Nuts and Volts - February 2009

>>>READER-TO-READER QUESTIONS AND ANSWERS

Peak Reading Voltmeter

How can I build a peak reading voltmeter or add-on unit for a VOM to read the peaks of a DC pulse? When troubleshooting ignition systems on motorcycles and marine engines, the peak voltage supplied by the various coils needs to be checked. A regular VOM always reads low. How can I catch the peaks?

With RPM as much as 14,000 and as many as 12 pulses per revolution, we are talking very short pulses. Testing is normally done at a cranking speed of 200 to 300 RPM. There can be as many as 12 magnets zinging by each coil per revolution.

#2098 Tony Altobelli

Grand Rapids

Figure 1

USB HDTV Tuner Hack

How hard it is to rewire or modify a USB HDTV tuner to work with a portable DVD player to watch HDTV programming from it or any other applications like making a portable HDTV tuner with a battery power source? I've written about this idea to many other sites and have had little to no responses positive or otherwise.

#2099 David Kichi

Inquiring Minds Want To Know!

Transformers, motor stators use stacks of thin (approx. 1/32") steel plates. Is this for manufacturing economics or is there a physics reason for using stacked plates?

Will using 1/16" plates instead of 1/32" plates affect performance? How much less efficient is aluminum wire versus copper for making transformers, stators, electromagnets?

Are there small solid-state relays capable of handling high voltage/high current surges through the switches (rail gun application)?

#20910 Jim Hicks DVM

Fernandina, FL

Figure 2

>>> ANSWERS [#12084 - December 2008]

Variable Brightness of LEDs

I have two sets of eight LEDs. The sets need to turn on alternately, with the brightness turning on gradually, then fading out.

As far as controlling the LED brightness, here are two possible solutions: one using PWM (pulse width modulation) and the other a variable DC power source. First, I'll describe the PWM circuit in

Figure 1

which is controlled by a PICAXE 08M microprocessor. The output from a PWM generator is programmed to run continuously. The timing duration is controlled by the ADC reading on potentiometer R1. The LEDs are controlled by FETs Q1 and Q2. All parameters are set by the programming code. Figure ²uses a combination triangle-square wave generator (U1) to control the variable DC voltage from Q3. R2 controls the duty cycle and R1 controls the minimum DC output from Q3. The setting of R1 will depend on the color and/or type of LEDs used. Basically, R1 is adjusted so the LEDs just shut off before the counter advances. The square wave output (pin7 of U1) is

used to advance the 4017 (U2) which is configured as a count to two and recycle counter. The square wave output switches low at each minimum crossing of the triangle generator. This is used to control the "clock enable" (pin 13) on U2. The clock input is connected to Vcc so the counter will advance each time pin 13 goes low. One nice feature about this circuit is the 4017 allows for easy expansion (up to 10) if more LED strings are needed.

There were no details mentioned about the LED strings regarding how they are wired, the color, or voltage needed. In this case, I decided to show both circuits operated with the LEDs in parallel with individual current limiting resistors. The RL values shown are only guidelines as the specs on the LEDs and the brightness required by the user will vary. Both circuits require a regulated 5 VDC power source.