Stupid transformer secondary VA vs current rating question - Page 1
Forum section: electronics / projects

author: SingedFingers
author_rank: Frequent Contributor
timestamp: February 06, 2017, 02:34:38 pm
content: Probably a stupid question. I need an 18v transformer after rectification and smoothing to kick out a solid 1.5A constantly at around 22v. Vigortronix do a relatively tiny 50VA 2x18v toroidal transformer that kicks out "1.39A" according to the datasheet apparently. I assume that is per secondary and that I can parallel these to get 2.8A input current to the smoothing circuit? This will be full wave rectified.Datasheet:https://www.rapidonline.com/pdf/82719.pdfPart # is VTX-146-050-218

author: DmitryL
author_rank: Regular Contributor
timestamp: February 06, 2017, 02:46:33 pm
content: Quote from: SingedFingers on February 06, 2017, 02:34:38 pmIf you have 2 identical windings, you can make full-wave rectifier using just 2 diodes. it will give you twice as less voltage drop on diodes comparing to using classic 4 diodes bridge.

author: capt bullshot
author_rank: Super Contributor
timestamp: February 06, 2017, 03:24:24 pm
content: The rectifier causes a quite high RMS current in the transformer windings, and the transformer VA rating relates to the RMS current in its windings. So you'll need a rather high VA rating for the transformer.As a rule of thumb, you may want to rate the transformers VA to double the outputs DC Watts.In your case, 22V * 1.5A -> 44W -> Transformer should be rated for about 75VA.Paralleling the secondary windings of your transformer will deliver twice the rated current of one winding (so 18V * 1.39A * 2 -> 50VA). Depending on your smoothing capacitor this may be sufficient for your power supply, you'll have to check it (measure the secondary current with a RMS Ammeter).

author: SingedFingers
author_rank: Frequent Contributor
timestamp: February 06, 2017, 03:26:26 pm
content: Thanks. 75VA is still in budget for this so will grab one of them.

author: Ian.M
author_rank: Super Contributor
timestamp: February 06, 2017, 03:41:08 pm
content: Hammond have a good guide to the transformer derating factors for various types of rectifier. See:http://www.hammondmfg.com/pdf/5c007.pdfA center-tapped two diode fullwave rectified secondary circuit can only deliver 80% of the power a bridge rectifier can if the two secondary halves are paralleled. Therefore it only makes sense for very low output voltages, below 5V, where the diode losses dominate. Using a Schottky bridge is usually a better alternative.

author: SingedFingers
author_rank: Frequent Contributor
timestamp: February 06, 2017, 06:09:19 pm
content: Thanks that's really useful. Will read tonight

author: james_s
author_rank: Super Contributor
timestamp: February 06, 2017, 06:36:12 pm
content: I'm a bit rusty in this area, but isn't the current load placed on the secondary of the transformer 1.414 times the current delivered to the load after a full wave rectifier?

author: johansen
author_rank: Super Contributor
timestamp: February 09, 2017, 05:53:25 am
content: Quote from: james_s on February 06, 2017, 06:36:12 pmthat is approximately right but it depends on the impedance of the transformer (the leakage inductance of the transformer is like a choke on the ac side of the rectifier) and the ratio of the capacitive impedance to the load impedance.for centertapped and rectified secondaries, you need 41% more copper in the secondary than the primary for the same copper losses in both the primary and secondary.but its only more efficient to go with a full wave bridge rectifier if the extra voltage drop in the diodes as a percentage of the output voltage is less losses than the copper losses that would be saved if the same transformer was rewound for a FWB instead of center tap.example:I have a 1500VA , 40vac transformer that was used to provide 56 vdc (peak) for a 0-36 volt, 0-30 amp linear supply. it is center tapped, and not because diodes were expensive back then, but because the total power loss would be more for a full wave diode bridge.keep in mind folks that small transformers are horribly inefficient.at 10Va, you have to "regulate away" the primary copper loss and primary coil leakage inductance by running the core completely saturated otherwise your voltage drop under load will be 50%.at 100Va you don't have to do that but power density is so horrible why not run beyond saturation. (not to mention at this level, the surface area to volume ratio is still rather high, so you can get away with wasting 20 watts in a wallwart sized transformer)at 1000VA, you have to run the iron at saturation or just slightly above so to keep the temperature rise below 40Celcius temperature rise under NO Load. --power density is still horrible at this level, around 10 kilograms per kilowatt.at 10000VA, you have to run below saturation and at this power level 98% should be minimum expected efficiency unless you have forced air cooling, if so then peak efficiency could be as low as that of a 10hp motor: 91%Quote from: SingedFingers on February 06, 2017, 02:34:38 pmyes the per secondary current rating is correct because it matches the VA rating of the transformer series.you should be able to get away with a 50VA transformer for your 33watt load.

author: SingedFingers
author_rank: Frequent Contributor
timestamp: February 09, 2017, 06:38:53 am
content: Thanks for the explanation. Very helpful.