I'm a newbie here but was trying to wire a setup and was running into some issues attempting to calculate my required resistance, and resistor tolerance to current.

I have a power supply that brings in a range of voltage, between [60 to 110]VDC @ [ 4.89Amps and approx 510 - 550watts ]

I'm trying to add a [ Shunt Resistor, Voltage Regulator, Voltage Stabilizer, Something?? ] in place to either drop 10 Volts from the power input, or a range where I can adjust the voltage coming in between dropping anywhere from 0 to 10 Volts.

The reason I'm attempting this is because I have a board that requires < 100 VDC to run properly, and I would like the ability to either switch on a regulator/shunt in the route (when necessary).


Voutmax = 100V <- what I've got Vin = 60-110 V. <- What I want

I tried calculating ohm resistance and I believe I wound up with something like, 100K Ohm resistance for a voltage regulator? and something like... 1.03 to 1.1 Ohm resistance on a 100amp shunt resistor?

Honestly really a bit confused still and just trying to find the most direct way to regulate or drop those 10 Volts from my input line.

  • \$\begingroup\$ Welcome to EE.SE! Please draw a schematic. Click on edit and the schematic symbol and a schematic editor will open up. \$\endgroup\$
    – winny
    Commented May 27, 2020 at 15:10
  • \$\begingroup\$ Sorry winny, I'm not skilled enough in electrical diagrams to know how to draw this... \$\endgroup\$
    – Edward H
    Commented May 27, 2020 at 19:29
  • \$\begingroup\$ Then you are out of luck and I would recommend you to start with something much simpler and learn to read and draw schematics. Schematics are the de facto language of EEs around the world. \$\endgroup\$
    – winny
    Commented May 27, 2020 at 20:51
  • \$\begingroup\$ Sorry, text descriptions of circuits are frowned upon in the EE community. \$\endgroup\$
    – winny
    Commented May 27, 2020 at 21:02
  • \$\begingroup\$ I'm struggling to understand what you're trying to do. Start with what voltage and current your board requires (just saying <100V is too vague) and what voltage and current your power supply is capable of producing. \$\endgroup\$
    – Finbarr
    Commented May 28, 2020 at 10:15

1 Answer 1


Allowed Voutmax = 100 VDC.
Available Vin is 60-110 VDC
Imax <= 5A
Want to limit Vout max to 100V so Vout is then 60-100 V.

A fuller description of the load and the source variation characteristics is necessary to give a good answer.

If you can ALWAYS tolerate a 10V drop then a 50 W zener diode (or bank of parallel lower wattage diodes) always in series between input and output would be the easiest and possibly overall lowest cost solution.

The "problem" is that doing this with a linear regulator will dissipate up to
P = 5A x 10V = 50 Watts. If you are prepared to dissipate this in a semiconductor (probably some or more FETS) then a simple circuit that meets you spec is possible BUT it may not meet you real (so far unspecified) needs.

Without knowing what value you place on this solution its hard to know what approach is suitable. In both the cases below for one off the design and build costs would far exceed the material cost.

eg Using 5 x 12V 1N2976B 10 Watt zener diodes in parallel mounted on a suitable heatsink and switched into circuit by a MOSFET when Vin exceeds 100V would cost about $US25 for the zener diodes plus the cost of the heatsink and glue electronics.

A PWM switched series resistor - perhaps a length of Nichrome (or similar) wire and a smoothing output capacitor and basic PWM controller would work.

An off the shelf ebay switching regulator (buck converter) may be the cheapest and fastest solution.

Q1 What is your budget?
Q2 How fast does Vin rise to beyond or below 100V?
Q3 Can you tolerate 10V drop at all times eg if Vin is 60V?
Q4 What output ripple voltage can you tolerate.
Q5 One off or how many?


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