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The base of the transistor is connected directly to the regulated output of the transformer, so when the transformer produces 9 volts the base voltage will be greater than or equal to the emitter voltage and the transistor will open the circuit. But when there is no power in the transformer, the output of the transformer is floating and the transistor’s base as well.
I tried to find a path that connects any negative voltage to the base so that it closes the circuit and the LEDs turn on but I didn’t find one!
Would you tell me from where the transistor’s base gets its negative voltage? Or at least a voltage that is less than the emitter voltage by 0. datasheeet
LM724 PDF Datasheet Search Results
This means that there’s no guarantee that there will be enough base current to turn T2 fully on. It would be a good idea to add a resistor from the regulator input to ground in order to supply datwsheet current to the transistor when the mains power is off.
If those are 20 mA LEDs, you’ll want to be able deliver a total of about mA, so you’ll need a base current of at least 2.
Home Questions Tags Users Unanswered. How does this automatic emergency LED lk724
LM IC Pinout, Specifications, Equivalent & Datasheet
PeterJ 16k 20 42 Michael George 1, 8 34 Does that mean a voltage regulator must be used in automatic emergency led circuits?
I can not use just a capacitor as a regulator, can I? A capacitor is not a regulator. You need a DC path for the base current. See the additional analysis above. What’s the purpose of the T1 plus Zener circuit?
Over voltage protection for the battery if someone adjusts VR1 too high? Looks like it would pull the adjust to 0V if I understand it right. Yes, it seems to be a redundant voltage limiter that will kick in at about 7. Ordinarily, I’d expect to see the LM wired as a current limiter, but datasheeet this circuit, is wired as an adjustable voltage regulator.
In actuality, to attempt at maintaining regulation, the LM will try to keep the original 1. Still not enough, but far from as bad as you estimated without helping resistor. Otherwise, since adding guarantees is good in all cases: Pm724 up or log in Sign up using Google. Sign up using Facebook. Ddatasheet up using Email and Password. Post as a guest Name.
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