The AM Forum

Greetings Gents!

Here is a really bizarre AC line phenomenon that has me
totally stumped.  Maybe someone has some insight into
what's going on. 

My wife got a new hair dryer; when she runs it on "low"
speed, a variac in my shack and a buck-boost transformer
in my AC line regulator start to hum like hell.  I looked at the
line voltage in the shack with a scope and cannot see any difference
with the dryer on or off.  A brute-force line filter has no effect, nor
does a ferrite choke with multiple turns of the dryer line cord wrapped
through it.

I've never seen anything like this!  Anybody out there have any ideas?

73,
Ken
W5KFS
ex-WB6VHE

Ken,

You checked the waveform with a scope, but have you checked the voltage to see if it is going up or down?  Only thing I can think of is if the neutral is open the from the breaker panel to the pole xfmr your line voltage from each 120 volt pole will change.  One side will rise, the other side will drop.

Only other thing is if the hair dryer has a SCR to control the speed, maybe that pulsing AC on that circuit is causing something to react with your equipment.

Maybe something to do with the dryer being on a ground fault breaker circuit.

Try the dryer on different outlets to see if that has anything to do with it.

What about when the dryer is on full high speed, what happens??

I'll have to think about this more.  Maybe someone else can come up with something.

Fred

Might be worth looking at the waveform again without the humming equipment in the circuit if you haven't already.

Check the voltage using a scope with the hair dryer running and not running. I'd bet that you have a neutral problem and the SCR current pulses are driving the brute fore regulator crazy.

Ken,

Try disconnecting most of the other items on the circuit and then look at the scope trace with and without the hair dryer.  Your humming transformers are probably helping to filter some of the line waveform distortion.

Several years ago I had finished restoring a Gonset G-28 and after lunch I went down for a final check before putting it into its case.   I found the most atrocious buzzing from the audio and thought the new filter caps must have failed.  Replacement of those and then the rectifiers made no difference and there was a tremendous amount of noise on the B+.  While thinking it over I unplugged the ceramic space heater and the noise disappeared.   The PTC ceramic element when running in low heat mode (probably via a SCR) created tremendous waveform distortion and I later found the heater created a buzzing in the surge protectors mounted at the breaker panels and could also be heard in my Viking 500 power transformer.  That heater was donated to goodwill.

 

The "low-power" option on many small heaters, including hair-dryers, heat guns, etc. is to insert a diode - thus giving half the heat.  However, this puts an unsymmetrical load on the power line, and any voltage drops in the line upstream from the heater will cause DC to appear on the power line.  Any transformers that have a small or no air gap in them will start to saturate, causing their magnetizing current to dramatically increase and to make a lot of noise.  Toroids are the worst in this regard, and a Variac is basically a big toroid.  I've seen this in audio installations where an expensive audio power amp with a fancy toroidal power transformer will start to horribly buzz when a light-dimmer is switched on.  I've also seen this on my own work bench when I put my heat gun on half-power.  Electrical devices should really not put an unsymmetrical load on the power line - but the cheapness of using a 5-cent diode to cut the power was probably irresitable to the hair dryer manufacturer.

- John

It's irresistible even for a $300 hair dryer although a triac is more common. They don't know any better.

I think I'm seeing the same thing. I have an old pole lamp. When I increase the brightness, my plate supply on my Westinghouse transmitter will increase by several hundred volts. It a non linear load that is causing havoc with my plate supply and I suspect your seeing the same thing.

  This sounds a little different to me. I wonder if keying your rig causes the pole lamp to get brighter? If so, your Neutral coming from the power pole is high impedance. That happened here, a tree branch whipped the neutral wire up the power pole until it fatigued and broke. I had plenty of strange stuff going on...  >:(

Jim
WD5JKO

I tend to agree with the bad neutral theory.  If you can, check the voltage from the neutral to a real ground [earth] ground.  This should prove or disprove the theory.

Consider the possibility of DC offset on the AC line.

There is a neat circuit that consists of some diodes and caps that eliminates this problem - it was developed as the result of this exact thing happening to transformers in audio equipment. I'd post the circuit, but I'd have to find it first, and that is not so simple or easy... but a search of the internet might find it, and fwiw, I think it was discussed on http://www.diyaudio.com

                 _-_-bear