120v led/fuse/lamp tester

[bremnerr]

I have been asked to find a way to test parts for a little mom and pop store that sells string lights. They would like to test the individual bulbs and fuses to make sure they work before selling them. Anyhow, here are the specifications of the project and what I came up with as a solution. Just wanted to get some feedback to make sure I am going about this correctly.

1. Needs to test single LED bayonet type bulbs, single Incandescent bayonet bulbs and mini glass fuses.

2. Needs to run off of standard US AC power as their products do.

3. Should have a momentary deadman switch and selector switch or knob to choose which product they are testing.

4. Should have an additional hardwired LED to determine fuse continuity.

5. Should also have a mains fuse.


https://www.digikey.com/schemeit/#fft

Does this look correct? Should I use a rectifier for the LED test socket? Im not sure what the specs are on the bulbs that they are giving me so I have not included any values.

Thank you so much for your time.

-Ryan

 

[blueroomelectronics]

Dangerous and will probably electrocute anyone that touches it. Use a multimeter to check fuses.

 

[ronv]

Yep. Maybe you could use a 12 volt DC wall transformer as the power supply. You may need to experiment with the lamps to find the right voltage and current.

 

[blueroomelectronics]

The old tube testers in the hardware store had a pushbutton to momentarily power them. Aren't bayonet lamps 12V?

 

[ronv]

String lights sounded like the Christmas tree lights you used to get where they were all in series.

 

[bremnerr]

Yes they are the push in Christmas light style strings. As far as using 120Vac goes, we are trying to roughly simulate the same voltage/current that they will see in use by their customers. I am assuming the 120vac LED strings are wired with two parallel 10 LED circuits in opposing directions with a 1.5k resistor wired in series. I do know there are no bridges or diodes in the LED string. I will be properly enclosing everything in a project box with a reed switch cover to make sure that contacts/sockets are not exposed to fingers when hot. BTW, the schematic has been updated from the above photo if you follow the link to the live drawing. Will this work? Thanks again.

 

[bremnerr]

I appreciate the feedback, and I understand the risks involved with using mains voltage/current, but is the consensus here that this is a dead project? Obviously a pure DC system would be simpler, but I was trying to come up with solution that matched the parameters that I was given. A secondary benefit was that I was hoping to demonstrate how a fully rectified LED should produce better output than the hokey AC strings that they are selling.

 

[bremnerr]

I tried telling my daughter about how we used to have to take the back off of the TV and replace the tubes with ones from the local variety store. She just looks at me crazy and is convinced that I am lying to her. I also recently found one of the old tube tester machines at a flea market, almost bought it as a cool piece of nostalgia, but common sense (read:my wife) talked me out of it.

 

[ronv]

No, the project is ok, we are just trying to make it as safe as possible. So here is a compromise:

Use like a 12 volt AC wall adapter as the power source rather than the mains voltage?

 

[Norator]

With 1700 people electrocuted each year in the US [out of 330M], mucking around with electricity is not the most dangerous thing you can be doing.

Polarizing the plug and adding a GFCI to your circuit would probably show Due Diligence and a Duty of Care.

 

[alec_t]

Single components, i.e. LED, incandescent bulb or fuse, can all be tested with relatively low voltage DC from a mains adapter. The only requirement for mains voltage AC would be to power the adapter and to test complete mains-rated strings of lights (of whatever type).
It should be possible with a few comparators and logic ICs to make a 'one-size-fits-all' tester which would both identify the type of component under test (if not obvious from its appearance) and indicate its good/bad condition.

Edit: It would be helpful if you could find out the range of voltage/current/wattage of the components you need to test, since there are so many variations of Christmas lights available.

 

[KeepItSimpleStupid]

If they are 1/2 way smart a $6.00 meter from harbor Freight could work. They don't have auto-off function, so power from a suitable 9V supply (I think). I can look at mine.

The diode test will do 90% of probaby what you need.

Fuses and LED's.

So, what you might do is:
1) make a 9V power supply
2) Make something that will hold the bulbs.
3) Add an easy reverse polarity switch.
4) That get's you fuse and basic LED testing.

5) For a little more testing, add a current source using an LM334 or an LT part and set the current to 50 mA.
Their little polarity tester, should make sure the voltage reads above 2.1 V.

Push another button and the bulb should light.

So:
1. we have a basic ceap meter always set to diode test.
2. We add an isolated 9V power supply because no auto off.
3. We add fuseholders and LED sockets in parallel.
4. A DPDT switch to set polarity.
e.g Insert bulb, flip DPDT swith
Reads 0 in both directions - bad
Reads 0 and >1.2, probably good.

Leave the DPDT switch in the >1.2 position
Push another switch and it would add an LED and the current source in series.
Light should light. If a blinker bulb is inserted, it should blink.

The first thought that came to me, was plug the set in.

I never quite developed a technique for finding a bad bulb in a string, One is easy. 2 or more is miserable.

I tried a contactless voltage probe. You must use a polarized connector. Never tried a TDR. That might work.

Measuring voltages and bisecting always sounds like a good idea,

Remember that some of the incadescent bulbs have a ring that shorts the filiment. Watch out for the blinker bulbs too.

Testing of 120 V strings should be done with an isolation xformer or voltages below 24 VAC. I've never tried that either.

 

[bremnerr]

Thanks you all very much for all input. I talked to the owner and they agreed to let me power this project with 12V. Whew, this simplified things significantly. The updated schematics can be found below. I also figured that both types of bulbs are 6V and the incand bulb burns .48W. I would appreciate one last look by some experts to check my math and design. Thanks again!

https://www.digikey.com/schemeit/#fft

 

[KeepItSimpleStupid]

It doesn't look too bad.

 

[alec_t]

The top circuit apparently does not provide mains isolation so would be DANGEROUS.
R1 and R2 are much too low in value. At present you would have ~100mA going through the LEDs and D1. They'd die

 

[bremnerr]

Check the bottom diagram. Is this better? I may have royally screwed this up.

https://www.digikey.com/schemeit/#fft

 

[alec_t]

Personally I would double the value of R3,R4. The C3 voltage will be ~16V and the LEDs are being pushed towards their ~30mA (assumed) max.

 

[bremnerr]

Will do, I was trying to compensate for the voltage drop due to the rectifier, but a higher value couldn't hurt anything.

 

[dr pepper]

Use a 9v battery, I used to do this before led xmas lights became popular, 9v lights a 12 lamp/led.
Use one of the 12v lamps as part of the fuse tester, the fuse makes the circuit and lights it.
No need for a push to test deadman, and no need for a mode switch, no risk of shock/fire and you can take it anywhere or lend it someone.