9 Volt Power Considerations

[Lizard King]

I'm just getting started in the roll your own stomp box thing.  I was looking at some Dann Green work and noted he generally uses a 78L09/cap circuit in his power supply section to guarantee 9V.  Since I saw this and realized what he is doing I measured some standard guitar stomp box 9V wall warts.  LOL.  The lowest I measured was 14V, some as high as 18V.  So I can see why one might want to include the 78L09 as SOP. 

However, I've since built a fairly well regulated power brick that I'm going to use to power my pedals.  I can dial it in pretty close to 9V.  It is my understanding that the common 78L09 will suck up some power anyway and give me a couple volts under 9V.  But if I give my boxes to my friends to try out I won't know what they're using for power.

It seems like I have several options.
1) Forget about it and leave the 78L09 out.  A few extra volts never hurt anyone.
2) Find a 78L09 variation that won't suck up any voltage when presented with 9V at it's input.  (Don't know if that animal exists.  Or maybe they all work like that and I'm worried over nothing.)
3) Add a switch - just what I need when trying to shoehorn a medium sized PCB into a 125B.
4) Use two power jacks - not much better than (3). 

Any suggestions?

[Mark Hammer]

The voltage read on a meter when a wallwart is plugged in but not attached to anything OTHER than the meter is normally higher than the voltage it provides when connected to a pedal.  Do not confuse this with the actual effective voltage the pedal gets.  The regulator will require at least 11VDC to operate properly, and the adaptor may not be able to provide that, despite all outward appearances to the contrary.

If you are worried about havng a dependably 9VDC supply voltage, then simply get a 9.1VDC zener (of suitable wattage), and lay it across the power lines (V+ and gnd). 

[Lizard King]

hmmmm....9.1V zener.....easy, peasy....why didn't I think of that?

I never thought of the 78L09 until I saw them written into most of the commonsound schematics...

[Gurner]

But if I give my boxes to my friends to try out I won't know what they're using for power.

So tell them what they need to use e.g. 9V battery, 12V DC supply (to allow  for the voltage lost across a regulator)....or  lend them the  correct wall wart along with the stompbox.

When I buy a stompbox it comes with the correct wall wart (it doesn't allow me to use whatever I have to hand...which is what you're saying your friends will do if you give them one)...if the wall wart fails, the spec is on it...I buy another identical one, etc...in other words the designer (you) sets the spec for the type of PSU to use.

IMHO it's not a great solution to just strap a zener across the rails without a series resistor preseding (it'll fry if the input voltage goes to high), then your faced with knowing the current draw of the circuit (& the input voltage - which you don't know of your fiends are gonna use whatever they have) to rate the right zener series resistor (also zeners aren't great for high current circuits).

If you need a dependable 9V, use a regulator & make sure the input voltage to the regulator is above the dropout voltage for that regulator (normally about 2V for a 7809 ...therefore supply with at least 12V DC)

[drolo]

Not so long ago i repaired a friend's EHX Pulsar (old one, Big Box) and exactly that 78L09 chip was fried.
I replaced it by a simple diode in series with the power rail.

That left me wondering about the same questions as you. What happens when the pedal receives less than 9v? Does the 78L09 just let whatever is below 9v through ? Or can this cause problems?

[R.G.]

... maybe.

You have to understand what a zener does. It acts like a normal diode, but with a carefully selected reverse breakdown voltage, that being the zener voltage.  At reverse voltages below the rated voltage, it doesn't conduct much at all. But when the reverse voltage hits the zener voltage, it goes from not conducting at all to letting through any amount of current the external supply has available.

In all non-transient, steady state applications, zeners cannot be used alone. They MUST be used with some external current limit which keeps the current through them from exceeding their power rating.  For the common 1/2W zeners, a 9.1V zener can only conduct 54.9 ma before it gets close to thermal death.

If you push more than the rated-power amount of current through a zener, then just like anything else, it gets hot, may possibly become a Light-Emitting Diode for an instant  , and then becomes a Darkness-Emitting Diode (or DED).

So Mark's comment "of a suitable wattage" is important. The "suitable wattage" is (for a 9.1V zener) equal to 9.1V times the current limit of the power you're feeding in. Zeners come in 0.5W, 1W, 2W, 5W and more. If your power supply can give you anything less than 109ma, then a 1W zener will suffice. If it's rated at 500ma, then you need a zener rated at 5W, plus heat sinking sufficient to get rid of the 5W into the surrounding air, or again you get the conversion to a DED.

Low dropout regulators (also called LDOs) exist, and may be able to regulate to less than 0.1V lower than their input power supply. You pay for this in them being trickier to get to be stable and not oscillate, but it's not too bad.

My suggestion? Leave it out, tell your friends to not put more than 9V into it unless you KNOW it'll take X volts, then tell them not to put more than X volts into it. All pedals have some voltage X where X will kill the pedal.

Even pedals with a regulator before them have this problem. The 78L09 will only take about 30V as an input before dying. If you have friends not willing or able to keep the power they feed in below X, don't lend them the pedals or supervise the use. If this is impossible, either don't lend the pedals or get new, higher-grade friends.  

It is in general not possible to make a circuit that is immune to all possible misuses. There is always a number which is a threshold for unexpected results.

Not so long ago i repaired a friend's EHX Pulsar (old one, Big Box) and exactly that 78L09 chip was fried.
I replaced it by a simple diode in series with the power rail.

There is a sudden-death scenario for all regulators where there isn't a protection diode across the regulator. All regulators I do have the $0.03 protection diode, and I've only rarely seen it on any other pedals. A sudden short on the INPUT voltage will kill a three terminal regulator if it's not so protected.

What happens when the pedal receives less than 9v? Does the 78L09 just let whatever is below 9v through ? Or can this cause problems?

All series regulators have a "dropout voltage" which is the minimum voltage they can achieve from raw DC input to output. For the 7800 family , this is about 2V. That is, if you feed a 7809 less than 11V, the output voltage drops under 9V and follows the input voltage down, but 2V lower. Since the regulator is as low as it can go, any noise and ripple on the input voltage rides right through too. Your pedal gets the lowest of 9V or two volts lower than the input voltage.

LDOs have drop out voltages in the 0.1 to 0.3V range, and were invented for just this situation. See above.

[Lizard King]

OK...so no to the zener.

Either source some LDO 7809's (arond $.80 on eBay) or lend out a power supply with the pedal if I want to be anal about my pedal voltages.  I'm not sure I need to be as there probably isn't much difference between 9V and 15V in most circuits.

I was just surprised when I started measuring my box of effect pedal wall warts....all rated at 9V and all delivered well over 12V. 

[R.G.]

Some effect pedal wall warts are pretty sloppy about what voltage comes out. There are counter examples with very good regulation indeed.

As side commentary:
- I no longer trust any semiconductor I by on ebay; I consider that there is only a chance they'll work. Ebay is swamped with counterfeits and scams.
- In some circuits, there is not much difference between operations at 9 and 15V. In some circuits with specific biasing requirements, there is.
- In circuits built with 10Vdc rated electrolytic caps, there can be a sudden-death difference.

I've gone to 25V or higher rated electros on all my commercial designs because people insisted on using 18V or 24V supplies on them because they had heard somewhere on the internet that it made them sound like lasers or flying saucers, or the ghost of Jim Morrison or something.

[Lizard King]

I've gone to 25V or higher rated electros on all my commercial designs because people insisted on using 18V or 24V supplies on them because they had heard somewhere on the internet that it made them sound like lasers or flying saucers, or the ghost of Jim Morrison or something.

ROTFLMAO....I always use 25 - 50V electros so I'm probably worrying about nothing....

[R.G.]

Good for you. Most opamps die at over 36V total. Many small signal transistors die at over 30-50V, although there are 300V versions in the same packages. 4000 series CMOS is good to 15V, or 18V for the later devices. 74C devices are the same. 74HC, 74AC and 74HCT dies at 6V.

The devil is always in the details. There is no substitute for knowing what's in there.