LED Tester

[Myauk]

Dear All,

I am now working on design of a PCBA Tester. The PCBA I am going to
test has a few LEDs. After assembly process, we will design a test
station where the LED in PCBA will be checked for open/short/reverse
polarity.
The basic concept of the design include pumping in the current into
LED via USB connector power supply (hope I could use up to 500 mA).The
idea is to use USB connector from PC as a power supply to pump in the
current
into the LEDs. There, I will add a 50 ohm resistor between the cathode
of the LED and the 5 V USB Gnd. The voltage will be read from the LED
to determine its condition.
Practically testing it, if the LED is worngly mounted in reverse
polarity, at least 50 mA is sinking into the PCBA. I wish to know
whether this amount would do any harm to the PCBAs.
Can anybody give me some advice on this?
please help. I am also looking for alternative ways.

Regards

 

[neon]

THAT IS GREAT IDEA USE A $1000 PC to tests $0.10 leds. if you get 500ma into a LED you probably have no more led to speak of. I am stupid i use a plain battery and a series diode and a resistor.

 

[Myauk]

Hello,

I really can't understand why you need to do this - if you are making just a
few then look at them to check the LEDs are the right way round and then put
the power on and measure the current. If you are making many then use a pick
and place machine to put the leds down and check the first off. Any decent
sub contract assembly house will use automated visual inspection which will
check that the LEDs are the right way round and well placed.

I hope you are not just going to treat the USB port on the PC as a dumb
power supply and just grab 500mA - that is not compliant with USB specs.

Michael Kellett

Thanks for your kind respond.

What would you like to mean by grabbing 500 mA is not compilant with
USB specs?

I know that it limits the current up to 500 mA. We are using just a
few mA round about 100 mA maximum. This won't create any problem,
will it?

 

[Myauk]

You're not supposed to just go and draw 500mA from a USB port -- you're
supposed to wait until the PC "enumerates" your device, prior to which you're
not supposed to draw more than 100mA. (The PC can also then tell you, no, you
shouldn't draw the 500mA because it's added up all the power requirements of
various devices connected to the hub and just doesn't have the juice to give
you full power.)

Going through enumeration requires the intelligence of, e.g., a
microcontroller, of course.


There are conceivably hubs that will limit you to 100mA prior to enumeration,
although (perhaps unfortunately) if such hubs exist, they're certainly not at
all popular: The additional cost to implement them isn't something the
very-cost-conscious PC market was willing to bear, it would seem.


In practice, probably not -- I expect there are very, very few machines where
it would. You won't be able to carry the official USB logo for such a device,
though.

Thank you so much for your kind explanation joel and michel.

I am thinking of testing LEDs installed DUT with automated test
station.

I will pump current into the LEDs and read out the voltage. If it is
not shorted, open circuited or reversely installed, it will give
readings in acceptable range and it will pass the test.

I think it is a feasible solution for it. What i am worried is that at
least 16 mA will be sinking into the DUT circuit.

I hope 16 mA sink current into the PCBA when there is no power
supplied won't damage the PCBA nor degrade any component in the PCBA
because of this test.

Any suggestions?

Thanks