Using a 7812 voltage regulator to maintain a constant voltage to a circuit (@ about 100ma) where the input can be up to 15 volts, what will happen if the supply voltage drops to 11 volts? Will the regulator pass the supply voltage through to the circuit being powered?
Sorta, maybe, but I wouldn't count on it. According to the datasheet it takes at least 14.6V for regulation and then the unit will give you a linear in to out V once you drop under that 14.6V.
See :
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They show the drop out data for the 7805 only, but I'm sure the 7812 acts similar.
You need more than that, the 7812 series require around 1.5 volts on the input side above the mark to provide a voltage of spec. anything below that most likely will be 1.5 volts less than the input. This would mean that if you have 11 volts input, your output most likely will be more more than 10 volts or less. What you need is a LDO type (Low drop out).. Something like a LM2940CT-12 LM2990T-12
That's just the tip of the ice burg.
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The L7812 uses a Darlington NPN as its pass device this accounts for some of the voltage drop and it will vary depending on temp, loading and manufacturing tolerances. A good experiment to see this is get yourself a power NPN in a TO-3 package and a thermal couple and vary Ic and measure Vbe note Tc .Tc is the case temperature of the TO-3 NPN.
The junction temperature can be calculated by Tj = Tc + (theta (jc) * Pd)
Tc is the case temp.
Theta jc junction to case thermal resistance see the data sheet.
Pd is Vce*Ic
Vbe will increase under load
Don't exceed Tjmax or you will fry the transistor.
Heres a data sheet on a NPN TO-92 Darlington showing Vbe in relation to Ic and temp.
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The 7800 series also has thermal protection. R11 in the schematic which is a TDR temperature dependent resistor. When VR11 hits about
0.7V q15 turns on cutting drive to the pass device. So r11 also contributes to the dropout voltage.
I've also seen a reverse-biased diode from output to input, in case the input drops abruptly, and some capacitance on the load side wants to discharge itself into the supply - the diode protects the regulator.
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