How to Step down from 12v - 9v?

[Hero999]

You can test a pedals current consumption by putting it in series with a multimeter set to measure current and connecting it to a power supply.

 

[audioguru]

Why would a guitar effects circuit draw 300mA? It probably draws only 3mA.
If it draws as much current as 300mA then maybe it has an incandescent light bulb/heater or motor in it.

 

[belpha]

Why would a guitar effects circuit draw 300mA?

I don't know why, but this is what we find on the label!

i.e. check this picture **broken link removed**

 

[audioguru]

The label says to use a 300mA AC/DC adapter, not that it uses 300mA. Its voltage is 9V when the load is 300mA but its voltage is probably 15V when its load is only 3mA.

If you use a 100mA adapter then its voltage will be much too high (maybe 20V) with a load of only 3mA and its voltage will vary all over the place.
If you use a 1A adapter then its voltage will probably be "only" 10V with a load of 3mA which might be too low.

 

[Hero999]

That's a point, it might use an unregulated adaptor.

Have you measured the voltage from the pedal's adaptor?

The chances are you can power it from a regulated 12V power supply without an LM7809.

 

[crutschow]

Since the power supply is already regulated the fluctuation will not be there. you could also just put a 3 volt light bulb in series with the 9 volt load to create a 12 volt load. An Incandescent bulb will take up the fluctuation as well. The bulb must be a larger current draw than the amplifier or it will simply act like a fuse.

An incandescent light bulb would have to carefully sized for this purpose since they have a very large resistance change, something like 1:10 from cold to their operating voltage.

 

[punkie]

Hi there!

I have a power supply taken from an old compact pc. It's 12 V - 2.5 A output.
I read the previous posts, so I tried to calculate the value of the "simple-resistor" version.

Could you confirm that I'm right?

from 12 V - 2.5A to 9 V - 2.5 A
R = (12 - 9) / 2.5 = 1.2 ohm
W = (12 - 9)^2 / 1.2 = 7.5 W

So for easly convert the the voltage I need a 1.2 Ω 10W resistor?

Thanks and sorry for my bad English.

hi belpha,

did you work it out? I'd like to do the same thing essentially (step down 12v to 9v from an old pc psu so i can run some fx pedals off it). I know next to nothing about electronics, and my math was never great, but a while ago i bought a step down regulator for a servo in a rc model, it takes 6v and puts out 4.8v, it seems to do this fairly simply using a rectifier i think, the printing on it is "ER503 GW", possibly something similar to this i assume.

Can anyone tell me whether it would be possible to use something like this to step down the 12v to 9v, and assuming that there is enough current, run some fx pedals on that? If so, can you point me in the direction of a rectifier that would do that? or what to look for?

one thing concerns me, as i wouldn't wanna fry a pedal, is that would too much available current would be bad news for an fx pedal...they only take as much as they need don't they?

 

[audioguru]

A recifier diode in series with the load reduces the voltage only 0.6V to 1V depending on the current the load uses.
A light bulb uses a fixed amount of current and a resistor can be used to reduce the voltage.
But an electronic circuit changes its current requirement nearly all the time. When it uses a low current then the voltage will be too high if a resistor is used to reduce the voltage calculated when it draws its max current.
A power supply that can supply more current than needed is fine because the circuit draws only as much current as it needs.

With a 12V supply and only 9V at up to 1A is needed then you can use a 7809 voltage regulator plus the input and output capacitors shown on its datasheet. If the load current is near 1A then the regulator will dissipate 3W of heat so a little heatsink will be needed for it.

 

[punkie]

hmm.. thanks

A light bulb uses a fixed amount of current and a resistor can be used to reduce the voltage.
But an electronic circuit changes its current requirement nearly all the time. When it uses a low current then the voltage will be too high if a resistor is used to reduce the voltage calculated when it draws its max current.

i like the sound of a light bulb, because it sounds simple, (like me!) does that mean if i stuck a 3v led in line i could reduce the voltage to 9v? my understanding of an led is that it can still operate with slightly more or less voltage than it's rating, so is it possible that it could act as a kind of buffer? just using up the extra volts?

A power supply that can supply more current than needed is fine because the circuit draws only as much current as it needs.

sweet... that's what i thought

With a 12V supply and only 9V at up to 1A is needed then you can use a 7809 voltage regulator plus the input and output capacitors shown on its datasheet. If the load current is near 1A then the regulator will dissipate 3W of heat so a little heatsink will be needed for it.

this sounds like the proper way to do it, although a bit more complex than my head can easily deal with (without someone to really dumb it down for me!) and I'd like the option of being able to run more than 1A off the psu...

do you think it could work with a 2.8v led in series with each fx pedal if the led had a 1.7 - 4.0v operating range?

thanks for your wisdom audioguru!

 

[audioguru]

Most ordinary LEDs have a max current of only 30mA. If your pedal draws more than 30mA then an LED in series will smoke then die.

 

[punkie]

Most ordinary LEDs have a max current of only 30mA. If your pedal draws more than 30mA then an LED in series will smoke then die.

duh yeah....that makes a lot of sense... how about something like this?
**broken link removed**

I'm really trying to avoid having to do anything that seems like it could be complicated!

 

[audioguru]

A light bulb is a resistor. If it is in series with a pedal then the voltage drop across the light bulb changes as the pedal's current changes. Then the voltage to the pedal will be too high sometimes and too low sometimes.

You need a 7809 voltage regulator IC.

 

[punkie]

I guess that the reason I'm resisting that thought is that i don't know how to read circuit diagrams and don't understand most of the information on datasheets. I've never really gone deeply into that side of electronics...

is there any way you could spell it out for me in laymans' terms exactly what i would need to do to make the circuit? Also, do you know of a good place that would sell LM7089 retail?

 

[audioguru]

I would simply post the schematic that is on the datasheet of the 7809 voltage regulator IC. But you can't read it so I won't.

I know millions of places that sell the voltage regulator IC but you don't say which planet you live on so I won't.

Sorry, but I have not been a layman for about 52 years and I can't remember what it was like.

 

[punkie]

ok...point taken, we can't all be electronics buffs, heck, if i was, i wouldn't be asking stupid questions here. thanks anyway for answering some of my dumb ass pluto questions.

 

[mneary]

If you want the part fast, Torrance Electronics 1545 West Carson Street, Torrance, CA probably has them. Just around the corner.

 

[punkie]

thanks...I'm gonna stop bothering you guys now.