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Reply to "Menards transformer question... How many.."

P paul 2OGR Forum Member SUPPORTING MEMBERMEMORIALIZED MEMBER

Hey Dad, does this help................son #?..........Paul

 
   
You don't, you measure it. – PlasmaHH Feb 3 '16 at 21:16
   
I don't want to measure it, I want to calculate it. I may have twenty LEDs I want to install and I don't want to install them, measure them, then find out I am not satisfied with the result. I want to know in advance. I know there are variables, PF, etc. I just want to know an approximate draw because I might want to put 20 wall warts driving 20 36 watt LED lamps. If so, what is the amperage load so I could size the correct breaker for the 120 volt AC circuit? – russT Feb 4 '16 at 22:08
   
Then we need the full schematics with all datasheets of all parts, since we must calculate the efficiency – PlasmaHH Feb 4 '16 at 22:09
   
You got to be kidding me! – russT Feb 6 '16 at 0:20
   
Do you know how to calculate the efficiency in another way without knowing anything about the device? – PlasmaHH Feb 6 '16 at 8:01

3 Answers 3

<input name="_id_" type="hidden" value="215175" /> up vote1down vote

What PlasmaHH said is the truth. You can only figure that the power was loss-less through the transformer. In other words since the power is (approximately) 3A X 12 VDC = 36 Watts I would estimate that the power on the primary would be the same which is to say:

W = E X I (without the power factor) rearranged 36W = 120 X I <=> 36W/120 = .3 Amps which again is 36W without knowing the power factor, eddy losses etc. You would need to measure the power and phase angle on the primary to know the exact measurements.

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