Power supply current :VS: Max input current....... help?

[anticpunk]

Hey,

I just built a power supply to feed 2 pedals, A vintage Rat and a Cool Cat chorus. The CCC takes 18VDC at a max of 300ma. The Rat is 9vdc and unspecified. The supply has a Triad Mag transformer (F20-1000) which feeds a 1A bridge Rec. and into a 1000uf smoothing cap. from there it splits out to the two LM78xx regs. and the 9v side also feeds the status LED via a 1K resistor.

Question is, If the chorus say it'll take 300ma MAX, Am I going to blow it to hell feeding it 1A?

Am I going to hurt something with the current available? and if so, is there a way to shed some of the current?

I was under the impression that loads only draw what ever current they required, but it's better to be safe than sorry.

Thanks,

Jay

[Scodiddly]

You're safe - make sure the voltage is the same, and that the supply current is greater than the input current. So 1A is fine to feed a 300mA effect - just as long as the voltage is 9v for both.

[ashcat_lt]

What he said.

Think of it as though the pedal pulls the current, rather than the supply pushing it. It will only draw what it needs. Make sure the voltages are correct and that there's at least enough current and you'll be fine.

[anticpunk]

Thanks for the replies!

The First pedal runs on 9VDC but the second runs off of 18VDC. Both of which I am pulling off of the same transformer but using 2 different solid state regulators to achieve the desired voltages.

so before the regs. I have 20VDC @ 1A which gets cut down to 9V and 18V.

am I still ok?

Thanks,

Jay

[philbo]

The 1 A rating on the rectifier is simply the amount of current the rectifier is guaranteed to handle before it vaporizes. That is not necessarily related to the maximum current the power supply can source - that is set by the current and power rating of the weakest component between AC input and DC output.

A lot depends on what kind of 79xxx regulators you are using - - are they also rated for 1 A each? or are they the 5 A version? or what? The 5A ones can handle a lot more abuse than the smaller types, though they are physically larger.

It might be worthwhile finding out how much current the 9 V pedal draws. Assume it's, say, 400 mA. That 400 mA will have to flow through the 9V regulator chip, which has 11 V across it (20V from the source, 9V used by the pedal, so the remaining 11V is dropped across the regulator (input to output).
This will make the regulator chip burn 4.84 Watts (11V x .44 A). If you don't have it bolted to some metal to suck up that heat, the regulator will die a painful but fairly quick death.

The 18 V regulator should be OK, though, since it only has to drop 2 V at 0.3 A, which works out to about 0.6 watts.

It might be a good idea to bolt both regulators to the case, or to a heatsink, or something to carry off the heat.

Don't forget to put a fuse in the AC line, to prevent your house from burning down if things go very badly.... The fuses amp rating should be 1.5 x (total watts of all pedals and regulators)/110. (110 for the USA. If not in the USA, substitute whatever AC voltage is used where you are.) If you use that formula, a slo-blo fuse will likely be appropriate. The 1.5 is a 'fudge factor' so the fuse doesn't blow from normal operation.
________
Drugtest

[anticpunk]

The 1A rating that I stated before is based on the specs of the Triad Transformer I built around.

The Regulators are rated for 1A each. the 9v one is an LM7809 unit from Fairchild. The 18v is an LM7818 also from Fairchild.

The 9v pedal is a Proco Rat. I have no specs on it. would guess it to be between a 300 and 500ma draw. so I will be drawing somewhere between 600 and 800ma total, of the available 1A.

I hadn't fused the line side, but I think I will as a precaution.

Heatsink wise, I am using sheet copper which I cut and bent to fit. Is that sufficient? with ample heatsinking, should I expect the 9volt reg to live or should I rethink my design? is there a way to lose some of the excess juice that the regulator has to burn without changing the transformer?

on the bright side, with my VOM I am reading a steady 9.02vdc and 17.99vdc which is right on target.

Thanks,

Jay

[anticpunk]

http://www.fairchildsemi.com/ds/LM%2FLM7809.pdf

on page 2 at the bottom, It give maximums. It says that max input voltage for these regulators (in the 5-18volt range) is 35volts. so at 20 I am well inside that.

Page 6 is for the 7809.

-Jay

[brianroth]

My 2 cents worth here.....

If the "raw" DC voltage you have available from the Triad xfmr. + rectifier/filter cap is only 20 VDC, you can have an "issue" deriving 18 VDC.

The "three-legged" voltage regulators have a "drop-out" voltage rating. IOW, you have to provide an input voltage which is several volts higher than the desired regulated output, and 20 VDC is a tad shy of what a garden-variety "3 legger" needs to properly function for an 18V output.

One trick, if you have a xfmr. with enough VA (ie wattage to handle the devices) is to use a centertap on the secondary of the xfmr. to provide a 1/2 VDC line to feed into your regulator(s).

Shrug... I would almost vote to come up with two different supply sources, BUT this one that is "on the cusp".

Bri