[NJARC] Current limiting Devices

Tue Nov 29 13:10:38 EST 2016

Hi JimA current probe will work and is easiest to use.  So would a small value resistor in series with the load.  You then measure the voltage drop across the resistor which is proportional to current.  Care needs to be taken since the scope voltage probe is referenced to ground.  This is where an isolation transformer comes in handy!  You have to power the radio from an isolation transformer so that the ground connection of the scope probe doesn't short the AC line voltage to ground.  
You could also put the resistor in the circuit near the common return of the AC line if you don't have an isolation transformer handy.  This should put the scope probe ground within a volt or two of neutral but it is still a crap shoot and not worth the gamble.
A decent o'scope has delayed triggering so you should see the whole pulse of inrush current.  You are correct, the old fashioned way to record a waveform was with a camera.  Later, storage o'scopes using special CRT's were used.  These would memorize a waveform for later analysis but the image will slowly fade over time.  I have an old HP scope that has this function.  Newer digital o'scopes of course write the waveform to memory and the image will last as long as you like.
You could possibly use your smart phone to record the waveform displayed on the o'scope CRT for later viewing if it is possible to either slow or stop the recording during playback.  I don't know if this is possible since I have never tried it.
Since voltage drop is proportional to current through a fixed resistance, any resistor will act to limit inrush current.  This is the purpose of the small value resistor used with Selenium rectifiers.  The resistor limits capacitor charging current to a value that the Selenium rectifier can tolerate without destruction.  Varistors or other devices that change resistance value with temperature just do this job better.
The wattage rating of the current limiting resistor just has to be sized for the normal current needed by the radio plus a safety factor of two or so.  Sizing the resistor for the peak inrush current is just over kill.  The Selenium rectifier current limiting resistor mentioned above was purposely sized on the low wattage side to also act as a fuse in case of rectifier or capacitor failure.Jim

      From: JIM HEARNEY via NJARC <njarc at mailman.qth.net>
 To: njarc at mailman.qth.net 
 Sent: Tuesday, November 29, 2016 5:37 AM
 Subject: [NJARC] Current limiting Devices

I believe the varistor should work.  This reminds me of a problem that I have not been able to solve: "how to you use voltage measuring equipment, aka analog oscilloscope, to measure a very short transient such as a current surge? " The first part I think is solved using a current probe, bench made or store bought, but how do you record the high speed transient.....the old fashioned way....poloroid camera on scope.. and single triggered trace?  What do you use for the trigger?  A scope with memory option makes second part a lot easier...I imagine! Jim
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