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How do you discharge a capacitor safely.

Thanks for looking.

How does one discharge a capacitor when working on a PCB, so that one
does not get electrocuted? Are there any other components that
maintain a charge after turning off power source. Thank you. please
answer to [email protected].
 
J

James Sweet

Jan 1, 1970
0
Thanks for looking.

How does one discharge a capacitor when working on a PCB, so that one
does not get electrocuted? Are there any other components that
maintain a charge after turning off power source. Thank you. please
answer to [email protected].



Usually just short it with a screwdriver. CRTs can store a charge too.
 
H

hr(bob) [email protected]

Jan 1, 1970
0
Usually just short it with a screwdriver. CRTs can store a charge too.

You may get a big spark from a large capacitor, don't jump when it
happens, you can do as much damage to yourself from ripping a hand on
nearby sheet metal as from the actual electrical charge. I know from
experience over manyyears and a few seconds of carelessness.

H. R. (Bob) Hofmann
 
T

THERES RAJ, BLR

Jan 1, 1970
0
Thanks for looking.

How does one discharge a capacitor when working on a PCB, so that one
does not get electrocuted? Are there any other components that
maintain a charge after turning off power source. Thank you. please
answer to [email protected].

the safe way to discharge capacitor are through resistor, use the
resistor in minimum 10 watts with resistor 10megaohms for AC capaitor
from 110v to 440v

for DC from 5 to 220v capacitor us 5 watts and 150kilo-ohms above 220v
DC use 10mega ohms resistor value,
use two long lead with insulation and one end ground to chase or PCB
ground, and other lead to discharge the capacitor.
take more care when discharging more than 110v capacitor which will
produced high sparking when discharging.
 
A

Arfa Daily

Jan 1, 1970
0
THERES RAJ said:
the safe way to discharge capacitor are through resistor, use the
resistor in minimum 10 watts with resistor 10megaohms for AC capaitor
from 110v to 440v

for DC from 5 to 220v capacitor us 5 watts and 150kilo-ohms above 220v
DC use 10mega ohms resistor value,
use two long lead with insulation and one end ground to chase or PCB
ground, and other lead to discharge the capacitor.
take more care when discharging more than 110v capacitor which will
produced high sparking when discharging.

I wouldn't argue with using a resistor to discharge caps, and also, on
occasion, just whacking a screwdriver across them - although this can leave
a bit of a metal deposit 'splat' on the PCB. However, the suggested 10 meg
10 watt resistor, or even the 150k, seems rather high to me. I think that
you might struggle to even find 10meg at 10 watts.

I have a metal film resistor of 2k at probably 3 or 4 watts, that has lived
for years on the bench screw-magnet, and serves to discharge any caps that I
ever come across that remain charged due to fault conditions. That includes
up to 500 volts in some PA amps. This resistor has never even got warm
during a discharge. If you work out the math, yes, at first the resistor may
be massively overloaded from a high voltage cap, but it is for such a short
period of time that the resistor never has a chance to get hot, which is
what its power rating is all about. A higher value resistor than this (
though not as high as 150k even ) will do a gentler job of it, with less of
an initial spark, but will take correspondingly longer to do it.

Note also that many high voltage cap circuits have bleeder resistors, or
voltage sharing resistors across them when they are stacked, and these
should discharge the caps for you. You should always check that they are
doing their job though, as they do have a tendency to go open. You can also
discharge caps safely with an older analogue multimeter, which has a much
lower input resistance than its modern digital counterpart. This has the
added advantage that you can see the voltage dropping, but is somewhat
slower than using a resistor of just a few k.

In general, the following circuitry should discharge the caps for you, and
caps remaining charged will usually indicate a fault condition such as a
switch mode power supply not starting up, and leaving the main primary
filter cap charged to peak line volts, or a filter resistor between two caps
open circuit, leaving the first one in line, charged.

Arfa
 
the safe way to discharge capacitor are through resistor, use the
resistor in minimum 10 watts with resistor 10megaohms for AC capaitor
from 110v to 440v

for DC from 5 to 220v capacitor us 5 watts and 150kilo-ohms above 220v
DC use 10mega ohms resistor value,
use two long lead with insulation and one end ground to chase or PCB
ground, and other lead to discharge the capacitor.
take more care when discharging more than 110v capacitor which will
produced high sparking when discharging.

Just how much wattage do you think you are going to get out of a
capacitor?
 
S

Sam Goldwasser

Jan 1, 1970
0
Thanks for looking.

How does one discharge a capacitor when working on a PCB, so that one
does not get electrocuted? Are there any other components that
maintain a charge after turning off power source. Thank you. please
answer to [email protected].

See the capacitor FAQ at the site below.

http://www.repairfaq.org/sam/captest.htm

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.
 
S

Sam Goldwasser

Jan 1, 1970
0
James Sweet said:
Usually just short it with a screwdriver. CRTs can store a charge too.

This is generally frowned upon both due to the pits it will produce in
the screwdriver, the noise, and potential damage to some types of
capacitors. See the info at the site below.

http://www.repairfaq.org/sam/captest.htm

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.
 
R

Ron(UK)

Jan 1, 1970
0
Sam said:
This is generally frowned upon both due to the pits it will produce in
the screwdriver, the noise, and potential damage to some types of
capacitors. See the info at the site below.


Rightly or wrongly, I use a 240volt BC pygmy lamp with two leads fitted
with prods soldered to the pads on the bulb. Works for me.

Ron(UK)
 
G

Guest

Jan 1, 1970
0
xcellent choice, lamp provides indication and nice slope for current rate.
 
D

Dave Plowman (News)

Jan 1, 1970
0
the safe way to discharge capacitor are through resistor, use the
resistor in minimum 10 watts with resistor 10megaohms for AC capaitor
from 110v to 440v

According to my calcs, the maximum power needed for a 10M0 resistor is
0.01936 watts.
 
Actually I'd like to get my hands on some 240V incandescent bulbs just
for that purpose.

I've seen some sets use as high as 940uF for the primary cap (2X470).
This is a considerable amount of power. I used to have a 75W 130V
(long life) bulb for the purpose until it got used. I actually put it
in the manifest as parts to be ordered but it is not going to happen.

When I see a 75 watt bulb take over a second to discharge it, what
goes through my mind is, just how long would my body take to discharge
it ?

Now I see that they make the PC so that the rectifiers and jumpers can
be used for 120 or 240, in the US, Canada and Japan at least, they
double the line voltage. For Europe it appears they just rearrainge
the jumpers so it is a simple full wave bridge. They just wind up
using higher voltage lower current devices.

As it is, I have to discharge each filter in the doubler seperately,
Going across the whole 320 DC would burn it out. I think a 240V bulb
would handle it, it is not 320 VDC for long.

Actually the bulb is the best choice, but a lazy choice. It is
available and cheap. I am totally against using a screwdriver to short
out the caps, it no doubt exceeds the filters' ripple current
capacity, evidenced by the metal deposits from the spark as noted by
one poster.

Ideally we could have a constant current source with a low wattage
bulb to give a visual indication. A regular lightbulb is less
desirable because it's resistance is low when cold, thus there is a
surge. If that filament burns out right away when you connect it, you
might erroneously think the cap(s) were discharged when they were
not.

Put your solder wick on the drain output of an SMPS IC that still has
320VDC (or even 160) on it and you'll wish you wore gloves (and maybe
goggles too !).

Certain Mits RPTVs and maybe some Hitachis come to mind. There are
plenty of direct view sets also but I generally don't work on them,
except for 32" and up. But I have seen where the main cap failed
because there was no load on it. All bulged, but it wasn't a power
surge. See some places the power is "dirty". It is essentially 120 VAC
but there are spikes and all kind of shit, it is not a perfect sine
wave to say the least. In some of these sets there is no bleedoff if
the SMPS doesn't start. Therefore the spikes will take the rectified
voltage quite higher than one would suspect.

They generally don't use high speed rectifiers for the AC line, which
means they have a certain effective series inductance. Well that is an
impedance, and even though a one meg resistor might keep the voltage
from climbing, it doesn't even have that. Also, if the rectifiers
outperform their specs, you might have a bit more.

Do not be surprised if a diod outperforms it's specs. I had a case not
too long ago, another tech had a set blowing the top damper diode. It
was a recall, but like an 8 month recall. Well, somehow at the time
the top damper was replaced with a 400V diode, like a 243636 (MI35 I
think) but the application clearly calls for a 1500V device.

I didn't do it, I have no idea how it happened but the 400V diode
worked for months. Eventually the set came to me. Once this was
figured out the set and customer have been happy together ever since.

That's why, I suspect, that sometimes I don't get many answers. I am
the dogshooter. And folks I got some big ones. But I can see as one
reads my post they think "Oh yeah, he needs to......" and in the next
few lines you see I already did that.

Then at the end of the post you might be thinking "Yup, this dude is
really fucked". That's my job, fucked. The newest part of my job of
fucked is those Sonys, the DX1 type chassis, the XBR400s and like
that. I have had to fix enough screwups on those sets that I am now
the alpha and omega of the DX1 at the shop. And I said it. Right after
the words came out of my mouth I got this jumping off a bridge
sensation, and rightly so. But in the long run, when I don't have to
follow someone else I'll learn alot more about those sets.

I would really like to get some 240V bulbs, any ideas for a source ?
My 120V bulb disappeared shortly after the light in the hall leading
to a bathroom burnt out, and now I am thinking, if I had a 240V bulb,
that wouldn't happen.

JURB
 
J

James Sweet

Jan 1, 1970
0
Actually I'd like to get my hands on some 240V incandescent bulbs just
for that purpose.

I've seen some sets use as high as 940uF for the primary cap (2X470).
This is a considerable amount of power. I used to have a 75W 130V
(long life) bulb for the purpose until it got used. I actually put it
in the manifest as parts to be ordered but it is not going to happen.


How about a couple of 130V bulbs in series?

If you want 240V bulbs they're easy enough to get, I've got some
contacts in the UK I get oddball lightbulbs from and these days you can
even get them with ES bases that will fit in a standard US socket.
 
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