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Universal Voltage LED Indicator

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LAN_403

HV Enthusiast

Sat Apr 22 2006, 03:52AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Curious is anyone has any thoughts on this. I'm attempting to design a discrete circuit (no active components preferred) that will power a single LED at 10mA with a voltage range from say 5V to 350V.

I've been playing with several current source topologies using BJTs but its not so easy with such a large and high voltage range. There are some universal voltage LED drivers out there such as Supertex HV9910, but they do require a lot of extra components. Looking for something relatively simple. Has to be an LED though.

Any other ideas???

...

Sat Apr 22 2006, 04:23AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

does it have to be at least somewhat efficient?

You could take like a 2v zener with the with a led/50r resistor, and put a 300r resistor in front of that and hook it up to the source... But then you are drawing like hundreds of watts...

That is about all that I can think of with no actives (unless you coun't a zener as active...). It could be made useable by using a large resistor (like 1M) and said zener going into a high indepedance (ie in the microamp range) gate/comparator/watever... You could use a little smd chip dead bug it on the chip and embed it all in eppxy... You might have to reduce the resistor to something considerably smaller depending on how much the zener draws... (I have now idea how much a zener draws when it is 'off')

Do I smell a new circuit chalenge? Or is that only for questions with known answers...

Sulaiman

Sat Apr 22 2006, 08:44AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Some kind of ptc thermistor?

A high value resistor charging a capacitor which is
periodically discharged into the led by a ujt ?
Flash-rate proportional to voltage.

As above, just a resistor;
e.g.
350V/18 kOhm 7W =20 mA, 5V/18 kOhm = dim but visible.

If this led is part of some equipment;
a safety-critical resistor from the dc to be measured
to the base of a transistor
switching led/resistor from local dc supply.

Bjørn

Sat Apr 22 2006, 10:50AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

I have seen that done in a voltage tester with a thermistor of some sort. It looked a bit like this: http://www.samstagsales.com/images/vag1527bx.jpg

It contained the LEDs and a resistor. When connected to high voltages the LED would take a fraction of a second to settle on the normal brightness.

Bored Chemist

Sat Apr 22 2006, 10:51AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

You could do it with a set of relays and zenners to switch resistors in and out of circuit as the voltage changes. Of course, then you need some way to protect the zenners and relay coils over a range of voltages....

Carbon_Rod

Sat Apr 22 2006, 11:50AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

Hmmmm, Perhaps 10 silicon diodes (5 if DC), a 300ohm resister, the LED, and your Power supply or load in series at the sacrifice of a slight line voltage

Or something like this in combination with a capacitor divider and a zener.
drop.

[Edit: Go here

to learn how to post pictures.]

HV Enthusiast

Sat Apr 22 2006, 03:01PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Cool. Some interesting proposals here. Thanks.

I did try the zener approach as proposed by ..., but power dissipation is pretty great, plus regulation is a bit difficult through the entire range as well. I'll take a closer look at some of the other proposals as well. Thanks!

Electroholic

Sat Apr 22 2006, 08:03PM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

mini smps led driver?
or is ti too active?

Marko

Sat Apr 22 2006, 09:43PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

With all passive conponents and few hundred V DC you simply can't avoid cookage few tens of watts.

For AC you could use capacitors and diode arross each LED to set the current.
You could also try to make some simple transistor oscillator and run the capacitive divider with it, but I fear that this includes already too much active components.

HV Enthusiast

Sun Apr 23 2006, 01:45AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Thanks. I think i'm going to just change course and using an optocoupler and power the LED from my control circuitry. Simplest way . . .

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