# Decrease a voltage from 24-27VDC into 18VDC using a 7812 and a bunch of other home supply parts

I have one 7812 1A linear regulator, bunch of resistors and zeners at home.

My input voltage is 27VDC from two 12V lead batteries. Desired output voltage is 18V and the maximum current consumption of the connected device will be 100mA.

I'm considering using a zener diode at the GND pin of regulator.

What adjustment current trought the GND pin on regulator I have to expect?

Do i need the 1uF capacitor when using lead batteries?

• I would add a resistor between the 7812 OUT and GND pins, to ensure that the zener will have enough current to maintain its voltage. – Peter Bennett Jul 23 '13 at 15:52

All 7812 implementations I know of have a current to ground < 10mA. I don't see a reason why that would change when you lift the ground pin using a zener.

However, zener voltage is definitely not constant with temperature. Be sure to take this effect into account and determine whether compensation is required when using a zener diode as a voltage reference.

I think you should move away from a linear regulator because of the heat dissipation. Power dissipation (Pd) from the load itself will be (27V-18V)*100mA = 900mW. The regulator itself is not 100% efficient, so it will draw current as well; thus, adding more heat output. Try to upgrade to a smps and use something like an adjustable AP1501 from diodes inc (link).

• I think the suggestion is good but I don't think a 7812 would be regarded as a Low Drop Out device. If you substituted the word "linear" in place of "LDO" in the first line and "regulator" for "LDO" in the second line this answer makes sense. – Andy aka Jul 23 '13 at 16:39
• @Andyaka Yeah you're right, 2V @ 1A is definitely not LDO. – NothinRandom Jul 23 '13 at 17:20

In this case, you should use adjustable voltage regulator as LM317, that you can set the output voltage with a range between 1.2 V to 37 V. It have the same package as the LM7812, the TO-220.

The output voltage will be:

Vo = 1.25 V.(1 + R2 / R1) + Iadj.R2


In the pin 1 (Adjust) should have a tension of 1.25V typically.

So, to get an output voltage of 18V, the R1 = 240 ohms and the R2=3.3k ohms, and you got a output tension of 18.6V supporting a current until 1.5A in the output.

• You can indeed use a 7812 to regulate voltages greater than 12V. The 78xx voltage regulators regulate the output voltage with respect to their ground terminal...lift ground above 0V and you get a higher voltage. – Joe Hass Jul 23 '13 at 15:13