“How to test LED screen?”

[budm]

Re: “How to test LED screen?”

Which one?


Hehe. We're not dealing with a true constant current circuit here, are we?

I can't understand all his effort, if, ultimately, a resistor will be doing all the work... All the "work" to arrive at the resistor value, oh but wait! Isn't this supposed to be a CCS?

Why bother? Once the load, or line voltages, changes, so will his "constant" current.

He could try for a switchmode CCS, but I ain't going there!

No matter how hard I try to explain what the constant current power supply is and how it supposes to work, he just does not get it and he wonder why things keep blowing up.

[kaboom]

Re: “How to test LED screen?”

No matter how hard I try to explain what the constant current power supply is and how it supposes to work, he just does not get it and he wonder why things keep blowing up.

I know. I wasn't the least bit surprised when I read about the blown LEDs. I knew I'd see it if I kept reading...

At least they're his parts.

[budm]

Re: “How to test LED screen?”

BTW, VERY simple questions for Peter:
1) How is that Common (Wiper) contact of the rotary switch is connected too?
2) and where is the 16K resistor for the 10mA?
3) The 3.3 Ohms is removed?
May be you should draw the diagram as how you have all the components connected, how the AMP meter is connected, the Volt meter is connected.

[budm]

Re: “How to test LED screen?”

BTW, your post 51 answer to the question I asked:
Now the load is changed to 25 Ohms and you still want 100mA of current flowing through it, then what what is the value of the series resistor needs to be to still give 100mA? What is the Voltage drops on the 25 Ohms load resistor?
10v/25=0.4A=400ma value of the series resistor needs to be to still give 100mA
A:100-25=75ohm
What is the Voltage drops on the 25 Ohms load resistor?
IxR=V A: 0.300A x25ohm=7.5v
I am curious, How did you calculate the 300mA?
The circuit has two resistor in series and connected to the 10V power supply source, one resistor per you answer is 75 Ohms (per your calculation) which is connected in series with the other is 25 Ohms resistor.

"but realty is, If ohm too low current too high ,like a piece of metal can't measure any voltage( with my meter)."
What?
If Voltage source of 1V with 0.1 Ohm load resistor will have 10A flowing = 1V Vdrops on the load resistor.
Voltage source of 1V with 1 Ohm load resistor will have 1A flowing = 1V

[kaboom]

Re: “How to test LED screen?”

BTW, VERY simple questions for Peter:
1) How is that Common (Wiper) contact of the rotary switch is connected too?
2) and where is the 16K resistor for the 10mA?
3) The 3.3 Ohms is removed?
May be you should draw the diagram as how you have all the components connected, how the AMP meter is connected, the Volt meter is connected.

Ok, that circuit.

I saw it, but was since there were other ones linked.

R2 does not directly limit the current, only the output current that Q2 turns on at. When it does, drive to the MOSFET is shunted away. The same principle is used in the 7800 regulators.

I'd think that should be a 65-70 ohm resistor. Q2 turns on with .65-.7V across its B-E junction. When it does, the pass transistor's drive is shunted away.

Since it takes .7V to for the current limiter/drive shunt transistor to turn on:

0.7V/.01= 70 ohms.

The bias to the pass MOSFET will be automatically and continuously adjusted to maintain (with a load connected) 0.7V/70 ohms= 10 mA. W/o a load, the MOSFET is fully on since no current flows thru, and no voltage appears across, R2. Or 7 ohms and 100 mA.

The FET bias ckt should be cleaned up though. Quick and dirty assumptions: 150V supply, 75V load, gate is 10V higher than source, leaving 65V to be dropped across bias resistor to B+. With a 10K resistor, we get 6.5 mA thru Q2 at steady state.

See how this isn't ideal? The FET bias resistor's current affects the load current. And this varies with the voltage across the FET! We need an interposing transistor, to give a seperate path for the shunted bias current.

I will elaborate on this at a later time.

[budm]

Re: “How to test LED screen?”

That is why I really like to see where that 16K resistor is, and if every thing else is shorted out ( the load, the MOSFET, ETC), the max current will be 10 mA, but since there will be unknown QTY of the series connected LEDs, he does not want to open up the LCD panel to find out how many LEDs there are inside the panel that is why he is building his circuit to drive these unknown QTY LEDs with 10mA up to 100mA per his design.
So 160V source connected in series 16K Ohms resistor to get 10mA per his calculation with the load which are a bunch of LEDs connected series (MOSFET FULLY ON with mOhms of resistance, I sure like to see what he will tell us when he hook it all up.
He sure is not putting in the load and other series elements in the equation, just 160V source and 16K resistor to limit the current to 10mA.
He got the circuit form the same guy that designs this circuit:
https://www.badcaps.net/forum/showpo...5&postcount=42
See the problem?

"I will elaborate on this at a later time." Oh I would not, I am waiting to see his design and see how many good LCD panel backlights and parts he will blow up, he is an EE you know. It is a good electronics lesson for other members of what not to do.

From Peter:
"Two capacitors or resisters put in series or parallel together, the values , I still make mistakes …. all the time........I'm not straight A student like most Asia's student do....I was a party boy....I's in a Band (called {120DB} U know what that means)........and I graduated by use cheating note …................. have fun......."
Now you know why, and according to Peter, this LED tester will have bigger market than ESR meters!

You should see some of his testing methods:
https://www.badcaps.net/forum/showth...t=45234&page=3