78xx Current Boosting: Circuit Design and Measurements

In the last video we developed an interesting model for linear regulators current boosting but... does it really work? In this video, I'll give a simple example of how to select the correct components values, and we'll take some measurements to see whether our model makes any sense.

[@RexxSchneider]

You formula for the power dissipated in Rb is wrong. No more than the current passing through the regulator passes through Rband you have designed that to be not much more than 150mA. In fact, the base-emitter junction of a power transistor will drop more than 0.6V when the transistor is supplying significant current, but not too much more. If Rb were really dissipating 4.7W, then it would be passing 1A and have a voltage drop of 4.7V, which is nonsensical given that the transistor Vbe is across it. Look at your own measurements and you 'll see that no more than 200mA was passing through the regulator and hence through Rb. Actual voltage across Rb was 4.7R x 0.2A = 0.94V, so power dissipation in Rb was 0.94V x 0.2A = 0.188W. Use a 0.5W resistor.

[@indepth_electronics]

You're absolutely right, I don't know how I let such a huge and simple oversight slip through. Thank you for pointing that out, I'll try to add a note on the video immediately

[@indepth_electronics]

Turns out annotations on videos are not a thing anymore, but I'll keep this comment pinned. Thank you

[@RexxSchneider]

@ You're welcome. Thank you for the effort you put in to making your videos. It is appreciated.

[@ashishh5138]

Your content quality is really high. Please continue making these videos. 🙌🙌

[@indepth_electronics]

Thanks a lot, I appreciate it!

[@indepth_electronics]

An additional comment: although in the video I mentioned the beta reduction at high collector currents as the reason for the change of the behavior, it turns out that the major contribution is given by the increase of the Vbe voltage! The beta still plays a role in it, but way less than the Vbe variation (just do a numerical example yourself and you can see it!)

[@bobbysparky4299]

definitely would like to see a video on the NPN.

[@navuthakur5264]

Be regular & keep uploading videos regularly. Your videos are interesting and informative 👍 👌 👏

[@indepth_electronics]

Thanks! I'll try my best!

[@jjsibo3802]

Funny that this video was a 2 points quest at my analogic electronics exam a week ago

[@indepth_electronics]

Really? Then sorry for being 1 week late, could have been useful 😂

[@ivolol]

Haha, woops. The point of the transistor is for it to handle the large current path, rather than the resistor. Although over-estimating the power requirements in an experiment is in some ways much preferable to the opposite mistake :D Uni-Byte channel recently blew up one of his TO-247 power transistors in a power supply he is designing. It is quite satisfying to see the real world results very closely follow the idealized theoretical graph one would hope for.

[@indepth_electronics]

I was so concentrated on the analysis, that I momentarily forgot Kirkchoff's current law :P Yes the experiment results were very satisfying, and after giving a better look at the VBE variation of the transistor in the datasheet, even the high load current behavior is perfectly expected

[@MahamoedShawky]

Yes please my country really don't have good pnp transistors so if you can make npn transistor it will be very helpful for me and thank you for this great video

[@indepth_electronics]

I already have another video in the making for the next week, but after that I'll address the NPN issue!