attempt to clone LED TV Backlight Tester

[budm]

Re: attempt to clone LED TV Backlight Tester

"At picture 2: When temperature rising to 125c, The power dissipation go to 0.0 w" It means at that point, there will be no current flow through the device to produce any heat, no Vdrops no heat, the higher the ambient the less current you can let it pass through the device otherwise it will be damaged.

[llonen]

Re: attempt to clone LED TV Backlight Tester

Are you not better off just using a variable precision bench power supply with adjustable current limiting.

[capwizard]

Re: attempt to clone LED TV Backlight Tester

RT7320 also provides a thermal regulation protection, instead of traditional thermal shutdown, to suppress the
rise of IC junction temperature and prevent LED lamps from flicker.


Page 3: Thermal Regulation Protection

When the IC junction temperature rises above 125°C (typ.), this
function starts to gradually reduce the regulated LED
current, depending on the rise of the junction temperature.
Meanwhile, the system power dissipation is also reduced.
Finally, the temperatures in the system will be well
controlled and enter their steady-state. The function can
achieve both of the two targets : to protect LED lamps and
to prevent them from flicker.

The maximum power dissipation can be calculated by the following formula :
PD(MAX) = (TJ(MAX) − TA) / θJA
where TJ(MAX) is the maximum junction temperature, TA is the ambient temperature, and θJA is the junction to ambient thermal resistance. For recommended operating condition specifications, the maximum junction temperature is 125°C. The junction to ambient thermal resistance, θJA, is layout dependent. For SOP-8 (Exposed Pad) package, the thermal resistance, θJA, is 29°C/W on a standard JEDEC 51-7 four-layer
thermal test board. The maximum power dissipation at TA = 25°C can be calculated by the following formula : PD(MAX) = (125°C − 25°C) / (29°C/W) = 3.44W for
SOP-8 (Exposed Pad) package The maximum power dissipation depends on the operating ambient temperature for fixed TJ(MAX) and thermal resistance, θJA. The derating curve in Figure 1 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation.

Is that mean IC never going to burn out?

At picture 2: When temperature rising to 125c, The power dissipation go to 0.0 w.

[capwizard]

Re: attempt to clone LED TV Backlight Tester

Yes, one or two LEDs voltage drop more on the MOSFET . I will Put a switch for low and high voltage usage. low switch for one to one led strip testing, high switch for Dled tv whole screen and Eled tv strip testing. Or variable power supply.

[budm]

Re: attempt to clone LED TV Backlight Tester

"OPAMP+ MOSFET +constant current control circuit ....Now, all that can be put inside one IC , in no time" That are what inside the RT7320 if you look at those links.

'Because, It is a LED tester for average 5 sec usage in small mount of leds maybe OK, for larger screen definitely need big heatsink." You will need bigger heatsink when testing one or two LEDs in series if the Voltage is not adjusted down ward because you will have more Vdrops on the MOSFET, it is constant current circuit. The more LEDs in series the less Vdrops on the MOSFET so less heat.

[capwizard]

Re: attempt to clone LED TV Backlight Tester

Thanks for your friendly reminder: you will still need variable Voltage power supply otherwise it will exceed the power dissipation of the IC.
Because, It is a LED tester for average 5 sec usage in small mount of leds maybe OK, for larger screen definitely will need big heatsink.

1996 presidential candidate Ross Perot said: I didn't care whether we make potato chips or computer chips. Well, anybody that thinks about it cares a great deal. Number one, you make more making computer chips than you do potato chips.

I also member he said: making computer chips is easy as making potato chips, all you need is money. Well, so damn right, for people as rich as him.

OPAMP+ MOSFET +constant current control circuit ....Now, all that can be put inside one IC , in no time. We will facing more variety ICs, like PFC power controller. For example, Samsung in different year , different model, different size of tv , use all different kind of PFC controller. make this simpler word more complicated.

A custom analog ASIC (application specific integrated circuit) can be developed in a very short time, push the boundaries in analog design lowering cost, or reducing power consumption, or increasing integration levels or circuit performance. We will see more and more this kinds of potato chips just for LED lighting application.

[budm]

Re: attempt to clone LED TV Backlight Tester

Using RT7320 for testing LEDs (depends on how many LEDs there in the string) you will still need variable Voltage power supply otherwise it will exceed the power dissipation of the IC.
The IC is just a higher Voltage low side (current sink) with built-in resistors for selectable constant current,
Basic OPAMP+ MOSFET constant current power supply:
https://cdn.badcaps-static.com/pdfs/...25bd060cc5.pdf
http://www.daycounter.com/Circuits/C...nt-Servo.phtml
https://cdn.badcaps-static.com/pdfs/...43730e4b17.pdf

[capwizard]

Re: attempt to clone LED TV Backlight Tester

Boy's & girl's LED tester


Cool Science Experiment - Potato Battery
https://www.youtube.com/watch?v=EFuF2Tsg9HY

We will need 10 lb to test LED strip and 100 lb to test LED screen

[capwizard]

Re: attempt to clone LED TV Backlight Tester

Ive found one mistake on the Richtek Technology company's product manual.

On page 6 Typical Operating Characteristics chart, Output Current vs. OUT Voltage, Output Current marks is wrong. Should I question about how good is their products? I hope not!

[capwizard]

Re: attempt to clone LED TV Backlight Tester

Muchas gracias compa.

richtek RT7320. Perfecto,exactamente lo que necesito.

I love the built in 2.8ma, 5.5ma, 10ma, 20ma, 40ma current switch. I will order from: http://www.digikey.com/product-detai...0-1-ND/4733241. I will post the result. Thanks aqain.

Programmable Regulated Current : 2.8mA to 78.3mA.
AC Input Voltage : 90 to 130VRMS or 200 to 240VRMS



Cant wait,so Ive ordered from Aliexpress. You make my day.

[cpt.charlie]

Re: attempt to clone LED TV Backlight Tester

After a quick research in google i found the richtek RT7320, maybe with one of these and a boost converter is all you need.

[capwizard]

Re: attempt to clone LED TV Backlight Tester

After 2 days breadboard with LM317, I still have problem with it.

The problems of LM317 are not not precision enough to do a TV LED TESTER job.

1: Good for fixed high current source (> 10ma) not a good current limiter. Maybe LM344 is batter.

2: LED is an active load - semiconductor device and can not consider as direct resistive load . I cant test one led then 10 leds because the LM317 act very unstable.

3: To stabilize the current, the input voltage must be at least the LM317 overhead voltage + Vref + combined Vf of all the LEDs and must of course be able to maintain that at the load current.

4: The LM317 is not a precision reference device, it will slightly change it's reference and regulation current as it waits rms up.

5: The current is decided by the voltage drop across the resistor, is it's value staying the same as it warms up?

6: The LM317 with the single series resistor between output and adjust input is actually a fixed current source, not a current limiter.

7: Maybe we focusing too much on the LM317. If we need a constant current we could use the LM334 which is a programmable current source for up to 10mA.

8: In Fig. 5 we have only a single LED yet the current remains 120mA. The load doesn't change the current from a constant current source in fact I could short out the output and maintain 120mA. Note the LM317 will get hot - and voltage not dropped by the load will be dropped by the pass transistor, LM317 etc. Don't go wild with a large Vcc powering a small voltage load.

http://www.bristolwatch.com/ccs/LM317.htm

So, we will need new version of LM317: That is LR8

High Input Voltag Linear Regulator
Commonly used 3-pin linear voltage regulators, for example the LM317, cannot handle input voltages in excess of about 30V. The LR8A from Supertex Inc is a new, adjustable three pin regulator that can accept input voltages up to 450V and can supply an output current of 0.5mA to 10mA. Using this device it is possible to work with rectified 230VAC. The LR8 has a wide input voltage range of +12 V to +450V. Two external resistors (R1 and R2) allow the output voltage to be adjusted from 1.20 V to 440 V provided that the input voltage is at least 10 V greater than the output voltage. The LR8 adjusts the voltage difference between the Vout and ADJ pins to a nominal value of 1.20V.

http://www.microchip.com/wwwproducts/en/LR8



BTW, Microchip Acquires Supertex, Inc.

[capwizard]

Re: attempt to clone LED TV Backlight Tester

After burn tons of LEDs. Problem of L6562A PFC has voltage error amplifier and current error amplifier , but the current is not precision. So all the current mode pwm controller ICs are not perfect for this purpose.

We will need Precision Current-Limiter.

Special thank to BUDM:
You will need constant current source (to limit the max current to about 20mA) for testing power supply that can put out >102V (3.4V x 30 White) to test the whole string or you can test each LED by utself.
http://www.maximintegrated.com/app-n...ex.mvp/id/3256

You have to be real careful testing the LED strip without current limiter or constant current.
So have 5 LEDs on the strip and you apply 17.6V directly without any current limitter, and if one of the LED shorted out, then the current flowing through the string will be so high that can damage the good LEDs. I use power supply that I can adjust the current.
If you do not have one, you can use LM317 with heat sink as constant current source for driving LED.
You can easily do the experiment and measure DCA through your LED string with your 17.6V power supply, then short out one of the LED to see what happen to the current and then look at the spec of the LED for the current handling..
LED is not linear device.
I do not understand why people keep trying to run the LEDs without any current limiting in place.

http://led.linear1.org/why-do-i-need...r-with-an-led/


Yes! LM317 is the choice. But! the voltage of LM317HV is about 57v. We will overcome this problem, we will need an output-voltage range of 0V to 300V.



Pictures were stolen from here.:
https://www.youtube.com/watch?v=vroEf_GtIowe

[gabiz_ro]

Re: attempt to clone LED TV Backlight Tester

Is not clear for me what happens when you apply high reverse voltage on single led testing.
For strings seems OK but in case of single led when you apply forward voltage it drops to lower value but in reverse still tester stay on high voltage.
I see some led's are protected by a diode or something in reverse
(tested on some LG strips,no sign of light on my multimeter set on diode measurement,but in reverse get 0.6-0.7 value,later I light them up on power source,was two series led in same capsule and diode in reverse position)
In that case no damage to led but if protective diode what is when you apply 300V on them,on datasheets reverse voltage is low enough.

[capwizard]

Re: attempt to clone LED TV Backlight Tester

Thank you very much, Maxim and linear technologies do have a lot of PFC to LED applications.

When i was young , I was amazed by one AAA battery can light up a fluorescent light tube.
After college,I found out, It's only an oscillator. For LED TV Backlight Tester was an oscillator+sophisticated OP amp+MosFET, basically is a 300v 2.5ma electronic oscillator.

[stj]

Re: attempt to clone LED TV Backlight Tester

there are chips from maxim and linear technologies designed for current-regulated led psu's
start with those datasheets

[capwizard]

Re: attempt to clone LED TV Backlight Tester

L6562A PFC has voltage error amplifier and current error amplifier (comparator). (similar to TL494)

Transition-mode PFC controller L6562A
https://cdn.badcaps-static.com/pdfs/...8e89c6f9ca.pdf

[stj]

Re: attempt to clone LED TV Backlight Tester

no chance, your comparing a voltage regulator to a current regulator.
the feedback is completely different.

[capwizard]

Re: attempt to clone LED TV Backlight Tester

Thank you, but I'm try to learn, is the tester using linear or digital to achieve voltage slowly goes up. I saw one microprocessor that is for the 7 segments voltage led display.

Hop! I could use two resistors to change AC adpter or PFC power to a LED tester.

[stj]

Re: attempt to clone LED TV Backlight Tester

seen those type of things on aliexpress / ebay etc.
they arent expensive