Being sure an NPN gets turned off.

[cupoftea]

Hi,
How many series diodes in the base do you think it takes to be sure of no NPN conduction when the switch is closed? Considering 1N4007 diodes and same diode used everywhere.
(LTspice and jpeg cct attached)

 

[dick_freebird]

Pedantically, you can't have that. There is always -some- non-
base-controlled collector leakage. Practically, seems like you
could dispense with all but one of the diodes if your switch
and wiring can hold less than 100-200mV on the base against
the 3mA or so through the 4.7K pullup (plus the 40mA for the
relay). In fact I question why you don't just put the 4.7K to the
bottom of the coil from the NPN base and lose all the diodes.

 

[Easy peasy]

a pnp across the relay coil driving the LED would seem an easier way to go ...

 

[crutschow]

Why all the circuitry just to control an LED?

(And please remove that strange jog in the wire below D5).

 

[crutschow]

Below is a simple circuit to light D2 when the relay is on, and light D3 when the relay is off.
Does that do what you want?

 

[cupoftea]

Thanks, looks good but i wasnt sure if the red led would be totally off, with 1n4148 across it?
ill have to pull the datasheet....
Nice ..less comps than the attached

 

[crutschow]

if the red led would be totally off, with 1n4148 across it?

My simulation shows only about a tenth of a microamp through a red LED when off.
If that's not sufficient, then add another 1N4148 diode in series with the LED.
Or you could use a small Schottky diode such as the BAT54, instead of the 1N4148, which has a lower forward drop.

 

[cupoftea]

Thankyou very much indeed for that, much appreciated.

 

[crutschow]

If you'd rather use a transistor to control the LED and eliminate the current through its 1k resistor when it is off, below is the circuit using a small N-MOSFET.

 

[BradtheRad]

Simple resistor network lights pertinent led. Values shown provide 9 mA maximum through either led.

Play with values if you wish to customize idle current through relay coil.