How to Connect 3 PSU’s to a Single Switch

[Larsson55]

I’m building a small control panel for my small cnc lathe which contains among other electronics 3 DC power supplies(PSU) of 48VDC/12A, 12VDC/1A and 5VDC/3A. I would like to see in a voltmeter the volts output of each PSU for troubleshooting.

I could not find a single voltmeter with 3 displays so I bought from Amazon a panel mount voltmeter with a 0-100VDC range and I was thinking if I could connect each PSU to one voltmeter with a 3way or 4way switch.

My knowledge in electronics is very basic so I’m asking if it’s possible and how to do it.

Thanks

[bob91343]

It depends on whether the voltages all share the same ground, or negative connection.  If so, a simple one-pole switch will work.  If not, you need a double pole switch.

Wire the meter to the switch wiper and the power supplies to the stationary poles.

[Benta]

Actually, it would be either a SP3T or DP3T switch.
Like these:
https://www.apem.com/us/index.php?controller=attachment&id_attachment=740

You need the ON-ON-ON option.

[CaptDon]

You will probably choose a rotary switch and very likely you can find one with two poles (two independent wipers) and anywhere from the three positions you need up to easily 10 positions. You could harvest the 3 position (sometimes 5 position) rotary switch from an old RS-232 DB9 serial port selector box. One thing to be very careful of, Your rotary switch MUST BE NON-Shorting which are the most common types. Some rotary switches 'make before break' as you switch from one position to the next. That would kill your project dead!!!

[inse]

As voltmeters usually have a high input impedance he could add some series resistors to the sense lines so a false type of rotary switch would not matter.

[Larsson55]

My thanks to all for the help

Searching I only found this switch in UK for GBP 4.24+3.00 for shipping
MINI TOGGLE SWITCH DPDT ON-ON-ON (1 X SWITCH) | eBay

Benta I like your solution and I also have the switch shown in the attached JPG which I don’t think will be any good because the switch it only has 2 positions ON/ON.

My PSU have a common ground (they all connect to the star point ground). Assuming I will get this switch how should I wire the terminals? Looks like there are 6 terminals on the switch and I will only have 3 + 1 for the ground.

Thanks

[CaptDon]

That is a double pole double throw and WILL NOT WORK, and whatever spec said on-on-on IS WRONG!! That switch at best has three positions ON-OFF-ON. Again you need a SP3T since your grounds are common. It will probably take on the form of a THREE POSITION ROTARY SWITCH. Rotary switches with up to 5 positions are very common and may be useful to you. Your solution WON'T be a toggle switch.

[Larsson55]

No problem CaptDon, I found this 3 position rotary switch in Amazon. Will it be good for the job?

Many thanks

[inse]

No, it has only two NO contacts, it is like the toggle switch with a neutral center position.
I would suggest something like Farnell 1123700.
But why don't you use one DVM for each voltage?
Those modules are cheap stuff.
By the way you have to pay attention to the power supply of your DVM module, there are different types: some need a supply separated from the signal, some allow GND to be shared and there are even types that can be supplied from the signal directly.

[Larsson55]

Sorry about it but I searched for a 3 position rotary switch and it came up with the one I posted. Searching for Farnell 1123700 I get nothing about switches. I also don’t know what DVM is but I would like to know about it.

How about this switch?

https://www.amazon.ca/Rotary-5-Position-Universal-Changeover-0-1-2-3-4/dp/B08JLCJ9YS/ref=sr_1_5?crid=2RHVSFI5SSP2C&keywords=rotary+switches+with+5+position&qid=1649399343&sprefix=rotary+switches+with+5+positions%2Caps%2C69&sr=8-5

Thanks

[inse]

The part i suggested is Lorlin CK1060
DVM is digital volt meter

[Larsson55]

Sorry inse but searching Amazon for Lorlin CK1060 brings nothing about switches

The DVM do you mean the handheld ones? I have one but what I want is a voltmeter build into my control panel

[HighVoltage]

Sine panel meters these days are small and cheap, why not using 3 voltage displays?
Then you have no need for switching.

[Ian.M]

Why bother?   Assuming your PSUs are reasonably modern switched mode ones, their reliability and accuracy is at least as good as a cheap Chinese digital panel voltmeter.   For diagnostic use, if you need to be able to check the rails without opening the case,  I would suggest providing 2mm wander sockets on each power rail + ground, so you can simply probe the power rails with a DMM when required.   Use 100mA polyfuses where you tap off the rails to protect against shorts at the wander sockets.

If you require 'live' monitoring use power supervisor ICs that offer a window comparator function to illuminate a 'power good' LED for each rail.  You could use comparators with push-pull outputs, bicolour LEDs and a resistor divider between the rail and ground feeding the LED to show red when the rail is present but out of spec and green when its within tolerance.   

[Larsson55]

It’s true that I can have 3 voltage displays but it’s not the cost, it’s the looks and space available.

My control panel will be about 12” square by 6”-8” deep and the space for all switches/displays will be a strip at the top of the panel 3” wide by 12” long. I will have other ON/OFF switches controlling the cnc + lights so 3 volt displays will take a lot of space which I don’t have.

Then it’s the looks and convenience, it’s much more convenient to turn a switch and see instantly if a PSU fail plus it looks better than having 3 displays.

The 3 PSU are brand new and I don’t expect any problems with them but It’s good to know the voltage when the machine is running. Above all it’s a matter of taste, I just like to have a switch with a single display. Having said that, if it’s too complicated or not safe to have the switch then I can live with that and if and when a problem arrives I will take care of it as needed.

[themadhippy]

https://www.mouser.co.uk/ProductDetail/Alpha-Taiwan/SR2512F-0403-19R0B-E9-N-W-159?qs=8%252Br4Hz5Xir%2F1zaBPQd8WiQ%3D%3D

[CaptDon]

The rotary switch suggested by MadHippy is perfect. These switches can be had with more positions also, 5 being very common and usually in stock. If there are other things you may want to monitor with you single digital panel meter you may want to buy a switch with more than three positions but for now let's at least get you looking at the rotary switch M.H. suggested. It will work perfect. You will need a knob to put on the shaft, the styles of knobs are endless but be sure to match up shaft diameter with knob bore. Cheers mate!

[Larsson55]

Thank you “themadhippy” & “CaptDon” no problem to buy this switch but a couple of things I don’t know.

First how do you wire the switch? Looks like it has a few connections so perhaps use a hand held voltmeter and establish which ones I need?

Then can I get a face plate so when I turn the knob it points to position 1,2,…..?

I will assume Mouser has the knob but I just search in Mouser for knobs?

[themadhippy]

Best to double check,but almost every rotary switch of this type  ive used has  been this way.
Theirs  2  sets of contacts,normally 12 on the outer ring marked 1 -12 and 4 in the center marked A-D,as its a 3 position switch contact A will make contact with either 1,2 or 3,B with 4,5 or 6 C with 7.8.9 and D with 10.11.or 12.Your volt meter connects to 2 of the center contacts,lets say positive  to A and negative  to C, your first power supply positive connect to pin 1  and the negative to pin 7,the second power supply positive to pin 2 and negative to pin 8 and the third psu positive to 3 and negative to 9

I will assume Mouser has the knob but I just search in Mouser for knobs?

Best to check the data sheet first  to see what sort of shaft the switch has,theirs multiple variety's  and whilst some knobs  will fit other shaft types  it don't all ways work

5 being very common

Dont think ive seen an off the shelf 5 position version,not without moving the stop any ways,i was led to believe  the poles and ways always made 12,something to to do with angles and things,hence  1x12 ,2x6,3 x 4 and 4 x3 being the most popular options, at least in the cheaper end of the market

[Larsson55]

Thanks for your time “themadhippy” but looking at the specs of this switch it has some specs which may not be good enough for me.

Rating: 125V AC 0.3A
Shaft Type: 15R0A (round shaft)
4 Poles/2 Positions

One of my PSU is rated for 12A and this switch is only 0.3A. I have no experience here but I think 0.3A is quite low?

Then it says 4 Poles/2 Positions. I thought I need a minimum of 3 positions?

Also it has round shaft and this kind of shaft may not hold the knob tight?

Would the switch below do the job better?

[themadhippy]

One of my PSU is rated for 12A and this switch is only 0.3A. I have no experience here but I think 0.3A is quite low?

But the switch isnt carrying 12A,its only carrying the current the voltmeter needs,probably less than a milliamp

looking at the specs

If you mean the data sheet then thats covering the whole range of switches,if you decipher the part number SR2512F-0403-19R0B-E9-N-W-159 the 0403 bit means 4 pole 3 position

it has round shaft and this kind of shaft may not hold the knob tight

use a knob designed for a round shaft,making sure its the correct diameter,and it will be fine

Would the switch below do the job better?

whats the contact arrangement ? it says 5 way it mentions change over but its only got 8 terminals and theres no clue to the number of poles.

[Larsson55]

Thanks again “themadhippy”, appreciated

As I said my knowledge in this field is quite limited so we forget my comments and I will give it a try.