diodes to isolate pedal power branches

[pd3]

Hello and nice to be here.   

My current homemade pedal power supply has been doing well, made up of 8 separate Mini power supplies but only provide a max of 100 milliamps per output. (It actually is 8 totally independent power supplies in a box, each having its own transformer and voltage regulator.

However, I am just now purchasing pedals whose current demand exceeds 100 milliamps so, will need to address that. 

What I have and would like to implement is a 24vdc regulated power module (.6A max).  I'll be looking to feed the output of this to a couple of different linear voltage regulators, the ubiquitous 78xx series.  I'd like to feed this 24vdc power module to a 9v and 15v regulator but wanted to ask you about anything to provide a better isolation or less potentially problematic arrangement.  It seems like it should work but, what do you think?  Perhaps use diodes to keep each regulator isolated from each other and the 24vdc module as well? 

Thanks for your input, it just seems a decent way to go since I have these components at home, and a good 4" x 3" space in the PS box to implement this.

Best,
PD3

[ElectricDruid]

24vdc seems like quite a lot to run the regulators from. They'll get hot if they're trying to get rid of that much voltage as heat. 15V won't be quite so bad (dropping 9V), but a 9V reg has got to lose 15V as heat, and that's a lot.
If you're using the metal tab T0220 package 78xx regulators, that's a good start, but you might need heatsinks too.
The diodes you suggest would actually help a bit, since they'd drop a little bit of voltage ahead of the regulator. I don't see any other reason to put them in, to be honest. You *do* sometimes see diodes around the regulators in power supply designs, which is protection of some kind - I forget against what. There's an example here with variable 317/337 regulators, but it's the same for the fixed regs:

https://www.circuitbasics.com/linear-power-supplies/


 

[Mark Hammer]

I don't know that it would address your needs, but Toshiba makes some little 3-pin regulators juuuuusssst a bit bigger than the more familiar little LM7809 devices, that can provide up to 150ma.  It's not 1A, of course, but it's 50% more current, per output.

[antonis]

double post..

[antonis]

You *do* sometimes see diodes around the regulators in power supply designs, which is protection of some kind - I forget against what.

Diodes across regulator IN-OUT prevent putting a voltage on OUT higher than the one on IN..
(usually due to large capacitance load when power shut down and reservoir cap decays)

Shunt diodes (across OUT & GND) are found in dual supply regulators preventing positive supply rail being dragged below 0V in case of negative rail starts up faster - and vice versa..
(the above can easily happen in case of +/- rails decoupling cap existence..)

There are also diodes between OUT and ADJ pin in variable regulators for noise and ripple improve capacitor (the one between ADJ pin and GND) discharge in case of shorted output..

[Rob Strand]

As Electricdruid outlined too much voltage drop is going to make the devices run hot.

Unfortunately there's more to it.

The labelled current rating for regulators only applies with a very small voltage across them.  They can quite often be limited by heat more than current.  Depending on which datasheets you use and you PCB construction that could be as low as 3V (0.45W) for a TO-92 package operating at 150mA, and 2V (2.0W) for a TO-220 operating at 1A.    Even then the IC's are running to edge of their temperature ratings, which is unwise, and ideally you want about half that amount of power dissipation.

With 24V in and 9V out the voltage drop is 24-9 = 15V you a will get less than 0.45W / 15 = 30mA for the TO-92 and 2.0W/15 = 130mA for the TO-220.  (That's with the regulators cooking.)

If you want to use TO-92 packages the only way to get more output is to make the input voltage close to 9V.  You can't make it too close otherwise the regulator won't work, something like 3V is minimal.  Meaning you need 12V in.  Even if you do the devices are near cooking at 150mA.  You might get 75mA to 100mA.

If you use TO-220 packages without heatsinks and set the input voltage to 12V then you can get about 2.0W/3 = 0.65A.  If the devices don't have enough air flow it's going to be less that.

You can make small improvements with small heatsinks on the TO-220's.

A different angle is to use a single high-powered regulator with large heatsink to drop 24V down to 12V.  Then power all the smaller regulators from that 12V output.    At end of the day it is just burning-up power and keeping that away from the smaller devices.  Sometimes you have to do things like that to use parts you have.  With the cost of a heatsink and a perhaps a larger case maybe the cost of building your own regulator is more than the cost of buying a 12V wall-wart!  For less power dissipation you could use a 24V to 12V DC/DC converter but that means addressing noise issues.

[R.G.]

As noted, diodes are not much good for "isolating" power supplies in the setup you're proposing. When they are conducting, as you would expect them all to be, their impedance to AC noise on the DC power line is perhaps a few ohms, which is inconsequential.
Multiple regulators are good for isolating positive-side noise from one pedal to another. However, regulators don't do anything at all for the ground/0v side of the power supply. Ground-current induced noise will still be there in full force.
On the other hand, full isolation of power supplies between pedals has been oversold in most cases. I have personally seen pedalboards with upwards of 30-50 pedals driven from a single (... high quality) 9V power supply with no ill effects. And I have seen single pedals that would make two-pedal connections hum and sing. It's more in the pedals than in isolation. The bad-actor pedals need full power and ground isolation, most pedals play well together.

[mozz]

At 1 time, Weber made a power transformer with 8 separate outputs, I think they might still make it.  I threw this in a Xmas present pedalboard with a custom made Reverb chassis and Ali xpress regulator boards. Got 9.1v and 9.6v, some AC and who knows what else. No idea what the current was but there's more than enough.  I always did think diodes would isolate enough but never got around to doing that experiment. 

 Top row was going to be all germanium pnp pedals and bottom regular pedals.  I'll finish this project after I'm in the nursing home 10 years and caught up with my other projects.  Short soldering span. Hey look, tonearm parts. Drop everything. I should finish this strat too.

[PRR]

Weber made a power transformer with 8 separate outputs, I think they might still make it. 

https://www.tedweber.com/wpdlxfmr-1/
Pedal and effects power supply transformer, Eight 11 volt, 300ma windings and one 9 volt, 2 amp winding. 120VAC input. This is a transformer, NOT a power supply. The output is AC, not DC. You must build a power supply that converts AC to DC in order to use this transformer.

[amptramp]

You can use resistors and additional capacitors to make a regulator operate off a high input voltage.  If you have a resistor in series with the input lead of the regulator and the usual capacitance at the input to the regulator, you can mitigate some of the power dissipation.

If you have 100 mA @ 9 VDC being delivered from a regulator operating from a 24 volt supply, a 100 ohm resistor in series with the input will drop 10 volts so the regulator only sees 14 volts at the input and only dissipates the 5 volts at 100 mA or 0.5 watts.  You must have enough capacitance to maintain stability but this can be a simple way to get a low-wattage regulator to work and stay within the dropout voltage limits.  One watt of the dissipation is transferred to the resistor.

[Knobby]

What do more expensive power supplies, like the Walrus Canvas ones, do? They claim to be fully isolated outputs, but run on something like a laptop power supply. Unless they've got an inverter circuit in there feeding multiple transformers, or one transformer with multiple secondaries, aren't all the grounds going to be common? Perhaps fully isolated in this case only means individual regulators for each output, so isolated positive volts only.

[mozz]

  One watt of the dissipation is transferred to the resistor.

Yes so you need probably a 3w resistor there. 

[antonis]

Preferably wirewound..

[PRR]

Unless they've got an inverter circuit in there feeding multiple transformers, or one transformer with multiple secondaries, aren't all the grounds going to be common?

That used to be unlikely, but modern economics has made it more practical, as long as you make many thousands at once.

[FiveseveN]

Unless they've got an inverter circuit in there feeding multiple transformers, or one transformer with multiple secondaries, aren't all the grounds going to be common?

They use individual isolated DC-DC converters, like the ones discussed here: https://www.diystompboxes.com/smfforum/index.php?topic=131639.msg1280100#msg1280100

[mozz]

Too expensive for home use if you need a bunch. Probably cheaper to go with the multiple secondary transformer ($28) and separate regulators ($1 each?) .

[PRR]

Too expensive

In that thread Baran reports "it costs around 1$ here.." I have seen similar but have to shop around a LOT to find 'isolated' with the in and out voltage and power-zone wanted. Avoids all wall-power splicing so seems safer for the inexperienced.

[mozz]

$8 each on Mouser.  He's looking for more than 100ma.

[Rob Strand]

$8 each on Mouser.  He's looking for more than 100ma

Also it doesn't address the 12V vs 24V issue or the heating issue.  (Don't know if that module will take 24V.)

The easiest way around 100mA is to use LM7805 TO-220 packages.


A lot of those multi-output power products for pedals use smaller packages regulators for the low current outputs and larger packages for the higher current outputs.   They also feed the regulators with lower voltages.   The isolation is done internally either with a multi-tap transformer or multi-output switchmode.  Probably some lowish DC voltage.  The switch-mode area will vary.  There's no fancy parts for the analog power:

Here's a rundown of the guts of a transformer based unit:

9V 100mA:
   BR1 to BR3: Small bridge
   Cap 1000uF 25V
   IC1 to IC3: L317LC, SO8
   Each IC has two resistors

18V 100mA:
   BR4: Small bridge
   Cap 1000uF 35V
   IC4: LM317M, TO-263
   Each IC has two resistors

12V 400mA:
   diode bridge
   Cap 3300uF 35V
   23.5V input no load
   IC5: LD1117V, TO-220, (800mA)

   475 ohm, 4.02k ohm   ?
   Vref = 1.25V
   calc 11.83V + 1uA*4.02k = 11.83V
   
9V 400mA:
   Fed from 12V 400mA rail.
   IC6: LM317M, TO-220

[pd3]

Whew. Thank you everyone, certainly a few things to digest here.   

What I'm doing is building this inside a long Hammond box that already harbors the 8 little 100mA power supplies that I build for my other pedals that are pretty much all draw much less than 100mA.   Hell, I could even use the box itself as a heatsink with added 220 heatsinks both inside and outside the box. 

So heres something I'm really curious about.   Are there any advantages (or disadvantages) to "piggybacking" an LM7809 off the output of an LM7818 that is "piggybacking" off the 24vdc module? Of course, the current draws I will be introducing are far below the max's of any and all regulators and modules.  I can also put a fan on the box.  Its my box.  I can do whatever I want to it! 
Thanks everyone, you've giving my a great deal to consider, more than I had bargained for!
thank you, Best
PD3