Question about minimum input voltage using Voltage Regulator L7809?

[Jasonmatthew911]

Hi guys, it's been awhile since I've posted...I have a design I'm working on, which is like a mini 2 input mixer for line instruments (Guitar/Bass or Synth in mono) with TL072 for Pre-Amps and a Buffer with CabSim on one of the inputs, both inputs have there own volume/Pre-amp and output to an unbalanced 1/4" jack to connect to an Amp, and they also both output to a balanced XLR DI with Jensen transformer to go direct out to a mixer or for direct recording, so this can be used as a Splitter for live and recording to play or record a speaker and send a direct signal simultaneously for recording or live situations...Lastly, I was adding in a simple headphone amp to also be able to use the same box for silent jamming through Phones...The whole circuit can work great on 9V - 18V except for the simple headphone amp, but the Buffer/Pre-amps/CabSim will all sound better with 18V...I was simply gonna make a simple LM386 headphone amp due to my lack of board space on my PCB design, so I was planning on putting an L7809 Voltage regulator w/ heat sink only for the LM386 so that I can use an 18V supply with no issues...But, I was wondering, since I see that an L7809 wants a minimum of 11V in order to output 9V...Will the L7809 still pass at least 7V to the LM386 if it's fed with a 9V power supply, or should I be opting to use the L7805 and limit the LM386 to 5V, in order to run my circuit in the 9V-18V range...I'd like everything to work properly whether I use a 9V or 18V supply for extra headroom...The LM386 usually gives out a lot of output for headphones, so maybe running it at 5V with the 7805 regulator could be better, not sure...Who here recommends I run the LM386 on 5V with 7805 or 9V with 7809?...Which could be cleaner for an LM386 as a simple headphone amp, 5V or 9V regulator?...Also will a 7809 work properly if it only receives 9V at the input?...If the 7809 passes at least 7V to the LM386, then I'd probably opt to use a 7809 for the extra voltage, but I'm really not sure how a 7809 reacts with a 9V input supply...Hopefully the experts here can clear this up for me, and recommend which regulator (7809/7805) may be cleaner/better for an LM386 Headphone amp so that I can run my entire circuit with a 9V or 18V supply, thanks.

[Rob Strand]

Yes 2V dropout is about right.

You can let the 7809 dropout.    Using a 7805 is probably wasting too much output from the LM386.

Slighty better is to increase the output of the 7805 with diodes,
https://www.deeptronic.com/electronic-circuit-design/converting-5v-7805-linear-voltage-regulator-to-produce-6-4v-output/

I'd suspect three diodes would give you about 7V.

It depends if your 9V rail is from battery (which could drop to 7V), an unregulated supply, or a regulated supply.

[antonis]

There are commercially availiable 7807 regulators, aren't they..?? 

An alternative to Rob's suggestion for raising 7805 output voltage should be to use 2 resistors and wire it as adjustable regulator using 5V for Vref in voltage calculation formula Vout = Vref (1+R2/R1)

[Jasonmatthew911]

[bugger]
hit Modify rather than Quote - my bad...
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[antonis]

IMHO, go for 7807..
(it's always better to use a regulator within its specs..)

[Jasonmatthew911]

Ok thanks, 7807 it is!...Cheers!

[Jasonmatthew911]

IMHO, go for 7807..
(it's always better to use a regulator within its specs..)

Actually the 7807 is hard to find, even Mouser doesn't have any...They do have 7806 for 6V, with 8V minimum supply and 7808 for 8V that's 10.5V minimum supply...So I guess I'm gonna have to compromise at 6V with a 7806 for the LM386...Do you know where to find some L7807?...I'd prefer the 7V now, and I have limited space for extra parts in order to bump up a 7806 to 7V.

[davent]

TO-220, seven volt regulators at Digikey.

https://www.digikey.ca/en/products/filter/pmic-voltage-regulators-linear/699?s=N4IgjCBcoGwJxVAYygMwIYBsDOBTANCAPZQDaIALGGABxwDsIAuoQA4AuUIAyuwE4BLAHYBzEAF9CAJgCsMxCBSQMOAsTIgAzHQAMYBCxAcuvQaImEw9egmiK0WPIRKRyUmmCkx5h45B78wmKSIAC0UgpK-ACuai7kPmyc-hLi4kA

Any reason not to use a LM317, probably much less expensive than any of the 7v fixed regulators.
dave

[PRR]

LM383/TDA2002 and like 32r 1/2w series resistor(s). I like the '386 but it sure is a weeny thing.

[Rob Strand]

Any reason not to use a LM317, probably much less expensive than any of the 7v fixed regulators.

It's probably a better solution.

Most 7805 etc devices quote 2V dropout at 1A.   Some LM317 datasheets imply a 3V dropout but others give graphs which give 2V dropout.    So you might lean towards the 7805.

However ...

Reading in between the lines.   The 7805's 2V is *typical*.    The LM317 graphs would be interpreted as typical.   So really both 2V.

The output stage circuits are very similar with similar current limits.    For a given *load* the current limit is a factor because the current limit circuit contributes to the drop-out,.  The contribution is about approx 0.65V * (I_load / I_limit).

Putting all that together the 7805's and LM317's are going to have similar dropouts at similar currents.

The way to cheat is to use a regulator with a higher current limit but that's going to get off the common parts route and it's just not worth it for *tiny* reductions in drop-outs.

[Jasonmatthew911]

Thanks for all the suggestions guys...It seems to me that 7806 might be my best option...I'd rather a fixed voltage regulator, I'm trying to keep it simple...I'll see if I can find some 7807 or similar, if not, I'll go with 7806 so that I'm working within part specs when I feed with 9V or 18V supply...Cheers!

[Rob Strand]

IMHO using weird arsed parts like 7806 or 7807 is just making life difficult.

[anotherjim]

When did all these incremental voltage regulators appeared. For a long time the only fixed regulators in 78xx/79xx were...
5v, 8v, 12v, 15v! Or was it that our UK distributers couldn't be bothered to stock the others? I still wonder what on earth needed an 8v regulator!

[antonis]

You can always make your 7.5V - 7.7V discrete voltage regulator using a BJT, a 8.2V Zener and a current limiting resistor..

[Rob Strand]

When did all these incremental voltage regulators appeared.

When TI bought National Semiconductors they found a big box of parts that were out of spec. 

Or was it that our UK distributers couldn't be bothered to stock the others?  I still wonder what on earth needed an 8v regulator!

I remember the 9V but after that I've got no idea when they appeared.  Even my local store has 6V and 8V and they don't really keep much.  Yes, why do they exist?

[antonis]

Maybe for those who like to have many stabilized outputs to their power supply.. 
(I built, once upon a time, a benchtop PS with 5,6,7,8,9,10,11,12,15,18,24 & 33 V outputs, just because I had plenty of space in front panel of an adjustable power supply..)

[anotherjim]

If a 6v is easy to get, but you want nearer 7v, isn't the 6v with a diode lift a cheap option?

If we need to order the part, isn't an LDO type an option? For example, a TI 750M08 can deliver 750mA, stand 26v input and won't drop out until input down to 8.6v. Also the 750L08 if you only need 150mA.
If the input falls below 8.6v, well it will just follow it down won't it?

[Rob Strand]

Maybe for those who like to have many stabilized outputs to their power supply.. 
(I built, once upon a time, a benchtop PS with 5,6,7,8,9,10,11,12,15,18,24 & 33 V outputs, just because I had plenty of space in front panel of an adjustable power supply..)

Maybe a keyboard power supply. where you press a key and the voltage appears on the output  .
Unfortunately it's not polyvoltic as pressing multiple keys result in disaster.   Nonetheless a circuit of internet quality.

[Jasonmatthew911]

If a 6v is easy to get, but you want nearer 7v, isn't the 6v with a diode lift a cheap option?

If we need to order the part, isn't an LDO type an option? For example, a TI 750M08 can deliver 750mA, stand 26v input and won't drop out until input down to 8.6v. Also the 750L08 if you only need 150mA.
If the input falls below 8.6v, well it will just follow it down won't it?

Thanks...I think I'm gonna try this recommendation...I'll use a 7806 with the Diode lift to get 7V...Where exactly do I put the Diode on the 7806 for the 7V output and what diode works best for this?

[MaxPower]

Have an unneeded phone/tablet charger? They usually include a variable voltage regulator chip.