Boost PFC Converter

[capwizard]

I have already read this and this. I don’t know why they have to use such a high voltage after I have been shocked frequently. So now I have a few questions?

(1) They use 375v is because of the input voltage AC 80 to 240v, So all the world can use this PFC?

(2) How do we discharge the power board when we are repairing the TV? what kind resistance value of the resistor should be used to discharge from the biggest capacitor?

(3)Is it suitable for LED tester applications after adding an isolation transformer?


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The line voltage varies from zero to some peak value typically 375 V; hence a step up converter is needed to output a DC bus voltage of 380 V or more. For that reason, the buck converter is eliminated, and the buck-boost converter has high switch voltage stress (Vin+Vo), therefore it is also not the popular one.

[Per Hansson]

Re: Boost PFC Converter

1: The higher the voltage the higher the stored charge in the bulk capacitor.
The ATX spec says you need 16ms holdup time, with a 1000uF 400v capacitor charged to 325v you have 53 joules, however if instead you charge it to 400v you have 80 joules.
Or looked at another way: it's enough with a 660uF capacitor charged to 400v to get 53 jolues.
When you have that info you can calculate the holdup time, as you understand the lower the capacitance the cheaper the capacitor is.
So it's s way to keep the size & cost of the capacitor down, while still attaining good holdup time.

2: A decent sized resistor is fine, at two or three kilo-ohms
If you want something more decent have a look here, it also shows what would happen with a low resistance resistor
https://www.badcaps.net/forum/showpo...6&postcount=86

3: Don't understand what you are asking here?

[gabiz_ro]

Re: Boost PFC Converter

2. In CRT TV era I used 220V 60-100W light bulb to test PSU part.But in some cases PSU won’t start becouse cold light bulb have low resistor value.
Then I make dumy load using (one switch) to one or two heating element for 60W soldering iron.
When I need to discharge capacitors I use old style bulb light or dummy load.

[capwizard]

Re: Boost PFC Converter

Thank you for your advice.

375*375/3000=46.875W if we use 3K resistor will be P= 46 watt. unfortunately, I do not have this big resistor. I was thinking use PFC circuits as an LED tester circuits.

Picture 1,2 shows PFC is 412v after 3-minute still has 219v, that's why I was charged.

Picture 3,4,5,6 shows, I used a soldering iron with the adapter to the capacitor, in one second, it drops down to 12.6v.

Thank you for saving my life.

[budm]

Re: Boost PFC Converter

https://www.digikey.com/en/resources...-time-constant
https://www.electronics-tutorials.ws/rc/rc_1.html
It will not be constant 46 Watts of power dissipation when you discharge the cap with the resistor. I.E. the cap is 100uf charge up to 375VDC so when you hook up the 3K resistor to the cap, at first time constant the Voltage will drop down to 375v x 0.37 = 138V after 1 TC which is 0.30 second.

[capwizard]

Re: Boost PFC Converter

You are right! because it (PFC circuit) has no more power. Maybe it will only need a 1w resistor.

This PFC has two 77uf capacitors parallel together. The capacitance value will be 154uf. With budm's formula to calculate is 13.07 joules.

my question is, it did not has a lot of power is that (be discharged)13.07 joules good for my health? Just a little tingling in my hand.

[eccerr0r]

Re: Boost PFC Converter

13.07J = 3.13 cal = enough energy to raise 1 teaspoon of water approximately 1 degree Fahrenheit.

You be the judge.

[capwizard]

Re: Boost PFC Converter

This is a very serious question, Can I use that energy to raise or lift a part of my body? As luigi galvani did to the frog legs?

[eccerr0r]

Re: Boost PFC Converter

There's a distinction between total potential energy and control energy. Like a transistor amplifier, you don't need much energy to activate its output, but if you're measuring the output, that's a lot more energy that's being controlled there compared to what was used to trigger it...

For a frog leg, since it's alive, it's actually using the ATP stored in it to contract muscles, so you're not necessarily using the electrical energy to do the work.

If I got the math right, 13J is ... okay I may have royally screwed up the math... more later.

[eccerr0r]

Re: Boost PFC Converter

I think this is right this time:

13J is about the same amount of energy as lifting a 10 pound weight one foot on the surface of the Earth.

Someone please check the math, especially someone who understands the system better than I do, some idiot decided to use pounds as mass instead of weight eons ago, because they never thought people could get off the surface of the Earth or do ballistic measurements... This is probably the sole reason why I like metric, because they got the definitions right. Kilogram is clearly mass!

(until the drug traders decided to use it wrong...)

[capwizard]

Re: Boost PFC Converter

For a frog leg, since it's alive, .....N0...it was dead frog legs...

In 1780, he discovered that the muscles of dead frogs' legs twitched when struck by an electrical spark.

How Luigi Galvani's Frog Leg Experiment Made a Dead Frog Jump & Invented the Battery
https://www.youtube.com/watch?v=xG6W8A3JYFA

How Volta Invented the First Battery Because He Was Jealous of Galvani's Frog
https://www.youtube.com/watch?v=f6wJfx0VYRY

[eccerr0r]

Re: Boost PFC Converter

I was aware of his experiment: In this case the frog cells are still alive. If you cooked the frog and tried the experiment again, it would not work.

[capwizard]

Re: Boost PFC Converter

Mr. eccerr0r, your older is ready to be served.

[Per Hansson]

Re: Boost PFC Converter

Thank you for your advice.

375*375/3000=46.875W if we use 3K resistor will be P= 46 watt. unfortunately, I do not have this big resistor. I was thinking use PFC circuits as an LED tester circuits.

If you read the post I linked in my previous post above I mentioned two things there.
The Fluke SV225 LoZ adapter is pretty nice, it uses a 1k resistor and two 1k PTC's.
This way it's protected from large energy dissipation.
That said it's very expensive, you can probably buy a Fluke 113 that has 3K input impedance to begin with, but it gives you the idea if you want to make a DIY device...
There was also Mr Carlsons capacitor discharger linked in the same post if you want to build something more fancy.
However for me the SV225 is nice because it's small so fits my toolbag well but I digress...

[capwizard]

Re: Boost PFC Converter

Thank you. Yes, I will DIY an SV225 LoZ adapter. You have already convinced me but where I can find one 1k PTC? Varistor, PTC and NTC thermistor, they all look the same but what is the difference? I mean I always Googled them but I still cannot memorize what's the difference and what will happen if the people don't use PTC like this video? Watch "How to make and use a Capacitor Discharge Tool" on YouTube
https://youtu.be/nvXH7RUn0KE

Watch "Fluke SV225 repair" on YouTube
https://youtu.be/IOoJmMBE5D8

Watch "Dual Impedance, LoZ and Ghost Voltages on the Fluke 117 Electrician's Digital Multimeter" on YouTube
https://youtu.be/dttUNvC9pUs

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Capacitance discharge calculator

[Per Hansson]

Re: Boost PFC Converter

If the resistor is large enough you don't need the PTC: It's just for safety since Fluke needs a CAT rating on the equipment they sell...
Here is one example: https://uk.farnell.com/epcos/b59774b...ded/dp/2285481
PTC means: positive temperature coefficient
It's a resistor that will be 1k at room temp but much higher at say 100°C.
That way if you connect it to mains or a very large cap the circuit will not blow up...
An advantage of using a multimeter with low impedance mode is that you can watch the voltage go down to safe levels.
Without that you have no idea if the circuit/component was energized or not...