Analog Delay/Zener Diode?

[Joe Kramer]

Hi Friends,

I've just finished perfing an analog delay and--it works!  Yay!  The circuit is mostly conventional--compander, filters, MN3205 BBD, MN3102 clock, etc.  My questions concern the zener diode that regulates the voltage to the BBD and clock.  With gratitude to GGG and godiksennet.com, here's the AD-3208 (Scott Swartz) and the Boss schemos for reference:

http://www.generalguitargadgets.com/index.php?option=content&task=view&id=140&Itemid=156

http://www.godiksennet.com/default.asp?main=Schematics

On the AD-3208, the designer has used a 1N4738 8.2 v zener.  This seems a wise choice since the MN3205 chip data specifies 9 volts as the max supply voltage.  The downside of this is, with a new battery  the zener is wrenching the voltage down and drawing a whopping 14 mA in the process.  Of course the current draw dips down to somewhere around 6 mA when the battery drains a bit, but since I intend to use batteries only, I'm trying to save every mA I can.  This seems a waste of valuable power.  Now take a look at the Boss schemo:  D4 is an 11v zener.  Since that's a good 2 volts over the BBD max, here come the questions:  Is the MN3205 really in danger of destruction from a little over 9 volts (i.e., a very fresh battery)?  Can I forego the power-wasting zener altogether, or at least go with a higher value, say 9.1 v?    Any help much appreciated!

Regards,
Joe

[Truman Barksdale]

Hello Joe.

The absolute maximum voltage of the MN3207 is rated at +11V.  Obviously, no one wants to run anything at the absolute max, but I'd say 9.1 or 9.2V wouldn't be too much of an insult to the IC. 

T. Barksdale, III

[johngreene]

I wouldn't sweat it. If you are really worried about it, you can do something like this:


It will give you the zener voltage - .6 volts but you can use a higher value resistor for the zener so it draws less current.

--john

[Joe Kramer]

Greetings, uh, Truman, and welcome, um, to the forum.  You are correct about the 3207, and the 11v max applies to the 3205 as well.  Typical of me to misread the datasheet--the 9v max refers to the Vdd and the 11v max refers to the Vcc.  Thanks.  I hesitate to say I owe you one, and only you, me, and our mutual friend would know why.   

Mr. Greene: Thanks for the nifty little circuit!  And you need not read anything into the immediately preceding exchange.  Barksdale will be Barksdale, and there's little we can do about it. 

Lab Report: took out the zener altogether with no smoke and no audible change in circuit function except that it may be a tiny bit quieter.  Current draw is now a very acceptable 7.5 mA with a fresh battery.


Regards,
Joe

PS: Be aware, Barksdale, the "Report to Moderator" button is easily accessible.  No funny business.   

[DiyFreaque]

Hey Joe,

Looks like Truman cracked the log-in on my computer while I'm stuck out of town.  I hope it doesn't turn out like last time.....

Anyway, yeah, I'd say John has a good solution there.  It's not uncommon for 78XX regulators to vary as much as you're talking about here, and I haven't been shy using those with BBD's.  Well, I should mention the BBD's were relatively cheap and easily replaceable MN/BL3207's. 

Can't wait to hear samples of the delay.  If you send any before I get back, I can still intercept them before TB gets hold of them and pirates those too.

Cheerio,
Scott

Stuck in Libertyville with the Wichita blues again.  Or is that As Falls Libertyville So Falls Wichita Falls?  No, that's not it.  Ton, I'm cruising for those postcards.....

[Joe Kramer]

Scott,

When I saw Barkdale's name on the post, an indescribable sensation went up my spine (hilarious laughter).

No delay sample as of yet.  I won't tease you with descriptions of how wonderful it sounds . . . oops.  At work on finishing the machining aspects of the pedal first.  Hope The Clock runs double fast for you while out of town and double slow on your return.

Joe