[R-390] An ON TOPIC ballast replacement question

[Bob Camp]

Hi

Ok, here's the "bridge" version of the ballast tube replacement:

You wire up a normal four diode full wave bridge rectifier to the two 
ballast tube pins (2 and 7 as I recall). The "ac" pins on the rectifier 
go to these pins.

On the "dc" side of the rectifier you put a 47  uf capacitor (to keep 
the regulator stable) and a nice normal three terminal regulator. Five, 
3.3 and 1.25 volt regulators all seem to have been used at one timer or 
another.

Another capacitor, also a couple uf goes on the regulator output (also 
for stability).

Finally you put a load resistor on the regulator output that will pull 
300 ma at what ever voltage your regulator puts out.

Since the load resistor is on the constant voltage side of the 
regulator it will always see the same voltage and thus always pull the 
same current.

The net result is a solid state DC constant current sink hooked up to 
do AC.

The down side is that the AC current flows in pulses rather than as a 
continous current. Thus you get RFI. You can play with the value of the 
47 uf capacitor to make the pulses wider. The lower the value of the 
capacitor the wider the pulses and the more ripple at the input to the 
regulator. You have to stop dropping the value of the capacitor when 
the ripple gets so great that the regulator drops out.

No matter how slick you get with the value of the capacitor you will 
still have pulsing current, it's only a matter of how much you get. 
Also remember that a proper AC current waveform goes from zero to 1.414 
times the RMS current during each half cycle. This little gizmo would 
be perfectly happy if the current was a square wave at the RMS value 
....

All that said the circuit does work. If you have significant amounts of 
time when your line voltage swings from 95 volts up to 130 volts then I 
would strongly recommend you use something as ballast ....

	Take Care!
		
		Bob Camp
		KB8TQ



On Jul 9, 2004, at 6:57 PM, Tom Norris wrote:

> A while back someone had come up with a
> ballast replacement that used a bridge
> and instead of a simple transistor arrangement
> it used a voltage regulator?
>
> Anyone have a recollection of that design
> or did it get mentioned and lost in the
> past weekend's clam bake?
>
> thanks
>
> Tom NU4G
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