Ratch please read this link: http://www.marcspages.co.uk/tech/6103.htm
It explains the basics
Regards,
tvtech
It explains the basics
Regards,
tvtech
Nor would I, and I didn't - it also wouldn't be much use as a surge limiter or 'fuse' if it was in parallel either.
This is one of the reasons we know you have limited electronics knowledge, it's a discharge resistor, while it obviously will leak a little current past the capacitor it certainly won't "let the surge current bypass the capacitor and be applied directly to the diode" with makes no sense at all, the surge is THROUGH the capacitor and caused by it (which is why a limiting resistor in series is always fitted in such designs).
Again, showing your lack of knowledge - there's no reason to mention joules anywhere in this design, it's not something which is very often used at all in electronic design. It also certainly won't 'de-energise' (correct term is discharge) is a fraction of a uS through a human body.
The resistor, again universally fitted, discharges the capacitor so a user can't get a shock from the plug pins, without such a resistor it could stay charged for a considerable length of time - while it's not going to be a fatal shock, it could easily be enough to cause you to have an accident, or throw the device and cause damage.
I have read your post above ^^
Have you read my Link yet here: http://www.marcspages.co.uk/tech/6103.htm
Before you post another thing..please read. Please
I am begging you.
The value is chosen using the R=P/I² i.e. 0.25W/(33mA)² = 229W
Ratch please read this link: http://www.marcspages.co.uk/tech/6103.htm
It explains the basics
Regards,
tvtech
I did burn few diodes with transients, so I believe in using the resistor, however I cannot come up with any scientific method to determine the necessary value. Just from my "feelings" 220 Ohm seems too much. If I were building this, I probably would consider something between 10 and 50 Ohm. I have read the quoted article, and this is the method they use to select the value of the resistor:
I don't understand where the 0.25W come from? Just because common resistors are 0.25W?
Why not 30 Ohm or 150 Ohm? With less resistance, power dissipation will be less. Will help Kyoto
How do they know that 229 Ohm is enough? Why not to upgrade to 800 Ohm 1W resistor?
Pity that article tends to use W for ohms though
But I agree, he's unlikely to read it, and even less likely to believe it.
Why use a resistor at all? Did you see my transient analysis and notice how short the time is and how small is the energy stored?
If you had problems, I would be interested in your capacitor values.
OK, I studied the single LED section thoroughly. Of interest, but not really pertinent is the fact that his mains are 230 vs my 115, his frequency is 50 vs my 60, and he made a mistake in transcribing the capacitor value as 0.0000001 instead of 0.000001, but his result was correct.
First of all, I have to wonder why he uses a full-wave diode bridge instead of two LEDs back to back like the OP did. Wouldn't that be much simpler? His calculation of the cap to limit the current is correct. The OP did not give me the max current of his diode, but with 1uf, it will need to be 60 ma. If that is too much, the cap will have to be reduced in value.
The 220 resistor has me scratching my head. He says it acts like a fuse in case the cap blows. I think the LED is going to act like a fuse and the resistor will be the last component left standing, right? I don't know what a resistor does for harmonics because it is frequency independent. If he is worried about line harmonics, he should get a filter. And what is a 220 ohm resistor going to do for a transient? Notice he did not do a transient analysis like I did, so he does not mention how quickly transients die out due to short time constants.
In conclusion, he uses components like the bridge and a resistor which I don't think he needs. His circuit uses more power than the OP's circuit does. Other than that, his calculations appear to be correct. By the way, notice he does not put a resistor across the capacitor.
Ratch
I understand your point that only the energy necessary to charge the capacitor will travel through the diode during the transient. You can easily calculate it without much of the analysis - C*V^2/2.
The resistor slows down the process. With 1o Ohm resistor it takes 100 times longer to deliver the energy than with 0.1 Ohm resistor (should really factor in capacitor ESR). Clearly, the speed of energy delivery increases the risk of diode failure. Faster delivery would be more damaging. I've never seen any formulae that would attempt to quantify the damage based on the speed of energy delivery, but it doesn't mean they don't exist.
Unfortunately, I cannot recall now.
Somebody help me 0.0000 to 0.000000
Cannot handle this anymore..Stop it Ratch....you are driving me insane
FFS = For Flipping Sake STOP TRYING TO BE CLEVER. Nobody is paying attention anymore
Bah
tvtech
Sometimes I will open up a good wall-wart for the fun of it. The last one was a Motorola phone charger. I must say that the quality of the PCB and components are very good. I was impressed.No, I didn't check them out. I assumed they're transormerless, which is a bad thing to do.
You're probably right. Most of them have enough room for a small high frequency isolating transformer. But some of them are so small, they don't seem to have enough space even for a capacitor.
After something breaks, I usually open it up to see how it works. So, I have pretty good idea how cofeemakers and toasters are built. But none of these little power supplies broke on me yet
We need a fabrication specialist to tell us if that kind of current will damage a diode.
I was impressed.
Jony130 said:
Ok, I did. I forgot that I took a few photos.You can post pictures!
tvtech said:Hi Guys
1. You need a current limiting Resistor. The resistor serves two roles....
Firstly as a device that limits inrush current into the X2 cap when plugging the PSU into Mains. This prevents too much arcing when plugging the plug in.
Secondly as a safety devise should the X2 Cap fail at any time (as in go shorted).
In my design it is a little 1W wirewound 6.8 Ohm Resistor feeding a 2.2 uf X2 Cap. No issues.
tvtech said:2. Referring to the post above, good practice is to have a discharge Resistor wired across the X2 Cap. I use a 1 Meg 1W for this. No issues.
tvtech said:3. Ensure you never loose the designed load. Probably the most important thing of all.... As I have stated here on another thread your output VOLTAGE from the X2 is going to go sky high if you do. Isolation Transformers don't help here....and you will still get belted if you are fiddling at the wrong time.
And that is all I have to say about Transformerless Power Supplies. They are reliable, interesting, unburstable and just need to be understood.
Really??????????What about the bond wires? Who is Cree?
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