18v power supply actually 28v - can I still use it?

[jhob]

I use this power supply for my pedal board.  The power adapter that came with it was noisy so I tried an old universal ac/dc adaptor which wasn't noisy (not sure of the exact difference) and that's what I currently use.

Someone gave me a decent 18v AC adaptor which I got a rectifier circuit for with the intention of making it DC to use with the pedal.  The rectifier works but the output is 28v.

Is this likely to be safe to use with the pedal power supply or is there a simple way that I can restrict the voltage? 

I want something with enough available current for 10 pedals, not exactly sure what the current requirements are for each pedal, but would guess less than 100mA for each?

[antonis]

Almost all unregulated power supplies exhibit voltage out higher enough than their nominal value..

That's an inherent issue of voltage versus current transformation, i.e. the lighter the load the bigger the dicrepancy between actual & nominal voltage value..

IMHO, safest method is by loading enough(*) output and measuring particular voltage drop..
(*) to a level of about 10% (arbitrary chosen.. ) of its maximum current capacity..

P.S.
Safest than safest should be the use of a voltage regulator..
(something like 7818 or similar..)

edit: I've just noticed that your adaptor is 18VAC, resulting into about 25VDC (in case of adequate capacitor filtering) making the use of voltage regulator oneway..!!

[antonis]

Ouch..!!!

Your pedal power supply also uses 18VAC adaptor - you don't have to rectify anything..!!


Or maybe I do need an extra cup of coffee..

[jhob]

I've just noticed that your adaptor is 18VAC, resulting into about 25VDC (in case of adequate capacitor filtering) making the use of voltage regulator oneway..!!

I don't understand what you mean there, could you explain?

Your pedal power supply also uses 18VAC adaptor - you don't have to rectify anything..!!

Yeah, new adaptor is AC, the pedal power block requires DC, so surely it needs recifying?  Am I missing something here?

[antonis]

OK.. Probably my bad..

DC voltage (after fullwave rectification and ripple filtering) is expected to be 1.414 times AC voltage (minus diode(s) forward voltage drop)..
Your 18VAC adaptor exhibits an unloaded output of about 20VAC so you've measured 28VDC after rectification..
The above measurement was probably taken under no-load condition (just under your DMM impedance) so you need to compare your specific measurement with the one taken from the old adaptor (if availiable..)

[jhob]

Thanks for explaining that, I'm still relatively new to all this and don't understand all concepts.

So the old adaptor, or at least the one I was using, measured at 18v DC, as did the noisy adaptor that it shipped with.

[antonis]

So the old adaptor, or at least the one I was using, measured at 18v DC, as did the noisy adaptor that it shipped with.

That results into regulated/stabilized PS adaptor..
(in case of higher than 18V measurement it should indicate "internal" voltage regulation at 18V..)

In your case, it's easier to use a voltage regulator (any commercially avaliable 3-pin IC, like popular 78XX series) of current capability at least that of your TOTAL outputs (which goes up to 2.2A in case of all outputs simultaneusly used)(*)

The above rise 2 basic issues..
1.(major): Your new "decent" adaptor is only capable to provide up to 1A..!!
2.(minor): You'll need a voltage regulator capable to carry more than 2.2A..

1.a. Can be settled in relation to (*)..
i.e. in case of total outputs marginally equal to 1A practically use (strongly depented on your specific pedals current consumption), you can use your new adaptor together with any 7818 1.5A regulator..
1.b. If not, you have to use another adaptor of 18(regulated)VDC/2.2A (or higher current output)
2. Self-vanished due to either 1.a or 1.b..