mA requirements For Pedals

AxeWieldingBastard

Dumb Question incoming:

If the mA on the power supply exceeds that of the pedal requirements, is it all good? Or are you risking pedal damage?

I want to power my AD999 (9v 200mA) using the Harley B Jr. So, im hoping if i double up 2 of the outlets for 9v 220mA ill be all set? Am i right?

EdGrip

Nah, it's fine.

All you need to know is that the power supply is capable of supplying all the current (mA) that the pedal needs. 

So you can run your 30mA pedal with a 500mA power supply - or indeed lots of 30mA pedals, as long as the total doesn't exceed 500mA.

With voltage, though, things are different. If your pedal is 9V, don't try powering it with an 18V power supply - you could easily fry the pedal. 

d8m

Pedals will only take the current(mA) needed.

If you over supply on current youll be fine.

Dont do it for voltage though as you may turn your pedal into a fancy paperweight!

stickyfiddle

Yup, as the others have said, the mA rating on the pedal is essentially how much it sucks from the PSU. The PSU rating is how much it can give out to the pedals in total, so as long as the "suck" is smaller than the total amount available it's all good. 

This gives me an idea for a thread though...

http://thefretboard.co.uk/discussion/15248/official-fx-pedal-current-and-voltage-requirement-thread

chrispy108

You should add your post to your new thread @stickyfiddle

A bit on isolated vs daisy chains would be useful too?

d8m

chrispy108 said:

You should add your post to your new thread @stickyfiddle

A bit on isolated vs daisy chains would be useful too?

Make sure you get the right daisy chain.

Had no end of problems trying to run my pedals of this:

AxeWieldingBastard

Nice one fellows!

Ta muchly for the infos :-bd

ecc83

There is a rare circumstance where the current rating of a PSU should not be very much greater than the demand of the pedal. This is in the case of an older, unregulated supply and an older, again pedal especially a delay type using Bucket Brigade devices.

A supply rated at 9V at say 1000mA is likely to have an open circuit voltage well above 9V and if connected to a pedal pulling only 10mA say might, in some cases cause harm. Very unlikely scenario I admit but IF you have a treasured old chorus pedal something to be aware of or stick to batteries?

Nearly a year ago I was tasked testing some 10 or so different brands of 9V,500mA PSUs (and 22V types but for a different issue). The purpose of the exercise was to check the output noise at various loads . I found a variation of at least 10:1 in the "muck" delivery. Even the noisiest did not bother the pedals under test but certainly did bother some other brands.

So, although it goes against the grain it would seem best to stick to manufacturers power supplies.

Dave.

ICBM

The other issue with massively high capability power supplies is that if something goes wrong, or you accidentally use the wrong one with the wrong pedal, the power supply will quite happily supply enough current to burn out the protection diode which most pedals have to prevent damage, then cook the rest of the circuitry as well. I've seen quite a lot of these in the last few years since 1000mA+ supplies became common. The pedals are almost always a write-off.

ecc83

ICBM said:

The other issue with massively high capability power supplies is that if something goes wrong, or you accidentally use the wrong one with the wrong pedal, the power supply will quite happily supply enough current to burn out the protection diode which most pedals have to prevent damage, then cook the rest of the circuitry as well. I've seen quite a lot of these in the last few years since 1000mA+ supplies became common. The pedals are almost always a write-off.

Heh! There is a situation where the pedal has "got its own back"! The early HT, AC fed pedals had a rare fault that caused them to pull about twice the normal current of about 800mA. This popped the internal heat fuse in the PSU. The insidious thing was the pedal carried on working fine, but with about 450V HT!

If anyone has an old AC HT pedal and the transformer fails, get the pedal checked by B's or a competent chap like ICBM.

BTW IC, your scenario above could be easily protected against with a wee, 1R fusible resistor pre the diode. Does no one do that?

Dave.

ICBM

ecc83 said:

BTW IC, your scenario above could be easily protected against with a wee, 1R fusible resistor pre the diode. Does no one do that?

Not as far as I know.

It could also be better protected against by using a series diode instead of shunt to block reverse polarity. I know that involves throwing away 0.7V, but that would not be an issue for most pedals, especially digitals.

Best of all, you could make the pedal almost completely immune to any incorrect voltage - in fact, make it compatible with any standard power supply - with only two small and fairly cheap components, a bridge rectifier and a voltage regulator. I'm pretty sure there is a small UK builder who does that. But the big boys don't seem to want to! Obviously it comes down to pennies.

ThePrettyDamned

ICBM;226910" said:

ecc83 said:BTW IC, your scenario above could be easily protected against with a wee, 1R fusible resistor pre the diode. Does no one do that?





Not as far as I know.

It could also be better protected against by using a series diode instead of shunt to block reverse polarity. I know that involves throwing away 0.7V, but that would not be an issue for most pedals, especially digitals.

Best of all, you could make the pedal almost completely immune to any incorrect voltage - in fact, make it compatible with any standard power supply - with only two small and fairly cheap components, a bridge rectifier and a voltage regulator. I'm pretty sure there is a small UK builder who does that. But the big boys don't seem to want to! Obviously it comes down to pennies.

Pedals broken by silly mistakes are replaced. Manufacturer saves pennies making them, and sells more because of that

Nova repeater uses about 240ma when running, but requires 300 on start up.

ecc83

Barely a penny!>>>>>http://www.rapidonline.com/Electronic-Components/S250-Diotec-Silicon-Bridge-Rectifier-50-3146/?source=googleps&utm_source=googleps&gclid=CLWTjN3mir4CFXHMtAodxWIAdQ. But if you fit a bridge or even a series diode you have to then fit a decent sized decoupler to keep the line impedance down.

Trouble is it always cost SOMETHING to fit "non-essential" but good practice components and the average Joe just does not understand or care. "Why should I pay another fiver? I ain't gonna 'king blow it up!"

'Tis a cutthroat market out there.

Err! Bit of arithmetic dyslexia crept in there! Still, I bet you COULD source an SM bridge for under 5p if you wanted 10,000?

Dave.

jpfamps

Polarity protection should be a mandatory design feature of compact pedals.

The cost of the additional parts is negligible.

The best way (in my opinion), of implementing this is to use a MOSFET in one of the power rails, either a P-channel in the +ve rail, or an N-channel in the -ve rail. If a MOSFET with very low on resistance is used then there is virtually zero voltage drop.

The series diode method (as ICBM suggested), is very cheap and easy to implement. You do reduce the voltage by the forward voltage drop of the diode. If you are concerned about this, then use of a Schottky diode will keep this to a minimum, typically 0.4 V. Incidentally this is the method I employ when retro fitting polarity protection to fx pedals.

A bridge rectifier would allow use of power supplies of either polarity, but you will now have two diodes in series and thus twice the voltage drop versus a single diode. I suppose you could implement a dropless rectifier with MOSFETs............

Voltage regulators drop further voltage (usually 2 diode junctions), so using one after a bridge will practically mean using a voltage significantly below the input voltage, possibly as low as 6VDC, to avoid regulator drop out. Of course you could use a pump charge chip to boost the voltage prior to the regulator. This would be wasteful of power (for many pedals you would be expending more energy in the power supply than in the rest of the pedal), as you would now be dissipating power in the bridge rectifier, the pump charger, and the voltage regulator. You have now a more complicated power supply. Indeed it's likely that the cost of the power supply components costs more than the rest of the electronics.

Re ICBMs point about over voltage on unregulated power supplies, I've seen a few pedal repairs where this has been an issue. These have all had ICs with maximum voltage rating around 10.5 VDC.

If you have components in your design that have such a slim over voltage margin, then I suggest either using alternatives or implementing some form of voltage clamp. This could be a simple zener shunt regulator, and needn't be used on the whole power supply but soley for those parts that could potentially be damaged by over voltage.

ecc83

Life was simpler when the small AC power supply was allowed. There had to be diodes in the power path! Then a + and - regulator so there was inherent polarity and overvolts protection. The latter since it is unlikely that a voltage source beyond the regulators ratings would ever be connected. I certainly never had such a case.

The DC version running at 22 V has proved equally rugged but these are quite expensive pedals and of course battery power is not possible.

TPD? DON'T talk to me about "startup currents"!

Dave.

jpfamps

ecc83 said:

Life was simpler when the small AC power supply was allowed. There had to be diodes in the power path! Then a + and - regulator so there was inherent polarity and overvolts protection. The latter since it is unlikely that a voltage source beyond the regulators ratings would ever be connected. I certainly never had such a case.

The DC version running at 22 V has proved equally rugged but these are quite expensive pedals and of course battery power is not possible.

TPD? DON'T talk to me about "startup currents"!

Dave.

Start up currents can be reduced by soft starting the power supply.

DC supplies can very easily be made soft start by using a MOSFET a cap and a couple of resistors. If you are using a MOSFET for polarity protection, this can be used for soft start as well, so you get plenty of benefit for the small additional expense.

You can make an AC soft start using MOSFETS but the circuitry is more complex.

3 terminal regulators can also easily be made soft start; there is a simple and cheap circuit in the application notes.

One of the problems you can encounter with the 3 terminal regulators is that if the in rush current is very high then the regulator will go into over current shut down and not turn on. This can seemingly occur at random, and is likely to be due to the part of the mains cycle you are on when the device is energized; if you switch on at the peak of the mains cycle the in rush current will be much higher than at say the zero crossing point.

We ran into this problem ages ago when using a 3-terminal regulator for the filament supply of a valve amp. Soft starting the DC supply prevented this problem. The soft start also is kinder to the filaments too.

Unless there is a very good reason not to soft start the power supply (and there maybe in some split rail applications where you don't want the supply rails coming up at different rates), then soft starting is a sensible and economic way of reducing inrush currents.

ecc83

jpfamps said:

ecc83 said:

Life was simpler when the small AC power supply was allowed. There had to be diodes in the power path! Then a + and - regulator so there was inherent polarity and overvolts protection. The latter since it is unlikely that a voltage source beyond the regulators ratings would ever be connected. I certainly never had such a case.

The DC version running at 22 V has proved equally rugged but these are quite expensive pedals and of course battery power is not possible.

TPD? DON'T talk to me about "startup currents"!

Dave.

Start up currents can be reduced by soft starting the power supply.

DC supplies can very easily be made soft start by using a MOSFET a cap and a couple of resistors. If you are using a MOSFET for polarity protection, this can be used for soft start as well, so you get plenty of benefit for the small additional expense.

You can make an AC soft start using MOSFETS but the circuitry is more complex.

3 terminal regulators can also easily be made soft start; there is a simple and cheap circuit in the application notes.

One of the problems you can encounter with the 3 terminal regulators is that if the in rush current is very high then the regulator will go into over current shut down and not turn on. This can seemingly occur at random, and is likely to be due to the part of the mains cycle you are on when the device is energized; if you switch on at the peak of the mains cycle the in rush current will be much higher than at say the zero crossing point.

We ran into this problem ages ago when using a 3-terminal regulator for the filament supply of a valve amp. Soft starting the DC supply prevented this problem. The soft start also is kinder to the filaments too.

Unless there is a very good reason not to soft start the power supply (and there maybe in some split rail applications where you don't want the supply rails coming up at different rates), then soft starting is a sensible and economic way of reducing inrush currents.

I am obliged for the information JPF but it was  a soft start third party SMPSU that was causing my problem!

If I might mention batteries again? I would dearly love to see the demise of the PP3 in pedals. AAs are much better and cheaper watt for watt. Four of them could be used with DC-DC converters and then we might get pedals with decent headroom.

Pie in Sky I know! The electric guitar market is FAR too conservative!

Dave.