Nuts and Volts - February 2009

TECHFORUM

This is a READER-TO-READER Column.

All questions ANDanswers are submitted by Nuts&Volts readers and are intended to promote the exchange of ideas and provide assistance for solving problems of a technical nature. Questions are subject to editing and will be published on a space available basis if deemed suitable by the publisher. Answers are submitted by readers and NO GUARANTEES WHATSOEVER are made by the publisher. The implementation of any answer printed in this column may require varying degrees of technical experience and should only be attempted by qualified individuals. Always use common sense and good judgement!

All questions and answers should be sent by email to forum@nuts volts.com All diagrams should be computer generated and sent with your submission as an attachment.

QUESTIONS

To be considered, all questions should relate to one or more of the following:

❶ Circuit Design

❷ Electronic Theory

❸ Problem Solving

❹ Other Similar Topics

■ Be brief but include all pertinent information. If no one knows what you're asking, you won't get any response (and we probably won't print it either).

■ Include your Name, Address, Phone Number, and email. Only your Name, City, and State will be published with the question, but we may need to contact you. ■ No questions will be accepted that offer equipment for sale or equipment wanted to buy.

■ Selected questions will be printed one time on a space available basis.

■ Questions are subject to editing.

ANSWERS
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Feedback section if space allows.
92 February 2009

>>> QUESTIONS

Meter Conversion

I am building a project that uses a 0-100 UA meter. Can I purchase a DATEL/MURATA 0-199.9 UA digital meter (LED) and use it in place of an analog style? I realize that I have to power it separately.

#2091

Mark Habian Conneaut, OH

USB Speakers

Can a USB speaker — like a Harmon Kardon iSub 2000 — be rewired to use a regular audio input while still using the built-in amp?

#2092 Jerry Williams

West Bend, WI

Power Pac for HO Railroad

Does anyone know of any plans for controls to run several trains simultaneously on the same line, independently of one anothe?

#2093 Terry Murphy

El Cajon, CA

Regulators or Zener Diodes?

Many of us use the regulator devices 7805, 7812, etc., in our designs. What is the difference between these and using zener diodes? An internal circuit diagram would be helpful! When do you use a low resistance resistor in series with these devices?

#2094

Tom King via email

Auto 12V to Laptop 19V Power

I’m looking for a circuit that can boost the 12V in a car to 19V, enough to run two laptops (batteries removed). It will have to provide at least 100W. I bought one off eBay but it is unstable with my laptops (shuts down randomly).

#2095 Craig

Hamilton, Ontario

Constant Current 30 VDC

I need a schematic for a 30 volt DC power supply running at a constant current of 800-900 microamps, adjustable.

I want to make colloidal silver of minimum particle size and the current flow through the solution must not exceed one milliamp. In the beginning, the resistance is very high through steam distilled water, but in time the resistance becomes progressively lower. The adjustable current flow must be held constant by the supply.

#2096 Phil Vogel via email

Sensing AC Fan Motor

I’m looking for an easy way to sense when my home AC/heater fan is running (110 volts).

I have an electronic timed pump air freshener that I want to spray into the AC/heater plenum ONLY when the fan is running so it will carry the fragrance throughout the house.