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Old 11th April 2013, 04:28 PM   #1
cancon is offline cancon  Canada
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Default DIY 25v power supply

Hi there,
I want to build a 25Vdc regulated power supply for a fender rhodes preamp.
I've got a 120v>31.4v (unloaded) transformer I pulled from a turntable. Could this do the trick? My plan was to put it through a full wave bridge rectifier (4x 1N4007), and some suitable filtering RC networks.

Any thoughts?

thanks in advance
adam
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Old 11th April 2013, 06:41 PM   #2
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That transformer could work assuming it can supply enough current. The problem will be finding a regulator that can stand almost 50V on the input. A standard three terminal regulator would need an additional transistor to limit the voltage the regulator sees on it's input.
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Old 11th April 2013, 07:54 PM   #3
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You can use a high voltage regulator like the LT783 (1,25 to 125 volt). Few caps and resistors, easy enough. Depending on the current required, use a suitable heatsink, because it has to drop quite a bit of voltage.
Use UF4002's instead of the 1N4007's, for far less switching noise.

Edit: ...and I like to bypass the main resevoir electrlytic cap with a 100nF film cap for better performance.

Last edited by funk1980; 11th April 2013 at 07:57 PM.
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Old 11th April 2013, 10:39 PM   #4
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Another high voltage regulator is the National/TI LM317HVT. They are under $1 each. Unlike the "LT783", datasheetcatalog.com has the actual datasheet available for download. Don't forget to buy an insulator and heat sink grease on the same order.
Preamps use such little current that you could look around and probably find a 24 VAC or 18 VAC transformer. 24 is common in old modems, furnaces, doorbells, and industrial equipment. The surplus houses like Apexelectronic usually have 120to24's but their shipping costs are a bit of an inhibitor. Transformers are sold by RMS voltage, whereas the DC voltage at zero current is 1.4 times that minus two diode drops (0.7 each). You can estimate the dc voltage out of an unregulated transformer by taking the peak voltage and measuring the internal secondary resistance. Voltage drop V=I*R where I is current. Magnetic design details may limit the current below that value.
When examining old junk for transformers, look at the the voltage rating on the filter capacitors on each winding. Usually the winding puts out DC at about 5/6 of the rating of the capacitor filtering it. Trace the circuit on the PWB and draw the windings on the transformer before removing it from the PWB. I pulled a projection television power supply board from trash beside the road last week. Lots of useful diodes and power resistors, plus the transformer; you have to assume the electrolytic capacitor seals are expired. The IC's had nice heatsinks but Toshiba house numbers, so the usual sources don't have IC datasheets.
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Last edited by indianajo; 11th April 2013 at 10:52 PM.
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