LED light up test jig - Page 1
Forum section: electronics / beginners

author: ocset
author_rank: Super Contributor
timestamp: January 07, 2018, 05:58:03 pm
content: Hello,We are producing an offline, non-isolated LED driver with 60V LEDs. We need our assemblers to be able to probe the leds to check they have assembled them correctly into the product. We are using the attached test jig. We have made the test circuit ground connected to earth via a 1Meg resistor. This is to prevent ESD issues being introduced by the test probes. Do you think this is a good idea? (ie earthing the circuit ground via a 1meg resistor)..We need the 1meg resistor in case someone accidentally touches the probes to the circuit when the circuit is powered up from the mains

author: capt bullshot
author_rank: Super Contributor
timestamp: January 07, 2018, 06:26:12 pm
content: I don't see an issue doing so.It's quite common to use a resistor or a parallel resistor / capacitor to loosely ground a circuit. Values depend on your application, most common I've seen is 47k to 220k and 1nF to 10nF.

author: floobydust
author_rank: Super Contributor
timestamp: January 07, 2018, 06:40:37 pm
content: Meh, good intentions but for production testing...MPSA06is 80V with no ESD protection, an ESD hit could kill it.The mains 5V wall-adapter will have Y-caps and I'd earth ground its output, and then float the 72V side with the 100k-1MEG and both outputs to Earth GND through 4.7nF caps to lessen ESD and stray (test jig) capacitance from damaging the LEDs under test.I find the small DC-DC converters blow up if series-connected and an output ever goes negative. So I would add a power-on LED as a dummy load on the 72V rail, with reverse diodes across each 24V DC-DC modules. For me they died during power on/off and that fixed it.

author: ocset
author_rank: Super Contributor
timestamp: January 07, 2018, 07:22:24 pm
content: QuoteThanks , for the 4n7 caps i presume you mean Y caps,? with say 250uA of leakage?...And not caps with only nanoAmps of leakage?QuoteThanks, to ESD protect this NPN, we could put diodes from E-B and from E-C (anode to emitter in each case)?

author: Zero999
author_rank: Super Contributor
timestamp: January 08, 2018, 02:37:16 pm
content: The tiny leakage current through Y-capacitors can damage some sensitive components.BJTs are generally more resilient to ESD, than MOSFETs, as there's no sensitive gate oxide to puncture.Yes, a diode in reverse parallel with the base-emitter and collector junctions, will protect against reverse voltage. A zener across the collector-emitter will help to protect against overvoltage.

author: mariush
author_rank: Super Contributor
timestamp: January 08, 2018, 03:23:03 pm
content: You could just use a small isolation transformer 230v to 2x120v or 230v to 230v and then use one or two linear regulators like LRN8 from Microchip to adjust the voltage and set a current limit as well.TO-243 : LR8N8-G (20mA max, 13.2v to 450v input, 12v dropout voltage) :http://uk.farnell.com/microchip/lr8n8-g/linear-volt-reg-0-02a-440v-to/dp/2448526TO-252 : LR8K4-G (20mA max, 13.2v to 450v input, 12v dropout voltage) :http://uk.farnell.com/microchip/lr8k4-g/linear-volt-reg-0-02a-440v-to/dp/2448524Set the test current first with one regulator, then set the voltage with another, if desired.

author: floobydust
author_rank: Super Contributor
timestamp: January 08, 2018, 09:24:55 pm
content: It's just I am used to Production test equipment getting abused and so I go an extra mile to ensure no drama.Did you determine if the LED's have built-in reverse-diodes, I remember your post about that long ago.The first danger I see is the mains adapter's internal Y-cap causing stray voltage on the 5V output. This is why I say earth-ground its output.Adding protection diodes across the current-source helps, but then the ESD hit goes to the DC-DC converters or elsewhere.For incoming ESD, having small capacitors to earth-ground on the DC output lessens the incoming ESD hit. They (caps) do not cause AC leakage currents, but add risk for the output capacitance of the current-source causing a small discharge pulse to the LED's when initially connecting the test leads. So some series-resistance on the output is desirable, also because the current-source has a finite time to come out of saturation.With no load, the current-source is full on, and once an LED is connected it must quickly throttle down to 1mA, which takes time. Enough time to damage an LED?A pair of 100-1k resistors on the DC output, this would limit output current spikes and ESD input spikes.