[HBR] Filament Voltage Dropping Scheme

[N2EY at aol.com]

In a message dated 12/2/06 8:45:42 AM Eastern Standard Time, 
Hopperdhh at aol.com writes:


> how I reduced the line voltage to 62.6 volts for 
> the filaments.
> 
> I used capacitive reactance to do it.  There is no heat dissipated in the 
> capacitor like there would be if the 120 volts were dropped with a resistor. 
>  
> 

3.89uF
> 
> 
>  I 
> used mylar capacitors rated at 250 volts to come up with the right value.  
> Computer circuit boards as well as solid state TV boards found at ham fests 
> often 
> have these types of capacitors.  Its easy to experimentally arrive at the 
> right 
> value after finding the target value from the equation.
> 
> The nice thing is that no power is dissipated so the capacitor runs stone 
> cold as it does the job.  I don't think the power company will mind the very 
> 
> slight power factor increase.
> 

Actually, the capacitive reactance may help! Most loads are inductive, and 
the utilities often have static capacitor banks on the poles to correct it. I've 
long been a fan of synchronous condensers but that's another topic...

> This should work just fine in line operated receiver circuits as well.  Of 
> course all the usual precautions apply for all line operated equipment. 

Agreed.

 Today, 
> 
> with houses wired for 3 wire outlets, line operated equipment is much safer 
> than it was back in the 50s and 60s.
> 

Only if the house is wired to code. Many aren't.

Some years back, we were thinking of moving to the point that we had a home 
inspection done on a house we were looking at. The inspector went around with 
an outlet tester - the three-light kind.

Besides finding numerous 2 prong outlets, he found almost every possible 
problem on the various 3 prong outlets:

- No ground
- Hot and neutral swapped
- Ground and neutral swapped.
- Completely dead outlet

About the only thing he didn't find was hot on the ground pin. 

We didn't buy the house!

> OK guys, what do you think?

It's certainly a useful technique, but it has some drawbacks:

First, the needed value will need to be found experimentally in almost all 
cases. Not really a problem if you are only making one or two units and have a 
handful of capacitors.

Second, the caps have to be rated to take the voltage and current. IMHO, 250 
volts is kinda close, because the peak value of a 125 volt RMS sine wave is 
177 volts, and if a surge comes along....pop? If a cap shorts, your precious 
bottles have full line voltage on the string. 

Third, the setup will only work right on one frequency. It is probably 
waveform-sensitive, too.

Fourth, when you turn on the set, there will be surge as the caps charge for 
the first time. The charging current is only limited by the cold resistance of 
the heaters.

Just be careful!

73 de Jim, N2EY