Re: Rewiring a ballast. Help needed.

From: Victor Roberts (xxx_at_lighting-research.com)
Date: 03/15/05 

Date: Tue, 15 Mar 2005 04:43:57 GMT

On Tue, 15 Mar 2005 03:43:06 GMT, "NSM" <nowrite@to.me> wrote:

>
>"Victor Roberts" <xxx@lighting-research.com> wrote in message
>news:pfkc315mks96ctlouddeu6tugu2ofu4n1j@4ax.com...
>
>> A triac can't be a resistor. It has two only states: on and off. The
>> triac is probably used as a switch to start the lamp.
>
>Obviously they use some sort of driver circuit to control current to the
>lamp via the triac. I would have assumed you knew that - from your own
>website.

Are you claiming that your "ballast" has no inductor or no capacitor?
That the only device connected between the lamp and the power line is
a triac?

The problem with a triac is that once it is on it will not turn off
until the current drops to zero. Triacs can be used to dim
incandescent lamps by cutting off the early part of the 50 or 60 Hz
power line cycle. However, the resulting on time, while a fraction of
the a normal half-cycle, is far too long for a fluorescent lamp, which
can run away in far less than 1 msec in less the current is
controlled.

There are circuits where a secondary triac is used to dump reverse
current through the main triac and therefore force it to turn off. I
have never seen these used for a lamp ballast, and I'm not sure they
can switch fast enough to keep the lamp from running away. It would
also require two triacs which is inconsistent with your message.

Are you sure it is a triac? The lamp current can be controlled by
quickly switching a BJT or FET, but that mode creates a lot of EMI. 

-- 
Vic Roberts
http://www.RobertsResearchInc.com
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