IEEE Consultants' Network of Long Island

Lethal Hazards of AC-DC Radios and Televisions - John Dunn, Consultant, Ambertec, P.E., P.C.

Remember the good old days before the CK722? Remember when your table model AM radio cast a warm, friendly glow on the wall from the filaments of its vacuum tubes? Remember the comforting warmth of the radio's cabinet? Remember how that thing had the capability of killing you?  

 
It's hard for some younger folks to believe, but these things could be lethal if you happened to come into simultaneous contact with such a radio's chassis, or in some cases the chassis mounting screws or control knob shafts and the nearest radiator.

There were television sets made this way too and they were just as dangerous, especially the AC-DC televisions that used metal cases.

If you ever happen to come across one of these oldies, BE CAREFUL. As Captain Kirk said to Ensign Chekov in one episode, "At close range, these things can be as deadly as a phaser."

 

 

High Voltage Corona Test - John Dunn, Consultant, Ambertec, P.E., P.C.

High voltage capacitors can exhibit problems with corona at voltages well below their ostensible voltage ratings. The problem can be qualitative tested for as follows:     

 
The high voltage power supply feeding this circuit can be the Spellman Model 205B-20R whose datasheet  is available at http://www.spellmanhv.com/~/media/Files/Products/205B.ashx .

Logic Vcc Power-On Switching Problem - John Dunn, Consultant, Ambertec, P.E., P.C.

Some troublesome digital chips were not starting up properly if their +5 Vcc was applied with too slow a rise time. (Hey, sometimes things like this can happen, right?) The minimum required rise time for that +5 Vcc rail was determined to be 45 mSec.

This was a problem which later resolved by the chip vendor, but while the problem still existed, a circuit was needed to delay Vcc application to the troublesome chips until the power supply voltage had risen to a particular threshold value, and not before, and to then apply the already risen power supply voltage to the chips with a relatively fast rise-time as sketched here:  

Aluminum Electrolytic Reclamation - John Dunn, Consultant, Ambertec, P.E., P.C.

An aluminum electrolytic capacitor that hasn't been in service for a long time may need to be brought back to service by very slowly raising its voltage up to rating. If you try to bring one up too rapidly, the capacitor can be destroyed.

This circuit enabled the resurrection of a very old, spider webbed, dust covered 14000 µF capacitor that had been sitting idle in a storage closet for how long, nobody knew.  

 The 0.1 µF, the two diodes and the 14000 µF capacitor itself form a half-wave, Cockcroft-Walton voltage doubler circuit. However, because the 0.1 µF is so small compared to the 14000 µF, the voltage rise time across the 14000 µF is something around fifteen minutes for the 60 Hz line frequency.

In use, you apply the input line voltage (120VAC at 60 Hz in my case) and turn the autotransformer up just a little bit. Maybe half an hour later, you raise the autotransformer setting upward a little bit, after another half hour you do it again and so on and so on.

Since the risetime is so slow, you don't need quick reflexes if you happen to raise the autotransformer a bit more than you had intended. You can just lower the setting back down again.

Also, if the 14000 µF were to short circuit itself, the AC line would simply be loaded at most by the 0.1 µF which poses no safety hazard.

Just do be sure to put the capacitor inside of something safe in case the capacitor does fail. An empty metal can might suffice.

 

Switchmode Dynamic Input Impedance - John Dunn, Consultant, Ambertec, P.E., P.C.

Let's assume an ideal, one hundred efficient switchmode power supply passing one hundred watts to its load and drawing its input power from an input line of one hundred volts. That ideal case would look like this:  

 

If we then vary the input line voltage up and down just a little bit, the input current responds by going down and up just a little bit:  

     

Unjustified CSR101 Test Failures - John Dunn, Consultant, Ambertec, P.E., P.C.

A modest number of watts switchmode power supply was undergoing a conducted susceptibility examination in the following test set-up. The power supply was occasionally showing instability, but it was not the power supply's fault: