AC power - 2 Pole switch? 3 Pole? - Page 1
Forum section: electronics / beginners

author: made2hack
author_rank: Frequent Contributor
timestamp: April 06, 2016, 11:00:20 am
content: Hi all,I've got a small mixer to which I want to add a power switch.The plug that comes with it has 3 pins. It seems that the transformer is wound to 36VAC with the middle pin being the center TAP and each pin respective to the middle having 18VAC and the two side pins 36VAC as respect to each other.How would you switch the power on off? Ideally I would use a 3Pole Single-Throw, but I don't have any in stock and don't want to wait for an order (if I can find one). Can I get away with a 2Pole Single-Throw? For example, powering off the Center tap pin and one of the side pins? I can't use a Single Pole as power will still get to the board that way.

author: Muttley Snickers
author_rank: Unknown
timestamp: April 06, 2016, 11:20:20 am
content: If it were me in your situation I would simply cut each of the 18 volt feeds via a suitable double pole switch, I don't know what the transformer looks like but you may find that the centre tap is at ground potential anyway if earth is involved.

author: made2hack
author_rank: Frequent Contributor
timestamp: April 06, 2016, 11:42:36 am
content: I haven't opened it up, to me it seems to be isolated from earth ground.The plug is a simple 2 pin plug without the ground lead. Just the live and neutral.I will check continuity to be sure, but I would extrapolate that the center tap is just tapped on the secondary.,

author: Muttley Snickers
author_rank: Unknown
timestamp: April 06, 2016, 11:45:21 am
content: No chance of just cutting the feed to the transformer ?

author: made2hack
author_rank: Frequent Contributor
timestamp: April 06, 2016, 12:31:01 pm
content: Not really. The idea is that they didn't add an on off button to this mixer and as a result to turn it on / off, I have to unplug the lead coming from the transformer. I want to add a button to the back of the mixer so that it is convenient.

author: macboy
author_rank: Super Contributor
timestamp: April 06, 2016, 01:29:26 pm
content: I would definitely leave the center conductor connected, and use the double pole switch to defeat the two 18 VAC lines. I think you will find that the center conductor is grounded (at least to the input/output connector ground/shields of the equipment) at the equipment side and floating/isolated on the transformer side.

author: Delta
author_rank: Super Contributor
timestamp: April 07, 2016, 07:07:55 pm
content: Double pole on the AC legs.

author: djspot
author_rank: Newbie
timestamp: November 07, 2016, 06:57:41 pm
content: Hi,So coincidentally I ran into this post and I'm looking to do the same as the original poster. Different model mixer, but powered exactly the same with no power switch.My question is this...and pardon the ignorance.I can not find the particular style switch as a 2 pole single throw (can only find SPST) so I was curious (insert said ignorance), would it be ok if I wired both 18V (as the OP shows in his diagram) from the power source to the single pole on the switch and then out of the single pole on the switch to each individual + soldering point on the board.I considered running two single pole switches, but I'm not too fond of the idea of mounting two switches. One of which will never really be touched.I've attached a quick sketch of what im thinking!Let me know if I'm likely to blow up my mixer and accidentally set my condo on fire.

author: Brumby
author_rank: Unknown
timestamp: November 08, 2016, 04:03:33 am
content: Quote from: djspot on November 07, 2016, 06:57:41 pmNOYou are going to short out your transformer.Look harder for the right switch - at least DPST.QuoteThe mixer will be fine. You transformer won't..... and as for the condo, thereisa fire risk if you don't have any protection on the transformer and you leave it powered up and shorted for too long.

author: Delta
author_rank: Super Contributor
timestamp: November 08, 2016, 12:14:00 pm
content: Quote from: djspot on November 07, 2016, 06:57:41 pmIf you do that, your mixer will be permanently switched off, and your power supply will melt due to the short circuit you have created...

author: djspot
author_rank: Newbie
timestamp: November 08, 2016, 01:16:31 pm
content: Thanks for the advice guys! Found myself the proper dpst switch. Appreciate the responses.If any of you don't mind, can you explain to me why the short circuit would happen? Just for my knowledge as I try to learn more about this stuff.Cheers

author: Delta
author_rank: Super Contributor
timestamp: November 09, 2016, 10:17:52 am
content: Quote from: djspot on November 08, 2016, 01:16:31 pmErm, well just look at your diagram! You have two power supply wires, and you were going to connect them together!

author: ovnr
author_rank: Frequent Contributor
timestamp: November 09, 2016, 10:37:00 am
content: Quote from: Delta on November 09, 2016, 10:17:52 amAnd since that's profoundly useless to someone who doesn't already know why it's bad:Your mixer power supply is configured to output -18V and +18V. Connecting these together is the same as connecting a 36V line to ground (at least when it comes to the amount of flames that's produced).

author: rch
author_rank: Regular Contributor
timestamp: November 09, 2016, 10:41:53 am
content: Quote from: djspot on November 08, 2016, 01:16:31 pmI don't think it will be very easy for us to do this until you study simple electrical circuits. I would start with the most basic book or website explaining about batteries and switches, conductors (wires) and resistance (like bulbs or other useful electrical devices). When you get the idea of current travelling round a circuit driven by a battery (or other kind of generator) you will be able to see that your two 36v wires are at a different voltage, ready to deliver a useful current to the mixer, which resists current flow. If you connect them together then there is no resistance to current flow in this part of the circuit and much too much current will flow. This is what we call a short circuit, a term which probably comes from the early days of circuit wiring.But all will become much clearer (including why you can't get the different voltages back on the other side of the switch once you have joined them together on one side) when you study the basic idea of electrical circuits.

author: Simon
author_rank: Unknown
timestamp: November 09, 2016, 11:20:20 am
content: Always have a mains switch on the mains side of the transformer. I really don't get all these designers that leave transformers live while powering off the load on the secondary.Sent from my phone so mind the autocorrect.

author: Zero999
author_rank: Super Contributor
timestamp: November 09, 2016, 11:28:24 am
content: Quote from: ovnr on November 09, 2016, 10:37:00 amNo, it's AC so it isn't +18V and -18V. It's split phase: 18V - 0V - 18V.Quote from: Simon on November 09, 2016, 11:20:20 amI agree but wouldn't advise someone will not much experience doing mains wiring.By the way, even 36VAC can give you a shock and needs to be insulated from the user, although it's very unlikely to cause a serious injury so doesn't need to be treated with the same precautions as mains.

author: Brumby
author_rank: Unknown
timestamp: November 09, 2016, 11:47:55 am
content: Quote from: djspot on November 08, 2016, 01:16:31 pmThe answer is visible in your original image:If you match up the two 18V power lines in your 'quick sketch' to this diagram, you will see that you will actually short out the wires that are marked as 36VAC.This arrangement from the transformer is exactly the same concept (but higher voltages of course) as a lot of US households with their mains supply. They get 120V on each side from the centre tap - but 240V when taken across the two 120V - like this:Your sketch would have the 240V shorted out.To understand what is happening, you need to get a handle on thephaseof AC sources.

author: djspot
author_rank: Newbie
timestamp: November 09, 2016, 02:51:40 pm
content: Thank you for all the responses. I'll definitely get to reading up on some beginner books, videos, etc.I was hoping to get an understanding of the issue I would have if I did what I mentioned and you guys did a great job of helping me out.Im the type of person who learns better by being pointed in the right direction and then left to do my learning (which I will do). Kinda like one of those things where I didn't know what I NEEDED to know to understand what I was asking...WOW that's a mouthful!Also, appreciate nobody tearing me apart for not knowing!we all know how forums can be!Cheers!

author: Wolfie
author_rank: Contributor
timestamp: November 10, 2016, 09:26:14 pm
content: Its probably not the DIY answer you are looking for but from the sounds of things, I kinda feel this is probably the best solution to the mixer power switch especially if you are reluctant to put the switch on the mains side of the transformer...Just go to the hardware store and get a switched pigtail. Call it a day. You don't need to solder anything. Yon don't need to mess with mains. It kills the power to the transformer so you are not leaving it hot (which itself is a bad idea).