Master-slave switch - Page 1
Forum section: electronics / beginners

author: kalnas
author_rank: Newbie
timestamp: February 17, 2023, 06:57:28 pm
content: I'm looking for simple master-slave switch for 230AC. Master load is 2kW table saw, slave is 1.2kW shopvac. I want the schematic to be totally disabled, while master is off and most designs found on internet have electronics all time connected to mains.Was able to find one design which does what I want -https://www.eeweb.com/master-slave-switch/However it uses 5A diodes which will not handle 2kW load. Looking for suggestions on replacement parts to make it work for my case.

author: james_s
author_rank: Super Contributor
timestamp: February 17, 2023, 07:11:16 pm
content: Use diodes that are rated for higher current, I don't see anything critical about the design.You could also use a current transformer monitored by a comparator.

author: wasedadoc
author_rank: Super Contributor
timestamp: February 17, 2023, 07:21:22 pm
content: What is the objection to a switch that can handle the total current to both saw and vacuum?

author: james_s
author_rank: Super Contributor
timestamp: February 17, 2023, 07:25:06 pm
content: He probably wants to use the existing switch that is part of the saw, controlling it remotely with some other switch could be dangerous. These sort of master/slave switches are very common for wood shop dust collectors, often there will be multiple tools, any one of which will cause the dust collector to power up.

author: themadhippy
author_rank: Super Contributor
timestamp: February 17, 2023, 08:43:21 pm
content: tap off after the switch to trigger a relay ,simples

author: Benta
author_rank: Super Contributor
timestamp: February 17, 2023, 09:06:31 pm
content: I find the circuit quite elegant, to be honest.Whether the LED indicator part is necessary is open, but apart from that it's good.

author: james_s
author_rank: Super Contributor
timestamp: February 17, 2023, 09:16:17 pm
content: Quote from: themadhippy on February 17, 2023, 08:43:21 pmThat still requires modification to the equipment. A current sensing slave switch is passive, you can move the saw around, you can replace the saw with something else, whatever, it just works. It's not like it's a complicated circuit to build.

author: Benta
author_rank: Super Contributor
timestamp: February 17, 2023, 09:35:04 pm
content: ^ +1. Exactly.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 17, 2023, 11:08:40 pm
content: Use a current transformer.In the simplest form, you can just use any toroidal inductor. Then for the primary winding use thick (insulated) copper wire and make a few loops trough the toroidal core. The original winding of the inductor then becomes the secondary winding of the current transformer. You should short circuit this with a low impedance (for example 10 to 50 Ohm) Then you can amplify the resulting signal with an opamp, rectify, peak detection, or do other further processing.You can also easily create double isolation by adding a heat shrink tube over the already insulated primary wire.This setup is ideal for hobby-ing and experimentation, as the exposure to mains voltage is very minimum.This does have always active electronics on the secondary side. This can be prevented, depending on the current transformer you use. It's possible to tap off enough energy though the current transformer to switch some other circuitry.You can also use a regular mains transformer, and use it as a current transformer by putting the secondary winding in series with your load. In that case you have plenty of energy to connect an external circuit (even directly putting an 24Vac relay on the 230V winding may work)There are a few gotcha's which make this approach non-trivial.* The secondary winding must be able to handle the current of your 2.2kW load (which is about 10A).* The High voltage output must be connected to a low impedance (12Vac or 24Vac relay may work).* You could have too much energy pushing though the transformer, which may overload your "relay".* There is more potentially hazardous wiring.* It may be necessary to rewind a transformer. For example a 230V to 6V 2A transformer, then remove the secondary and put thicker wire on it to handle 10A.With a toroidal core the energy output will be much lower, but it's probably enough to drive an "optomos" relay. And the output of an optomos relay is strong enough to drive the gate of a triac.But using an "always on" power suppply would make it easier, and these can be quite low power too.

author: kalnas
author_rank: Newbie
timestamp: February 17, 2023, 11:23:29 pm
content: Quote from: Doctorandus_P on February 17, 2023, 11:08:40 pmI was thinking about it, however all schematics I found were using 'external' power supply, meaning sensing circuit is always on. Is there a reason current transformer can't power the triac directly ?

author: Benta
author_rank: Super Contributor
timestamp: February 17, 2023, 11:28:50 pm
content: Oh dear.Why keep it simple when it can be over-engineered?

author: IanB
author_rank: Super Contributor
timestamp: February 17, 2023, 11:36:26 pm
content: Quote from: kalnas on February 17, 2023, 06:57:28 pmI recall Matthias Wandel has made a solution to this for his workshop. Take a look at that for inspiration?

author: boB
author_rank: Frequent Contributor
timestamp: February 17, 2023, 11:54:41 pm
content: Quote from: kalnas on February 17, 2023, 11:23:29 pmIt should be able to drive that LED in the SSR by itself if the burden resistor and/or LED resistor is chosen correctly.I would try a full bridge rectifier and at least some capacitance so the LED turns on with each polarity half cycle.I suppose the LED and its current limiting resistor could act as the burden resistor too but you might want to make sure the secondary current doesn't go sky high by adding resistor in series with zener diode well above the LED turn on voltage.boB

author: Benta
author_rank: Super Contributor
timestamp: February 17, 2023, 11:59:07 pm
content: I'm not certain that the split-phase US wiring is relevant here...Also that the idea of NOT cutting cables really applies. Not certain, though, didn't watch the full video.I still find the original solution elegant.

author: IanB
author_rank: Super Contributor
timestamp: February 18, 2023, 12:06:55 am
content: Quote from: Benta on February 17, 2023, 11:59:07 pmIf you are referring to the video, you are right, the split phase US wiring is not relevant.What Matthias did was drive a relay off the primary circuit (table saw) and use it to switch the secondary circuit (dust collector). Then he put the whole thing in a project box.Everything used off-the-shelf components with no custom electronics needed.

author: james_s
author_rank: Super Contributor
timestamp: February 18, 2023, 01:10:27 am
content: I don't know if they're still a thing, but they used to make electromechanical current relays too. They were used for hermetic compressor starters and incandescent emergency lamps in HID fixtures. Current flowing through the coil placed in series with a load would keep the armature pulled in.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 18, 2023, 03:22:59 am
content: Quote from: kalnas on February 17, 2023, 11:23:29 pmNo problem at all. You do need a bit bigger transformer so it can steal some power from the sensing side. Linearity will probably also suffer, but that is not important in this application. I thought I explained this well enough in the "photomos" description.Quote from: Benta on February 17, 2023, 11:28:50 pmEach and every project is some compromise between complexity and functionality (and a handful of other factors)...

author: wizard69
author_rank: Super Contributor
timestamp: February 18, 2023, 03:52:16 am
content: Quote from: IanB on February 18, 2023, 12:06:55 amYes this can be very simple.