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Old 1st November 2005, 12:05 PM   #1
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Default Quick way to drop 2V?

Hey, all!

I need a 1A 10V power supply for MIDI equipment (so audio concerns don't apply here), and don't want to build a regulated PS from scratch. I also don't want to use a store-bought "brick", as these don't play nice with the power bar in my equipment rack.

I have a 12V 2A regulated SMPS out of some cisco gear that almost fits the bill, and will fit nicely where I want to put it.

Can I just use three big diodes in series to drop the voltage? (e.g. MUR860s or similar).

I also seem to recall a trick from high school electronics, where we floated the ground of a 78xx regulator. Could I use a resistor to float the ground of a 7809 to achieve the regulation desired? Obviously, at 1A, the 7809 will need a heat sink.

Any obvious gotchas in either of these plans? (the most obvious one for me is -- will the SMPS be upset that I'm ignoring the 5V and -12V lines..

Thanks,
Wes
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Old 1st November 2005, 12:24 PM   #2
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Check once again if 12 volt is possible. Maybe the MIDI thing has a internal voltage regulator?
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Old 1st November 2005, 12:32 PM   #3
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Hi Wes,

if you know that your load is taking quite constant current of 1 Ampere you can just add a 2 Ohm resistor in series and get your 2 Volt dropp, easy isn't it!?

Cheers Michael
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Old 1st November 2005, 12:39 PM   #4
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If it is a SMPS then it may have an output voltage adjustment trimmer. Also, the output voltage may be probably changed by playing with some resistor divider that feeds a fraction of the output voltage to a comparator, but finding it and understanding how it works may be a quite hard if you don't have any experience in that matter.
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Old 1st November 2005, 02:06 PM   #5
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Peranders -- geez, why I didn't I think of that? Too early in the morning, I guess. Either that, or I got scared by the warnings in the user manual. "The power supply model XXX is the ONLY power supply that will work!". (Yeah right)

As soon as it shows up, I'll crack it open and see if it does indeed have an internal regulator. I have used that trick more than once, but having recently ended a 10+ year break in the electronics hobby, I seem to have forgotten a lot of simple things!

Michael -- Thought about the resistor, even have some 2 ohm monsters around (50W?). But you can be fairly confident the load won't be constant, as it is essentially an 80's microcomputer. Load will vary (at least somewhat) with data transmission and CPU load. Oh, yes, and it will definately go up when it spins up the disk drive.

Eva -- interesting, I hadn't considered that. I have a hard time getting my head around SMPS, mostly because I lack the requisite background and have forgotten most of the higher maths I once knew. If I crack open the power supply and find some trim pots, what are the odds I can damage the SMPS by carefully fiddling with them? I would imagine that critical things like switching frequency would be fixed with crystals or something.

Thanks,
Wes
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Old 1st November 2005, 03:40 PM   #6
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My fax had a power supply which was specified as 24 V, 3 A and got broken (hopeless to fix ) but the new sparepart was 24 V, 1.5 A! So specifications of a power supply says only how much you'll need to be sure. If you MIDI consumes 1 A at 10 volts it would be pretty warm in the enclosure is small. My guess is that most of the stuff inside are 5 volt circuits.
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Old 1st November 2005, 03:43 PM   #7
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Quote:
Originally posted by wes-ninja250
...switching frequency would be fixed with crystals or something.
... hardly.. I have never seen a crystal in a SMPS not that I'm saying it doesn't exist.
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Old 1st November 2005, 04:16 PM   #8
Eva is offline Eva  Spain
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If there is any potentiometer, it will usually be either for output voltage trimming or for current limit trimming. Critical parameters are almost always fixed, especially in small SMPS.

Also, if you post detailed pictures of both sides of the SMPS PCB and/or the MIDI equipment, we may be able to trace the circuit and find out how to adjust output voltage and/or wether the MIDI equipment will handle a 12V PSU.
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Old 1st November 2005, 05:45 PM   #9
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Thanks Eva. I'll try and crack the PSU tonight; I have to hang on 'till the other piece of equipment arrives (doing the PSU first because I need to start using it as soon as it gets here!)

Thanks,
Wes
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Old 1st November 2005, 07:21 PM   #10
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Two 1N4001 (or any other thru 1N4007) diodes will make a 2-volt drop. And their current rating is supposed to be exactly 1 Ampere.
This is perhaps the cheapest and most painless method, because the diodes are often sold by kilos and can be easily implanted in the power cable with a heat-shrinking tube.
But I'd use it only if no adjustment is possible or the device actually draws maximum 10V.
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