Attempting to diagnose the problem with fairy seed lights, and why they don’t light.
have a string of these lights that won’t light, power unit works test tested on others check. I’ve gone along the whole string with a multimeter and get the same voltage everywhere and have used a continuity test which shows continuity throughout. What am I missing?

 

Are they LEDs, recheck the polarity.

 

What happens if you test reverse continuity?

 

If the polarity has been reversed then there would not be much current draw. What voltage is measured across the two lines???

 

The supply says 3.5 volts Judging by the shape i am guessing it is regulated And it does state a reasonable voltage for direct across the LEDs. BUT the polarity might be wrong. At that low voltage a reverse should not be destructive.

 

If the polarity has been reversed then there would not be much current draw. What voltage is measured across the two lines???

Testing two identical sets. Set on the left, (A) not working, set on the right (B) working.
Continuity using an alligator clip is bright, prongs and anywhere across the wires on set A. And barely visible on set B.

Voltage measured at the plugs (A & B) is 7.2 volts. Voltage measured (A) anywhere across the two lines is 3.4 V and 0.0 when probes are reversed. On (B) it measures 5.3 V & 0.0 when reversed.
The (B) set that lights; is working, has 15 dead seeds that do not light.

 

1. Not sure if I am understanding correctly.​
2. What is meant by reverse continuity?
3. If I am thinking correctly when you say reverse polarity-as in flipping the prongs on the plug? Which is prevented by the regulated plug
4. 
   
   
5. And further help is greatly appreciated.

 

The power to the LEDs is DC, meaning that one wire is positive and the other is negative. LEDs only light when the polarity is correct, because they are diodes and the current can only pass in one direction. So if somehow the plug were reversed none of the LEDs will light.
I have seen a few strings of LED lights like that, and I see them as being very fragile and easily damaged. And the insulation on those wires appears to be quite fragile as well. That is, easily damaged so as to allow short circuiting between the two wires.