AnthonyL wrote:
On Wed, 13 May 2020 13:48:11 -0400, Paul
wrote:
AnthonyL wrote:
I have an old battery (6 x D cells) torch which has a fluorescent
tube, a spotlight and a flashing light behind a red (plastic) glass.
The torch also has a DC power input of unspecified voltage which then
runs into some circuitry.
1) The battery compartment has got corrosion, half of a spring has
gone (there are two springs at the bottom, one for each set of 3
batteries), but there is continuity between the spring connections. I
plan to take a spring off a defunct 6v lantern battery to replace the
damaged spring. What would be the best way to fix it?
2) The fluorescent tube is a Sylvania White F6W/W which has probably
been superseded. 210mm x 16mm. Grey/black at one end. Is there
anyway to test if this works outside of the torch?
3) I've put a 9v supply into the torch and at least the spotlight
functions though nothing else. All the connections appear to be
accessible. What should I be measuring for the fluorescent tube?
Whilst this is one of numerous "Covid" projects I have on the go are
modern LED torches a better bet? Though I don't like the light from
my head torch or even any that I've put into lamps
Tubes work best with sine wave (AC) drive. In this example
web page, the "working" voltage listed for the tube is 70V.
https://www.electroschematics.com/fl...t-lamp-driver/
"After start, the voltage collapses from several hundred volts
to the operating voltage, which is ca 70V for an 8W rod."
It's highly likely there is an inverter of some sort, inside
the plastic housing. And with any luck, glued plastic pieces
to keep you out.
"Pure flyback single-transistor inverter, makes voltage spikes
in the kilovolt range in idling condition, so that the lamp
will start. Disadvantage: One filament evaporates and blackens
the lamp, makes it electrically unsymmetric and shortens the lifetime."
Well I can get to the connections that the tube fits in. So with
power connected I should get an AC voltage reading with my
multi-meter?
Are you going to make the measurement with a load, or with no load ?
First of all, this web page is *not* about a Johnlite 6W fluorescent lantern.
What it does show though, is the "ignite/burn" curve. If the tube runs
at 70V, it ignites at 200V.
https://www.edn.com/electronic-balla...ower-supplies/
If the tube did not ignite, the voltage could rise above 200V.
My objective here, is to prevent two things from happening.
1) Avoid having AnthonyL zapped by the voltage across the tube socket pair.
And thrown across the room. Now, it's not going to do that, as long
as the frequency remains high. At 20KHz, you might not feel it, but
it might cause a flesh burn if you don't rapidly remove your hand.
At 100KHz, it would definitely be a "skin effect" thing, rather than
hitting a nerve. If the frequency were to drop (due to loading caused
by human dead short across socket), then that aggravates the shock hazard.
2) Prevent multimeter AC 1000V range from blowing out, taking out
measurement chip. I've only seen one meter blown out that way,
a Fluke with about 2kV across it. That was in a physics lab, a
Xenon flash tube driver, which a masters student destroyed by
being not clever. When I saw him reach for the meter, I thought
to myself "he's not that stupid, he's a genius". Oh, well, never
assume anything.
I don't want you to get hurt fooling with this stuff.
If the filaments are not hooked up (circuit uses two pins of
the four pins total on the tube), then it's not using the filaments
to heat the mercury, and it's relying on a little extra voltage
to establish the initial plasma. You could place a resistor in
place of the tube, using the two pins that are connected.
What value should you use ? Is it a 820 ohms 6 watt
power resistor ? Is it a 1 megohm resistor ? The circuit has a high Q,
and the trick is intuitively picking the right load to prevent
runaway behavior. 70V 6W would be 820 ohms (V^2/R = P = 6W = 70v*70v/820 ohms.
That's a rather obscure value for the parts cabinet, and by the time
you were done, you could likely purchase a replacement tube instead :-)
And measure with that present. Or, don't even bother to measure
and just use it. I have power resistors, but they're in the 1 ohm
to 10 ohm range or so.
I have one of these Johnlite lamps, but I don't know where it's got to.
It's not on my junkroom list, which means I have no hints
to work with. It's one of the black plastic ones.
Paul