Nuts and Volts - May 2014

READER-TO-READER TECHFORUM

>>> QUESTIONS

3D Printer Materials Reference

I’m designing an RC dune buggy using a 3D printer, but I can't decide which material to use. I want it to be as light and strong as possible, but I need a reference that shows (for example) the strength of an I-beam made of ABS vs PLA. Does anyone know of such an engineering reference?

#5141 Peter Murphy Pensacola, FL

Thermocouple Switcher

I want to read and log temperatures from multiple points in my (unnecessarily complicated) central heating system. I want to use thermocouples because: 1) they are easy to attach to my hot water cylinder (stabbed under the insulation); and 2) I already have them — from IKEA meat thermometers.

I would like to select each thermocouple in turn, use a single amplifier /analog-to-digital stage (Max31855 ??) to obtain the reading, and move on to the next thermocouple.

I need ideas for switching between the different thermocouples without introducing unwanted switch resistances, etc. Would an array of reed switches driven by a shift register be a sensible solution? Is there a better solid-state solution?

#5142 Mike O'Hagan Aberdeen, Scotland, UK

Dimmable LED Bulbs

I've seen replacement LED light bulbs in the hardware stores that claim they are 'dimmable.' Is this a real feature that has a different internal design or is it just a way to get me to pay extra for an LED bulb? If there is a difference, what are they doing circuit-wise to make them dimmable?

#5143 Robert Parsons

Detroit, MI

What Wall Wart?

While on vacation, I managed to lose the power supply wall wart to a cheap vintage portable no-name brand shortwave radio. On the back of the radio, the power jack says 9 VDC and has a C shaped circle and a dot in the center with a plus sign on the dot. I’m pretty sure this means nine volts; positive tip. However, what it doesn't state is the milliamp rating. If I use a power supply that has too high or low an amperage rating, am I in danger of damaging the radio? How can I determine the right supply?

#5144 Lyle Gardner

Vista, CA

Pot Cleaning

I recently purchased a small "pig-nose" guitar amplifier at a neighborhood garage sale. I put batteries in it and it works, but the volume knob is very "scratchy" and at certain places in its rotation, the sound cuts out entirely. Is this something I could fix by cleaning the potentiometer and if so, what would I use? If it's time for a replacement, does anyone know where I could find a schematic for this unit?

#5145 Emanuel Estrada Denver, CO

[#3141 - March 2014]

Motor Kickback

I am using an Ametek 965922-101 brushed DC motor (rated 38V nominal, 12A peak) in a project that runs it as both a drive and a brake. (Interestingly, the 38V rating is a "bus rating," i.e., with the motor stalled at 38V, it will use 12A and get nothing done.) In drive mode, the controller typically runs it up to 60V; current limited to 10A. Works great.

For the brake, I am using an IRFB260 MOSFET with a 0.6 ohm spring in series to limit the surge current. The PWM rate is 120 Hz. (NOTE: When sinking 1/2 HP of energy, the motor generates 30V.) However, each time the MOSFET turns off (at 120 Hz), the motor provides a huge kickback, and the MOSFET's reverse-protection zener diode probably will not survive that for very long.

At first, I paralleled the motor with an Elite 100 µF 400V capacitor [marked PM 105°C, (M 9305)] which calmed down the splash nicely. Then, one day after some generous usage, a huge cloud of smoke boiled out of a severely melted capacitor. It was a very abrupt introduction to the (unmarked) ripple current rating on capacitors. At the moment, I'm just using the same 4,700 µF 100V bolt-mounted huge capacitor that filters the drive circuit. (When using the 100 µF capacitor, one of the relay contacts would switch the 4,700 µF capacitor out of the circuit.) However, the 0.64 ohm brake circuit discharges the capacitor so quickly

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