Being sure an NPN gets turned off.

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cupoftea

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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)
 

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  • INRUSH RELAY MANUAL.zip
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  • Switching cct.jpg
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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.
 
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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
 

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  • red or green.jpg
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cupoftea said:
if the red led would be totally off, with 1n4148 across it?
Click to expand...
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.
 
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