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'[buy] multimeter with ~5V diode test voltage.'
2019\06\28@200332 by peter green

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It has recently come up as a requirement to troubleshoot boards with LEDs on them of various colors. Unfortunately the diode test voltage on my meter is too low for some wavelengths of LED.

Currently I have been using a lab PSU instead, but that is undesirable due to high output capacitance, I could certainly build something custom but i'd rather just have a multimeter that could do it.

Can anyone suggest an affordable and readilly available (something from farnell/rs/rapid/cpc/mouser/digikey would make it much easier to get my employer to buy it) multimeter with a diode test voltage of 5V or so.

I am aware of the EEVBLOG 121GW, but it's expensive, hard to get and 15V seems excessive.

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2019\06\29@031650 by Clint Jay

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A few random thoughts.

Are you saying you want to test LEDs in circuit?

Have you seen the Atmega based LCR/Semiconductor testers ?

Ate you looking for a device that will give you numbers for forward
voltage, current etc or just a functional test of the LED?

On Sat, 29 Jun 2019, 03:00 peter green, <spam_OUTplugwashTakeThisOuTspamp10link.net> wrote:

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2019\06\29@031650 by peter green

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On 29/06/2019 08:17, Clint Jay wrote:
> A few random thoughts.
>
> Are you saying you want to test LEDs in circuit?
Yes
>
> Have you seen the Atmega based LCR/Semiconductor testers ?
I've seen them, but I don't think they are what I want, I don't want to hook up a component and wait for it to be analysed, I want to stick probes on and get a near instant indication.
> Ate you looking for a device that will give you numbers for forward
> voltage, current etc or just a functional test of the LED?

Mostly I am trying to identify LEDs that are either fitted backwards or have slightly incorrect placement (unfortunately it seems when you are picky about wavelength you don't get to be picky about the exact size of package and the PCBs were designed before the wavelengths were chosen, so the footprints are suboptimal for some of the LEDs) leading to open circuits. The LEDs are in strings, so functional testing of the board as a whole only reveals that a string is down, not which LEDs in the string are wrong.

I do need some indication of whether current is flowing because some of the LEDs on the board are invisible to the naked eye. My current multimeter works great for the longer wavelength LEDs, but for the shorter wavelength ones it doesn't have enough voltage.

I guess I could probablly jerry-rig something with some 4mm shrouded sockets, a red LED a 9V battery and a resistor, I'll probablly do that if I don't get an off the shelf solution.

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2019\06\29@043300 by Richard Prosser

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Can't you use a simple power supply (or battery), a resistor & a voltmeter?

How many to test? - is it worth setting up a video camera & look at the
red, green, blue output?

RP

On Sat, 29 Jun 2019 at 19:50, peter green <.....plugwashKILLspamspam@spam@p10link.net> wrote:

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2019\06\29@064504 by AB Pearce - UKRI STFC

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>I guess I could probablly jerry-rig something with some 4mm shrouded sockets,
> a red LED a 9V battery and a resistor, I'll probablly do that if I don't get an off the shelf solution.


You don't need shrouded sockets  - or are you looking at using standard multimeter leads as your probes?

I would knock it all up on a piece of veroboard with a pair of brass wires as probes. I would add a push button and zener to ensure the red led doesn't light on the current of a reverse LED.

So from battery have switch, resistor, zener to battery return (voltage chosen to be a little higher than forward voltage of red led plus LED under test) then red LED and LED under test to battery return.

Without the zener an LED under test may provide enough reverse breakdown to light the red LED, but such reverse breakdown would hopefully be at a voltage somewhat higher than the worst case forward voltage drop. If you want to be pedantic another LED in series with the zener gives a 'fail' indicator, so one LED should always light when the button is pressed - helps with testing by non-technical people. No meter needed.





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2019\06\29@141909 by Jon Chandler

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How about configuring an LM317 as a constant current source powered from a
9 volt battery with an LED in series to drop some voltage and to indicate
current flow?

On Sat, Jun 29, 2019, 3:48 AM AB Pearce - UKRI STFC <
alan.b.pearcespamKILLspamstfc.ac.uk> wrote:

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2019\06\29@150927 by Clint Jay

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Or indeed a 78xx device, you've just reminded me of a job I have to do.

On Sat, 29 Jun 2019, 19:21 Jon Chandler, <.....jon.chandlerKILLspamspam.....gmail.com> wrote:

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