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I need to lower a 12v, 100mA signal voltage to a 5v signal voltage of any current rating. Here's the catch: I need that 7V drop to be constant with any input voltage. For example, since it is a signal voltage, if I drop the signal to 10V, it needs to output 3V. Likewise 7V to 0V, etc. what device would accomplish this?

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  • \$\begingroup\$ Seven 1V diodes maybe? \$\endgroup\$ – Chris Fernandez Jan 3 at 22:26
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    \$\begingroup\$ A zener diode.. \$\endgroup\$ – Oldfart Jan 3 at 22:30
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    \$\begingroup\$ When you asked this question on Physics.SE you specified 1 A current rather than 100 mA. Have your requirements changed? \$\endgroup\$ – The Photon Jan 3 at 23:00
  • \$\begingroup\$ BTW, this change makes it a lot more likely you'll find a zener diode solution. \$\endgroup\$ – The Photon Jan 3 at 23:00
  • \$\begingroup\$ Thank you, it seems that it's a zener solution, straight up! \$\endgroup\$ – Praesidium Jan 3 at 23:30
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Here is a possible way to do this. Note that it cannot supply negative voltage, so Vout = Vin - 5.0V), and current should be more than 1mA so there has to be a relatively low load resistance at low output voltage. If you do not draw at least 1mA+ the output voltage will rise. R4 below is the output load.

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Here U1 is a TL431 shunt reference. The transistor will be okay up to an ampere or two with proper heat sinking.

Edit: Vdrop = 2.495V * (1+R1/R2) so by changing R1 the drop can be changed to whatever is required from about 2.5V to about 30V. The graph shown is for 5V drop. If you change R1 to 18K you'll get about 7V as @Jasen comments below. It still will be fine from 1mA to 1A with proper heat sinking on the TIP32.

A simple 1W zener will work too- but you cannot be adjusted and won't be as stable or accurate.

Here is an example for comparison (using a 7.2V nominal SMT 1W zener):

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And, finally, and in order of poorer performance, a string of 10 1N4148 diodes:

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  • \$\begingroup\$ should be 7V not 5V drop. increase R1 to 18k \$\endgroup\$ – Jasen Jan 4 at 0:05

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