12V to 9V adaptor

Frequently asked questions regarding powering your pedal.
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Marko9801
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Post by Marko9801 »

I'm using a 12V (5A) adaptor that's OEM of a laptop, I use it only for overdives and distortions such as DS-1, SD-1, TS9 because I know I won't burn anything... I've made a simple project that I've found on Beavis audio research, a simple pot that sags the 12V all the way to 0V...
Can you tell me what to build to make the 12V voltage always 9V?
Is there a verified project because I don't want to vurn my modulation pedals... TNX!

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Post by mictester »

Marko9801 wrote:I'm using a 12V (5A) adaptor that's OEM of a laptop, I use it only for overdives and distortions such as DS-1, SD-1, TS9 because I know I won't burn anything... I've made a simple project that I've found on Beavis audio research, a simple pot that sags the 12V all the way to 0V...
Can you tell me what to build to make the 12V voltage always 9V?
Is there a verified project because I don't want to vurn my modulation pedals... TNX!
The device you want is a 7809 voltage regulator IC. Bolt it to a metal case to get rid of the heat, and it's good for 1 Amp. You need to fit decoupling capacitors to the IC legs - 100n fron input to ground and from output to ground. Also, at the output side, fit a resistor and LED to show "on". This will provide loading if the effects are all unplugged - some early 78XX regulators had to be loaded or their outputs would wander upwards, though modern ones should be OK.

You can get these in lots of voltage ratings, and their outputs are really accurate and regulated. common ratings are 3V, 5V, 6V, 8V, 9V, 12, 15V, 18V, 24V etc. Obviously they're 7803, 7805, 7806 and so on.

The rules of the game are that you need at least 2V more on the input than you want at the output, so you're OK with 12V in for 9V out. The other rule (that doesn't apply here) is not to try to drop too many volts. The IC will do it (a 7809 has a maximum of 30V input) bt the energy has to go somewhere, and the IC will get HOT!!!
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Post by Marko9801 »

This helps, but you say it's good for 1A, does this mean I can't use it with 5A adaptor? I have like 20 of this adaptors and I only have them... Can I use it with 5A or will it burn ?
This i a new subject to me, my main goal is to make a power supply with enough A to power my pedalboard with 8-10 pedals and to make use ot of this 12V adaptors... It would be great if I could make an option of 12V or 9V.

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Post by Marko9801 »

I guess if I want more than 1A of output I can do more than one of this ICs?

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Post by Marko9801 »

I've researched the subject and ordered PCBs for this project on ebay:
Image
Do the values for the el. capacitors seem too high or does it matter? Other projects I've found use only 10uF capacitors http://www.bpesolutions.com/gadgets.ws/ ... mpower.pdf Are the large values el.capacitors only for use with AC input power?

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Post by mictester »

Marko9801 wrote:I guess if I want more than 1A of output I can do more than one of this ICs?
Yes! However, DO NOT connect the outputs in parallel - the output voltages will be fractionally different, and you'll have some fail or get really hot and die. They are really cheap, so you could use one per effect. This would also minimise power-borne interference effects from one effect to another. One commercial pedalboard power supply I saw used this approach and had a 4 Amp 12 Volt SMPSU feeding eight 7809 ICs with their own associated filter capacitors. Each output went through a 20mm 500mA fuse, so that it wasn't possible to overload the supply.

The circuit above has Electrolytic caps that are too big - the input side of the 78XX is OK, but the output side could be 220µF or even as low as 100µF. C4 needs to be as close to the pins 2 and 3 of the 78XX as you can get it. I usually solder it right to the IC pins! I'll hack your diagram about in a moment, and give you a multiple output version....
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Post by mictester »

Marko9801 wrote:I guess if I want more than 1A of output I can do more than one of this ICs?
Here you go:
78xxSCH.png
I would suggest individual fuses for the outputs, and put the LEDs and their resistors after the fuses to show "power good". This is especially important if you're going to use the PSU to power home made or experimental effects. If something goes wrong, the worst you'll do is blow a fuse!

Also, the capacitors are now more sensible. The 1000µF can be a 25V working type, and the output side can be 16V rated - this will save you both space and money! The 100nF can be any low voltage rating (as long as it's higher than 9 Volts!)

You might have to heatsink the 7809s - if you're building it into a metal box, just screw the tabs to the case with bolts and nuts - the tabs are at 0V and connected to the centre pin of the ICs.

Remember - you said you were going to use a 12V 5 A SMPSU. Switched-mode PSUs have big capacitors inside them that remain charged to very high voltages for a long time after the power is unplugged - mind your fingers! At the very least it will hurt, and in extreme cases, you won't be posting here (or anywhere else) again! Please be careful!
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Post by Marko9801 »

Thank you very much for the schem, I've bought 2 of these PCBs on eBay https://cgi.ebay.com/ws/eBayISAPI.dll?V ... K:MEWAX:IT so I'll first try to build a one output 1A 9V PSU and daisy-chain power a few pedals, and after that try to build more complicated version with many ICs like you said and power each pedal from one regulator...

I just have one more question, since I'm going to plug a DC adaptor on the input of my unit, do I have to use the bridge rectifier component or is it only for AC input power? I've came across on some schems that don't include the bridge or the diodes in front of the 78xx, what does the bridge do?

Thank you!

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Post by mictester »

Marko9801 wrote:Thank you very much for the schem, I've bought 2 of these PCBs on eBay https://cgi.ebay.com/ws/eBayISAPI.dll?V ... K:MEWAX:IT so I'll first try to build a one output 1A 9V PSU and daisy-chain power a few pedals, and after that try to build more complicated version with many ICs like you said and power each pedal from one regulator...

I just have one more question, since I'm going to plug a DC adaptor on the input of my unit, do I have to use the bridge rectifier component or is it only for AC input power? I've came across on some schems that don't include the bridge or the diodes in front of the 78xx, what does the bridge do?

Thank you!
The bridge is there to convert AC to DC (crudely - the 1000µF smooths out the rough DC from the bridge). You can omit it if you're sure of the polarity of the output of your 12V PSU (wire it backwards and you'll do damage). Looking at that board you've ordered - you'll see that it has space for a bridge and the inputs are labelled "AC". Just for simplicity, include the bridge, and connect your 12v DC supply to the "AC" terminals (it won't matter which way 'round!). Also - investigate the LM317 - it an adjustable version of the 78XX series, and the outout voltage is set by means of a resistive divider. The 317 is good for unusual, accurate stabilised voltages.
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Post by Marko9801 »

I've ordered two of these PCBs to try the LM317 version! Thank you very much, this is all the info I need to be sure what I'm doing! TNX again!

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Post by rocklander »

tried to reply to this topic (https://www.freestompboxes.org/viewtopic ... 69&start=0) but guess I can't cos it's in the how to's?
anyhoo... I put together a 7809 filter (220uF on the 9v output and 100uF on the input - I 'think' that's right?) but am getting 10.4V out of the regulator... my question is, is that acceptable? seems a lil high.. but is it likely to damage anything it goes into? one thing that does concern me is the 220UF cap is rated at 10V.. will the half a volt more damage it?

thanks in advance and sorry if this is in the wrong place :-|
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Post by mictester »

rocklander wrote:tried to reply to this topic (https://www.freestompboxes.org/viewtopic ... 69&start=0) but guess I can't cos it's in the how to's?
anyhoo... I put together a 7809 filter (220uF on the 9v output and 100uF on the input - I 'think' that's right?) but am getting 10.4V out of the regulator... my question is, is that acceptable? seems a lil high.. but is it likely to damage anything it goes into? one thing that does concern me is the 220UF cap is rated at 10V.. will the half a volt more damage it?

thanks in advance and sorry if this is in the wrong place :-|
Make sure that the centre pin of the 7809 is connected to ground! This is probably the fault.
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Post by rocklander »

yup, the centre pin is defo common/ground. I pulled the 7809 out of a piece of other kit so perhaps it's faulty?

doesn't change the question though.. is 10.4VDC going to be a bother?
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Post by Gerry de la Sel »

rocklander wrote:yup, the centre pin is defo common/ground. I pulled the 7809 out of a piece of other kit so perhaps it's faulty?
What voltage do you actually have going into the 7809? If it's less than 12V then it may be dropping out of regulation.

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Post by mictester »

rocklander wrote:yup, the centre pin is defo common/ground. I pulled the 7809 out of a piece of other kit so perhaps it's faulty?

doesn't change the question though.. is 10.4VDC going to be a bother?
The 7809 is probably faulty, then. I wouldn't risk using the 10.4V - you might find that it's completely unregulated, and could rise far enough to do damage. Get a new 7809. They only cost pennies!
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Post by Hides-His-Eyes »

10.4 into a 10v cap is a bad idea. You should really over-rate your capacitors by around a third to be totally sure. I personally don't like the idea of smoothing caps that won't take an accidental 12 or even 18v...
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Post by rocklander »

Gerry de la Sel wrote:
rocklander wrote:yup, the centre pin is defo common/ground. I pulled the 7809 out of a piece of other kit so perhaps it's faulty?
What voltage do you actually have going into the 7809? If it's less than 12V then it may be dropping out of regulation.
it's 18V
mictester wrote:
rocklander wrote:yup, the centre pin is defo common/ground. I pulled the 7809 out of a piece of other kit so perhaps it's faulty?

doesn't change the question though.. is 10.4VDC going to be a bother?
The 7809 is probably faulty, then. I wouldn't risk using the 10.4V - you might find that it's completely unregulated, and could rise far enough to do damage. Get a new 7809. They only cost pennies!
yup, am in a smallish town so the local may not have them, and I had this lying round... delivery would be several times the cost of the component, but I'm thinking the same thing.
Hides-His-Eyes wrote:10.4 into a 10v cap is a bad idea. You should really over-rate your capacitors by around a third to be totally sure. I personally don't like the idea of smoothing caps that won't take an accidental 12 or even 18v...
ok, will do this also..

thanks all.
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Post by rocklander »

FYI, put in a new lm7809 and replaced the lower voltage cap and all is working as expected now :-)
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