Wall Wart current question

[Guite Lectrique]

Hi, I'm about to build the preamp show on this site:

http://www.redcircuits.com/Page120.htm

It is said that the thing draws about 8.5mA and needs about 24V. I also want the thing to supply voltage for the rest of my stompboxes (which are supplyed by one 9V 500mA wall wart) with a "DC out" plug. My question is: would a 24V 600mA wall wart be enough/too much/safe?

Thanks

[gez]

Your question is a little ambiguous.  Presumably you mean that you want to build a 24V supply that can also power your 9V pedals?  If so, that's doable if you stick a 9V regulator (or 'bump up' an 8V reg with a couple of diodes) after your 24V regulator/24V DC PSU.

Is 600mA enough to power your pedals?  How long is a piece of string!  Most probably, but it all depends on how much your pedals consume.  It's a simple matter to measure the current consuption of a pedal (FAQ/search should cover it), so tot it all up for all your pedals and you'll have your answer.

[Paul Perry (Frostwave)]

24v supply dropping to 9v at 500mA?
That means at least a 15V drop across the 9V reg - that's 7.5 watts right there......... gonna get warm!

[gez]

24v supply dropping to 9v at 500mA?
That means at least a 15V drop across the 9V reg - that's 7.5 watts right there......... gonna get warm!

Fair Point Paul (wasn't thinking).  However, there is a little trick you can do that I picked up from Elektor magazine.  You can stick a small value resistor (with suitable power rating) between the output of the 24V reg and the input of the 9V regulator to drop some/most of the voltage.

The other thing to bear in mind is that his pedals might all be low power, in which case the max current rating doesn't come into it.  As you rightly point out, though, power ratings need to be factored into the equation.

[darron]

why not just setup two voltage regulators with a voltage higher than the desired 24v?

[Guite Lectrique]

You can stick a small value resistor (with suitable power rating) between the output of the 24V reg and the input of the 9V regulator to drop some/most of the voltage.

Thanks everybody!

How low should the resistor be. I have a 18 ohm 10w resitor at home, would that be ok or should I need a higher value? I would like to have about 400-500mA to supply my pedalboard.

The whole thing is a small standalone preamp powered with a 24v, 600mA wall wart. I would like to add a "DC out" that would supply my pedalboard.

[km-r]

AFAIK...

[ 24 volts minus, say, 11 Volts ] / [ 600 mA, thats 0.6 Amps] = Resistor Ohms

[gez]

How low should the resistor be. I have a 18 ohm 10w resitor at home, would that be ok or should I need a higher value? I would like to have about 400-500mA to supply my pedalboard.

The whole thing is a small standalone preamp powered with a 24v, 600mA wall wart. I would like to add a "DC out" that would supply my pedalboard.

It's a simple case of using Ohm's law.  For example:

L7808CV 8V 1A REGULATOR + 2 Si diodes from ground pin to actual ground = 9.2V (approx)

Max dropout voltage of regulator (according to data sheet) = 2.5V max.  So, a minimum of 9.2V + 2.5V = 11.7V is required at the input of the regulator.  Lets say 12V.

From a 24V supply, we need to drop 24V - 12V = 12V. 

With a max current draw of 500mA (use a fuse from the '9V' regulator's output to make sure you don't exceed this rating), the ideal resistor required to drop 12V = 12/500mA = 24 Ohms.  22 Ohms is the nearest and would drop 11V (good enough).

Power rating of required resistor = 11V X 500mA = 5.5W.  Use a resistor with a higher rating.  You could probably get away with 6W, but I'd still go for higher.  Alternatively, parallel two 6W 47 Ohm resistors.

Don't forget heatsinks.

[Guite Lectrique]

Wow! Thanks Gez for teaching me all this!

One last question: I found out after some research that the 24V power supply I have is made for a printer. I bought it at a surplus electronic parts store. What are the chances this wall wart is not filtered nor regulated? The fact is, I get some loud hum from the prototype I made on breadboard, which I suspect is from the PS. My voltmeter reads 29.5v out of the PS, so I guess it's not regulated right? If so, do I just need to stick a 24v regulator + diodes + caps in front of it?

[gez]

My voltmeter reads 29.5v out of the PS, so I guess it's not regulated right? If so, do I just need to stick a 24v regulator + diodes + caps in front of it?

The diodes in front of regulators are only required if the power supply is AC (they rectify the raw AC), and I take it that your supply is DC.  Yes?  If you're measuring 29.5V DC, this will probably decrease when under load.  If so, then sticking a 24V regulator after it isn't going to work as the regulator will have a 'drop': the input voltage has to be a volt or two higher than the rating of the regulator.  Although it's possible to get 'low-drop' regulators, they may not work too well - a lot will depend on how well your supply behaves under load.

An 18V regulator should work fine (do you really need 24V?), but I'd stick it in parallel with the 9V regulator in this case, rather than cascade the two.