Need help adding a power transistor to my 555

[adown]

Yes, kinda of. That is the reason for the 100 Ω resistor in my drawing. It is a voltage drop.

 

[crutschow]

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Yes, a diode does need to be added to protect the transistor on shutoff. But the diode drops I was referring to at the collector are for something else.

An NPN can only turn on if the base is at a higher voltage than the emitter. Higher by the amount of B-E voltage drop (about 0.7V). This is why NPN's are used for low-side switching, not high-side switching like being done here. If the NPN is "full on" then the emitter voltage is only slightly below the collector voltage. So almost 12V. This means the base has to be at least 12.7V, which won't happen in your setup. And definitely won't happen with R1 and R2 creating a voltage divider.

So my thinking was to drop some voltage at the collector with diodes so that the emitter is low enough for the base to work.
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An emitter follower with 12V on the base with have about 11.3V at the emitter, which is sufficient to operate the LM7805. The means the collector emitter voltage will be about 0.7V but that's not a problem.
You don't need to do anything with the collector voltage for that to happen.

 

[cowboybob]

I just like to limit the current to the base of the NPN (in this case a 2N2222), hence R1, which I'd put at 330Ω. R2 (in this case, 22kΩ) is just a habit.

This will drive (8.5VDC on the Base) the NPN quite handily (with an initial peak current to the Base of ≈25mA as it charges the cap, then immediately dropping to 11mA nominal) and with 7.2VDC on the input to the VR (a 7805 in my test sim circuit) will easily provide 500mA to a 10Ω load, with NO drop in the nominal output voltage of the VR.

The 100μF smoothing cap (or whatever value) will discharge through the voltage regulator and is not a danger to the NPN - no need for a protective diode. No harm, though, if you feel the need to add one anyway. Also, at startup, the current thru the NPN peaks at about 870mA (for ≈3mS) as the 100μF cap charges.

There is NO point to adding the 100Ω resistor in the emitter leg of the NPN. It will reduce the VDC feeding the VR by ≈30%. Your just bleeding off current better sent to the VR and its load.

 

[crutschow]

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The 100μF smoothing cap (or whatever value) will discharge through the voltage regulator and is not a danger to the NPN - no need for a protective diode. No harm, though, if you feel the need to add one anyway. .
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The 100uF cap will indeed discharge through the voltage regulator when the NPN is turned off, with a time interval depending upon the load, but until it's discharged, the base-emitter will be momentarily reverse-biased with a voltage greater then the typical Vbe breakdown voltage. Thus it is a danger to the NPN and could damage the junction, which is why you need the diode.

 

[cowboybob]

The 100uF cap will indeed discharge through the voltage regulator when the NPN is turned off, with a time interval depending upon the load, but until it's discharged, the base-emitter will be momentarily reverse-biased with a voltage greater then the typical Vbe breakdown voltage. Thus it is a danger to the NPN and could damage the junction, which is why you need the diode.

I stand corrected.

I went back and saw that in the sim there's an ≈77VDC spike (at the collector) from the 100μF cap when the 2n2222 is shut off. Sort of curious why the sim does not, apparently, flag that over voltage as an error.

That said, still don't see the need for a cap, given the OP's application.