PNP transistor as a switch

Thread Starter

Denesius

Joined Feb 5, 2014
106
Having studied the forums on BJT as a switch, I thought the following would be straightforward: I need a simple (very simple) circuit to flash an LED when the battery pack is no longer being charged. The pack is 48v, 47.5v nominal when off the charger and under normal load, and 54v+/- when charging. So the attached circuit using a 48v zener should have worked. But the LED refuses to behave, flashing above 48v, and on steady with a flicker below. The voltage at the zener-R2 junction varies between 0v and 6.4v, when the circuit is powered from 40v to 56v. The transistor is a BC556 with a tested hFE of 420-450. I've replaced the transistor and the LED to no avail.
Am I missing something fundamental here?
Would a low power MOSFET (I have J182 P-channel devices on hand) be a better choice?

Circ-1.jpg
 
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KeithWalker

Joined Jul 10, 2017
1,222
In that configuration, the transistor will not turn off until the battery is almost dead. To make it work, exchange the positions of D1 and R2. R3 is rather high vaue to turn the transistor on hard. 22K would be a better choice.
Regards,
Keith
 

ci139

Joined Jul 11, 2016
1,696
Am I missing something fundamental here?
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(( no shit ? ))
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. . .
depending from what you'll lit your LED you may need a different "sensor" for the task
the one could be determining the direction/lack of current to the battery
. . . something like ... http://tinyurl.com/yykgwc4j ... a /!\ dangerous /!\ transistor only v. http://tinyurl.com/y34sh5js -- because of it's ultra high gain it may be not a safe option
+ a "voltage detector" http://tinyurl.com/yxeqqshp
 
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Thread Starter

Denesius

Joined Feb 5, 2014
106
Correct but need another transistor to invert the logic. TS wants flasher ON when battery is not connected to charger.
SG
Thank you "sghioto" - worked like a charm. If possible I would dearly appreciate an explanation of why the original setup did not: The transistor base should have been at 0v with charger off line (which should have turned on the transistor); and 4-6v with the charger on line, via the zener-R1 feed, which should have presented the base with a 0.004ma positive current, resulting in 20ma EC current.
 

sghioto

Joined Dec 31, 2017
1,453
You turn on a PNP transistor by connecting the base resistor R3 to ground. In your circuit the zener is "open" when the battery is at 48 volts so the transistor turns on. At 54 volts the zener is conducting and the voltage at R2 and D1 is 6 volts. But this is well below the supply voltage so the transistor is still on. Does this make sense?
SG
 

Thread Starter

Denesius

Joined Feb 5, 2014
106
You turn on a PNP transistor by connecting the base resistor R3 to ground. In your circuit the zener is "open" when the battery is at 48 volts so the transistor turns on. At 54 volts the zener is conducting and the voltage at R2 and D1 is 6 volts. But this is well below the supply voltage so the transistor is still on. Does this make sense?
Thank you!
 
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