EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: d4n13l on August 27, 2018, 09:46:59 pm
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Hi,
I've been working on a power supply design. I have a 24V 5A transformer from which I can get ~35V once rectified. However I understand I won't be able to get the 35V 5A.
Can I calculate the max power is possible/safe to draw from this transformer?
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Almost certainly *No*, because you don't have any data on how much the voltage will drop when loaded, including rectifier losses, nor do you have data for its short-term overload ratings.
However, the current derating factor for a bridge rectifier directly feeding a reservoir capacitor is 0.62, so your 5A RMS secondary is only good for 3.1A DC out if that's your configuration. See Hammond (transformer division) Design Guide for Rectifier Use (http://www.hammondmfg.com/pdf/5c007.pdf) for derating factors for other configurations.
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24 volts times 5 amps means you can get a maximum of 120 watts from the transformer. If the output is bridge rectified and connected to a capacitor you will get a maximum voltage of about 35 VDC, no load. Once you put a load on the output the voltage will drop and because of voltage drops across diodes, etc., you cannot expect to get the maximum 120 watts out. As a rough guess I’d say you might get 30 VDC at maybe 3 amps but an unregulated supply like this is of little use for a workbench supply.
If you need a bench supply and don’t have a lot of experience building stuff, it might pay to buy a used or new supply from Ebay or Amazon that will do what you want.
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24 volts times 5 amps means you can get a maximum of 120 watts from the transformer.
Careful -- you're correct up to here, when PF = 1. Otherwise, the general truth is 120 VA of capacity.
If the output is bridge rectified and connected to a capacitor you will get a maximum voltage of about 35 VDC, no load. Once you put a load on the output the voltage will drop and because of voltage drops across diodes, etc., you cannot expect to get the maximum 120 watts out. As a rough guess I’d say you might get 30 VDC at maybe 3 amps but an unregulated supply like this is of little use for a workbench supply.
A FWB rectifier, with cap-input filter, typically has PF ~ 0.5, so you should expect 120 VA * 0.5 PF = 60 W available output. About 2A DC would be reasonable.
Think of it as the price you pay for getting extra voltage out. :) (Indeed, the price is even worse if you use a doubler, say. There, PF can be ~0.2.)
You can always run more, of course, either for higher operating temperature*, or for short durations since the temperature is effectively averaged over a long period of time (minutes?). A momentary load of 4 or 5A DC might be acceptable, say for CNC controller or audio amp applications, since the average will tend to be lower.
*Given suitable considerations. Like, will ambient temperature always be modest? Or is there a fan? Or is lower operating lifetime acceptable? That sort of thing.
Tim