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I came across an interesting problem in one of the hobby group discussions - Following problem statement describes the design requirement 1) Input voltage range can be anywhere between 0-500 Volts (It is most likely EV battery pack etc.). They needed a prominent indicator, such as a LED that will illuminate whenever a voltage greater than 60V DC is present across the battery pack. 2) The voltage being present at the connectors must directly control the indicator using hard wired electronics (no software control is permitted). 3) The 0-500V is the only power source available (No external supply / battery can be attached.)

There are 2 design directions I could think - 1) based on TVS diode and a series resistor, which activates an LED after and onward 60 V. 2) Use resistor divider to bring down 0V -> 500V in range of let's say 0 - 5V. I guess even at 500V, if Rdivider value is high as 500K, even a 0.5 W resistor should work. then use any over-voltage detection circuit for LV systems such as this Am I over complicating/ simplifying things here? Critical feedback / suggestions appreciated.

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If you want something really simple where the 60V threshold is not stunningly accurate then try a Constant current sink in series with a Zener diode .The constant current sink could be set up at say 6 mA .This value does not need to be accurate so a 100 ohm resister across the base and emitter of a garden variety npn BJT like BC547 would suffice for LED current sensing .The pass transistor must be rated for the voltage and a safety factor.You could use an old fashioned N channel mosfet like BUK454 800 which is rated at 800V and comes in a TO220 package .You do not need low on resistance so why pay more money .You will be wasting some power at high input voltages so the TO220 package is sensible .You will need a heatsink if extended operation at high voltages is intended.The older mosfets are better in the linear mode when it comes to even die temperatures.You could use a NPN BJT as the main device in your constant current source .The issues of power dissipation and voltage rating still apply .Safe operating area should also be checked.The zener is chosen to give the 60V threshold .It will be of course a little bit less than 60V because of the LED drop and the Drop of the constant current sink.You may want to use 2 Zeners in series to get the desired performance .Zener power dissipation is not a problem .It will be less than 360mW with a 6mA LED current.I have used diacs in circuits like this to give hysteresis meaning that the LED is either on or off .I placed the diac in series with my 2 Zeners .The diac is less accurate than a Zener but I did not have to have high accuracy .I used lower LED currents to allow SMD devices.

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  • \$\begingroup\$ Transistor here needs to be beefy though... 500V * LED current = HOT HOT HOT \$\endgroup\$ – Trevor_G Sep 18 '17 at 21:06

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