Power a single LED with.......... 1000 VDC!! - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: June 15, 2015, 12:06:46 pm
content: Take a common or garden LED, lets say, 3Vfwd and 10mA for the sake of argument.Now assume i want to have that LED lit from a DC Bus that varies between 50 and 1000VDC.What's the simplest way to do this?(linear reg made out of a HV transistor is simple, but large power loss/heat (~10W ), some form of switcher is much more complex, but less heat to worry about)System needs to be powered only from the DC bus, and be self contained.

author: muvideo
author_rank: Frequent Contributor
timestamp: June 15, 2015, 12:16:26 pm
content: You pretty much answered yourself...if it had been AC, you can use some other trick

author: amyk
author_rank: Super Contributor
timestamp: June 15, 2015, 12:31:22 pm
content: Note that there are HV LEDs available with forward voltages > 50V, so even though you'd probably need a custom, multistage, buck converter you don't need such a low output voltage.

author: digsys
author_rank: Unknown
timestamp: June 15, 2015, 12:58:07 pm
content: If you can afford to lose ~3.0V on the bus, or have a voltage drop around 3-5V, you can use an Isolated DC-DC converter, as cheap as $5That way you'll have an Isolated 3,5,12V with at least 150mW+ to do whatever you like. Otherwise it'll be tough.

author: calexanian
author_rank: Super Contributor
timestamp: June 15, 2015, 02:33:19 pm
content: Is this just for a small indicator or are you actually trying to light an area?

author: max_torque
author_rank: Super Contributor
timestamp: June 15, 2015, 03:38:07 pm
content: Just needs to indicate the presence of voltage on the DC BUS, so over 50V = LED ON

author: TerraHertz
author_rank: Super Contributor
timestamp: June 15, 2015, 03:59:00 pm
content: Is that '1000V' an absolute maximum, or can it sometimes overshoot?What kind of bus ranges from 50 to 1000 V ?Do you absolutely not have any other power source, at lower voltage?Does the LED have to be continuously lit, or is blinking OK?Asking, because your problem as given is actually pretty hard.

author: station240
author_rank: Unknown
timestamp: June 15, 2015, 04:26:12 pm
content: Why not just a tiny switch mode supply, say a high voltage input rated version of the CCFL inverters.I've seen some that were nothing more than a single transistor, tiny radio coupling sized transformer, and output cap and a few other cheap random parts.

author: langwadt
author_rank: Super Contributor
timestamp: June 15, 2015, 04:36:56 pm
content: Quote from: max_torque on June 15, 2015, 12:06:46 pmThere was a long thread about exactly that on sci.electronics.design

author: max_torque
author_rank: Super Contributor
timestamp: June 15, 2015, 06:19:02 pm
content: 1000VDC is peak, intermittent maximum.Normally, the DC bus will sit about 700Vdc to 800VdcI guess LED can flash, to reduce average current a bit. I did wonder about some sort of "trickle" resistor into a capacitor, that every sec or so somehow pulses the LED?

author: ajb
author_rank: Super Contributor
timestamp: June 15, 2015, 07:19:18 pm
content: With a suitable LED you may only need half a mA or so, as long as you don't need blinding brightness. That makes a linear solution more reasonable, as you'd only need to dissipate a watt or so.The flashing idea is interesting. Could be as simple as a three-terminal voltage supervisor IC, then it's just a matter of sizing the capacitor to suite the hysteresis of the detector IC so that you get a suitable flash duty cycle/frequency. The frequency will drop off with voltage, though, and you won't get a firm cutoff at your stated 50V lower theshold, unless you do some additional trickery.

author: wraper
author_rank: Unknown
timestamp: June 15, 2015, 07:51:40 pm
content: I would consider neon lamp instead of LED too. Also 10 mA is excessive unless using it outdoors. Just take some ultra bright high efficiency led and run it < 1 mA.

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 15, 2015, 07:53:25 pm
content: Quote from: wraper on June 15, 2015, 07:51:40 pmNeon will not start at 50V

author: ivan747
author_rank: Super Contributor
timestamp: June 16, 2015, 03:08:53 am
content: What about a 240VDC relay that can switch between a 240V indicator bulb (with a resistor in series) and a lower voltage solution when the voltage on its coil reaches a certain value?

author: ivan747
author_rank: Super Contributor
timestamp: June 16, 2015, 03:11:59 am
content: Isn't it easier to have a panel meter?

author: TerraHertz
author_rank: Super Contributor
timestamp: June 16, 2015, 03:13:23 am
content: Quote from: PA0PBZ on June 15, 2015, 07:53:25 pmI'd asked about blinking, because I was thinking of the old neon blinker circuit. Tektronix used that as a warning indicator on some of the HV rails in the 7000 series scope power supplies.It is possible to get neons that fire at lower than 50V. Around 35V I think.If it had to be an LED, then there's equivalent blinker circuits using UJTs or SCRs.Anything to avoid a small switchmode supply that can withstand 1000V (plus safety margin, say another 200V) and work over such a wide supply range. Though I guess, since the output could be a crudely current regulated 5mA into the LED and who cares about ripple, the control aspect would be OK despite the wide supply range..But the main switching component would have to be rated at least 1200V, and be at least a TO220 or bigger, for creepage distance between legs. Also, there'd be real problems overcoming G-D / B-E miller capacitance, to achieve sensible efficiency.Edit to add: By the way, if this indicator is for safety purposes, there's a serious problem with the concept since the indicator fails to the 'safe' indication, ie off. You *really* should have a manual test state, where an alternate supply forces the circuit to light, proving it's functioning correctly.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 16, 2015, 03:23:33 am
content: What about use "LED filaments", which are a lot of little LEDs in series?On a related note, there was a very interesting way to make a isolated HV sensor - a series of high value resistors, a small capacitor, a SIDAC or similar, and an optoisolator or pulse transformer to make a relaxation oscillator. Then measure the output frequency and it's nearly proportional to voltage as long as the input voltage is much larger than the breakover voltage of the SIDAC or whatever.

author: c4757p
author_rank: Super Contributor
timestamp: June 16, 2015, 03:45:47 am
content: Quote from: TerraHertz on June 16, 2015, 03:13:23 amTHIS.

author: Mr.B
author_rank: Unknown
timestamp: June 16, 2015, 04:09:24 am
content: Quote from: c4757p on June 16, 2015, 03:45:47 amDOUBLE THIS.I did a similar thing years ago.Bi Colour LED...Green for under 50V.Red for over 50V. Bus maximum was only 100VDC though.And...A big sign next to the indicator saying "If this indicator is off, assume the indicator has failed and not that the bus is dead!"

author: calexanian
author_rank: Super Contributor
timestamp: June 16, 2015, 05:43:40 am
content: Very simple solution. series 5 watt zener diode of say 60 to 100 volts with a large resistor. Drive your small LED from a divider network from across the zener. Work the divider network where the LED just turns on as the circuit gets above 50 volts. it will get brighter until it hits your 60 or so volt zener limit, then stay consistent up to your 1000 volts. Its not perfect, but its very simple.

author: Icarus
author_rank: Regular Contributor
timestamp: June 16, 2015, 07:43:39 am
content: How about blinking the LED ?If you are okay with this, try that:R + C + DIAC = Blinking LED. So you can choice any resistor value and doesn't required more than 1W.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 16, 2015, 07:47:31 am
content: 1) Use a neon,2) Use a white LED, which will give more light for less current.3) If it's for safety, use 2 Neons and 2 LEDs

author: mikerj
author_rank: Super Contributor
timestamp: June 16, 2015, 08:37:00 am
content: Quote from: calexanian on June 16, 2015, 05:43:40 amThat's simply a linear regulator, albeit shunt rather than series. Power dissipation is the main problem here.

author: Zero999
author_rank: Super Contributor
timestamp: June 16, 2015, 09:11:45 am
content: Keep the current low and spread the power dissipation.A linear constant current source may do.

author: SeanB
author_rank: Super Contributor
timestamp: June 16, 2015, 10:09:41 am
content: Going to be hard to get ultra low leakage and greater than 1kV rated transistors though, you might want to burn off 200V or so in a resistor as well, just for safety. Running a modern LED at 1mA will be bright as an indicator in any case, visible in anything other than direct sun.

author: Fungus
author_rank: Super Contributor
timestamp: June 16, 2015, 11:17:47 am
content: Quote from: SeanB on June 16, 2015, 10:09:41 am...except he wants it to start at 50V.It's not the 1000V that's the problem, it's the variability - 50V to 1000V is quite a wide range to work with.Me? I'd be thinking of a big voltage divider made with a chain of resistors (for safety) and the last resistor feeding into a comparator. Use the comparator output to light up the LED (with a transistor or whatever).Yes, it needs a separate low voltage power supply but maybe the OP's device already has one somewhere.As usual, we don't really know enough about the device to provide good answers.

author: TerraHertz
author_rank: Super Contributor
timestamp: June 16, 2015, 12:51:06 pm
content: Quote from: Fungus on June 16, 2015, 11:17:47 amIf there was a secondary power source, it's an _easy_ problem. Since the bus sensing can be through a very high impedance divider, to a FET input comparator.Another way would be to use a rotating mechanical vane electrostatic chopper, with a sense plate feeding a high impedance amp. AC signal out proportional to the bus voltage, with zero loading of the bus at all.But the design requirement says 'no alternative power source'.

author: max_torque
author_rank: Super Contributor
timestamp: June 16, 2015, 01:44:05 pm
content: Indeed, the device MUST be entirely BUS powered, no external power sources etc.As long as it uses a solid state light source and circuit, and probably 2 in series for some redundancy, i'm happy with the safety case.Daily checks on the system include checking that warning lamps work when the system is powered, and there are also analogue panel meters showing a voltage indication on the HV Bus as well.

author: Gyro
author_rank: Super Contributor
timestamp: June 16, 2015, 02:38:27 pm
content: Would it be sufficient to know whether the load is pulling current when the rail is on (ie, is it always loaded when on)?If so you could maybe use the voltage drop over a couple of series diodes (big or small) to drive the LED.Just a thought.

author: TerraHertz
author_rank: Super Contributor
timestamp: June 16, 2015, 06:57:51 pm
content: Quote from: max_torque on June 16, 2015, 01:44:05 pmWait...Are the meter(s) always connected to the bus?Are they moving coil meters, with series resistors to the bus? Resistors that probably get quite warm?Meter movement is 0-1mA? Or something like that?If all those are yes, just put the LED in series with the meter.Well, plus a few additional parts to ensure failsafe. Like a few chunky diodes in series, in parallel with the LED so that the meter still reads, if the LED went open circuit.That doesn't quite match your requirements, since the LED will light (dimly) below 50V.If dissipating 20W in a resistor at peak bus voltage is no problem, here's a simple circuit that works.The LED turn-on is fairly sharp, around +/-10% of the ideal point. Then LED brightness for therest of the range is relatively constant.If you need a very sharp turn on at the threshold voltage, it takes a 2 transistor circuit.This is far from a precision detector, and also I didn't attempt to temperature compensate it. For instance the Vf of the LED is part of the sensing system, and uncompensated.

author: BennVenn
author_rank: Regular Contributor
timestamp: June 17, 2015, 12:11:36 am
content: PTC's? You'll be wasting power dropping voltage regardless. This should give you a wider voltage envelope?

author: TerraHertz
author_rank: Super Contributor
timestamp: June 17, 2015, 02:16:59 am
content: Quote from: BennVenn on June 17, 2015, 12:11:36 amBut I think a PTC would make it very difficult to get a reasonably accurate LED turn-on threshold around 50V. Since the V-current relationship becomes non-linear and also affected by ambient temp.

author: BennVenn
author_rank: Regular Contributor
timestamp: June 17, 2015, 04:45:26 am
content: It can turn on below 50v though, right?

author: Simon
author_rank: Unknown
timestamp: June 17, 2015, 11:25:12 am
content: how about a comparator circuit to detect the ballpark voltage so that appropriate dropping resistors or zeners can be switched before a buck converter or linear current source.

author: Marco
author_rank: Super Contributor
timestamp: June 17, 2015, 12:03:52 pm
content: Going to need one hell of an inductor for a buck converter ... it's not like your transistor is going to switch very quickly.

author: Kalvin
author_rank: Super Contributor
timestamp: June 17, 2015, 12:12:48 pm
content: Dual-stage buck-converter?- 1. stage converts the 1000V ... +50V -> 48VDC.- 2. stage converts the 48VDC -> what ever the led requires.

author: Marco
author_rank: Super Contributor
timestamp: June 17, 2015, 04:08:26 pm
content: Nahh, don't bother for something this low power ... going to need a boost circuit though, to get drive voltage for the NPN/IGBT (which has to be slightly above the buck voltage, but preferably not 1000V above it).All annoyingly complex for something which feels like it should be simple.PS. I guess a GDT could work too, or SEPIC.PPS. to simply burn the power find one of those linear use IGBTs and make a constant current source.

author: TerraHertz
author_rank: Super Contributor
timestamp: June 17, 2015, 10:41:55 pm
content: Quote from: Marco on June 17, 2015, 04:08:26 pmYes.But... here's the inescapable logic:Design requirement #1: No secondary power is available. Just the 0-1000V bus.Implied requirement: circuit must have breakdown safety margin, so say 0-1200V intermittent.If you're going to go with simple, that means a resistor to convert the 0-1000V to current, and some circuit to sense the ON point and run the LED, given just that current and nothing else.But the indicator LED must definitely turn ON at 50V, and be definitely OFF below 50V.Lets say for a typical (white or blue) LED, ON requires 1mA.So that resistor must be no greater than 50V/1mA = 50K ohms.At 1000V that's going to be dissipating 20W. That's the unavoidable cost of going with 'simple'.The difficult part is getting the LED to go from OFF to ON sharply at 50V, given you have no spare current beyond the 1mA that needs to go through the LED once it's ON. The threshold sensing circuitry has to be powered by that same 1mA. So somehow it has to be in series with the LED. But the LED has to be OFF right up to the threshold point.Heh... it's a nice problem.If anyone can do better than the circuit I posted (last item on previous page) I'd be impressed.With more transistors, etc, a sharper threshold could be achieved. But is it worth it?If you go with 'complex' to try for better efficiency, then you are inescapably going to need a switching device that can withstand 1200V. But the circuit also has to run from _below_ 50V, if you're going to be properly sensing the 50V LED turn-on point. Also, with anything complex the sense point had better be sharp, accurate and drift-free, or what's the point?If I did a complex version of this, it would be a simple switchmode supply generating a power rail (say 12V) used to run a FET comparator. Bus voltage sensing via a precision high value resistor and precision reference voltage. An adjustable '50V' threshold, and small adjustable hysteresis. LED drive from the 12V supply. Since we've gone with complex, it should be able to drive really bright indicator(s) so say 50mA available.The painful part would be the switchmode. Providing adequate gate/base drive at startup (say 40V), and getting it to run over a supply range of say 40V to 1200V. While providing that 50ma at 12V. Hmm... or say 10mA at 12V while the bus is below 50V, since the indicator will be off. (Not counting current required by the switchmode itself.)Oh, and also it has to properly start up with sudden application of any voltage from 40 to 1200V, since the purpose is to provide a safety warning LED. And it has to be reasonably failure proof.The original request didn't mention space available, or desired manufacturing cost, but I imagine those would ideally be something like "This is for a control panel indicator. Small and cheap please." Good luck with that.Personally I'd chose simple, and live with the wasted 20W at peak voltage. Unless there's something about the application that makes that undesirable.

author: BennVenn
author_rank: Regular Contributor
timestamp: June 18, 2015, 12:19:03 am
content: Moving away from resistive current limiting, how about...(I’m not an E.E. so please correct me if my assumptions are incorrect)Heartley oscillator, ferrite coupled secondary - fed via a very large inductance to limit the current/voltage seen by the oscillator. Oscillator would need to be tuned to provide as close to a reactive? load as possible (when driven with higher voltage) and becoming less reactive as voltage drops - perhaps by core saturation techniques/biasing/frequency? to maintain a reasonable secondary voltage. From that point we have a low voltage that can be used to power traditional voltage sense/discrimination circuits?My assumption is, the inductor will be resistant to current flow assuming the load has very little dc current draw.Just thinking out loud...

author: TerraHertz
author_rank: Super Contributor
timestamp: June 18, 2015, 02:17:11 pm
content: Quote from: BennVenn on June 17, 2015, 12:11:36 amActually... on second thoughts PTCs could work well, if the sensing didn't rely on that current path.How about this? (see pic of general idea.)Uses a norton current mirror amp for the sensing, with a PTC in the supply current chain for the amp and the LED.But the actual threshold comparator is comparing precision current through a 1M resistor from the bus, against another current from the 5.6V zener (less the Vin of the amp.)The PTC and ordinary series resistor need to be worked out to give enough current at 50V to run the amp and LED. But then as Vbus rises, the PTC self-heats and doesn't allow current to rise very much. Just need to find a suitable PTC.So say the current can be kept below 5mA at 1000V; that's only 5W. Not too bad.One problem would be that while the PTC was hot, the circuit wouldn't light the LED for low bus voltages 50V and higher.Also of course, the "1M" resistor should actually be a series-parallel set of resistors to get 1M total but with redundancy and not stressing the resistors.Other things that circuit needs:- Threshold adjustment.- A small amount of hysteresis.- Use the other half of the 359 to drive a 'Voltage below 50V' LED.Or the same output with another LED to the +5Vrail.Oh, and the zener would have to be rated to survive sudden connection of the 1000V rail, through the cold PTC.Ha ha... Earlier today I had a go at doing a simple "few transistors" circuit to try and get a snappy, accurate transition. It's not so easy... Better than that first single transistor one, but still crap.

author: muvideo
author_rank: Frequent Contributor
timestamp: June 18, 2015, 03:57:01 pm
content: I would build a (crude) constant current sink using a mosfet to power upmore than one high brightness 5mm leds.Something like 1.5mA trough 3 leds probably is enough to have a good visibility.Stacking a pair of mosfets allows the use of cheap 600V parts, the powerdissipation would be something in the order of 3W at 1kV.The mosfets are exposed to full voltage (or half if there are 2 of them stacked),also some of the resistors will be exposed to full voltage and need to be dividedin 2 or more in series.Something like this:R1 Q1 set the current trough the leds.D2 summed to threshold of the mosfet (around 3 to 5V) and forward voltageof the leds set the starting voltage (around 50V).R5 R6 R8 R9 is the biasing network for M2 that puts the drain of M1 to abouthalf of the input voltage.R1R7 should be a fusible resistor rated for full voltage, or a series of resistorthat will blow up in case of short of the mosfets, saving the circuit from catching fire.

author: Marco
author_rank: Super Contributor
timestamp: June 18, 2015, 07:13:23 pm
content: Isn't running high voltage MOSFETs anywhere near their rating for any significant percentage of the time a bad idea due to cosmic rays? (ie. you have to derate them by half.)

author: muvideo
author_rank: Frequent Contributor
timestamp: June 18, 2015, 09:02:48 pm
content: Quote from: Marco on June 18, 2015, 07:13:23 pmInteresting, do you have any references?650Vx2=1.3kV, so they should be not near their limit, usually 80% derating is enough.AFAIK mosfets are able to cope with overvoltage breakdown fairly well, but you are rightthat the part chosen should have the SOA in the datasheet specified for DC operation,just to be on the safe side. The partnumber in my shematic was the first i found in LTSpice,FQPF2N70 or NDD03N80 for example have SOA specified up to DC.It should be noted that breakdown voltage goes down with temperature, so things get worseat low temperatures.

author: Marco
author_rank: Super Contributor
timestamp: June 18, 2015, 11:11:22 pm
content: Nothing other than what'son the web.Don't know of any actual numbers for the 600V range parts though. For 1kV range parts the numbers are scary enough I would just use IGBTs or BJTs out of principle though.