[sdiy] Vanilla electronics question

Thu Feb 22 10:24:09 CET 2024

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

[image: image.png]

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
>> >>> <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 <
>> 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
>> >>> <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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