(HELP) 3V Relay Short Circuit Protection

Thread Starter

Rod Zero

Joined Nov 6, 2014
9
Hi, everyone! I'm designing a 3V Relay Short Circuit Protection to protect 2*AA Parallel Batteries Holder. The goal is to light a LED when someone makes a shortcircuit in it. Since the the relay is rated at 3V, I've noticed that sometimes the input Signal(3V DC) doesn't triggered the relay coil. It makes a magnetic sound trying to close it, but it doesn't close the contact. Maybe it's the coil sensitivity or due to internal resistance that makes the short-circuit input signal to drop around(2.8-2.9V DC). I'd like to hear from you alternatives to make it work correctly. I've looked up some tips to use npn transistors, but have no clue how to connect it. What should I do? Would it be better to step-up the input signal using transistors?

I've attached the circuit image and the YouTube tutorial that I followed the instructions:
 

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sghioto

Joined Dec 31, 2017
1,470
I've attached the circuit image
From what I see with voltage connected on the Input side you have the red LED on and no voltage on the output. Press the button the red goes off and the yellow LED comes on stays on after you release the button and now you have voltage on the Output side. When you short the Output the relay should drop out. Is that what is happening at this time? What is the model # of the relay you are using?
SD
 

Thread Starter

Rod Zero

Joined Nov 6, 2014
9
Yes! When you short the output, the RED LED turns on and it disconnects the '2x AA batteries' to the load. I'm having some trouble because sometimes when you short-circuit it, the relay doesn't trigger. It happens when you connect two or more push-buttons or switches in series to simulate the short circuit in parallel with the AA batteries. I believe it's voltage drop and as it doesn't reach 3V, it doesn't drop out. What can I do?
 

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Thread Starter

Rod Zero

Joined Nov 6, 2014
9
From what I see with voltage connected on the Input side you have the red LED on and no voltage on the output. Press the button the red goes off and the yellow LED comes on stays on after you release the button and now you have voltage on the Output side. When you short the Output the relay should drop out. Is that what is happening at this time? What is the model # of the relay you are using?
SD
 

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sghioto

Joined Dec 31, 2017
1,470
I believe it's voltage drop and as it doesn't reach 3V, it doesn't drop out.
I don't see how the relay would not drop out if you short the output. Relays have a minimum "holding" current or voltage before they drop out but a dead short should be zero volts. Can you post a wiring diagram on how you have those switches wired that simulate the short?
SG
EDIT: Where did you find that relay?
 

Sensacell

Joined Jun 19, 2012
2,598
That's an inherently bad circuit design.
It assumes that the internal resistance of the battery will cause the output to drop when a high current is drawn from the output.
The problem is that the relay "drop-out" voltage is very unpredictable, and so is the internal battery resistance.
The current needed to trip it out will vary over a huge range.

Any time you see a schematic presented in cartoon graphics, you can also assume the design is cartoonish as well.

"Designed by Wile-E-Coyote"
 
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