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Maximum power from a transformer?
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d4n13l:
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?
Ian.M:
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 for derating factors for other configurations.
ArthurDent:
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.
T3sl4co1l:
--- Quote from: ArthurDent on August 27, 2018, 10:27:43 pm ---24 volts times 5 amps means you can get a maximum of 120 watts from the transformer.
--- End quote ---
Careful -- you're correct up to here, when PF = 1. Otherwise, the general truth is 120 VA of capacity.
--- Quote ---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.
--- End quote ---
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
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