High voltage in TV's
Andreas Sikstrom
been pulled out, but where is the voltage and how do you get rid of it??? Is it
just to short-circuit the capacitors???
Andreas (s...@p33.quad.ct.se)
( UUCP Dialup connection )
Martin Daly
andreas....@p33.quad.ct.se (Andreas Sikstrom) wrote:
>I know that there is dangerous high voltage in TV's even after the cord has
>been pulled out, but where is the voltage and how do you get rid of it??? Is
it
>just to short-circuit the capacitors???
CRT second anode voltage is generally in the order of 25KV to 30KV. The CRT
will hold this charge for a long time after the power is disconnected.
Discharge the second anode lead(HV lead) on the CRT with a ground lead before
trying to disconnect it.
Marty
Sam Goldwasser
> I know that there is dangerous high voltage in TV's even after the cord has
> been pulled out, but where is the voltage and how do you get rid of it???
> Is it just to short-circuit the capacitors???
> Andreas (s...@p33.quad.ct.se)
There are two places:
1. CRT Anode - this is the fat red wire leading from the flyback transformer
to the CRT. If you will not be working on the flyback or CRT, just
don't get to chummy with this wire (it is usually adequately insulated).
2. Capacitors in the power supply. Usually there are one or two large
electrolytic capacitors in the line-connected power supply. You won't
be able to miss them.
Discharging Capacitors:
The technique I recommend is to use a high wattage resistor of about
100 ohms/V of the working voltage of the capacitor. This will
prevent the arc-welding associated with screwdriver discharge but will
have a short enough time constant so that the capacitor will drop to
a low voltage in at most a few seconds (dependent of course on the
RC time constant and its original voltage).
Then check with a voltmeter to be double sure. Better yet, monitor
while discharging.
Obviously, make sure that you are well insulated.
So, for the main capacitors in a switching power supply which might be
100 uF at 350 V this would mean a 10K resistor (10W). RC=1 second.
5RC=5 seconds.
Reasons not to use a screwdriver:
1. It destroys screwdrivers and capacitor/circuit board terminals.
2. It potentially can damage the capacitor as well.
3. It will reduce your wife's (or husband's) stress in not having to
hear those scary snaps and crackles.
--- sam
BOB WEBB, SNA, ELECTRONIC TECHNICIAN, (714)434-2243
> In article <ftn_2.205.603.33_2fbf...@p33.quad.ct.se>,
> andreas....@p33.quad.ct.se (Andreas Sikstrom) wrote:
>>been pulled out, but where is the voltage and how do you get rid of it??? Is
> it
>>just to short-circuit the capacitors???
>
>
> will hold this charge for a long time after the power is disconnected.
> Discharge the second anode lead(HV lead) on the CRT with a ground lead before
> trying to disconnect it.
>
> Marty
it. I would like to say- be careful. I wouldn't touch the ground wire while
discharging, high voltage can arc even if the wire has insulation. I use a
thick jumper wire with alegator clips. One side connected to the metal chassis
(ground) and the other side connected to a plastic handled screwdriver. Holding
the plastic handle of the screwdriver, stick the other end under the anode cap
once or twice until there is no more spark. Use a screwdriver with a bad tip
because the spark might make it worse. Of course, this is done with the power
off. I would also touch solder connections on the flyback if you intend to work
in that area. After this is done, the high voltage charge should be gone. This
is the way I was taught in my TV repair class about 20 years ago. If anyone has
a safer way, feel free to add.
Sam Goldwasser
<snip - question about safely discharging caps in TVs/monitors>
>> Marty
>Marty is correct, you need to discharge the anode lead before disconnecting
>it. I would like to say- be careful. I wouldn't touch the ground wire while
>discharging, high voltage can arc even if the wire has insulation. I use a
>thick jumper wire with alegator clips. One side connected to the metal chassis
>(ground)and the other side connected to a plastic handled screwdriver. Holding
>the plastic handle of the screwdriver, stick the other end under the anode cap
>once or twice until there is no more spark. Use a screwdriver with a bad tip
>because the spark might make it worse. Of course, this is done with the power
>off.I would also touch solder connections on the flyback if you intend to work
>in that area. After this is done, the high voltage charge should be gone. This
>is the way I was taught in my TV repair class about 20 years ago.If anyone has
>a safer way, feel free to add.
If you are not going to be removing the CRT anode connection or replacing
the flyback, I would just stay away from the fat red wire and what it is
connected to including the focus and screen wires. Repeatedly shoving
a screwdriver under the anode cap risks scratching the CRT envelopey which
is something you really do not want to do.
The other area to discharge are the main filter capacitors in the power
supply. For these, I prefer to use a high wattage resistor of around 100
ohms/working volt of the capacitor (appropriately insulated). You will
need to keep it in place for a few seconds to fully discharge the cap
but it will be better for the capacitor and your (and your wife's) nerves
not to produce those really loud zaps. Then test with or monitor with
a voltmeter to be sure.
Theoretically, discharging at the flyback can damage components upstreadm
between the flyback and the main capacitors due to the current spike.
--- sam
Stefan Huebner
> >thick jumper wire with alegator clips. One side connected to the metal
> >chassis (ground)and the other side connected to a plastic handled
I already killed some CMOS devices by discharging the tube against
the metal chassis :-(
The cathode of the tube was not directly connected to ground because
there were some resistors (for measuring the beam current I suppose)
Since this little mishap I always discharge the tube directly against
its own cathode, never against the metal chassis !
> connected to including the focus and screen wires. Repeatedly shoving
> a screwdriver under the anode cap risks scratching the CRT envelopey which
I always use a relatively thin wire (0.5mm^2) to discharge picture
tubes to prevent scratching the glass
zapp
>Date: Thu, 08 Jun 1995 19:43:00 +0200
>From: Stefan_...@rookie.fido.de (Stefan Huebner)
>Subject: Re^2: High voltage in TV's
>Hi there !
>bye
>stefan_...@rookie.fido.de
Like the service manauls say "NEVER arc the anode to chasis ground!".
and a thin flat insulated screwdriver works best. (with and alligator lead of
course ) :-)