteristics look like (Figure A); the charge current dropping off markedly as end-of-charge conditions are approached. Thus, a lead acid battery charger needs to be a two step device, delivering current (without regard to battery terminal voltage) at the onset of charge, and a constant float voltage at the end-of-charge point.

Transformer T2 operates from the unregulated AC line voltage. The voltage delivered to the rectifiers is the sum of the T2 secondary voltage plus the two secondary winding voltages on T1. We can surmise that T2 is designed such that float voltage will be provided out of the rectifiers with little or no contribution by T1. On the other hand, at the start of charge, the battery terminal voltage is significantly smaller than the voltage out of the rectifiers, and without the moderating influence of T1, this could result in forcing excessive charge current into the battery.

T1 is effectively fed by a current source — the series-connected inductor L1 — and this limits the battery charging current because T1 is a series-connected member of the charging circuit.

T1 also provides a degree of line regulation. L1 and C1 form a 60 Hz resonant tank circuit and for constant load, the output voltage from T1 remains relatively constant. If the AC line voltage decreases below design center, the output voltage from T2 will decrease proportionally. The battery current, however, will remain nearly constant during charge. Once the charging event has completed and float conditions are obtained, T1 will compensate for variations in T2's secondary voltage caused by corresponding variations in AC line voltage.

Inductor L2 makes the battery appear as a current fed load. L2 also acts to remove the 120 Hz ripples from the battery bus, which suggests that this charger might be part of a telephone system in which it's important to isolate a hum voltage component from the 36 volt DC.