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7809 vs 9.1v zener?

Started by Father Haskell May 22, 2012
Low current (< 100 mA) power supply regulator.  Any reason I'd want to
use 7809s instead of cheaper 9.1v zeners?
On Tue, 22 May 2012 09:27:27 -0700, Father Haskell wrote:

> Low current (< 100 mA) power supply regulator. Any reason I'd want to > use 7809s instead of cheaper 9.1v zeners?
Because the 7809 will give you better regulation and significantly less current wasted to achieve it than the Zener will -- particularly if your current requirements vary. And it's easier to use, once you've verified that you won't burn it up. A 1N4739A from DigiKey will run you $14.40 if you buy 100 of 'em. An MC78L09 will run $22.00. That's a $0.076 difference, you still have to buy the resistor to make the Zener work, you have to _always_ present it with about 105mA, even when the circuit downstream isn't pulling any current at all, and the regulation won't be nearly as good as the 7809. And if your input voltage varies much then you won't be able to reliably hold the Zener current down enough to use a 1W diode, which means you'll have to spend even more. If you can stand 9.3 to 9.4V, then a 1N5240 is only $7.32/100, a PMBT2222A is only $7.42/100, and a resistor is something like $0.48/100 -- that gives you much better regulation than a Zener alone, about the same or better dropout characteristics than a 7809, and should cost less than a penny more than your bare nekkid Zener, once you buy the resistor to bias that honkin' big thing. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
On Tue, 22 May 2012 11:44:36 -0500, Tim Wescott <tim@seemywebsite.com>
wrote:

>On Tue, 22 May 2012 09:27:27 -0700, Father Haskell wrote: > >> Low current (< 100 mA) power supply regulator. Any reason I'd want to >> use 7809s instead of cheaper 9.1v zeners? > >Because the 7809 will give you better regulation and significantly less >current wasted to achieve it than the Zener will -- particularly if your >current requirements vary. And it's easier to use, once you've verified >that you won't burn it up. > >A 1N4739A from DigiKey will run you $14.40 if you buy 100 of 'em. An >MC78L09 will run $22.00. That's a $0.076 difference, you still have to >buy the resistor to make the Zener work, you have to _always_ present it >with about 105mA, even when the circuit downstream isn't pulling any >current at all, and the regulation won't be nearly as good as the 7809. >And if your input voltage varies much then you won't be able to reliably >hold the Zener current down enough to use a 1W diode, which means you'll >have to spend even more. > >If you can stand 9.3 to 9.4V, then a 1N5240 is only $7.32/100, a PMBT2222A >is only $7.42/100, and a resistor is something like $0.48/100 -- that >gives you much better regulation than a Zener alone, about the same or >better dropout characteristics than a 7809, and should cost less than a >penny more than your bare nekkid Zener, once you buy the resistor to bias >that honkin' big thing.
A BCX70J is only 4 cents, and has a huge, bracketed beta! -- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
Father Haskell wrote:
> > Low current (< 100 mA) power supply regulator. Any reason I'd want to > use 7809s instead of cheaper 9.1v zeners?
Better regulation Lower power dissipation Better ripple rejection, probably Lower output impedance. Otherwise, not really. ;) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
On Tue, 22 May 2012 11:44:36 -0500, Tim Wescott <tim@seemywebsite.com>
wrote:

>On Tue, 22 May 2012 09:27:27 -0700, Father Haskell wrote: > >> Low current (< 100 mA) power supply regulator. Any reason I'd want to >> use 7809s instead of cheaper 9.1v zeners? > >Because the 7809 will give you better regulation and significantly less >current wasted to achieve it than the Zener will -- particularly if your >current requirements vary. And it's easier to use, once you've verified >that you won't burn it up. > >A 1N4739A from DigiKey will run you $14.40 if you buy 100 of 'em. An >MC78L09 will run $22.00. That's a $0.076 difference, you still have to >buy the resistor to make the Zener work, you have to _always_ present it >with about 105mA, even when the circuit downstream isn't pulling any >current at all, and the regulation won't be nearly as good as the 7809. >And if your input voltage varies much then you won't be able to reliably >hold the Zener current down enough to use a 1W diode, which means you'll >have to spend even more. > >If you can stand 9.3 to 9.4V, then a 1N5240 is only $7.32/100, a PMBT2222A >is only $7.42/100, and a resistor is something like $0.48/100 -- that >gives you much better regulation than a Zener alone, about the same or >better dropout characteristics than a 7809, and should cost less than a >penny more than your bare nekkid Zener, once you buy the resistor to bias >that honkin' big thing.
Simple zeners are cheap in quantity- you can probably get all three parts for 2-3 cents. But a small transistor does not have the power dissipation capacity of a TO220. If you have a 12V input and need 9V at 100mA, that's 420mW at +10% in, which is too much for an SOT-23. If you're making a Chinese-assembled gewgaw that doesn't have to work over a wide temperature range or for decades, you can stick an 8050 in there and still stay at less than 3 cents, which is probably less than 1/5 the cost of a 7809.
Father Haskell wrote:

> Low current (< 100 mA) power supply regulator. Any reason I'd want to > use 7809s instead of cheaper 9.1v zeners?
You can use a 78L09 which is a small 100 ma regulator however, you do need a little head way before that. These types of regulators require a minimum above the regulated output voltage to work. Like 2 volts or more to be on the safe side. A zener is ok of you can live with the wasted power. you'll find a integrated solution is more stable and uses less power why it's laying around doing nothing. (quiescent current). Now I like something like this.. http://www.ti.com/lit/ds/symlink/mc33063a.pdf It'll handle up to 40 volts in and start at very low voltage. It has its own internal ref 2.5V that you can use with the voltage divider to configure for the voltage you need. You can use it as a step up, step down, voltage to voltage. etc.. It handles up to 1.5 A on chip and it's cheap! It's up to you. Most people these days are very conscientious about power usage, especially when it comes to batteries as the supply. Jamie
On May 22, 5:27=A0pm, Father Haskell <fatherhask...@yahoo.com> wrote:
> Low current (< 100 mA) power supply regulator. =A0Any reason I'd want to > use 7809s instead of cheaper 9.1v zeners?
This suggests you want a regulated voltage, but presumably don't care about energy waste. So probably mains power, but what needs regulated 9v? reg v zener is already covered, so what about other options. Would smoothed unregulated do? Is it an option to tweak the circuit so it doesnt need a clean supply? NT
On May 24, 9:34=A0am, NT <meow2...@care2.com> wrote:
> On May 22, 5:27=A0pm, Father Haskell <fatherhask...@yahoo.com> wrote: > > > Low current (< 100 mA) power supply regulator. =A0Any reason I'd want t=
o
> > use 7809s instead of cheaper 9.1v zeners? > > This suggests you want a regulated voltage, but presumably don't care > about energy waste. So probably mains power, but what needs regulated > 9v? reg v zener is already covered, so what about other options. Would > smoothed unregulated do? Is it an option to tweak the circuit so it > doesnt need a clean supply? > > NT
Guitar pedal supply -- pedals are supposed to sound better if you run them from separate regulators. Most draw less than 20 mA, so cheaper zeners should power them fine and allow more branches per dollar.