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Old 6th February 2008, 05:22 AM   #11
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Join Date: Mar 2005
Location: killeen, texas.
thanks for the reply.

these are the measurements i got from the transformer i currently have.
its a dual 45v, so ~44.2Vac 0 ~44.2Vac , thats what it measured with the DMM with out a rectifier.
with the rectifier installed the dc voltage measured at: +39.7Vdc 0 -39.7Vdc , it lost 4.7Vdc because of the diodes.
with the filter capacitors installed it measured: +58.2Vdc 0 -58.2Vdc.

I also measured the voltage with a 4ohm resistor load wire in series to five 12v light bulbs in parallel to each other, just to see how much the voltage dropped, it dropped down to 44vdc and with the amperage meter hocked up in series with the load it read 4.5amp draw and the transformer didn't sound like it was hurting so it may have some more juice left in it.

would you say its safe to use a 60v filtering capacitor or should i go with the 70v?

laters
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Old 6th February 2008, 06:09 AM   #12
hermanv is offline hermanv  United States
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Join Date: Apr 2004
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To pick a capacitor for your transformer, you need to know the power line voltage. Good practice allows for a variation from nominal of +/-10% although +/-5% is more typical.

So to answer your question you need to know if the power line was high, low or average at the time you made the measurements.

It used to be that designers used 115VAC as the most likely nominal, over the years that has increased to 117VAC and now days 120VAC isn't uncommon. The highest I've personally measured was 126VAC.

It is safer to err with capacitors rated higher than the voltage you expect, getting it wrong that way has a tiny impact on life. Using an electrolytic rated too low will result in gases escaping the aluminum can, sometimes explosively. Even if they don't explode, they leave a messy caustic goo all over your circuit

I'd go with 70 or 75V
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Old 6th February 2008, 08:15 AM   #13
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Join Date: Mar 2005
Location: killeen, texas.
thanks to you hermanv and AndrewT for your explanations. now i pretty much understand how to select the proper capacitance voltage rating and more.

some of the instructional files that i found on the internet about power supply design, although very informative, they lacked this necessary pieces of information.
thanks you guy for showing me the missing parts that i needed to know to finish this section of the project.
even though it was a simple thing, using a too low of a voltage or using a capacitor rated way too high, was only going to cost me more and more money. so thanks for saving me some cash. lol

tomorrow ill measure the input voltage to see if it was at low 110v or at high 120v and ill adjust from there. im sure ill end up using the 70v capacitor, but ill check just to make sure.

cant wait to get the power supply built, so i can start designing and building the LME49810 chip amp.

thanks again for all the help.
talk to you guys laters
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Old 6th February 2008, 02:24 PM   #14
hermanv is offline hermanv  United States
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That's what's so great about DIY projects, the opportunity to learn new skills.

Good luck on your amplifier project, hopefully you can enjoy both the building of it and the results.
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