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I'd like a relatively efficient way to regulate a voltage at say 6.5 volts less than the Vin to my circuit. The circuit requires a Vin range of 7 to 60 VDC. So for example if the Vin was 45 VDC the voltage regulator would output 38.5 VDC and if the Vin dropped to 12 the voltage regulator would output 5.5VDC. Ideally this regulator would be able to supply a maximum continuous current of say 150 mA but would not use that much when the circuit wasn't using it... Perhaps this device (and/or circuit) is common and I just need help with the jargon/terminology.

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  • \$\begingroup\$ A zener diode might work, not efficiently though. \$\endgroup\$
    – Colin
    Jan 5, 2018 at 18:47
  • \$\begingroup\$ A Zener diode and NPN voltage follower could be a good choice. If operation down to such low output voltages was not required, using a low-side regulator like the 7908 followed by an LM317 might be a good way to get 6.75 volts (the 7908 would generate a voltage 8 volts below the upper rail, but wouldn't be able to source current; the 317 would pass through enough current to the load to keep its voltage just above the reference voltage pin, which would feed about 50-100uA into the output of the 7805. \$\endgroup\$
    – supercat
    Jan 5, 2018 at 18:59
  • \$\begingroup\$ I hate terms like "regulate a voltage at say 6.5 volts". Is it 6.5V or is it a voltage of your choosing in operation? Some fixed voltage is very different from an adjustable one. \$\endgroup\$
    – Trevor_G
    Jan 5, 2018 at 19:22
  • \$\begingroup\$ 6.5 V is the value I need for the current application. If, in the future, I was tasked with making a version that didn't require the low 7 Vin operation I would increase this value (one of several things the 6.5 V will be used for is switching a large P-channel MOSFET so more drop would be better). \$\endgroup\$
    – CuForMiles
    Jan 5, 2018 at 19:48

4 Answers 4

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You merely have to connect a negative voltage regulator of the desired "drop voltage" to the positive rail. Then, connect your load between the output of this negative regulator and the "ground" or DC Common or negative rail, of your voltage source.

Look at negative regulator ICs such as 79M05 (-5 VDC), 79M12 (-12 VDC), etc. for fixed voltage drops. There are also adjustable negative voltage regulators which can make almost any (reasonable) drop you need.

The 79Mxx series will work up to a certain supply voltage. Eg. from Vin = 6 up to maybe 35 volts or so. For higher Vin voltages you will have to identify a more appropriate family. There is a wide variety of both positive and negative IC voltage regulators in the marketplace. Check DigiKey and Mouser.

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    \$\begingroup\$ A negative linear regulator can only sink current, not source current, so how will positive current be provided to the load with this arrangement? \$\endgroup\$
    – The Photon
    Jan 5, 2018 at 19:03
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    \$\begingroup\$ Of course you can make it work with a small modification, but 1. You should explain that to OP, and 2. The efficiency is atrocious for light loads. \$\endgroup\$
    – The Photon
    Jan 5, 2018 at 19:14
  • \$\begingroup\$ In your first paragraph where do you connect the reference / GND pin? A schematic would be better. \$\endgroup\$
    – Transistor
    Jan 5, 2018 at 23:12
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schematic

simulate this circuit – Schematic created using CircuitLab

Depending if you can accept reasonable tolerance values, this could be selected for 6.5V +/-0.2V ~150mA using 1.4V for Darlington and 4.1V with two yellow or Red 5mm LEDs for D1 D2 . With 1W max (6.5V*0.15A) dissipating in the device , it should have a heatsink.

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It is not a common requirement, and it is actually fairly difficult/complex to do accurately with small current draw and capable of 7-60V input.

Part of that reason is that it has to work to 0.5V out. The power dissipation at 150mA out is also a bit high for a simple and easily available Zener diode.

Here is a simple (but not very accurate) approach:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Could use a darlington or Sziklai pair on the output if you need more current and don't want to dissipate too much at R1 due to high voltage drop. \$\endgroup\$ Jan 5, 2018 at 21:52
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What you need is a shunt regulator -- those kinds of regulators are designed to maintain a constant voltage drop, just like you want.

Two important things to keep in mind:

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