Need help with relay circuit.

[MikeMl]

...

But, I've always wondered about ground plane. Your diagram has 2 separate grounds.

Is there a difference in the 2 grounds Appliance vs Wall wart or Are they just both at 0vdc and not to worry ?

Actually, LTSpice lets you connect two nodes "by name" without having an explicit wire drawn. So there actually is only one global ground (which Spice requires), but it just happened to be called "Cathode"

Actually, I was trying to show that the relay circuit is not grounded until the bottom clip lead (Cathode) is attached to the appliance. Suppose that for some reason, the Cathode of the LED was not connected to "ground", but say the series resistor and the LED were reversed inside the appliance. By having the relay circuit "float", it would still work... Note the current through the relay coil, which is all that matters...

 

[killivolt]

Actually, LTSpice lets you connect two nodes "by name" without having an explicit wire drawn. So there actually is only one global ground (which Spice requires), but it just happened to be called "Cathode"

Actually, I was trying to show that the relay circuit is not grounded until the bottom clip lead (Cathode) is attached to the appliance. Suppose that for some reason, the Cathode of the LED was not connected to "ground", but say the series resistor and the LED were reversed inside the appliance. By having the relay circuit "float", it would still work... Note the current through the relay coil, which is all that matters...

Gottcha.

Thanks again.

kv

 

[jjcf89]

Is there anyway of instead of using a wall wart to pull the relay use the A/C sources in the appliance which are 6.3V, 18V, 28V.

 

[MikeMl]

Not unless you have intimate knowledge of how the common side of the ac source is connected to the circuit driving the LED. They could be connected to each other or not. If not connected, there could be large common-mode voltage differences between the two.

The circuit I presented operates on DC, and was "floating" by virtue of the isolation through the wall-wart transformer. This way, you do not have to know anything about how the LED is connected inside the appliance; it just operates on the voltage difference between the two ends of the LED. As an alternative, you could use a small transformer, the primary of which is connected to your AC source; the secondary of which drives a full-wave rectifier bridge and a filter capacitor to make the DC to power the relay circuit.

Enough free consulting. It seems to me that you need to hire a competent engineer.

 

[jjcf89]

Ok so I finally built the circuit but it won't unlatch. After measuring the voltages I think I see the problem. Attached is the circuit with the voltages when the LED was on and when it was off. The cathode is used as ground.

The problem I see is that there is a voltage difference across the coil even when the LED is off. This doesn't seem to be enough to pull the relay in but it is enough to keep it pulled in. My question is how do I fix it?

Heres the parts I used:
Relay
3904 Transistor replacement
2n2222 transistor replacement?

EDIT:
Pictures didn't attach. Here is link to them.
**broken link removed**

 

[blueroomelectronics]

What is the wall wart rated at?

 

[jjcf89]

Power supply

 

[blueroomelectronics]

Now that's a supply! A simple and cheap 12-15VDC wall wart would have worked equally well.

 

[jjcf89]

Any idea whats going wrong?