# How to drop voltage to give a constant DC output

I have a load that drives current 1.8A[Max] @26V DC. It has to be supplied with a DC voltage of 30V[max]. I have a DC voltage 36V coming from another Power Supply Unit.

In order to supply a constant DC voltage of 30V to the load from 36V DC supply, would it be okay if I use a zener diode and a resistor as shown in the circuit or just a resistor would be sufficient? If yes, how to choose their values to drive this load. If not, what would be the best way to have a constant 30V DC?

I'm more concerned about the power rating.

Thanks in advance for the suggestions.

• Use a voltage regulator and preferably a buck voltage regulator so that heat dissipation is minimized. Commented Jul 27, 2020 at 11:30
• Why are you trying to avoid PWM - a chip will manage this for you - you don't need to control it with PWM. Commented Jul 27, 2020 at 11:45
• If you want to go linear (no PWM) then use a pass FET with enough power dissipation for the job. 6v at 2A is 12W. Probably that will be a TO220 plus heatsink. Then a linear regulator that supports controlling external FET (you can do it with a shunt regulator). Look at fig 34 of the tl431 datasheet ti.com/product/… Commented Jul 27, 2020 at 12:18
• It only drops 6 volts when there is current flow. This of course may be fine for the load if it takes a minimum current of several mA. However, at full-load the zener would be doing the job but dissipating 10.8 watts. If that's acceptable then it's a fairly simple modification. You will of course need space around the zener to be able to dissipate the 10 watts. In fact more space than a buck regulator takes up I'll wager. Commented Jul 27, 2020 at 13:27
• a zener isn't really meant to regulate a load, it's used as a voltage ref. you can use an adjustable voltage regulator like the LM317 to turn 36v into 30v. Commented Jul 27, 2020 at 15:24