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Old 11th September 2010, 06:40 PM   #1
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Default cheap simple way to drop 3-5v

I have 2 projects that i need a simple way of dropping 3-5vdc while maintaining 3-5a of current, source voltage is less then 35vdc. I was thinking about a series of diodes, but the high current ones are 100x more expensive then the 1amp versions, and parallel diodes is just asking for trouble.

While helping another member he pointed me to this guy:

Buffered LM3886 + overthetop regulated power supply

Using a lm317 with a 2SA1265 (but its 3 bucks and only at partsexpress) or some TIP36C ( i am not up on my transistor knowledge to pick a non over the top alternative)that cost 3 bucks that i can get at mouser/digikey.

Any other simpler cheaper ideas? This isnt for regulation.

Hmm i can get a 5amp LD1084V from mouser for 3 bucks...

Last edited by nightanole; 11th September 2010 at 07:07 PM.
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Old 11th September 2010, 06:58 PM   #2
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If you're looking to drop that many volts off the output of a rectifier filter, and you're using a toroid transformer, it's usually practical to take a few turns off the secondary. Untape and go, most times the secondaries are wound bifilar. Just find the ends, unwind, re-tape. You can always add some bucking turns over the outside of the whole mess, but that's not very clean for a finished setup.

If your application is more complicated than that you'd get farther with a more complete circuit description.
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Old 11th September 2010, 07:23 PM   #3
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In case that would be a solution for you, all you have to do is stuff one of your DVM leads through the core, short them together, and measure the volts per turn. Divide how many volts you want to lose by that number and that is the number of turns that need to come off.

Last edited by Andrew Eckhardt; 11th September 2010 at 07:27 PM.
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Old 11th September 2010, 07:39 PM   #4
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Well for a more complete problem. I have a 41hz amp3 that has 35v caps in it and its sweet spot is 24volts. The power supply is 32-35v unloaded after rectification and power supply caps. I need to knock this back down to at least 28volts.

So the voltage drop has to occur after the rectifier and power caps.
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Old 11th September 2010, 07:51 PM   #5
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So you're using an EI core power transformer, or what?
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Old 11th September 2010, 08:29 PM   #6
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I cant unwind the transformer. Its also used in another project that needs the 35 volts. This is just me using 1 high quality power supply for 2 amps (since im incapable of listening to 2 amps at the same time).

Hmm the amp4 uses a tp2050 which has a max voltage rating of 35 volts. I might be able to just get away with replacing the 35 volt on board caps with higher voltage units.

Lets see, 2 10k high ripple caps, 1 300uf low esr cap, 2 1500uf low esr caps.
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Old 11th September 2010, 08:39 PM   #7
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I'm not sure whether you're admitting the existence of a toroid or not. If that's what you have, the bucking widings are fairly easy. Just add them in series with the load that needs them and leave the transformer alone. It can be done nice and tidy. I've added turns on many toroids over the years to get higher voltages for driver stages, threw some tape over it and left it permanently. I don't think that if the board were to do well with a higher voltage they wouldn't have afforded caps to handle it. Probably not a good plan to change the amp.
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Old 12th September 2010, 01:00 AM   #8
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.....other options are resistor, transistor, MOSFET,.....but calculate your power dissipation. You need 7V voltage drop (35V-28V=7V) at 5A.
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Old 12th September 2010, 03:11 AM   #9
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In the end, might be a good excuse to make a simple regulator for the Amp4. I looked up the datasheet on that chip and it looks like it really can't handle the current for lower than 8 ohms at the highest voltages. While 36V is given the go, 40 is maybe blow up maybe not land. If you make sure no caps launch off the board in a few minutes, you're still pushing it with an unregulated 35 volt supply. A little far from uncommon surge voltage and the chip could easily short. If I were to recap the board for higher voltage, I would at least fuse the supply after the rectifier and put some big zeners after the fuse that start drawing lots of current at 40 volts. A few amps worth of zeners and a fuse would qualify as cheap protection. You don't need to put the zeners or fuse after the main filters, so supply impedance shouldn't be greatly affected. You could use a big transistor as the shunt, driven by a fairly small zener, but ironing that out might not be worth it since you'd be half way to a regulator by then.

Last edited by Andrew Eckhardt; 12th September 2010 at 03:18 AM.
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Old 12th September 2010, 03:44 AM   #10
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Digi-Key - 3KP36A-TPMSCT-ND (Manufacturer - 3KP36A-TP)

If you decide to put some wire on the transformer, which is certainly the most simple and reliable angle, you can use anything good for the current. It doesn't need to be magnet wire. 16 or 14 solid THHN is nice and doesn't need additional protection. Much bigger than the secondary gauge would be a waste.
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