modified 9 years ago

Npnelite

02:00:46

NPN switched emergency light..an example if using NPN transistors in an inverter config so that when the switch is opened the LEDs turn on instead of the usual way. The switch is supposed to represent a power outage and the DC supply would be a wall wart adapter plugged into the light.I see a problem with the circuit where the outage unlike the switch would cut power to the 2nd npn base and turn off LEDs..see if u have a solution.

published 9 years ago

agentaero

9 years ago

I am not sure if the problem you see really is one...unless the outage is a short circuit failure of the primary power supply, which is def a failure mode but not representative of an outage. An outage usually means the power supply (12V) is in some sort of high impedance state. The switch should be moved to properly represent a power outage. Put the switch directly in series with 12V supply.

agentaero

9 years ago

Aftet the switch is movrd in series with 12V put a relatively large resistor, 10kohm should be good, from +6V to the base of the second npn to keep the circuit biased the way you want. If you are worried about the 12V circuit being shorted, then put some overcurrent protection in. Simplest is a fuse, but there are electronic topologies which can achieve it as well.

jamlynx

9 years ago

Hi Thanks for the information..I was thinking about a resistor from +6v to npn 2 base but I never saw the thing about moving the switch to better simulate the outage..I have made the changes to the circuit and the switch works better but the circuit is still not working properly..somehow npn 1 is still on even when switch is off.Here is the new circuit: http://everycircuit.com/circuit/6462479035269120

jamlynx

9 years ago

Finally fixed the circuit..problem was a tiny current flowing in the wrong direction keeping npn 1 base on..a diode solved it.See the link I posted in the last comment for fixed circuit.

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