Re: Calculating DC Output Current From Unregulated AC Transformer


<emailaddress@xxxxxxxxxxxxx> wrote in message
news:c66df173-a1f2-4fbe-a446-c3b0b0b353a9@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I have an application in which I need to determine if a transformer is
suitable.

Transformer is rated for 12VAC, 1.67A but I need DC. I could put a
bridge rectifier and capacitor after it easily enough but how does one
go about calculating the DC output this is capable of?

I've previously used a x 1.41 factor to convert which would give
16.92V, minus the forward drop of a couple silicon diodes in the
bridge rectifier estimated at roughly ( 2 * 0.7V, ignoring changes in
diode forward drop at different current) which would leave (16.92 -
1.4) = 15.5V, but am I correct in thinking this is peak DC output and
there is a different calculation needed to arrive at the output
voltage if the load were drawing 1.67A?

Perhaps a little more info about the project is in order. Following
the transformer there'll be a bridge rectifier, smoothing cap... then
a linear regulator suppling constant current to charge batteries. The
circuit is a bit more involved than only this (protection diodes,
charge controller, etc) but this is the subsection in question and the
rectified output of the transformer will need to stay above roughly
12.3VDC so I'm trying to figure out what constant current this
transformer (or others) can supply. I've alread accounted for other
looses like that through the regulator when coming up with the 12.3V
figure.

How much rectified DC voltage can that transformer maintain at 1.67A?
How much current can it maintain while staying at or above 12.3V, or
if there is another voltage to consider because we don't know the
other properties of this transformer, what voltage would that be and
at that voltage what is the DC current capability?

Please I'm asking to learn how to calculate this myself instead of
only the numerical answer.

I find it easier to just simulate the circuit using LTSpice and adjust
values until they are close. I used a voltage source with 18 V peak at 60
Hz, and internal resistance of 0.8 ohms, which is probably about right for
a 20 VA transformer with about 10% regulation at full load of 1.67 A. I
added four rectifiers in FWB, a 2000 uF capacitor, and a 12 ohm load. I got
1.71 amps RMS from the voltage source, which dropped to 11.5 VRMS under
load. The output power is 13.2 watts, with 12.5 VDC as you require, and a
little over one amp. So in this case the 20 VA tranny is derated by
13.2/20, or about 65%. That's right in the range of 0.5 to 0.7 suggested by
Phil. Some of this is due to rectifier losses, which I estimate as about 1
or 2 watts. Schottkys will help a little.

The ASCII file follows. You can take it from there.

Paul

==========================================================================

Version 4
*** 1 880 680
WIRE 240 144 128 144
WIRE 288 144 240 144
WIRE 384 144 352 144
WIRE 416 144 384 144
WIRE 512 144 416 144
WIRE 528 144 512 144
WIRE 528 160 528 144
WIRE 128 192 128 144
WIRE 416 192 416 144
WIRE 240 256 240 144
WIRE 288 256 240 256
WIRE 416 256 352 256
WIRE 416 304 416 256
WIRE 528 304 528 240
WIRE 528 304 416 304
WIRE 640 304 528 304
WIRE 640 336 640 304
WIRE 128 368 128 272
WIRE 288 368 128 368
WIRE 384 368 384 144
WIRE 384 368 352 368
WIRE 128 480 128 368
WIRE 288 480 128 480
WIRE 416 480 416 304
WIRE 416 480 352 480
FLAG 640 336 0
FLAG 512 144 V+
SYMBOL diode 288 160 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName D2
SYMATTR Value MUR460
SYMBOL polcap 400 192 R0
SYMATTR InstName C2
SYMATTR Value 2000µ
SYMATTR Description Capacitor
SYMATTR Type cap
SYMATTR SpiceLine V=63 Irms=2.51 Rser=0.025 MTBF=5000 Lser=0 ppPkg=1
SYMBOL voltage 128 176 R0
WINDOW 3 -11 133 Left 0
WINDOW 123 0 0 Left 0
WINDOW 39 24 44 Left 0
SYMATTR Value SINE(0 18 60 0 0 0 100)
SYMATTR SpiceLine Rser=.8
SYMATTR InstName V1
SYMBOL res 512 144 R0
SYMATTR InstName R1
SYMATTR Value 12
SYMBOL diode 352 272 M270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName D3
SYMATTR Value MUR460
SYMBOL diode 288 384 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName D1
SYMATTR Value MUR460
SYMBOL diode 352 496 M270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName D4
SYMATTR Value MUR460
TEXT 184 528 Left 0 !.tran 1


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