Zener design question

Frequent question abouts LED and other types of diodes.
Post Reply
User avatar
mlp-mx6
Information
Posts: 23
Joined: 22 Jan 2016, 16:43
Has thanked: 8 times
Been thanked: 1 time

Post by mlp-mx6 »

I'm questioning the value of using a zener as the polarity insurance diode in a pedal, as is done with any design I've seen that has the zener as the way to "regulate" the voltage to X volts (esp. with charge pumps). I see this with the cathode on the power input and the anode to ground. There is no current-limiting resistor to enable the zener to actually control the voltage - because (I'm guessing) you don't want the voltage drop when the input voltage is close to (or below) the zener voltage (like battery power). But without that CLR the zener *cannot* dump any material voltage to ground - for example, if your external power supply supplies 10.8V with *any* appreciable current capacity then the zener *cannot* keep up, and the input voltage will be the voltage. Where this can really hurt is any circuit with a charge pump. Every charge pump I've looked up that is in regular use in pedals has 12V (or less) as the MAX voltage. If your 12V regulator is on the high side of tolerance (I've seen many 7812 regulators output 12.9V) then your zener will probably fail and/or the charge pump will get HOT because it is being operated out of range.

Is this analysis accurate? What am I missing? Is there a good solution?

Thanks,
Michael

User avatar
mictester
Old Solderhand
Information
Posts: 2921
Joined: 11 Sep 2008, 20:29
my favorite amplifier: Mesa Boogie, Roost Sessionmaster, AC30
Completed builds: Hundreds! Mostly originals, a few clones and lots of modifications.
Location: Somewhat closer to Amsterdam than before!
Has thanked: 32 times
Been thanked: 841 times
Contact:

Post by mictester »

Just for the sake of amusement, I got a small pile of various manufacturer's 7812 and 78L12 regulators. I fed them from my bench supply at 16V, loaded the output with an LED in series with a 2k2 resistor (the first value that came to hand, and measured the output voltages with an accurately calibrated Solartron desk multimeter.

Out of 40 tests (I got bored) none gave an output of more than 12.13V. None were lower than 11.97V.

I don't know where you got your "12.9V" 7812s from, but I suggest that you return them as faulty or you get your meter recalibrated!

A Zener is a really poor way to give over-voltage and polarity protection. I use an isolated input socket feeding a bridge rectifier input, so idiots can't plug in a "reversed" supply. The output of the bridge goes into a 7812 (or whatever other voltage I require, with the body tab screwed to the effect case, and the screw used as the star point for earthing inside the box. If some idiot connects up an AC supply, there won't be any damage, but there will be a loud hum at the output!
"Why is it humming?" "Because it doesn't know the words!"

User avatar
deltafred
Opamp Operator
Information
Posts: 1654
Joined: 06 Apr 2010, 16:16
Location: England
Has thanked: 813 times
Been thanked: 306 times

Post by deltafred »

mictester wrote:get your meter recalibrated!
This ^

Hobby multimeters give you an indication not a measurement. If you want to measure things you need to get them calibrated to a traceable standard, and then get them recalibrated regularly.

On 2 of mine if the battery voltage starts to fall then the readings go up. It caught me out the first time it happened.
Politics is the art of so plucking the goose as to obtain the most feathers with the least squawking. - R.G. 2011
Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012

User avatar
mlp-mx6
Information
Posts: 23
Joined: 22 Jan 2016, 16:43
Has thanked: 8 times
Been thanked: 1 time

Post by mlp-mx6 »

Sorry, that was a typo - should've been 12.1V. I'd correct the original post but that would make your reply seem bizarre. I'm certain that my meter is fine, though probably not to the standard of your desk meter.

As to the zener for polarity question - that is part of what I was asking. More than that, though, is the question about charge pumps and max voltage sensitivity. It seems to me that if you're going to run a charge pump then you should use a real regulator as you mention, or just stick to batteries.

Thanks for the education - I'm quite experienced in tube amps but new to pedals.

What triggered this question was I built a "Catch-22" from Grind (Timmy-ish circuit, but with a charge pump). I have an old DOD supply (form the very early 90's) that gives over 11V and it was getting the charge pump chip quite hot. It has a 9.1V zener for polarity protection but that was doing nothing. I'm not even sure if the zener is still good. So it seems to me that I have to be careful w/r/t the input voltage. Time for a new power supply.

User avatar
mictester
Old Solderhand
Information
Posts: 2921
Joined: 11 Sep 2008, 20:29
my favorite amplifier: Mesa Boogie, Roost Sessionmaster, AC30
Completed builds: Hundreds! Mostly originals, a few clones and lots of modifications.
Location: Somewhat closer to Amsterdam than before!
Has thanked: 32 times
Been thanked: 841 times
Contact:

Post by mictester »

You might not need a new PSU - just get a 7809 regulator IC. It'll give (almost) exactly 9V out for anything over about 11V in. Remember to bypass the regulator very close to the pins (I usually put 100n from input to ground and output to ground). I've even been known to use surface-mounted capacitors and solder them to the legs of the regulator IC where they emerge from the body of the chip!
"Why is it humming?" "Because it doesn't know the words!"

User avatar
Dirk_Hendrik
Old Solderhand
Information
Posts: 4189
Joined: 03 Jul 2007, 08:44
Location: Old Amsterdam
Has thanked: 232 times
Been thanked: 886 times
Contact:

Post by Dirk_Hendrik »

mictester wrote: Out of 40 tests (I got bored) none gave an output of more than 12.13V. None were lower than 11.97V.
I take may hat off. I would have gotten bored after 2 or 3 already.
Sorry. Plain out of planes.

http://www.dirk-hendrik.com

User avatar
mictester
Old Solderhand
Information
Posts: 2921
Joined: 11 Sep 2008, 20:29
my favorite amplifier: Mesa Boogie, Roost Sessionmaster, AC30
Completed builds: Hundreds! Mostly originals, a few clones and lots of modifications.
Location: Somewhat closer to Amsterdam than before!
Has thanked: 32 times
Been thanked: 841 times
Contact:

Post by mictester »

I just lashed up a test rig on a plug-in breadboard, and plugged in some of the 7812s (and 78L12s) that I had in stock. Each test took just a couple of seconds - plug in, note voltage, unplug, next! I did it because I'd never really considered the accuracy of the 3-terminal regulators - I just slap them into a circuit and assume that they're pretty accurate, especially since the absolute voltage doesn't usually matter: +/- 0.5V (or even more) doesn't really matter.
"Why is it humming?" "Because it doesn't know the words!"

User avatar
Dirk_Hendrik
Old Solderhand
Information
Posts: 4189
Joined: 03 Jul 2007, 08:44
Location: Old Amsterdam
Has thanked: 232 times
Been thanked: 886 times
Contact:

Post by Dirk_Hendrik »

Which makes me think.
I dont care too much about the exact outpu voltage of that regulator if it's within say +/- 10% of the stated voltage. I would care however, if that output voltage would fluctuate. Because that's the situation in which one cannot base voltages like biases on that supply voltage.
Sorry. Plain out of planes.

http://www.dirk-hendrik.com

User avatar
mictester
Old Solderhand
Information
Posts: 2921
Joined: 11 Sep 2008, 20:29
my favorite amplifier: Mesa Boogie, Roost Sessionmaster, AC30
Completed builds: Hundreds! Mostly originals, a few clones and lots of modifications.
Location: Somewhat closer to Amsterdam than before!
Has thanked: 32 times
Been thanked: 841 times
Contact:

Post by mictester »

Just for my amusement, (and my education) I tried a random sample of various 12V regulators (several different manufacturers) fed from a 16V supply, and tried varying the load on the output to see how much the output voltage changed. I'm happy to report that the 10 7812 devices I tried all stayed within 20mV over the output current range of 30mA to 900mA. The biggest differences between devices of various brands was the output noise - usually well filtered by the bypass capacitors. Other than that, they all performed pretty much the same.

I also tried varying the input voltage, but as long as I was over the specified operational threshold, the output Voltage remained constant.
"Why is it humming?" "Because it doesn't know the words!"

User avatar
Ice-9
Degoop Doctor
Information
Posts: 1094
Joined: 31 Mar 2009, 15:29
my favorite amplifier: Marshall
Completed builds: Far to many to afford the enclosures for them all. :)
Has thanked: 134 times
Been thanked: 359 times
Contact:

Post by Ice-9 »

Buy or build one of these 'High Precision Voltage Reference circuit' to check and set your DMM , you can buy or build one for less that £4 using an AD584 IC
http://www.ebay.co.uk/itm/Brand-New-AD5 ... SwcOFWajuO
It's fairly straight forward, if you want to start it , press start. You can work out the rest of the controls for yourself !

No silicon heaven ? preposterous ! Where would all the calculators go ?

Post Reply