AC Transformer output current - Page 1
Forum section: electronics / beginners

author: 21miki21
author_rank: Newbie
timestamp: November 24, 2022, 05:38:20 pm
content: Hi, I need a little advice about AC output current of transformer. So here is my setup:Got transformer with output 24VAC 6,7A (160VA). Iam not sure of anything in AC circuits, not designed anything in AC circuit. But now I plan to build simple linear DC power supply and have to determine how much power can I draw from that transformer for long time without burning it.So, I put bridge and 3x2200uF capacitors at the secondary. Loaded that setup with dummy load at 5A. Uni-T and clamp meter are measuring AC current before bridge rectifier, Aneng measure DC voltage on capacitors and "oscilloscope" measure DC voltage ripple at capacitors.Clamp meter says 8,37A of AC current and Uni-T 7,51A. And ripple is very high, more capacitors did not change the game so much. My maximum output voltage after stabilisation will be at the lowest point of the ripple, that is not problem, but problem is that AC current before bridge rectifier. Iam not sure what maximum DC current can I draw after bridge and capacitors without burning out the transformer coil in long operation time.Thank you for your advice and help!

author: TimFox
author_rank: Super Contributor
timestamp: November 24, 2022, 05:45:19 pm
content: I recommend PSUD2 freeware from Duncan Labs to simulate "traditional" rectifier circuits.https://www.duncanamps.com/psud2/Note that this software requires the no-load voltage and effective secondary resistance of the transformer, while transformer manufacturers specify the loaded output voltage at a maximum load current.The freeware includes a discussion of how to calculate the former values from the latter specifications.The software will calculate the transformer current, ripple current in filter capacitors, current in rectifiers, etc.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 24, 2022, 05:56:27 pm
content: this may shed some light

author: tunk
author_rank: Super Contributor
timestamp: November 24, 2022, 06:08:18 pm
content: No expert, but I think a rule of thumb is to use ~2000uF/A,and that this will give you ~4V ripple. So in your setup youshould get around 6-7V ripple. And remember that the ACcurrent draw isn't continuous, it only draws current when thecapacitors are charged. This could explain the currentdifference on the load and multimeters.

author: wasedadoc
author_rank: Super Contributor
timestamp: November 24, 2022, 06:14:55 pm
content: For a given load current the more you reduce the ripple by using larger value capacitance the shorter the diode conduction time becomes. That means the peak current in the diode(s) and the transformer becomes larger. More loss (and more heat) in the transformer. Also higher ripple current in the capacitor which means more necessary to use a low ESR one and (whatever ESR) a shorter life.

author: jonpaul
author_rank: Super Contributor
timestamp: November 24, 2022, 06:18:19 pm
content: 160 VA xfrm ratingload w/efficiency % is power deliveryUse lytics about 10x 30x less ripple, caps must be rated for ripple current.See 1934 General Electric paper for Schades curvesJon

author: tunk
author_rank: Super Contributor
timestamp: November 24, 2022, 06:23:09 pm
content: You could try a web search for "unregulated power supply".The first one that shows up:https://www.electronics-tutorials.ws/blog/unregulated-power-supply.html

author: 21miki21
author_rank: Newbie
timestamp: November 24, 2022, 06:42:14 pm
content: Thank you for your replies, appreciate it. I realized why clamp meter was showing a lot more of current, it was simply very near the transformer. Near transformer it shows current even when not attached to wire. Placed it at wire near Uni-T meter and they measure +- the same. And also added 3300uF capacitor and Vpp is now about 2,8V which is acceptable but in final will add a little bit more. I use low ESR capacitors (got plenty of them, using to replace vented caps in SMPS).... And at output DC current 4,3A it draws +- 6,7A from transformer which is ok for its rating.

author: floobydust
author_rank: Super Contributor
timestamp: November 24, 2022, 06:59:12 pm
content: Quote from: TimFox on November 24, 2022, 05:45:19 pmI didn't find it accurate at all- I can't remember but the rectifier diode drops or transformer properties seemed to be missing some numbers. Nowadays I just end up using LTSpice. I was going to let Duncan Amps know but who's got time for thatOP's surprise I think will be the no-load DC voltage he gets.

author: TimFox
author_rank: Super Contributor
timestamp: November 24, 2022, 07:15:36 pm
content: Since the no-load AC voltage at the secondary is often quite a bit higher than the spec (loaded) value, the no-load DC voltage will be higher than anticipated from the name-plate AC voltage.Personally, I have found PSUD2 accurate for power-supply design (with careful estimation of the transformer voltage and resistance) at 60 Hz, with good values for capacitor ESR and the rectifier diode models supplied.I believe that the computation engine inside it is Spice in .tran computation mode.I especially appreciate the built-in calculations of RMS, mean, and peak values, depending on which is more relevant.Since the leakage inductance of the transformer is not included, the software is not useful for predicting transients due to diode switching and the snubbing networks used to reduce those effects; these can be computed either analytically (if the leakage inductance is known) or in a separate SPICE simulation.