[sdiy] Vanilla electronics question

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Thu Feb 22 11:40:11 CET 2024 

I would throw it in the WEEE waste and buy a replacement 12V LED rear 
light!  There's a lot of things that just don't add up about that 
circuit...  Why they put the two LEDs in parallel for a start.

If you really want to try to make it work somehow I'd re-wire the two 
LEDs in series (assuming each LED package really is just a single red 
LED die) and then fit a suitable resistor to set the current through the 
series pair from your 12V supply.  Also make sure the LEDs and the 
series resistor have ample heatsinking to keep them at a nice low 
temperature.  If they get hot reliability will suffer.

-Richie,



On 2024-02-22 09:24, Rutger Vlek via Synth-diy wrote:
> Thanks everyone for sharing your views and hypotheses. Let me answer
> some of the recent questions that popped up:
> 
> - yes the design I described has been working for several years, and
> is a steady light (nothing slowly blinking)! It started failing
> intermittently and then completely during a rainy ride home.
> - as for the direction of D1, the diode marking on the device itself
> and on the underlying PCB are consistent with each other and as
> represented in the schematic.
> - even if D1 would mysteriously work backwards, making it a bog
> standard LED circuit with reverse polarity protection, the resistor
> value and power dissipation would be unfit for the circuit. The LEDs
> would run beyond their maximum current rating and the resistor would
> have to dissipate more heat than it's rated for. Both would burn out I
> presume. In fact, this might be what has happened, as D1 is close to
> the power inlet of the enclosure, where moisture might have creeped in
> and caused a short.
> - another clue to the mystery is the print on the enclosure: suitable
> for 6V up to 12V supplies. In all these conditions the LEDs should
> emit enough light to be legally accepted as a safe rear light, which
> means the circuit might need some kind of way to regulate voltage
> internally.
> 
> So I currently feel the most likely hypothesis is that D1 is some kind
> of zener or shunt regulator, even though it's labelled as a common
> Schottky Diode (see picture from the web with identical markings).
> 
> Anyway, the path to making this light work again is unclear to me.
> I've fixed the LED's themselves and I believe I could modify the PCB
> to operate them purely on the available 12V in a traditional approach
> with current limiting resistors. My only challenge: getting sufficient
> power dissipation for the resistors in the small format of the PCB.
> I'm thinking about parallelling resistors to increase their total heat
> dissipation to around 1W, which I believe is needed to accomodate
> about 65mA through both power LEDs. Any other suggestions?
> 
> Rutger
> 
> And then there's the print on the enclosure: suitable for 6V to 12V
> supplies. Om
> 
> Op di 20 feb. 2024 17:29 schreef Chromatest J. Pantsmaker
> <chromatest at chromatest.net>:
> 
>> Flashing the LED quickly can make it much more visible.  For a
>> safety device (bike light) this would be desirable.
>> 
>> On Tue, Feb 20, 2024 at 6:23 AM René Schmitz
>> <synth at schmitzbits.de> wrote:
>> 
>>> That's probably what was intended originally, the schematic symbol
>>> for
>>> schottky and zener are somewhat similar after all.
>>> 
>>> Absolutely nothing hints at a switching or oscillation here at
>>> all.
>>> 
>>> And again, a 60V device should not exhibit any form of negative
>>> resistance at 12V. (I also think it would not do that anywhere
>>> outside
>>> its reverse voltage range.)
>>> 
>>> I wonder, however why they would not do the dead simplest, lowest
>>> parts
>>> count thing: just a resistor (per LED).
>>> 
>>> Best,
>>> 
>>> René
>>> 
>>> Am 20.02.2024 um 13:13 schrieb rburnett at richieburnett.co.uk:
>>>> Most likely a Zener diode to drop some of the excess voltage.
>>>> 
>>>> -Richie,
>>>> 
>>>> 
>>>> 
>>>> On 2024-02-19 22:34, Ben Bradley via Synth-diy wrote:
>>>>> From the schematic I'm pretty sure that one "diode" or whatever
>>> it is
>>>>> has to have a range of negative resistance for this thing to
>>>>> oscillate, which I'm sure is how it works. I've not heard of
>>> schottky
>>>>> diodes/rectifiers having negative resistance in their reverse
>>>>> direction,  but then I don't know everything. I don't see
>>> anything on
>>>>> the SS16 datasheet that looks like a negative resistance.
>>>>> 
>>>>> I wonder if this could be an SCR with an internal resistor
>>> between the
>>>>> gate and anode, so that it triggers at something below 12
>>> volts. The
>>>>> current through the LEDs would have to be below the "turnoff"
>>> current
>>>>> of the SCR, and that seems questionable.
>>>>> 
>>>>> Decades ago (1950s-1960s)  a tunnel diode was popular as an
>>> oscillator
>>>>> in the VHF/UHF region, but apparently went obsolete when
>>> transistors
>>>>> were made to operate at those frequencies. Electrically this
>>> could be
>>>>> a tunnel diode, but looking on Digikey and Mouser shows no
>>>>> possibilities for anything called a tunnel diode.
>>>>> 
>>>>> As mentioned, a unijunction transistor is another possibility,
>>> but
>>>>> Mouser only shows a few, at over a dollar each in any quantity,
>>> and
>>>>> made by Central Semiconductor, which is apparently a source of
>>>>> otherwise-obsolete semiconductors much like Rochester.
>>>>> 
>>>>> Something like an SCR would be my best guess, but I'm not
>>> confident
>>>>> in that.
>>>>> 
>>>>> On Mon, 19 Feb 2024 at 11:07, Tony Mowbray
>>> <tmowbray at ihug.com.au> wrote:
>>>>>> 
>>>>>> Actually I think it runs as an oscillator so the operation of
>>> the
>>>>>> LEDs is pulsed to allow high current for a short period of
>>> time.
>>>>>> 
>>>>>> But then again, I could be way off
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> Regards Tony,
>>>>>> 
>>>>>> Sent from Samsung tablet.
>>>>>> 
>>>>>> 
>>>>>> -------- Original message --------
>>>>>> From: René Schmitz <synth at schmitzbits.de>
>>>>>> Date: 19/2/24 23:41 (GMT+10:00)
>>>>>> To: synth-diy at synth-diy.org
>>>>>> Subject: Re: [sdiy] Vanilla electronics question
>>>>>> 
>>>>>> 
>>>>>> Only: SS16 has 60V reverse voltage capability. Probably with
>>>>>> considerable slack until it actually breaks down. I can't see
>>> how it
>>>>>> would get into a breakdown with 12V.
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> The reverse leakage current of this device would be in the
>>> order
>>>>>> where you can see the LED light up. (0.4mA)
>>>>>> 
>>>>>> IMO it's an attempt to build a current limit.
>>>>>> 
>>>>>> 
>>>>>> Best,
>>>>>> 
>>>>>> René
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> Am 18.02.2024 um 00:34 schrieb Mike Bryant:
>>>>>> 
>>>>>> Pure guess ... more like a breakdown current.  Sort of like a
>>> Zener
>>>>>> ... but not quite.
>>>>>> 
>>>>>> Conversely there may be a better explanation ... just I can't
>>> think
>>>>>> of one.
>>>>>> ________________________________
>>>>>> From: Chris McDowell <declareupdate at gmail.com>
>>>>>> Sent: 17 February 2024 23:16
>>>>>> To: Mike Bryant <mbryant at futurehorizons.com>
>>>>>> Cc: grant musictechnologiesgroup.com [1]
>>>>>> <grant at musictechnologiesgroup.com>; Rutger Vlek
>>>>>> <rutgervlek at gmail.com>; SDIY List <synth-diy at synth-diy.org>
>>>>>> Subject: Re: [sdiy] Vanilla electronics question
>>>>>> 
>>>>>> wacky. Is it the leakage current of D1 that allows C1 to do
>>> anything
>>>>>> in the first place?
>>>>>> 
>>>>>> Cheers,
>>>>>> Chris
>>>>>> 
>>>>>> On Feb 17, 2024, at 4:59 PM, Mike Bryant
>>>>>> <mbryant at futurehorizons.com> wrote:
>>>>>> 
>>>>>> 
>>>>>> Yes fairly sure D1 is operating in some form of avalanche
>>> mode,
>>>>>> turning the LEDs on for a short while before it recovers and
>>> turns
>>>>>> them off again.  Could be at 100Hz, 1Hz or even 0.1Hz though,
>>>>>> depends on C1.
>>>>>> 
>>>>>> As for it not working, it's possibly because D1 is borked from
>>> being
>>>>>> used like this.
>>>>>> ________________________________
>>>>>> From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of
>>> 
>>>>>> Rutger Vlek via Synth-diy <synth-diy at synth-diy.org>
>>>>>> Sent: 17 February 2024 18:35
>>>>>> To: grant musictechnologiesgroup.com [1]
>>> <grant at musictechnologiesgroup.com>
>>>>>> Cc: SDIY List <Synth-diy at synth-diy.org>
>>>>>> Subject: Re: [sdiy] Vanilla electronics question
>>>>>> 
>>>>>> Here's a schematic of the circuit. These are really all the
>>>>>> components. The entire PCB is really no more than twice the
>>> size of
>>>>>> my finger tip. The capacitor value is unknown.
>>>>>> 
>>>>>> <image.png>
>>>>>> 
>>>>>> 
>>>>>> Op za 17 feb 2024 om 18:58 schreef grant
>>> musictechnologiesgroup.com [1]
>>>>>> <grant at musictechnologiesgroup.com>:
>>>>>> 
>>>>>> Once you all get consensus on the circuit, any chance us in
>>> the
>>>>>> peanut gallery can get some kind of schematic so we can follow
>>> 
>>>>>> along? Thanks!
>>>>>> 
>>>>>> GB
>>>>>> 
>>>>>> 
>>>>>> ------ Original Message ------
>>>>>> From "Rutger Vlek via Synth-diy" <Synth-diy at synth-diy.org>
>>>>>> To "SDIY List" <Synth-diy at synth-diy.org>
>>>>>> Date 2/17/2024 7:48:43 AM
>>>>>> Subject [sdiy] Vanilla electronics question
>>>>>> 
>>>>>> Dear list,
>>>>>> 
>>>>>> Against my wishes, I'm working on repairing my eBike's rear
>>> light,
>>>>>> instead of building something synth related. However, I'm
>>> stumbled
>>>>>> by a seemingly simple circuit that just does not behave as I
>>>>>> expected. Of course I considered buying a new rear light, but
>>> for
>>>>>> the sake of the planet (and my own knowledge gathering), I'm
>>> trying
>>>>>> to revive the original one.
>>>>>> 
>>>>>> The circuit is simple, I'll try to describe it. Positive
>>> voltage
>>>>>> (appox. 12V) from the bike's battery enters two red,
>>> parallelled
>>>>>> power LEDs (in forward direction), goes through a 150 ohm
>>> resistor,
>>>>>> and then through what seems like a Schottky diode (SMD package
>>> 
>>>>>> labelled SS16) in reverse (!), non-conducting direction to
>>> ground.
>>>>>> The point before the SS16 is also connected via a capacitor to
>>> the
>>>>>> positive supply.
>>>>>> 
>>>>>> I replaced the LEDs and they are testing fine now (didn't
>>> before).
>>>>>> From the start I assumed the SS16 (presumed Schottky diode) to
>>> be
>>>>>> there for reverse voltage protection, until today... when I
>>> looked
>>>>>> at the direction in which it's placed. Moreover, the
>>> resistance and
>>>>>> power dissipation capacity of the resistor don't seem to fit a
>>> 
>>>>>> traditional LED circuit. In other words, the resistor would
>>> run the
>>>>>> LEDs very close or over their maximum current rating, and
>>> would burn
>>>>>> out due to the heat it had to dissipate while doing so.
>>>>>> 
>>>>>> So....can anyone tell me, what is this circuit? If I had to
>>> guess,
>>>>>> it's either:
>>>>>> (1) an attempt at a traditional LED circuit by someone who
>>> messed-up
>>>>>> badly
>>>>>> (2) some kind of switching voltage regulator (the rear light
>>>>>> indicates to be capable of handling a supply anywhere between
>>> 6V to
>>>>>> 12V). So I'm guessing it's the latter.
>>>>>> 
>>>>>> If moderation feels this is way off topic, I can completely
>>>>>> understand. Although I'm interested to learn about this
>>> circuit, as
>>>>>> it perhaps could find its way into synthesizers too.
>>>>>> 
>>>>>> Rutger
>>>>>> 
>>>>>> 
>>>>>> 
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>>>>>> 
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>>>>>> --
>>>>>> --
>>>>>> synth at schmitzbits.de
>>>>>> http://schmitzbits.de
>>>>>> 
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>>>>> 
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>>> --
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>>> 
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> 
> 
> Links:
> ------
> [1] http://musictechnologiesgroup.com
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