[sdiy] 3340 oscillator - current limiting resistor equation is wrong?

[Tom Wiltshire]

Yeah, there are several examples of synths that just run the chip on +V/-5V supplies instead. SH101 is another. It’s a perfectly valid way to go.

T.

==================
       Electric Druid
Synth & Stompbox DIY
==================

> On 19 Nov 2017, at 01:09, Michael E Caloroso <mec.forumreader at gmail.com> wrote:
> 
> FWIW, the Memorymoog voicecard replaced the series resistors with an
> onboard (-)5VDC rail.  The pads for the resistors were always on the
> voicecard.
> 
> Oberheim also elected to use a (-)5VDC rail.
> 
> MC
> 
> On 11/18/17, Tom Wiltshire <tom at electricdruid.net> wrote:
>> Ok, so this is why you choose the next value down, rather than the next
>> value up.
>> 
>> Hence 820R not 1K.
>> 
>> Thanks Neil.
>> 
>> Tom
>> 
>> ==================
>>       Electric Druid
>> Synth & Stompbox DIY
>> ==================
>> 
>>> On 18 Nov 2017, at 20:38, Neil Johnson <neil.johnson71 at gmail.com> wrote:
>>> 
>>> Tom,
>>> 
>>> If you read the datasheet carefully you see that all the resistor is
>>> doing is limiting the current for the internal zener diode.  So the
>>> actual current is not that critical, it just needs to be enough to
>>> bias the diode into its zener region.  The equation in the datasheet
>>> suggest this needs to be at least 8mA, more (but not too much) is also
>>> ok, but less may lead to problems.
>>> 
>>> I would go with the E12 series explanation.
>>> 
>>> Neil
>>> 
>>> On 18 November 2017 at 17:44, Tom Wiltshire <tom at electricdruid.net> wrote:
>>>> 
>>>>> On 18 Nov 2017, at 16:20, Florian Anwander <fanwander at mnet-online.de>
>>>>> wrote:
>>>>> 
>>>>> On 18.11.17 16:17 , Pete Hartman wrote:
>>>>>> 
>>>>>> Could it be as simple as the closest E12 value without going over?
>>>>>> 
>>>>> 1k would be much closer to 975R than 820R
>>>> 
>>>> I agree, and for a current *limiting* resistor, I’d have thought you
>>>> wanted a slightly larger value to be on the safe side, rather than a
>>>> slightly smaller one.
>>>> 
>>>> So no-one has ever come across this before?! Seems hard to believe.
>>>> 
>>>> I notice both the V3340 and AS3340 data sheets say only “current limiting
>>>> resistor required for supplies greater than -6V” without actually
>>>> providing any equation or committing themselves to a figure!!
>>>> 
>>>> Tom
>>>> 
>>>> ==================
>>>>      Electric Druid
>>>> Synth & Stompbox DIY
>>>> ==================
>>>> 
>>>> 
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>>> 
>>> 
>>> --
>>> http://www.njohnson.co.uk
>> 
>> 
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