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I would like suggestions to build a very simple circuit to supply a Vout of 1.8-2.0 V with a Vin of 3 V supplied by 2 D cell batteries. Load is 20 Ω. I think 2.4 V Zener with a regular diode could bring the voltage into range (2.4 V - 0.6 across the diode = 1.8 V. I'd like Vout to remain constant as Vin decreases as the batteries age.
(obviously I am very far from an electronics design genius) The output is a 3 ms pulse every 1500 ms so battery life should be good.

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  • \$\begingroup\$ A Zener diode can not be used as a voltage regulator. At most it provides a voltage reference that can be used as part of a voltage regulator. \$\endgroup\$
    – asdfex
    Jul 7, 2023 at 14:38
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    \$\begingroup\$ How stiff (output impedance) and how efficient does it need to be? The simplest scenario is obviously a single 10 ohm series resistor. \$\endgroup\$
    – tobalt
    Jul 7, 2023 at 14:48
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    \$\begingroup\$ You won't get the full life of the batteries with a simple linear regulator, maybe 1/2 to 3/4, depending on current profile, the output voltage you choose and the regulator performance. End of life for an alkaline battery is typically taken as 0.8V, so 1.6V for two. \$\endgroup\$ Jul 7, 2023 at 15:03
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    \$\begingroup\$ @asdfex Who told you that? Zener diodes have been used as shunt regulators since the 1950s. One wouldn't work very well in this particular case, but there are plenty of applications where they make perfectly adequate regulators. \$\endgroup\$
    – GodJihyo
    Jul 7, 2023 at 15:49
  • \$\begingroup\$ Battery life will be very bad with Zener regulator as it consumes current constantly despite no load. \$\endgroup\$
    – winny
    Jul 7, 2023 at 16:33

2 Answers 2

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Perhaps the simplest solution is something like the LD39015M18R from ST, a low voltage, low drop-out linear regulator. It will provide exactly 1.8V at up to 150mA, all the way down to about 2V input. From their datasheet:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Very nice find. \$\endgroup\$
    – AnalogKid
    Jul 7, 2023 at 22:16
  • \$\begingroup\$ Agreed ! Thankyou Simon Fitch \$\endgroup\$
    – no name
    Jul 8, 2023 at 0:26
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Here is a simple LDO regulator that may meet your needs, although of course there are many specific devices that work better, for less than a dollar. But this can be built with any old NPN and PNP transistors.

schematic

simulate this circuit – Schematic created using CircuitLab

Simple LDO 1.8V out with 3.3-1.6V input

Here is an LTspice simulation of a circuit using a PMOS series device, and the temperature effect at 0, 25, and 50 C.

LDO using PMOS, and temperature effect

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    \$\begingroup\$ This answer is great! Proper old-school, cereal box and shoelaces. \$\endgroup\$ Jul 8, 2023 at 2:26
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    \$\begingroup\$ Its main issue is temperature coefficient of -2 mV/C. If Vbe is 650 mV at 25C it will be 600 mV at 50C and 700 mV at 0C. So for a nominal 1.8V output will change about +/- 10% over that range, or 1.98 to 1.62 VDC. A thermistor (or maybe another diode) might be used for compensation. physicsforums.com/threads/… \$\endgroup\$
    – PStechPaul
    Jul 8, 2023 at 3:54

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