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Either way, the first circuit is bad because the saturation voltage of the TIP2955 will increase as the current is increased which will ruin the regulation.
The collector saturation voltage doesn't add to Vbe. I also got the part number wrong, the fist circuit uses the 2SC2681 which is Japanese so the datasheet makes little sense to me.
Vbe still varies pretty widely so it's bad practise you use the first circuit.
The second circuit only uses one extra part, a 100R resistor which is really cheap.
100R means that the transistor will turn on whenever the 7812 is using any more than 6.5 mA of input current. Since some 7812s could have 10mA input with no load, this might cause the output to be too high. When operating, the transistor would be carrying almost all of the load current which means the 7812 has no idea when there is an overload condition.
IMHO this sort of step-down is not very suitable for 4A. You could be burning up to 48W of power in that transistor- that's possible, with a big transistor and heatsink, good ventilation and probably a fan, but it's pretty clunky. Also if this is a battery, losing half your power is costly.
There are DC/DC converters for this job, but they're MUCH more complicated for a beginner. There are some 24v-to-12v DC/DC converter modules on the new and surplus markets.
Then Oznog's concern is even more correct. A linear regulator is possibly the worst way to drop 24V to 12V 5A.
What is it for? If the load doesn't need an absolutely flawless 12V, there are many ways to do this. I would consider a Black regulator if cost is a concern and regulation is a little relaxed.
100R means that the transistor will turn on whenever the 7812 is using any more than 6.5 mA of input current. Since some 7812s could have 10mA input with no load, this might cause the output to be too high. When operating, the transistor would be carrying almost all of the load current which means the 7812 has no idea when there is an overload condition.
Yes, I missed that, the resistor should be 1R because it will start to turn the transistor on when the current to the regulator is between 600mA and 700mA. It's not a good idea to use a much lower value, because Vbe can be as high as 1V when lots of current is flowing through the transistor which would mean 1A is flowing through the regulator which is its maximum current rating.
I got away with using 0.47R before but that was for an LM317 and really it should've been 0.68R.
thanks for the information bro.... do you have any dc to dc converter design sir.. 24v inputto 12v output with high current requirement for about 5A to 10A? thanks in advance...
iam sorry guys but i wanna know why we use transistor here??
we got vr 7812 is already regulate o/p voltage to be 12 volt why we sue those extra componat in both of circuits ??
Google "Web Bench" and sign up. They have a neat program where you just enter your input voltage, output voltage and current and it gives you a design and all the parts needed.
Google "Web Bench" and sign up. They have a neat program where you just enter your input voltage, output voltage and current and it gives you a design and all the parts needed.
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