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Zener Ckt HELP

Posted: Wed Jan 26, 2005 11:01 pm
by mikeman
I,ve been looking to design a LOW current Zener reg ckt...the more I read the muddier the Equasion gets. I'm looking to use a WallWart(rated 9v 500ma) to power a mic. phantom power supply that is 9v batt pwr'd. The open ckt V of 'wart is 13-14v...I only need [email protected]~25ma at batt connector...Pls Help...
Mike :confused:

Re: Zener Ckt HELP

Posted: Thu Jan 27, 2005 7:10 am
by Engineer1138
You need to drop ~13V to 9V, so that's about 4V across a dropping resistor. That resistor needs to carry at least 25mA for your mic, so its maximum value should be 4/0.025= 160ohms.
A 150 ohm resistor is a close standard value and will allow 27mA to flow.
The resistor will also need to dissipate V*I watts, so that's 4 * 27mA= ~107mW. A 1/4W resistor will be fine.<p>Note that since the wall wart isn't actually rated at 13V, this calculation will be a bit off (at 13V it may not be able to supply the current), but experience tells me it's close enough to work. Worst case you may have to use a 120ohm resistor instead.<p> <blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by mikeman:
I,ve been looking to design a LOW current Zener reg ckt...the more I read the muddier the Equasion gets. I'm looking to use a WallWart(rated 9v 500ma) to power a mic. phantom power supply that is 9v batt pwr'd. The open ckt V of 'wart is 13-14v...I only need [email protected]~25ma at batt connector...Pls Help...
Mike :confused:
<hr></blockquote>

Re: Zener Ckt HELP

Posted: Thu Jan 27, 2005 7:17 am
by philba
out of curriosity, why do you want to use a zener? <p>Here's the basic circuit:
Image
Wattage calcs are:
Power = ((Vin-Vout)/R1 - Iout)*Vzener
some current needs to flow through the zener so pick R1 such that (Vin-Vout)/R1 > Iout. In this case R1 needs to be less than 150. I'd use the next smaller standard value.<p>Pick a zener that has at least 25% (and preferably more) over that calculation for safety. Make sure the resistor wattage is ok, too.<p>If it was me doing this, I'd just use a 7809 family device in a to92 package. They are usually very cheap. a couple of smallish caps and you'd be good. Of course, the wall wart needs to have a 25mA loaded V of about 11V though you could probably squeeze by with 10.5 or so. Use a 1/4 watt 360 Ω resistor to load test the WW.<p>edit: fixed resistor info...<p>[ January 27, 2005: Message edited by: philba ]</p>

Re: Zener Ckt HELP

Posted: Thu Jan 27, 2005 7:24 am
by viveguy
You can use a fixed, 3 terminal regulator for this. Then hang a pre-load resistor about 560 ohms @ .5W across the output. This should keep the voltage from floating high.

Re: Zener Ckt HELP

Posted: Thu Jan 27, 2005 7:25 am
by philba
engineer, we past in the postings...<p>The problem with a dropping resistor (which will work in many cases) is that the WW may not be filtered and thus introduce 60/120 cycle hum or worse. <p>Check the WW, if it has a dashed line over a straight line somewhere on it that means it is just a transformer plus a rectifier. You will want to use more than droppping resistor. <p>Oddly enough, that kind of WW is called a "pulsed mode" adaptor. Kind a fancy way of saying "crappiest power supply you can get". :D <p>Phillipe<p>edit to fix raging stupidity...<p>[ January 27, 2005: Message edited by: philba ]</p>

Re: Zener Ckt HELP

Posted: Thu Jan 27, 2005 7:51 am
by Chris Smith
Because wall transformers are fairly steady, and because your draw is 1/20 of the total available current, you can set up a simple voltage divider to drop and limit the current draw to your source. Center tap these two resistors for your source.<p> If there is noise or ripple, you can introduce filters after the circuit. <p>Two resistors in a bridge is all you should need.<p> If your worried about voltage spikes, you can place a zener in with your output leg to ground, at the divider out put to ground. This limits the zener to only spikes, and keeps it off when its not needed.

Re: Zener Ckt HELP

Posted: Thu Jan 27, 2005 8:20 am
by Engineer1138
Good point, philba. I automatically put input power filter caps on everything I design, so I didn't even think of it here. <p>I guess there can be a downside to good habits ;)

Re: Zener Ckt HELP

Posted: Thu Jan 27, 2005 8:58 am
by philba
lol, I completely forgot the obvious. Look inside the device you want to power. Is there a regulator already? If so, you can ignore all the above stuff and hook it up directly with maybe the addition of a filter cap.

Re: Zener Ckt HELP

Posted: Thu Jan 27, 2005 11:19 am
by jimandy
Here’s a naive comment on my part, but perhaps worth considering in the design criteria.<p>If the mic requires power it most likely is a “condenser” mic and, besides powering a small internal amp, the source is used as a potential across elements of a capacitor which when vibrated by sound pressure causes fluctuations that translate sound into audio signal. Might this affect parameters of the DC supply, like maybe source impedance???<p>Ouch, composing this made my head hurt!

Re: Zener Ckt HELP

Posted: Sat Jan 29, 2005 9:28 am
by L. Daniel Rosa
mikeman, as everyone is pointing out there are lots of ways to do what you want, few of which are what you actually asked for. It sounds like you just want something that will do the job. Perhaps the best method is one that uses only the parts on hand (which may be why you asked about a zener regulated supply).<p>Do you have a 9.1V zener and a mess of resistors? A 220uF capacitor, diode(s), smaller capacitors, maybe an NPN transistor (an improvement)? If you don't have the zener but you have a 7805 and few LEDs and ordinary diodes then that'll work too. The greatest design in the world won't thrill you if it's more trouble than just not having it at all.

Re: Zener Ckt HELP

Posted: Sat Jan 29, 2005 11:31 am
by peter-f
<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by philba:
out of curriosity, why do you want to use a zener? <p> ... <p>If it was me doing this, I'd just use a 7809 family device in a to92 package. They are usually very cheap. a couple of smallish caps and you'd be good. Of course, the wall wart needs to have a 25mA loaded V of about 11V though you could probably squeeze by with 10.5 or so. Use a 1/4 watt 360 Ω resistor to load test the WW.<p>[ January 27, 2005: Message edited by: philba ]<hr></blockquote><p>[my thoughts, exactly]
What's in the 'vampire'? How does it control 13 Vdc?
I've rebuilt a few old automobile Cigarette lighter power supplies by replacing the 7809 w/ 7805 (for example) with little work and great success.<p>Wall warts are plentiful... can you fine an old or surplus or returned (ex: Nokia) battery charger at the local cell-phone shop? That's 3/4 of what you want! Probably well filtered and regulated.<p>(I often tell myself: re-examine the path you're taking to the solution... forget the junk you have, no matter how seemingly useful!)<p>Enjoy- and hope your results are good!