LED Tester  - Page 1
Forum section: electronics / projects

author: mrlorin
author_rank: Newbie
timestamp: September 25, 2015, 03:30:53 am
content: I have upgraded a led tester from just a led, 9v battery and a resistor to a constant current circuit. I use the tester at work for repairing led video display panels. I'm concerned that the 9v battery powered circuit could damage a led when accidentally probing reverse biased? (hasn't happened yet)The circuit doesn't seem to work lower than 5v. I need about 13mA to get the blue leds to light up. Is it just the case of selecting the correct transistors for this? (see pic for circuit diagram)

author: KL27x
author_rank: Super Contributor
timestamp: September 25, 2015, 05:05:35 am
content: QuoteHow low do you want it to go? There is some overhead voltage that will be dropped by the circuit. So you're going to need FVD of the LED plus something like a couple volts. Swapping transistors can drop the overhead a little but not anything particularly significant. The closer you cut it, the more the "constant" voltage is going to vary (drop). You probably should just run it at 6+V.

author: Fungus
author_rank: Super Contributor
timestamp: September 25, 2015, 03:20:13 pm
content: Quote from: mrlorin on September 25, 2015, 03:30:53 amYes, that circuit has a lot of voltage drops in the transistors. They'll soon add up to 5 with some LEDs in there as well.The solution would be to increase R but you say "I need about 13mA to get the blue leds to light up". Why is that?

author: technix
author_rank: Super Contributor
timestamp: September 25, 2015, 04:08:07 pm
content: I would use a high-side current mirror circuit based on TL431 and a pair of PNPs - works down to 3V. The example board is a 0.65-inch square single-sided SMT board with two SOT-23 chips, a 0603 resistor, a trimpot and two connectors.

author: nrxnrx
author_rank: Unknown
timestamp: September 25, 2015, 04:26:21 pm
content: Quote from: mrlorin on September 25, 2015, 03:30:53 amYou can use a series diode (like the 1n4148) to protect the leds, but that would add another ~0.6V to your minimum voltage. A 3.x V zener in parallel may be better (but then you'd need a power switch).--Mihai

author: technix
author_rank: Super Contributor
timestamp: September 25, 2015, 04:26:44 pm
content: Even smalle

author: edavid
author_rank: Super Contributor
timestamp: September 25, 2015, 05:38:06 pm
content: Quote from: Fungus on September 25, 2015, 03:20:13 pmIt drops 2 VBEs = 1.4V. It should work OK at 5V with any single LED. The drop does not change with R.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 25, 2015, 05:56:59 pm
content: Check out the AP2502 linear constant-current sink. This single SOT-23-6 component can regulate the current at 20 mA each to 4 separate LEDs, or the sinks can be paralleled, so a single LED can be driven at 20, 40, 60, 80 mA. I am presently testing AP2502+LumiLed LUXEON 3535L LEDs, it works a treat. It has an enable pin so that the LEDs can be dimmed using PWM if desired.

author: Zero999
author_rank: Super Contributor
timestamp: September 25, 2015, 06:04:11 pm
content: A reverse bias of 9V shouldn't damage an LED, regardless of what the datasheet says. Most LEDs specify a maximum reverse voltage rating of 5V but in practise more than double that is required to cause any damage.The current mirror circuit will work but it has the disadvantage of needing a switch.

author: akis
author_rank: Frequent Contributor
timestamp: September 26, 2015, 09:59:26 am
content: I have blue and green leds that light bright at 0.5mA and others that need 10mA to match the same brightness. They drop from 1.7V to 2.3V depending. I am not sure what reverse voltage they can sustain before getting damaged. But I do not think that a reverse biased LED would break out of a 9V battery. As a matter of fact you can use an LED in 9V battery circuits for reverse polarity protection.

author: mrlorin
author_rank: Newbie
timestamp: September 28, 2015, 10:17:49 am
content: Thank you all for the great responses to my first post- I'm usually shy online but I'm tentatively lifting my foil hat after noticing how helpful people are on this forum. Dave's videos and this forum has taught me a lot about electronics but I've also learn't that making a fool of yourself and being wrong at times are important and necessary parts of "getting it right". Dave's great attitude and passion seems infective. I've had a long career as a "technician" so it is embarrassing to admit gaps in my understanding and knowledge of basic electronics.I'll look into all the hardware suggestions. Thanks for the reassurance regarding led reverse bias voltage.Your questions have gently reminded me that I have failed to regard the post: NEWBIES PLEASE READ BEFORE POSTINGIn video displays the pwm led driver ICs sink about 8 leds on each leg. Each driver IC only does one color and has an external reference resistor to set the current. The leds are powered sequentially by pulses from a mosfet that powers a full row.I was keen on low voltage (<5v) for a tester as I'm using it on RGB leds "in circuit" when repairing video display pcb's -I worry about damaging parts by clumsy probing?Video display RGB leds are usually 4 pin with a common anode. I've just taken some current measurements that I should have ages ago and discovered that the current required to turn different leds "just-on" can vary a lot.Red: 2mA , Green: 12mA , Blue: 21mA all within a single package.Red: 15mA, Green:43mA , Blue:47mA -another led that I tested in a 3.75mm pixel pitch screen.I guess there is some current leaking when testing in-circuit because occasionally other leds on the pcb glow dimly when I use a higher test current.The bezel mounted blue led I use in the tester has a very low current requirement (dumb luck) so it ended up quite handy as a continuity tester and well as a led tester - even when the current is too low to turn the led on, if it worked as a diode it's usually ok.So now I've learned that a tester with a fixed current that would light up all of the leds that I work with is just wishful thinking!My arbitrary 13mA current target came after testing just one blue led that came on at 10mA and then assuming 13mA would be a happy medium. I also assumed 20mA is the normal operating current for most leds?My first tester upgrade had a constant current circuit that always powered the transistors, so I was happy to find this variation of the circuit that turned on only when I was using it, negating the need for a switch that I would forget to turn off.The next version of the tester I suppose will need selectable current, so the hardware suggestions from this forum may better for doing that.I just googled "led tester" and found many commercial testers - all with selectable current from about 1 to 50mA. e.g.http://www.farnell.com/datasheets/106321.pdfI could just go and buy one but it seems that I'm learning a lot more by doing it the hard way.I guess a good tester for me would be one that defaults to a low current when first connected, then gently ramps the current up over a few seconds.Cheers. - I have many more led related questions that I hope will be interesting to you.