Discharging and General Capacitor Information for LNS4051 LCD TV

[MTatersKT738]

I have been trolling and reading the forums here for a few hours and trying to figure a few things out.

1. How long for the specific capacitors on my board to discharge?
2. Good SAFE method to discharge said capacitors.
3. SAFE procedure that will prevent a SHOCK to me or the board.

I have read around and feel 100% confident that I can solder and replace the caps just need to learn more about the specific caps I see and need to replace.

Like most people on the boards my bad caps are the ones situated in an assortment of 5 near the power supply area. They are each 1000uf 10v. How long after being unplugged will they be safe to remove? What is a good method to discharge such a rated capacitor?

I also have noticed a MUCH larger capacitor in the middle of the board that is a 400uf 350v. This is the one that really scares me? How long to discharge? and best SAFE method to discharge?

I have NO idea about the electronics portion of this project. I have fixed other small solder issues in the past but REALLY want to avoid a shock of any SIZE at all costs.

I want to educate myself with the proper knowledge and tools before removing the board and frying my finger.

TIA,
Jason

TV: Samsung LNS4051/XAA
Purchased: Oct '06
Symptons Started: 5/11
# Bad Caps: 3
Bad Cap Ratings: 1000uf 10v

You will see in the attached picture the bad caps as well as components that I have questions about that are marked with "High Voltage" warnings.

[b700029]

Re: Discharging and General Capacitor Information for LNS4051 LCD TV

The 10v ones won't hurt. (10v is not high enough to make any significant current go through your skin.)

The large one is primary-side and will hurt, a LOT, if you touch it. It has up to ~340V. Measure the voltage on it with a multimeter, discharge if it's above 30V with a resistor (5W, 10-20K). It'll probably discharge on its own though.

[MTatersKT738]

Re: Discharging and General Capacitor Information for LNS4051 LCD TV

The 10v ones won't hurt. (10v is not high enough to make any significant current go through your skin.)

Well that's good to know.

The large one is primary-side and will hurt, a LOT, if you touch it. It has up to ~340V. Measure the voltage on it with a multimeter, discharge if it's above 30V with a resistor (5W, 10-20K). It'll probably discharge on its own though.

To measure with a multimeter (which I have) I just need to set it to a reading level and touch the probes to the bottom solder connection points?

Will Radio Shack have said resistor if I need it ? TV has been unplugged for almost 24 hours now, so hopefully its discharged.

What about the other areas of the board with the "?" once the "big one" is discharged we should be good?

Also I have read that when replacing I should up the voltage a little on the caps (10v -16v) is this good thought? Should I replace with same or slightly higher?

Thanks

[bipolar]

Re: Discharging and General Capacitor Information for LNS4051 LCD TV

1. How long for the specific capacitors on my board to discharge?

Depends. Essentially, how much juice (uF-V) and how much load (R) for the juice to pour into. It's right up there with the "RC Time constant". If there is no load, then you're talking about the self-discharge (leakage) rate, which for some serious caps, can be months.

2. Good SAFE method to discharge said capacitors.

Through a high-enough (use Ohm's Law) resistor. For safety's sake, use the voltage rating on the cap. For a typical input filter, we'll use 450V (the uF value only matters for how long we'll be appying the resistor to get it discharged). We'll also try for a safe current, say 100mA or so. Now, plug that right up in there...

R=E/I --> R=450/.1 --> R=4500 ohms (closest value is 4700).

Now, for the wattage...
P=IE --> P=.1 x 450 --> P=45W (Whoa!)

Not many of us have a 4700 ohm 45W cement block lying around (I don't), so all we have to do to bring this down is ramp up a factor (or two). So, let's make that a 45K resistor, which will bring the current down to 10mA, and the wattage at 450V will drop to 4.5W (much better). So, a 47000 ohm 5W resistor should do the trick. Factoring again gives us 470K .5W, which is even better, as even I've got some of those in my spare parts.

After getting your value/size, just run some alligator/hook clips to the leads of the cap(s), and measure with the DVM until it reads zero.

Will Radio Shack have said resistor if I need it ? TV has been unplugged for almost 24 hours now, so hopefully its discharged.

Maybe, if you do the math right. A pair of 1Meg .5W twisted in parallel will give you a 500K 1W equivalent - just about right! RS should have those.

To measure with a multimeter (which I have) I just need to set it to a reading level and touch the probes to the bottom solder connection points?

Yup. Most DVMs use "Auto-ranging", which means you click the dial to "VDC", and touch your probes/clips to the cap's leads. If you have an older style DVM with range selection (two dials, one for measurement (AC/DC/Ohms/Cont/Diode) and one for range (2-20-200-2000)), then set it to VDC AND set it to the highest range FIRST, and work your way down (saves having to change fuses all the time).

3. SAFE procedure that will prevent a SHOCK to me or the board.

Well, you could solder leads to your resistor, shove it into a non-conductive tube (PVC pipe?), run the clip out the back end, and use the lead out the front as the probe. Clip to ground, probe the positive, wait, test voltage, probe again, wait, test, ad nauseum...

Like most people on the boards my bad caps are the ones situated in an assortment of 5 near the power supply area. They are each 1000uf 10v. How long after being unplugged will they be safe to remove?

Since these are the output caps on the SMPS, these are usually tied to a load already (most SMPSs need to "see" some kind of load to even start up), so they tend to discharge really quick (a few seconds). It's those (as Monty Markum said in the movie "Midway") "Big Bastards up front" you gotta watch out for. This is because the load is switched in/out by the MOSFET, and if the supply is turned off when the MOSFET has the charge still in that cap(s), then it doesn't see much of a load, and waits for you to come sniffing around. If that MOSFET went *poof*, chances are good there's no charge left.

What about the other areas of the board with the "?" once the "big one" is discharged we should be good?

Maybe. Use your DVM to make sure.

Also I have read that when replacing I should up the voltage a little on the caps (10v -16v) is this good thought? Should I replace with same or slightly higher?

That, my friend, is a 55-gallon can-o-worms you're opening up. I was taught that the designer should allow for 150% of rating, minimum. So, if my circuit runs at 12V, then I'll design it using 18V caps as minimum. Since I don't see any of those at Digikey, I can go a step up (25V), or cross my fingers and use the 16Vs. Now, if you're the poor SOB fixing what I just under-engineered, then you might want to use the 25V and send me an e-mail telling me how much better you are than me. IF I gave you the room to do it, as the 25V cap may be quite bigger than the 16V, and won't fit. In that case, I'll just politely give you the finger. If I use Teapos or L-Tecs, I'd be using both fingers.
So, the short answer (too late!) is to up the voltage if it'll fit, keep the voltage if it doesn't, and stay with the same uF rating if at all possible (+/-20% is okay). Lastly, try to keep within the same type/family/series (e.g., KZG, FC/FM, MBZ, etc.). If you're not sure, ask.

I want to educate myself with the proper knowledge and tools before removing the board and frying my finger.

The black/grey end of the soldering iron is the "cool" side
Wear safety glasses/goggles, as solder/flux LOVES to splatter/pop.
Get a "flux fan" or open a window to keep the fumes out of your nose.
Keep calm. Be patient. It helps to take a step back, inhale (to get those f*cking fumes out of your nose), and calmly re-approach the problem.
Invent strawberry-scented flux.

(Boy! You sure do ask a lot of questions...)
bipolar

[bipolar]

Re: Discharging and General Capacitor Information for LNS4051 LCD TV

Believe it or not, I forgot something...

All those High-Voltage! warning stickers? They're there to warn you that the HEATSINK is attached to the tabs of the mounted components, and that they may be at a high-voltage (I was taught anything over ~50-75V) potential while powered.

So, don't touch the big metal "radiators" with your fingers, or use them as ground points for your DVM or scope, while plugged in and you'll be fine.

bipolar

[Longbow]

Re: Discharging and General Capacitor Information for LNS4051 LCD TV

A good rule of thumb is to never use a screwdriver to discharge a capacitor. This method can damage the cap very easily and it will take a bite out of the screwdriver. The larger the uF value, the more like a welder the cap can be. Have a convenient 100 ohm 5 watt resistor handy for discharging caps. For tube equipment you can use a 1K resistor due to higher voltages. Put probes on the resistor if you want.

[popeye]

Re: Discharging and General Capacitor Information for LNS4051 LCD TV

I have been trolling and reading the forums here for a few hours and trying to figure a few things out.

1. How long for the specific capacitors on my board to discharge?
2. Good SAFE method to discharge said capacitors.
3. SAFE procedure that will prevent a SHOCK to me or the board.

I have read around and feel 100% confident that I can solder and replace the caps just need to learn more about the specific caps I see and need to replace.

Like most people on the boards my bad caps are the ones situated in an assortment of 5 near the power supply area. They are each 1000uf 10v. How long after being unplugged will they be safe to remove? What is a good method to discharge such a rated capacitor?

I also have noticed a MUCH larger capacitor in the middle of the board that is a 400uf 350v. This is the one that really scares me? How long to discharge? and best SAFE method to discharge?

I have NO idea about the electronics portion of this project. I have fixed other small solder issues in the past but REALLY want to avoid a shock of any SIZE at all costs.

I want to educate myself with the proper knowledge and tools before removing the board and frying my finger.

TIA,
Jason

TV: Samsung LNS4051/XAA
Purchased: Oct '06
Symptons Started: 5/11
# Bad Caps: 3
Bad Cap Ratings: 1000uf 10v

You will see in the attached picture the bad caps as well as components that I have questions about that are marked with "High Voltage" warnings.

You could use a light bulb with 2 crock clips then you will know when the mains smoother is discharged light will go out. simples

[mockingbird]

Re: Discharging and General Capacitor Information for LNS4051 LCD TV

I thought most modern PCBs have "bleeder" resistors that discharge the primaries when it's powered off?

[momaka]

Re: Discharging and General Capacitor Information for LNS4051 LCD TV

I thought most modern PCBs have "bleeder" resistors that discharge the primaries when it's powered off?

Keyword here is *most*.

I have been zapped once by a 400V, 330 uF capacitor charged up to 385V. This was on a faulty 36" HP LCD TV (don't remember model #). Felt like being stung by someting, maybe a bit worse (but not too bad - still, I'd now rather avoid it if I could). I performed a test on the TV after this - it took that cap about 10 minutes to discharge to a level where you wouldn't feel the voltage, which was about 30-40V or so.

An old CRT monitor of mine also didn't have any bleeder resistors across the main cap. That one would stay fully charged for about 4 days. Nearly got zapped on that one as well. Luckily, I slid a screw driver by accident across the terminals and the cap discharged itself with a loud pop. Like Longbow mentioned, this left a nice rough spot on my screw driver.

When discharging such caps, I now use a 5W, 1.5 KOhm resistor. Works great.

[randtek]

Re: Discharging and General Capacitor Information for LNS4051 LCD TV

Also I have read that when replacing I should up the voltage a little on the caps (10v -16v) is this good thought? Should I replace with same or slightly higher?
Thanks

In switrching regulator cicruits. which are what the power supplies in most modern TVs are, I was taught that it is best to use the voltages the designer intended, unless it is absloutely necessary to increase them. If you cannot obtain the correct voltages, it is probably OK to increase by one voltage rating level (IE 10v to 16v, 16v to 25v, etc.). The potential problem with increasing the voltage rating of the capacitors in switching regulators is that electrolytic capacitors are essentially a coil of aluminium foil inside the can. Being a coil, they do have an inductive component. Switching regulators rely on the proper capacitance/inductance ratios within the circuit to properly regulate. If you throw this way off by using higher voltage rated capacitors, you can affect the regulation and efficiency, as well as increasing the loading on the regulator components. I have experience repairing switching regulators where using higher voltage rated capacitors (in this case, double the rated voltage) caused the supply not to run.

[bipolar]

Re: Discharging and General Capacitor Information for LNS4051 LCD TV

Being a coil, they do have an inductive component. Switching regulators rely on the proper capacitance/inductance ratios within the circuit to properly regulate. If you throw this way off by using higher voltage rated capacitors, you can affect the regulation and efficiency, as well as increasing the loading on the regulator components. I have experience repairing switching regulators where using higher voltage rated capacitors (in this case, double the rated voltage) caused the supply not to run.

Seriously? The inductive differences are... what?... a few nano Henries? If your SMPS design can't handle the equivalent of a couple of loops on your output leads, then you're in the wrong fscking line of work. Heaven forbid that your load will be a motor, solenoid, or relay.

Nowadays, most SMPS design falls down to just a few controller ICs, some MOSFETs, Schottkys, and our familiar LESR caps. They've become something of a commodity. So, I find it hard to believe that using a 25V cap in a 16V design is going to fsck it up that badly, if at all. Next one I re-cap, I'll shove a 50V in there, just to see what happens...

Although it's a linear supply, my home-made (I literally laid out and etched the PCB in my basement) PSU has lasted since 1980 by using the aforementioned 150%-200% rule. I have since moved the guts to a new case, re-using an old ATX PSU shell, but have not had to replace anything, even a fuse, in 31 years.

Yeah, I'm pissed off at all this "just enough" design mentality. Using 6.3V caps on a 5V rail? Putting 1N4001s on the output of a 36VAC transformer? Hell, the original design of my also-31-years-old clock had 1/2W resistors to drive the LED segments - I had to replace every one of them with 2Watters, as the coating on the 1/2Ws were baking off in little flakes (yes, I could've used 1Ws, but the local store had some neat RCA 2W flameproof jobbies on clearance).

bipolar

[randtek]

Re: Discharging and General Capacitor Information for LNS4051 LCD TV

Bipolar, I really don't have any idea who you are, but your response to my post demonstrates considerable ignorance. I have been a full time service technician in electronics since 1980. (That's 31 years coincidentally). I was trained in the Air Force. I currently work on 15-20 TVs per day. I don not profess to know everything in this field, but my information is accurate, and much of it comes from training I continually receive from current manufacturers. You are just speculating on what the inductive difference may be, but you obviously don't know. Whether you choose to accept my input or not is a personal choice, but I did not deserve, and do not appreciate your immature flaming. By the way. I do not design professionally, I only fix the results of the flaws in the designs of others.

[popeye]

Re: Discharging and General Capacitor Information for LNS4051 LCD TV

Bipolar, I really don't have any idea who you are, but your response to my post demonstrates considerable ignorance. I have been a full time service technician in electronics since 1980. (That's 31 years coincidentally). I was trained in the Air Force. I currently work on 15-20 TVs per day. I don not profess to know everything in this field, but my information is accurate, and much of it comes from training I continually receive from current manufacturers. You are just speculating on what the inductive difference may be, but you obviously don't know. Whether you choose to accept my input or not is a personal choice, but I did not deserve, and do not appreciate your immature flaming. By the way. I do not design professionally, I only fix the results of the flaws in the designs of others.

Nice one Randtek

Tony

[bipolar]

Re: Discharging and General Capacitor Information for LNS4051 LCD TV

I have been a full time service technician in electronics since 1980. (That's 31 years coincidentally). I was trained in the Air Force.

Navy, here. If it's any consolation, I was born at Wright-Pat

I don not profess to know everything in this field, but my information is accurate, and much of it comes from training I continually receive from current manufacturers.

Agreed. When our profession develops thousands of new components every day, keeping up with things is monumental. I profess that you may have the advantage over me in regards to updated materials. However, the basics still apply - it's the smaller (inductive?) differences that matter here.

You are just speculating on what the inductive difference may be, but you obviously don't know.

True. I admit, I have never measured the inductance of a cap. Have you?

Whether you choose to accept my input or not is a personal choice, but I did not deserve, and do not appreciate your immature flaming.

Oops. I now see what went wrong, here - and for that, I offer my apologies. My wording has made it seem that I was yelling at you. This was not the case, and I offer a better wording of my post, with the changes in bold:

Seriously? The inductive differences are... what?... a few nano Henries? If the SMPS design can't handle the equivalent of a couple of loops on the output leads, then the designer is in the wrong fscking line of work. Heaven forbid that the load will be a motor, solenoid, or relay.

See? Much better... and "my bad"

By the way. I do not design professionally, I only fix the results of the flaws in the designs of others.

As many on this board do. This site could have been called "Bad Things Those Guys Design (And How to Fix Them)".

Bipolar, I really don't have any idea who you are, but your response to my post demonstrates considerable ignorance.

Then enlighten me. Tell me what my two HS instructors, NEETS, and 30+ years forgot to teach me. Hell, how about making a new thread discussing just this topic? We can then combine our 60+ years of experiences, actually measure the inductive differences in caps, and figure out how/why they do the voodoo that they do so well. I'm up for it, and I really do want to know these things you know - I never stop learning. Top started this site to enlighten folks about the importance of good caps and what "LESR" means. Maybe, based on your expertise, we can expand that to include induction values of various voltage ratings - as if it wasn't confusing enough.

And popeye, Randtek has made clear his concerns. Hopefully, I have addressed them. Fanning what was not a flame war isn't cool. Now, if you would be so kind as to figure out an effective way of measuring inductance in a cap without all that... capacitance... getting in the way, maybe we can all turn this into something productive.

My apologies also go to Top - I have never started (or fanned) any flame wars on any board, and I'm sure as hell not going to start now.

bipolar

[Longbow]

Re: Discharging and General Capacitor Information for LNS4051 LCD TV

In fact, most "modern" equipment, meaning solid state gear does not use bleeder resistors. This concept came from the high voltage tube days with simple unregulated power supplies. The bleeder's main function was to put a minimum current load on the supply, and the secondary function was to discharge the high voltage capacitors. Another design feature that was dropped for the most part is the use of decoupling resistors and capacitors between stages. Some well-designed gear will have them, most do not.