Does 'LED Tester' really work?

Does \'LED Tester\' really work?

I've seen LED testers like this one (it's roughly halfway down the linked page) at several online stores. They claim to allow you to plug an LED into one of the several distinct fixed-current connections and push the button to apply the labelled current to the LED. The tester runs with a 9V battery.

The question, of course, is do they work as advertised? Put another way, do they really drive the LED at the currents specified at the various connection points, and at a voltage somewhere around Vf, which is typically between 3V and 4V?

Anyone actually purchased this tester and willing to share your impressions of it?

Re: Does \'LED Tester\' really work?

That's the first I've seen of this, but it is inexpensive, isn't it?

Re: Does \'LED Tester\' really work?

I would guess that for that price what you are getting is a box with some resistors in. A 275 ohm resistor at 3v Vf gives 21.8mA and at 4v Vf gives 18.2mA so the inaccuracy is probably tolerable for general purpose use.

Re: Does \'LED Tester\' really work?

My local Electronic's Store (Al Lashers) has one of these they use all the time. I strongly suspect Ralph is spot on, it's a switch, socket, battery and a few resistors.

Yes, current isn't all that well controlled, but it's 'close enough for jazz'. Good for go/no go checks and screening to find matching units.

For myself, I use a 9 Volt battery clip, LM317, 51 ohm resistor and two small clip leads for the LED. It delivers a *regulated* 25 mA and is useful for screening for Vf (hook a DMM up at the same time). Cheaper, smaller, more precise and home made.

Doug Owen

Re: Does \'LED Tester\' really work?

I use a multimeter. Just put a resistor on the line in series and measure the voltage and current. Cheap multimeters go for $20 or so and are adequate.

Re: Does \'LED Tester\' really work?

Agree with Ralph. I like Doug's setup though. And I like what he said about it:
[ QUOTE ]
Doug Owen said:
Cheaper, smaller, more precise and home made.

Doug Owen

[/ QUOTE ]

That's the CPF spirit!!!

Re: Does \'LED Tester\' really work?

I have one and have used it for 2 - 3 years. It made life so much easier not having to deal with clips and frayed wire ends. I've never tested it for accuracy, but it has been a real Godsend for comparison. I think that there's probably one manufacturer and many electronic suppliers and manf that buy them in quantity imprinted with their name on them. Then they sell them or give them away. Originally they were $20-$30. Prices have been going down and I've seen them for $10-$15. Hosfelt's price is great as it usually is and I highly recommend it for anyone that plays around with LEDs. After having one, I couldn't do without it. Mine was given to me by an engineer at Sloan with some sample 5mm white LEDs. God Bless that man wherever he is today.

Re: Does \'LED Tester\' really work?

I' built up a simple LED tester a few years ago, using an LM317 regulator and a 62 ohm (for 20 mA) resistor. connect the resistor betwen the output and adjust pins, feed the DC + into the input pin, and use the adjust pin (not the putput pin) to source the anode of the LED under test. The battery minus goes straight to the LED cathode. I use a 9V battery, but any Dc voltage from 6 to 12 is OK.

This uses the LM 317 as a simple current regulator, way it works is that it likes to see a 1.25 V delta between the putput and adjust pins, and by applying a simple formula of:

1250/R (ohms) = current (in mA).

So, a 62 ohm resistor will deliver 20 mA, etc. For odd value resistors, try series or parallel arrangements, two resistors in series just add the values, for two in parallel use
R1 * R2 / R1 + R2, for 3 or more in parallel, use
1 / (1/R1 + 1/R2 +1/R3...)

For 30 mA you would need 41.7 ohms, 50 mA, 25 ohms, 100 mA, 12.5 ohms, etc.

/ed brown