BK-50VA Transformer wiring help needed. *SOLVED* - Page 1
Forum section: electronics / beginners

author: tahp
author_rank: Newbie
timestamp: October 12, 2016, 01:22:47 pm
content: Hello, I am hoping someone here could make some sense out of this transformer for me. Instructions are in Chinese and google only shows where I can purchase the unit. Do I plug which I am assuming is the primary side to mains and just probe the secondary pins? I need to get 48v DC out of it for a bigger circuit that will eventually power a 300 watt CNC spindleThanks for lookingEDIT:4:20AM friday oct 20th.Figured it out. Working with 110v input on pins 0 and 1 - Outputs 50.4v on pins 12 and 15.

author: singapol
author_rank: Frequent Contributor
timestamp: October 12, 2016, 06:57:49 pm
content: 36V after rectification to DC will give 36 X 1.4 =50.4Vdc.From the label it states: primary input 220V connect terminal 0-1.Secondary : 11-12 =6V11-14 =36V0rPrimary input380V terminal 0-2.Secondary: 11-13 = 24V11-15 = 110V

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: October 12, 2016, 07:41:24 pm
content: Since OP is in the US and mains power in the US 110-120v @ 60hz, this would mean OP will get perhaps just slightly more than half of that. Either a step up transformer will be required, or a voltage doubler circuit could be used.http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/voldoub.htmlHowever, the current requirements also need to be taken into account. Since the CNC spindle requires 300W to operate, a 50VA transformer would fall far short of being able to deliver this. If the 48v is required only to power a control circuit, with the CNC spindle motor being powered separately, then this would probably be OK.

author: tahp
author_rank: Newbie
timestamp: October 12, 2016, 08:00:06 pm
content: thanks for the replies, Yes, the voltage is the biggest concern i have also. it always throws so much off. have to think that it's more hassle then it is worth. Just because the transformer isn't the project this time around. maybe someone else have any other ideas? I will probably just give it to someone that doesnt live in the electronic ghetto known as the states. back to work though..

author: tahp
author_rank: Newbie
timestamp: October 14, 2016, 11:23:37 am
content: Quote from: singapol on October 12, 2016, 06:57:49 pmYou are spot on. Just had to shift the output pins over one for 110v, you know why that would of worked?

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: October 14, 2016, 01:33:02 pm
content: From your comment I assume you mean AC voltage reading across pins 12-15? Doesn't make much sense unless all secondary voltages are given for a 220v input at pins 0-1. All taps would be in series with pin 11 being ground so:11-12: 6v11-13: 24v11-14: 36v11-15: 110vSince you are supplying 110v, i.e. half of that, you would expect to get:11-12: 3v11-13: 12v11-14: 18v11-15: 55vHowever since you are measuring the AC voltage at pins 12-15, you have to subtract the 3v across 11-12, in which case you arrive at approximately 52v across 12-15 which is pretty close. The actual readings you get will also depend on the accuracy or tolerance of the transformer windings, the efficiency of the transformer as well as mains voltage fluctuation.In any case, bear in mind that if you are getting a 50v AC output, then by the time you have rectified this to DC you will get nearer 70v, which may cause damage to your CNC .You might have been able to get closer to the desired DC voltage by taking the output from 14-15, in which case you would get 55v-18v = 37v. After rectification 37v * 1.4 = 51.8vDC. As I mentioned though, I don't believe the transformer will handle 300w.

author: Zero999
author_rank: Super Contributor
timestamp: October 14, 2016, 02:55:16 pm
content: Also note that with under half the rated primary voltage, the power rating should be more than halved. If it's a 50VA transformer, then the 110VAC secondary winding would be rated to just over 450mA, which will remain the same with 110V applied to the primary and just over 50V on the secondary.