Over voltage protection for LEDs in sculpture - Page 1
Forum section: electronics / projects

author: Factotum
author_rank: Contributor
timestamp: July 16, 2023, 08:56:15 am
content: Hey everyone.Over the past few years I've been building / refining / exhibiting a large scale sculpture called Whisper Tree. You can read all about it herehttps://whispertree.netThis forum post focuses on the basic electronics on each of the speakers. In essence it's two pairs of LEDs with current limiting resistors in parallel to small speaker which also has a resistor in series to increase the over all impedance (and get enough voltage to drive the LEDs at low volume). This board is mounted on the back of the speaker with the goal of generating some light output when there is audio. There are 8 of these units in a series / parallel arrangement on a small TPA3116 D-class amplifier module (go to the website for much more info, details etc).I eventually have the LEDs fail over time and know the circuit design could do with some revision. I suspect over voltage of the LEDs is the kiss of death as is exposure to reverse voltage.Adding a diode to each LED would solve the reverse voltage exposure.Is a zenner the optimum way to prevent over voltage?Figured there's enough info here to start a discussion!

author: tszaboo
author_rank: Super Contributor
timestamp: July 16, 2023, 09:10:12 am
content: LEDs usually fail due to high temperatures, because they are not 100% efficient, the rest of the electricity is turned into heat. So take a board, run it for a few hours and measure temperatures.That being said, with the long wires, it's possible that overvoltage is the issue. You don't want Zener diodes, you want special diodes called TVS diodes to protect it. They are designed for this. A 5V bidirectional TVS like a SMAJ5.0CA would protect an LED from surges and ESD.

author: Zero999
author_rank: Super Contributor
timestamp: July 16, 2023, 09:28:19 am
content: How can it be reverse voltage spikes? The LEDs are connected back-to-back, so the reverse voltage will be no more than the forward voltage, which will be below the maximum voltage rating.It will be overheating, or current spikes.

author: Someone
author_rank: Super Contributor
timestamp: July 16, 2023, 09:47:43 am
content: Quote from: Zero999 on July 16, 2023, 09:28:19 amOr regular old fashioned operating outside the device absolute maximums.Quote from: Factotum on July 16, 2023, 08:56:15 amA zener, in combination with other changes to the circuitry, might be a way to solve it.What are the supply voltages of the amplifier, and maximum voltage expected across the speaker?What LED parts are used?What thermal solution is used for the LEDs? (even 5mm LEDs can require thermal management)

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: July 16, 2023, 11:06:05 am
content: Hi Factotum,R1 and R3 look quit low (imho).When i read "Class-D" the neck-hairs start to stand up. =:-OMaybe there is too much hf-garbage on the wires.Do you have a scope on hand? Post some pictures of the output-signal (min./max.).A nonlinear Resistor (PTC) could be your sollution.Depending on your audio-signal might be not fast enough.Good luck

author: mikerj
author_rank: Super Contributor
timestamp: July 16, 2023, 12:31:28 pm
content: Those 10R resistors seem far too low in value, do you know what the peak-peak output voltage from the amplifier in? Even if you are powering the amplifier from it's minimum operating voltage I suspect it would be capable of driving many times more than the maximum LED current. Regular 5mm LEDs which are typically rated for 20-25mA continuous.

author: Factotum
author_rank: Contributor
timestamp: July 16, 2023, 01:05:08 pm
content: Additional info :In regard to reverse voltage - I figured if one LED (in a pair) failed than the other one getting exposed (to a reverse voltage) was inevitable. I also wasn't entirely sure that with the back to back configuration that it was perfect / ideal in removing the potential of reverse voltage.LED's are SMD2835 150mA (examplehttps://www.lcsc.com/product-detail/_TOGIALED-_C1322331.html) on large filled plane 1 oz FL- boards.As for final component values - I did initial empirical testing and found that I couldn't get enough voltage (to drive the LEDs) with 8 x 8R speakers in a series / parallel configuration (for a total of 4R which was the ideal impedance for the TPA3116 amp modules on a 24v DC supply) without deafening volume.So I built a set of resistor ladders and tried a few different configs (I don't have my notes at hand).Basically by bringing the impedance of each driver assembly to 40R (32R resistor and 8R driver) the total network was 20R which have me enough voltage (about 5v from memory) to drive the LEDs which I estimated with (Peak Voltage - LED Forward Voltage) / 0.150 to give me about 10R for the current limiting resistor.Again the "weakness" in this setup is the ease in which one can supply too much audio signal to an amp module thus delivering over voltage at the output.I don't remember doing in depth scope analysis but can do...There's 16 amplifier modules in total in the amp cabinet driving 112 speaker assemblies, a monitor unit and a subwoofer...

author: HwAoRrDk
author_rank: Super Contributor
timestamp: July 16, 2023, 05:49:33 pm
content: I think you are over-driving those LEDs.Not sure how you managed to calculate that 10 ohms value, but if your supply voltage is peaking at 5V (although is that +/-, pk-pk or RMS?) then 10 ohms will give you - with a typical LED forward voltage of 2.1V (per datasheet) - a current of 290 mA! And the LED will be dissipating 609mW! Themaximumratings for those LEDs is 150mA current and 390mW. You are driving them with almosttwicetheir rated current and power.Also, I hope those resistors are beefy, because they'll be dissipating 0.84W each too.I would suggest changing those 10 ohm resistors for something like 30 ohms. And ensure they're 0.5W rated parts (e.g. 1206/1210 size SMD).If you want to protect the LEDs against voltage spikes (either supply spikes or one failed-short LED in a pair), then you could use a bi-directional TVS diode in parallel with the LEDs - perhaps something like an SMAJ5.0CA. This will clip voltage spikes in either polarity.

author: DavidAlfa
author_rank: Super Contributor
timestamp: July 16, 2023, 07:03:33 pm
content: Class-D amplifiers are basically PWM, so you're not feeding smooth waves to these leds, more like hammering them, specially with such low resistors.A speaker would filter out the high frequency pulses, but leds won't, even when dim the leds are taking small pulses at max power.So you must take the max output voltage from the amplifier and calculate proper resistor values.

author: Someone
author_rank: Super Contributor
timestamp: July 16, 2023, 09:55:37 pm
content: Quote from: Factotum on July 16, 2023, 01:05:08 pmQuote from: Someone on July 16, 2023, 09:47:43 amQuote from: DatasheetIf any value in the absolute maximum ratings is exceeded, then you have no guarantee of function/performance. Even without exceeding those ratings you may damage the device (particularly for LEDs and other semiconductors as they are typically specified at unrealistic temperatures).Quote from: Factotum on July 16, 2023, 01:05:08 pmStep 1 is to know what worst case voltages could be present at the connection to your LEDs/speakers, then work backwards to ensure the devices are within their maximum ratings in that worst case.Quote from: Factotum on July 16, 2023, 01:05:08 pmWhich is probably evidence that driving LEDs directly from the same signals as speakers will not work aesthetically for your application. Easiest solution for you is to drive the LEDs with some other voltage you can control within a well defined range. LEDs are not linear in their output so won't match the variations in intensity of the sound coming from the speakers.There are a range of complicated solutions that would allow the LEDs to be driven from the same signal as the speakers but that's a fair bunch of engineering and needs specifications of what the input ranges could be and the desired LED intensities at those levels.

author: Factotum
author_rank: Contributor
timestamp: July 17, 2023, 03:14:27 am
content: Resistors are 2512 footprint and 2W each.I had a scan of the sheets of the other coloured LEDs used - Vf is 2.8-3.0 for most. 3.2-2.4 max.To be clear - 5v would be right up on the upper bound. The system was run outside in extreme heat (43C some days) for 18 hours a days for 20 odd days with only a small number of failures.My main goal is to deign a revision where I can easily drive the volume up (a recent exhibition had thousands of school kids creating a massive noise floor during some parts of the day) without overdriving the LEDs, yet still have them bright enough at the regular lower volume.Normally when I build commercial projects with light / sound they get driven with completely separate systems for this reason. This piece was a case of extremely limited budget and wanting to utilize the one amplifier channel / cable to do both an audio and visual component.Sadly most of my workbench (and sculptures) are away in storage (predatory landlords) so I've got limited gear. I do have my scope - will see if I can get an amp module and see how well / poorly the final LC stage deals with the PWM.Thanks to all for you input so far!

author: ArdWar
author_rank: Frequent Contributor
timestamp: July 17, 2023, 03:31:33 am
content: It might be easier to just use 3 wire with dedicated speaker and led drive lines. Audio/music program usually have quite large dynamic range (at least one not terribly compressed to oblivion) with more than 1:10 between RMS and peak power, that's why it's hard to strike a balance between presentable lightwork, usable SPL and manageable voltage excursion. You probably also want a peak detector circuit to modulate the lightwork instead of feeding the audio waveform directly.Using separate driver also relieves you from using ballast resistor for the speaker, allowing for a single amplifier to drive larger amount of speakers (you need to make it series-parallel connection to keep them within amplifier's output load impedance range)

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 17, 2023, 05:27:56 am
content: Quote from: Factotum on July 16, 2023, 08:56:15 amNo, you need to limit thecurrent, as the voltage across the LED follows a predictable current-voltage curve.As already mentioned 10R is a low series R for LEDs especially from a undefined peak voltage.There are LED current regulator diodes that start regulating from relatively low voltages, behaving like a J-FET.They do not reverse block, but two in reverse series would current regulate in both directions.They come in various current preset values, and some are adjustable.https://www.onsemi.com/products/power-management/led-drivers/linear-led-driversaddit: Parts like the Diodes inc AL5801 are examples of a BJT and FET as a current regulator.

author: Berni
author_rank: Super Contributor
timestamp: July 17, 2023, 05:35:52 am
content: Quote from: Factotum on July 17, 2023, 03:14:27 amWhat you need is a current source:https://www.electronics-tutorials.ws/transistor/fet-current-source.htmlThis way you can have a LED that turns on at a low voltage and keeps lit evenly even if you increase the voltage 10xMaximum voltage range is completely dependent on the specs of your JFET or depletion MOSFET. I used this kind of circuit to make a HV warning indicator LED for a 400V DC bus that stays lit for voltages between 10V to 400V. There are some depletion MOSFETs that handle 800V and up.The circuit definitely would not work for AC tho, so you would need to add a bridge rectifier to the input, but those can be had as a single 4 pin chip.

author: Factotum
author_rank: Contributor
timestamp: July 17, 2023, 09:26:04 am
content: Quote from: Berni on July 17, 2023, 05:35:52 amHaving the LEDs flash with the audio is part of the sculpture's atmosphere

author: Siwastaja
author_rank: Super Contributor
timestamp: July 17, 2023, 09:32:55 am
content: As others have said, just increase the 10R for lower LED current, try 100R for example. If the LEDs are too dim, buy brighter LEDs - either more efficient ones with better lumen (or candela, but then keep in mind that relates to beam angle, narrow giving more candelas out from same amount of light), or some which can handle higher current.

author: AndyBeez
author_rank: Frequent Contributor
timestamp: July 17, 2023, 10:41:33 am
content: @Factotum: Whisper Tree, loving the conceptFor the EEs. Is there a significant back EMF on the speaker winding when driven with harsh low frequency transients? Would input filtering or different sound design make a difference? Does it help if the resistors are on both sides of the LEDs at a lower value? Or how about a small capacitance across the diodes?+ one other thought, are those LEDs suffering leg corrosion? I have seen LEDs fail when corrosion passes along the legs and into the sealed head. It is not obvious to the unmagnified eye. The steel of the leg rusts underneath the tin plating.

author: Berni
author_rank: Super Contributor
timestamp: July 17, 2023, 12:02:40 pm
content: Quote from: Factotum on July 17, 2023, 09:26:04 amExactly, the LEDs do have a minimum forward voltage, so the circuit still can't magically turn on a LED at <2V. So they would still blink with audio (Unless you are playing a bass test over the speakers where it is full max volume the whole time). The FET also has a soft knee, so it needs a fair bit of voltage across it before the current becomes stable, so you would still get the LED slowly turning on as it gets louder just like with a resistor. Just that at very high volumes the current stops increasing as the current source comes into regulation. So from 0 to 2V you might get no LED, it might slowly turn on between 2V and 4V and from 4V to 100V it is lit constantly with the same current.If you want the LED to linearly go from 0% brightness to 100% brightness as the audio goes from 0V to 5V or from 0V to 20V when loud then you will need a lot more electronics. You would need a power savaging circuit that boosts the audio to a stable 3V where you have a MCU running and monitoring audio trough the ADC and then driving LEDs. Along with a storage capacitor to keep the supply stable trough the dips etc..

author: strawberry
author_rank: Super Contributor
timestamp: July 17, 2023, 05:02:04 pm
content: if PSU is 12V /10R = ~1.2Apk . 2WLED should be fine

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 17, 2023, 07:55:43 pm
content: Quote from: Factotum on July 17, 2023, 09:26:04 amYes, your simplest approach is to get some of the OnSemi LED current regulator parts I linked above.Those behave like simple resistors, up to the current regulation knee, above that, they protect the LED.examples:NSI45035JZ Adjustable Constant Current Regulator & LED Driver 45 V, 35 − 70 mA 15%, 1.5 W Packagefixed:NSIC2050JBT3G Constant Current Regulator & LED Driver 120 V, 50 mA 15%, 3 W PackageThe AC of Audio means each LED will average half that peak current, at highest volumes.

author: Zero999
author_rank: Super Contributor
timestamp: July 18, 2023, 05:36:26 pm
content: Quote from: Berni on July 17, 2023, 12:02:40 pmThe LED turn on voltage isn't an issue here, because the amplifier has a class D output, which gives PWM, which is filtered by the speaker's inductance. The LEDs just need the appropriate value series resistors, to limit the current to a safe level.

author: Someone
author_rank: Super Contributor
timestamp: July 18, 2023, 09:45:09 pm
content: Quote from: Zero999 on July 18, 2023, 05:36:26 pmDepends on the implementation as the specific amplifier chip used here has reference designs for fully filtered outputs that block energy above audible frequencies. The underlying issue, even if is was idealised PWM (which class D generally isn't), is that the intensity of the LEDs does not follow the audible intensity of the sound when they are connected simply in parallel, the OP wants to balance the intensity for their specific conditions (and then possibly be able to adjust that?).

author: Berni
author_rank: Super Contributor
timestamp: July 19, 2023, 05:05:28 am
content: Quote from: Zero999 on July 18, 2023, 05:36:26 pmA lot of the time D class amplifiers have a LC filter on the output, turning it into a glorified buck DC/DC. Sending the raw unfiltered 100Khz to 1MHz square wave out onto the speaker wires would emit a lot of RF noise and so it would make the product fail FCC and CE certifications. Also some tweeters might not like being fed that since those are typically connected in parallel with the main speaker but with a series capacitor to block low frequency. So as a result you do get a nice smooth looking analog waveform out of them.Typically they use unfiltered D class amplifiers for things like headphones where the powers are low, or when the speaker is installed in the same enclosure as the amplifier so you can grantee the wires being short and routed in a way that doesn't make them an antenna.

author: Factotum
author_rank: Contributor
timestamp: October 21, 2023, 02:19:26 am
content: Hey all - I'm back on this trail.Any thoughts on the attached schematic? The NSV50350AST3G is a 350mA unit, was going to used 100mA LEDs or similar. Generic bridge rectifier. Have left the "pad" resistor for the speaker (will test if needed). Was going for minimum part count (easier for a bridge than multiple current drivers).One failure mode I can see is if one of the LEDs is damaged there will be an over current scenario for the remaining units.I also started doing some testing with my 'scope on the output. Wasn't nearly as ugly as I thought (on the the current design) even on 15m+ leads. Will upload some captures if I get a chance.Thanks in advance!

author: Dan123456
author_rank: Regular Contributor
timestamp: October 21, 2023, 02:42:49 am
content: I have nothing to offer other than what others have already said. Just wanted to say that’s a pretty cool sculptureHope you get her sorted and would love to see it if it ever heads to the east coast

author: Factotum
author_rank: Contributor
timestamp: October 21, 2023, 03:09:12 am
content: Quote from: Dan123456 on October 21, 2023, 02:42:49 amI couldn't afford the cost (time or money) to bring it to Sculpture By The Sea Bondi 2023 - maybe another year. There's also been several stalled attempts to purchase it / exhibit it over East but sadly none have worked out so far.Another variant of this piece is planned to head to Burning Man in 2024 - again if we can get some funding...

author: Zero999
author_rank: Super Contributor
timestamp: October 21, 2023, 05:19:17 pm
content: R2 isn't needed, now the current regulator is there.

author: Factotum
author_rank: Contributor
timestamp: October 22, 2023, 07:30:27 am
content: Quote from: Zero999 on October 21, 2023, 05:19:17 pmMy plan was to simply leave the PCB pad there (and bridge it) in the possible event I did want to pad the speaker output so that there is a more pronounced light output from the LED relative to the speaker volume...