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Recent Questions

2020 Issue-6


VSWR Value Change

Last night I was testing my 145MHz Yagi antenna for VSWR value when I realized something. When I kept the antenna close to ground, the VSWR value would change. Also, when I stood in front of the antenna the same thing happened.

I searched about it on the internet but I'm not satisfied. If any one knows the reason for this, could you explain it to me please. It will be really appreciated.

#06202
Emre Eroglu
KONYA


Sump Pump Monitor

My basement is prone to seepage. When the sump pump runs so often that there is only a 15 second off-time the seepage is eminent. I am looking for a stand-alone circuit that gives a digital read-out of the off time in seconds. An adjustable alarm output would be a big plus.

I am not concerned with sensing the pump power status, that I think I can do. The off time range of interest would fall between 120 and 2 seconds. I am sure this can be easily accomplished with an Arduino or Raspberry Pi circuit but I have no experience with either. Thank you.

#06201
Allen Goodcase
Brookfield, IL


2020 Issue-5


Bench Power Supply

I would like to build an inexpensive AC power supply for my workbench. I want something much smaller and lighter than a variac, 0-30 VAC, and maybe one or two amps would be fine. Can anyone point to a good schematic or even a well-written circuit description?

#05201
Jeff Bowles
Columbus, OH


In-Line speaker Amp

I recently moved into a home that has in-ceiling speakers. I have them connected to an A/V receiver and in one room they work great. In the other room, the sound is very muted. There’s a volume control in that room which I’ve replaced and checked. I’m looking for some kind of amplifier that I can purchase or build that can just increase the volume level on that pair of speakers (there’s a pair leaving the receiver which goes into the volume control and then splits into four speakers). I have checked obvious issues and swapped the A/B pairs just to make sure my receiver hasn’t failed.

#05202
Michaeljon Miller
Trabuco Canyon, CA



Answers

Because you swapped the A/B speaker leads and got the same audio results, the culprit might be speaker-impedance mismatch. Check the output impedance of your A/V receiver and of the low-volume speakers. The receiver manual should specify an impedance, which in most cases comes to, 4, 8, or 16 ohms. If not in the manual, check for a label at the outputs. 

Also, find the impedance of your speakers in the manufacturer's information or on a speaker label.  You want the same impedance at both ends.  A mismatch can cause reduced volume and even distortion. If you want to measure impedance, here's a link to a helpful article: https://www.wikihow.com/Measure-Speaker-Impedance. If all else fails, look for an impedance-matching transformer. More information here: https://www.electronics-tutorials.ws/transformer/audio-transformer.html.

Jon Titus
Herriman, UT


Assistance Against Cyber Attack

What materials make a good phased array antenna (i.e., efficient transmission and reception and the shape of the individual components)? What frequencies go through earth and seawater above 10 GHz also?

#05203
William Zimmerman
via email


2020 Issue-4


Component Footprints

Can someone please explain the standards for the footprint of electronic components? I’m trying to figure out the the best way to lay things out on a circuit board. I’m new at this and any advice would be appreciated.

#04201
James Devera
Scandinavia, WI


Fan Indicator

Good day to all you experts!  I have a plywood basement floor that is suspended like any other floor in the house (bentonite soil in my area requires this construction). The actual dirt ground is about two feet below the wood floor, covered by a rubber tarp.

To prevent a build-up of mold and stale air, this space has a 6” duct vent fan that turns on via a humidity sensor rheostat. The supply side duct is on one side of my basement and the evacuation duct is on the other.

In the past, I could hear this fan running, so I knew when the bearings were wearing out. It was an easy job to buy a new duct fan and replace it. We just had our basement finished, putting drywall around the perimeter wall. Now I can no longer hear this fan when it kicks on.

Does anybody have a suggestion for some sort of sensor that detects when the fan is turned on by the humidity sensor but drawing too large of a current supply, so on the verge of bearing failure? Ideally, I would like some sort of an indicator light that I can make part of the access panel that is over the fan. Even an AC ammeter movement would be adequate.

At the location of the fan, I have both the switched 120 VAC power supply and a constant 120 VAC available if needed. I don’t have the specifications on this exact fan available, but a quick search online found several that had operating currents of 0.35-0.40 amps. I know the start-up amps would be a little higher but not too much because the motor is small and has very little inertia to overcome. Thank you for any suggestions!

#04202
Bill Young
Denver, CO


Temperature Sensing LEDs

I’m looking for a temperature sensing circuit that will light each of three LEDs at approximately 80°F, 90°F, and 100°F.

#04203
Matthew Seltzer
via Internet


Capacitor P&Cs

What are the pros and cons for using electrolytic capacitors in a voltage divider circuit to provide about 24 volts AC to a heater cable from the 120 volt AC line?

Is there a possibility of having a capacitor explode from overheating? If so, could that be prevented by stringing several capacitors in parallel to provide for additional heat dissipation?

#04204
Robert Gotts
Madison, IN



Answers

Electrolytic capacitors are polarized and every half cycle of the powerline their polarity will be reversed. Depending on the values and types, they may get hot, or more exciting, blow out their pressure relief attended by a puff of smoke and fumes. In any case, their life is rapidly shortened if reversed. Non polarized caps are available but it all depends on the values needed. Most other capacitors are non-polpoarized and they should work for you.

Len Powell
Finksburg, MD

Mr. Gotts seeks information on employing the reactive property of a capacitor to reduce AC line voltage to 24 volts.

The short answer is “Don’t do it.”

Been there. Done that. Didn’t know any better. In my case, I had a small circuit comprising one vacuum tube having a 12-volt filament drawing 0.15 amperes. Dropping the voltage from 120 volts required a series impedance of 720 ohms. Like you, it occurred to me that the reactive impedance of a capacitor might provide the needed voltage drop, eliminating a large (and hot) series resistor. A capacitor of 3.7 uF at 60 Hz provided the necessary 720-ohm impedance.

The technique worked and nothing blew up. I was lucky. Seventy subsequent years of experience, however, lead me to consider the reasons NOT to use this technique:

  • Most importantly, no isolation from the AC line is obtained, so that the possibility of shock hazard always exists. Furthermore, because the reactance of a capacitor varies inversely as the frequency of the applied voltage, and lacking any line-to-load isolation, it follows that any high-frequency line-voltage transients will be coupled through to the load without attenuation.
  • The technique does not, in general, work where the characteristics of the load are unknown. If the load is resistive and the current drawn is constant, — as in my vacuum-tube example — the technique works. If the load varies, however, then the voltage across the load will vary accordingly.
  • In any application with this technique, phase shift of the load voltage occurs relative to the AC line voltage. This phase shift is unimportant to a resistive load. If, however, the load is inductive or capacitive, this phase shift may produce wild swings in load voltage due to oscillation. Heater control circuits usually employ relays, and relay coils are inductive loads.

One would never use an electrolytic capacitor for this job. Film-dielectric non-polarized motor-start and motor-run capacitors are available with operating voltage ratings suitable for the job. But the off-the-shelf tolerances of such devices is relatively large, running to 6% for motor-run capacitors and 10% for motor-start capacitors. For a 24-volt resistive load supplied through an off-the-shelf motor-run capacitor, the load voltage may be anything from 22 to 25 volts.

A small transformer is less expensive than a motor-start or -run capacitor, it provides safety isolation from the AC line, and the load voltage won’t oscillate.

Peter A. Goodwin
Rockport, MA

That is not at all practical. Use a 24 volt transformer from a sprinkler timer or thermostat.

Richard Cox
Thousand Oaks, CA

Most aluminum electrolytic capacitors are not suited to having large amounts of AC voltage on them. Also they have to be used in pairs, to handle both polarities of voltage. And yes, you may have them overheat and "rapidly disassemble."

Also the heater cable will not be isolated from the AC line, which may be hazardous under fault conditions. You didn't say how much current you needed at 24VAC, but I assume it might be more than an ampere. My first choice would be a 120:24V transformer. You'd get decent efficiency and isolated power.

Jonathan Wexler
Los Angeles, CA


Battery Woes

I have some brand new lead-acid batteries that have never been used. They have been stored in my garage for a while (1-1/2 to 2 yrs). My smart charger errors and won’t charge them. Why is this and is there anything that can be done to revive them?

#04205
Reva Pino
Charlotte, NC


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