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I am currently working on a battery powered project (4 x AA rechargeable batteries) with an Attiny (max vin = 5.5V), which works perfectly so far.

But if I would insert normal batteries into the holder instead of the 4 AA rechargeable batteries (the voltage of a normal battery is much higher than that of a full rechargeable battery), the maximum input voltage of the Attiny would be exceeded, which wouldn't be so great.

For this reason I am looking for the most efficient, cheap and above all power-saving method to lower the total voltage of the 4 AA batteries or rechargeable batteries by about 1V, so that it doesn't matter anymore if I use rechargeable or non-rechargeable batteries for the project.

Should I simply use a diode (voltage drop around 0.7V) or resistors? What are your suggestions?

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You are in need of a voltage regulator.
A diode drop is not ideal (no pun intended). It's only 0.7V for a certain current for a certain diode make for a specific diode. i.e. Given several diodes, they will all measure a little different.

I would suggest adding another battery in series, then use a buck regulator to turn it into the low voltage (5V or 3.3V) that you need. Buck regulators are very efficient nowadays and can commonly be found in the upper 80%.

If you don't need 5V, then you can skip the extra battery.

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  • \$\begingroup\$ Thanks for the tip, but my circuit only uses a few u amps, so a voltage regulator like the lm2596 would be a total waste (I think it wouldn't work in my circuit anyway, as a few u amps would be far too few for it). Or am I wrong? \$\endgroup\$ Jul 28, 2020 at 5:45
  • \$\begingroup\$ It does change things. I think it rules out using a simple diode, as the diode curve will be near zero for Vf of most diodes at that low of a current. I'd probably go with the voltage reference as Voltage Spoke mentioned. \$\endgroup\$
    – Aaron
    Jul 28, 2020 at 14:45
  • \$\begingroup\$ ok, thanks for your help. I will try this \$\endgroup\$ Jul 29, 2020 at 8:26
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It really depends on the accuracy needed, and how stable (temperature affects most electronic devices and can cause them to vary)

There are three ways to create a voltage reference:

  • Resistor and diode (least stable and accurate, but least expensive)
  • Voltage regulator (more expensive stable in the ~1mV range depending on the IC)
  • Voltage reference (built to provide a voltage reference that is stable and accurate below 1mV and sometimes into the uV's)

If you need something that is more stable than 1mV, I'd use a voltage reference.

This article goes into more detail: https://www.nutsvolts.com/magazine/article/build_a_01_accurate_voltage_reference

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A diode is the easiest place to start. A 1N4004 drops about 0.8 V, and a 1N4148 drops about 0.7 V (both dependent on the actual current through the diodes). Those calculate out to above 85% efficient.

The downside comes when the batteries are partially discharged. Now you don't need the voltage drop, and it is subtracting from the useful battery life. An LDO (low drop-out) regulator circuit can get you the regulation at higher voltages, but only about 0.1 V drop as the batteries discharge. However, it is a more complex circuit.

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