220v Transformer on 240v. Will it break? Korad KA3005D - Page 1
Forum section: electronics / repair

author: cybertronicify
author_rank: Regular Contributor
timestamp: September 25, 2014, 02:08:51 am
content: Hi everyone!I have a Korad KA3005D and its rated at 220v. However the mains im getting here at my house is 240v, is that 20v difference enough to break my power supply? The Korad is a linear supply and has switching taps.

author: Monkeh
author_rank: Super Contributor
timestamp: September 25, 2014, 02:13:13 am
content: It'll live.

author: cybertronicify
author_rank: Regular Contributor
timestamp: September 25, 2014, 02:14:08 am
content: Ok great, Thanks!

author: sacherjj
author_rank: Frequent Contributor
timestamp: September 25, 2014, 02:16:10 am
content: Power tolerance is often spec'ed at +/- 10% so 198-242V of 220V.

author: cybertronicify
author_rank: Regular Contributor
timestamp: September 25, 2014, 02:45:43 am
content: Does that mean if i have 126 VAC on my mains it is counted as out of tolerance? Im in the US and we receive 3 lines -110 neutral and +110. On the +110 i get 126 VACCould that be why the step up transformer is giving out too high of a voltage? If i ever want to bring the voltage down to 220v, what is the best way? The step up is a 500w model.

author: neuraxon77
author_rank: Newbie
timestamp: September 25, 2014, 02:46:44 am
content: Quote from: sacherjj on September 25, 2014, 02:16:10 amThat. However did cybertronicify measure 240V from the mains, or assume it? Here in Australia the mains was originally nominally rated at 240V, but since the 80's it's been 230V, however I get an actual 252V.+10% of 240V is up to 264V and could mean kablammo.

author: cybertronicify
author_rank: Regular Contributor
timestamp: September 25, 2014, 02:48:28 am
content: Quote from: neuraxon77 on September 25, 2014, 02:46:44 amThe actual output from the step up transformer is 240.4 VAC, round that down and its 240 VAC

author: ignator
author_rank: Regular Contributor
timestamp: September 25, 2014, 04:32:09 am
content: Cybertronicify: Here is a link to a quick description of the US and Canadian power system;http://www.quick220.com/page/cp-voltage.html

author: cybertronicify
author_rank: Regular Contributor
timestamp: September 25, 2014, 08:03:25 am
content: Ok, so i guess 240v into this power supply is fine.

author: tggzzz
author_rank: Super Contributor
timestamp: September 25, 2014, 08:23:41 am
content: Quote from: cybertronicify on September 25, 2014, 02:45:43 amThat is a seriously strange supply - where in the US are you?Can you point to any reference indicating the specification of the supply?Don't you have an earth/ground connection?

author: cybertronicify
author_rank: Regular Contributor
timestamp: September 25, 2014, 09:01:01 am
content: I have done a mini review of it on EEVBLOG FORUMS. (pictures too)https://www.eevblog.com/forum/testgear/220v-lab-power-supply-on-110v-mains-korad-ka3005dI bought that in China where they only have 220v, but who cares i got it for 50 USD!! Here in California, USA we get 126VAC Mains and after a step up transformer (500W), i get 240VAC (all are true RMS measurements) Korad rated 220v 3a input, the transformer says "220V/36V/5A"

author: mikerj
author_rank: Super Contributor
timestamp: September 25, 2014, 09:16:04 am
content: Quote from: tggzzz on September 25, 2014, 08:23:41 amThat's pretty normal in the US as far as I know. There is a local transformer on the power pole that provides a centre-tapped 240v (nominal) supply, 120v is used for most portable appliances, and 240v is used for larger consumers such as ovens, clothes driers etc.

author: tggzzz
author_rank: Super Contributor
timestamp: September 25, 2014, 10:37:26 am
content: Quote from: mikerj on September 25, 2014, 09:16:04 amWell I can quite understand the desirability of halving the current for large appliances, but this is the first time I have heard of such an arrangement.

author: sacherjj
author_rank: Frequent Contributor
timestamp: September 25, 2014, 11:24:10 am
content: Quote from: tggzzz on September 25, 2014, 10:37:26 amAs mikerj pointed out, the feed to the building is 240 with two powered sides and a neutral. The breaker panel in the US has alternating hot legs for each breaker.Pos 1 - phase 1 (120V)Pos 2 - phase 2 (120V)Pos 3 - phase 1 (120V)Pos 4 - phase 2 (120V)If you want 240V power, you install a breaker that takes up two slots and it has two hot lines and neutral. So approx half of the 120V in a building are phase 1 and the other half are phase 2, to balance to a 240V feed to the building. Any 240V loads are already balanced.Picture on the right is a typical panel. Notice the breakers that have two slots with a joining bar between them, vs the 120V that just have a normal toggle switch.In businesses, you often get into 208V and 120V, where the 208V is 3 phase and 120V is each leg of that phase. That is our primary wiring here at work. This allows lower voltage 3 phase power. We have many 220V devices that we run at 208V with no problem.

author: cybertronicify
author_rank: Regular Contributor
timestamp: September 25, 2014, 08:05:50 pm
content: The 240v is for central air. It is connected with 2 circuit breakers each on one of the 110v lines.

author: denelec
author_rank: Regular Contributor
timestamp: September 25, 2014, 09:13:16 pm
content: Quote from: cybertronicify on September 25, 2014, 02:45:43 amIn Canada, CSA Standard C235 specifies that the voltage at the outlet can vary from 104V to 127V.In the US, standard ANSI C84.1 specifies a similar range.http://www.pge.com/includes/docs/pdfs/mybusiness/customerservice/energystatus/powerquality/voltage_tolerance.pdf126V is a little high but still tolerable.Both your 120V lines should be at similar voltage. If one is several volts higher, you may have neutral problem.