Poorman's LED TV Backlight Tester.

[capwizard]

Poorman's LED TV Backlight Tester. use multimeter as voltage meter to saving $.

Finally working product: Add on type.

I use LR8K4-G IC , power use US AC 110v double to 220V. Current limit resistor use 270 Ohm.

[capwizard]

Re: Poorman's LED TV Backlight Tester.

Take A long time is because for safety reason needs Isolation transformer, I do have three big Isolation transformers, cant find a tiny one, then I found a post over Badcaps , solved my problem right away, Thanks again.

[capwizard]

Re: Poorman's LED TV Backlight Tester.

test.............

Pic1: 6 leds strip

pic4: Sharp lc60-c6400u edge led strip

[Dumah Brazorf]

Re: Poorman's LED TV Backlight Tester.

There's no resistor to discharge C1 cap.

[stj]

Re: Poorman's LED TV Backlight Tester.

yes, nasty that.
needs a 470k or higher high voltage resistor.

[ReeceyBurger123]

Re: Poorman's LED TV Backlight Tester.

Just use a ddm in diode mode lol these testers are so much faff.

[capwizard]

Re: Poorman's LED TV Backlight Tester.

Both of You are right, took long time to discharge the C1 cap, I did thought about that. and Zapped me twice.

ReeceyBurger123, because 330v DC this babe can test 50" whole screen series dLEDs.

PS: This is A high voltage, low current power supply, up to 450v and current limit, I set 6.8 ma.

[ReeceyBurger123]

Re: Poorman's LED TV Backlight Tester.

Lol shit happens

[capwizard]

Re: Poorman's LED TV Backlight Tester.

more.....

Pic2..............top: 18 LEDs .......... 94.9v/18=5.2722222

Pic3................bottom: 27 LEDs ........ 143.6v/27=5.3185

Pic 4,5............Total :45 LEDs............. 239.8v/45=5.3288

each LED is 6v. we call twins LED.

PS: with the meter read out voltage we can estimate how many LEDs inside the unopened screen.

[capwizard]

Re: Poorman's LED TV Backlight Tester.

OLED display for voltage ,current, wattage meter and battery powered Tester still under development....stay turned...

[capwizard]

Re: Poorman's LED TV Backlight Tester.

Just for fun, tested LG 60LN5400, A Direct-Lit LED LCD tv, it's amazingly accurate.....

DC 106.8v / 2.65v (the tester shows 2.65v with 1 TV LED)= 40.301 LEDs

has 4 loops =40 LEDs x 4 = 160 LEDs .....i don't believe have so many LEDs

googled picture shows Im right.....

8 long strips +12 short strips = 20 strips x each has 8 LEDs = 160 LEDs...... each loop has 5 led strips, but I don't know the combination of short or long strips, I never open the tv screen.

PS: LR8K4-G $1.21 each, PLEASE!! DIY youself A Poormans LED TV Backlight Tester.

PS: pic1 show 32.82v is wrong polarity lids 32.82v / 0.8 = 41.02 Diodes (reality is 40 diodes)

[capwizard]

Re: Poorman's LED TV Backlight Tester.

Now come in, one LG 47LN5700 sound no picture, with this A Poormans LED TV Backlight Tester, I know one loop leds not work, how many x LEDs, then need to open it. At least i know x LEDs <27 LEDs, can give estimate right away....

PS: Bad LEDs= X LEDs

Tested:
71.8v / 2.65v =27.09 LEDs , 27 LEDsx 2 loops= 54 LEDs Total
22.25v / 0.8v =27.81 Diodes = 27 diodes = this loop all LEDs are good. 27 LEDs = 27 Diodes
If LEDs < Diodes, then have some LEDS not light up in this loop, when open it and measure the LEDs will be shorted, and can be predict how many X LEDs by the difference.

the other loop shows 323v : open loop, so I know x LEDs <27 LEDs, one of the LED 100% open, or connector open.

Googled picture: 6x5+6x4=54 LEDs

[capwizard]

Re: Poorman's LED TV Backlight Tester.

I wrote A voltage-leds calculator on spreadsheets, It will be a plus for counting number of LEDs by input the meter readout voltage.

If anyone can use microcontroller and A LCD display write some program will be A+, I did learn Assembly language but now Im A dinosaur.

[capwizard]

Re: Poorman's LED TV Backlight Tester.

from this post #12 "LG 47LN5700 sound no picture, the other loop shows 323v : open loop, so I know bad LEDs <27 LEDs, one of the LED 100% open, or connector open."

Update: One LED open as I predicted, after changed TV work OK.

[Agent24]

Re: Poorman's LED TV Backlight Tester.

It's not a bad tester, I'd put it inside a case though, at least!

[capwizard]

Re: Poorman's LED TV Backlight Tester.

now is at the prototype testing final stage, because of the precision constant current source the accuracy is amazing, but I will need a miniature 20mA isolation transformer or battery-powered boost power supply to finish this project......otherwise AC live circuit will keeps shocking me............

[budm]

Re: Poorman's LED TV Backlight Tester.

You can use two small back to back transformers.

[capwizard]

Re: Poorman's LED TV Backlight Tester.

Thank you, Still too big to fit in!

PS: maybe use stereo 5v standby transformer:............

[mariushm]

Re: Poorman's LED TV Backlight Tester.

For a relatively low voltage tester, you could use A23 / 23A alkaline batteries, they're chea at 5$ for a 4 pack or 11$ for a 12pack : http://amzn.to/2n82UXs

You have 10v to 12v per battery, and each battery can do up to around 10mA (but you'd deplete them fast at such high current). With around 1mA load they'd last about 10 hours, with around 3-4 mA they'd last for about 2 hours. Considering each test lasts maybe 10-20 seconds, they should last a long time.

So you can put 4 in series and you have up to 48v. Put up to 12 in series and you have up to around 120-140v DC.
You can reuse battery holders for these, their diameter is smaller than AA batteries ... here's an example .. 2 of these for each pair of batteries : http://uk.farnell.com/keystone/5212/...-aa/dp/1650703
You can find them cheaper on eBay or other stores, just giving that link for the picture, so you can understand what I mean

--

Microchip has a linear regulator that works with up to 450V DC and can output up to 30mA as long as some conditions are met:

LKR8K4-G : Adjustable, 1.2V to 440V/20 mA out, TO-252AA-3

Link : http://uk.farnell.com/microchip/lr8k...-to/dp/2448524

The TO-252 has a θja of about 81C/w so basically it can pretty much dissipate up to about 1.5 watts safely.
So you wouldn't be able to do 24v at 10mA output from a 350v DC (rectifier 230v) for example, because that would mean a power dissipation of (350v-24v)x0.01A = 3.26 w and the regulator would enter its thermal protection.

But it could work at 1-2mA, or you could use two or more transformer taps to limit the input voltage depending on the output voltage desired. Basically, switch taps to keep the input voltage above output voltage by 12v.

The regulator can also work as current limiter, like any plain linear regulator, just the same story about 12v difference between input and output.

So as long as you're willing to have a 24v drop on the regulators, you can chain two of these linear regulators, once to lower the voltage from big voltage to test voltage + 12v , then second regulator just to limit the current to desired amount



So for example, you could get a 115v to 2x48v transformer, or two of these, or a 230v to 4x48v secondaries and use a slide switch to switch from 48v AC to 192v AC (4x48v). Then a basic bridge rectifier converts this up to around 260v DC and a smoothing capacitor can get the minimum voltage up to a minimum desired level.
For example, let's say 20mA maximum, so you'd have C = 0.02A / (2 x 60hz x (270v dc peak voltage - 250v minimum voltage)) = 0.02A / 120x20 = 0.02 / 2400 = 8.3333e-6 Farads or 8.33uF ... and naturally, the voltage rating should be at least 350-400v.
So basically a 10-15uF 450v rated capacitor would be enough for about 20-25mA test currents.

Then if you want you can use just one regulator to have your voltage regulation ... output voltage is whatever input voltage - 12v and then further adjustment by two resistors (you can use a potentiometer, and you have the formula Vout = 1.2 x ( 1+ R2/r1) ... use maybe a 470k or 500k potentiometer as those are common, and a 2.4k resistor and that would give you up to around 230v output.

For you in US, something like this would work : http://www.digikey.com/product-detai...794-ND/4915208 115v in, 60/120v out ... so you could use a switch to switch between 60v AC and 120v AC ... and you could buy 2 and put the secondaries in series to have up to 240v AC ... and after rectification and whatever, you'd still have about 10mA of current (you usually have Idc = ~ 0.62 x Iac when rectifying and smoothing to DC, but 0.62 is a generic value which works better with bigger VA transformers)
For lower secondary voltages you have something like this at 5$ each : http://www.digikey.com/product-detai...2130-ND/285639 (28v AC / 56v AC) per transformer... so you could buy two and have a switch between 56v ac / 110v ac or a more complex 28/56/84/110v ac switch

[capwizard]

Re: Poorman's LED TV Backlight Tester.

"The F120-020 : you could buy 2 and put the secondaries in series to have up to 240v AC" This is what I've been looking for, Thank you.

Now I try to use a 3.6v battery oscillate to 400v, to omit bulky transformer, by using laptop CCFL ballast + a OLED display for voltage and current = portable LED Tester.