I have to design a self powered high voltage (several kilovolts) indicator. A simple resistor ladder to measure high voltage + n stacked MOSFETs cell, i.e all MOSFETs in series (each arranged as a current gen) for low power electronics (opamps, comparators...etc.).
SYMBOL res 688 576 R0 SYMATTR InstName R8 SYMATTR Value 1k SYMBOL zener 544 656 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D1 SYMATTR Value BZX84C15L SYMATTR Description Diode SYMATTR Type diode SYMBOL voltage 704 688 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value 2.2 TEXT 8 640 Left 2 !.dc V1 0 3000 1 TEXT 744 744 Left 2 ;LED
Something a little simpler, like this, ought to work....
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It could maybe use a little more protections, like some source-gate zeners, depending on how radically the HV might change. Two more of the 15 volt guys maybe.
Use more stages if you can't find a suitable 1KV mosfet.
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John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
Good point indeed. I saw somewhere in the spec that Vin could change very rapidly (something like dv/dt = 500V/0.1ms) i think it's a bit problemeatic for MOSFETs.
The series LEDs improve efficiency and get the mosfet power dissipation down.
The three (or more!) top resistors are sacrificial, in case the fets short. The cap reduces dv/dt into the fet stack. The zeners are 8 volts roughly; 8 or so volt zeners work well at low currents.
Of course, you'd have to find a way to use multiple LEDs in the display, or find an illumination-type multi-chip LED, 12 volts maybe.
Depletion fets, like LND150s, would be interesting, especially at lower input voltages. The gate protection would have to change, but the lower zener can go.
Or a hybrid, with just the bottom fet being depletion. That's cute.
Interesting problem.
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John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
Does your circuit light the LED's for a voltage just greater than 50V? Which was how the OP spec 'd it. ...Jim Thompson
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| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
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"> What do you want the indicator to do? Indicate that a Voltage > 50V is present ... assuming a 3KV is the rated voltage is the equipement" ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
What is wrong with a neon bulb, AKA NE-2; hold it near the HV wire and the bulb will light up. The illumination will indicate AC(both elements light), DC positive or DC negative (only one element light - which one is polarity sensitive).
Hell, if the voltage was high enough, connecting one lead of the NE-2 to the HV wire, and letting the other lead dangle for corona, will do the job (and show if AC or DC polarity).
Yes it might light the LED for Vin >= 50, although the schematic here in is a bit simplistic, the main problem is the thermic and dv/dt on stacked MOSFETs,
BTW you are understanding the topic what would you do in such a case ?
Thanks for the name, could NOT remember it. I remember accidently applying way too much voltage only to see the leaf be 'blown' away. Those things are NOT easy to repair.
yeah, don't even know what you're referring to re 35V [guess there's some kind of 'new' constraint, in addition to lighting an indicator!
I responded to the original request, which made no mention of 35V, nor accuracy, nor lighting something, nor .... To me, a semi-mechanical solution was an interesting suggestion, but evidently not.
AFTER responding, I find there's a trail of 'extra' details. As, in oh by the way, I need to also...
Although self powered is interesting, I'll bet after everyone here solves the problem, there'll be another constraint, like how much impedance appears to the HV that one is measuring. 100MEG? 1G?
IIRC you cited 500V/0.1ms as "fast"? That's only 5V/us! There are _many_ OpAmps faster than that.
As for a solution... a >50V detector buffered to withstand a 3kV source?
"1 or 2mA" isn't going to light the LED very brightly... unless you had it blink... like in that "Miser" chip from National (Bob Hirschfeld, RIP).
Back when I had to measure 36kV (when I was a student at MIT), I used a sinusoidally-variable capacitor... bottom plate moving, and top plate connected to the 36kV...
I = V*dC/dt ;-)
I'll think on it. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
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