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

[Richie Burnett]

I guess if you keep the negative supply voltage under the zener voltage then 
you don't waste supply current driving the zener diode in each oscillator 
chip?

-Richie,

-----Original Message----- 
From: Tom Wiltshire
Sent: Sunday, November 19, 2017 10:51 AM
To: Michael E Caloroso
Cc: synth-diy mailing list
Subject: Re: [sdiy] 3340 oscillator - current limiting resistor equation is 
wrong?

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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