Multiplexing a Panel meter with a switch. Will it work ?

Thread Starter

noweare

Joined Jun 30, 2017
109
Hello friends,

I am using a power supply from a desktop for a lab power supply.
I have a panel meter that I want to use to monitor the 3.3, 12, and 5 volt outputs & current from the supply.

When using more than one supply will I get the correct current measurement from the
supply if connected as shown in the schematic ?

Another thing, it seems that a 4 pole, 3 position switches are not cheap. Is it possible to use mosfets as
switches if I can not find a economically priced mechanical switch ?
Schematic_Bench-Power-Supply_Sheet-1_20191120154203.png
 

AlbertHall

Joined Jun 4, 2014
8,551
No, not if it is wired as in the diagram.
You have all three common terminals connected together and SW1 and/or SW2 connect that common point to GND effectively shorting the current meter.
 

Reloadron

Joined Jan 15, 2015
4,594
I would just power my meter off a single supply like 5V or 12V. Rotary switches start getting expensive as you add decks or positions. All you really need to monitor voltage is a single pole 3 position rotary switch, 4 pole if you want an off position. Companies like Electro Switch, NKK and Grayhill to name a few make them as well as I would guess Chinese versions. A simple Google of "3 position rotary switch" will bring up a dozen hits with some about six dollars US. Measuring the current gets a little more involved. You can go with current shunts and amplifiers or for example 50 Amp 50 mV shunts so it scales conveniently to 1.0 mV / Amp. There are also hall effect current sensors which are not expensive, The Allegro ACS723 comes to mind. You need to determine in advance what max current you expect to see? Just make sure as to voltage that any small panel meter you use can measure the same voltage it is powered by.

Your drawing as drawn won't work per Albert's post above.

Ron
 

Thread Starter

noweare

Joined Jun 30, 2017
109
What I wanted to do was measure the different currents put out by the individual supply voltages. If I understand correctly I can only measure the total current from all 3 supply voltages ?
 

crutschow

Joined Mar 14, 2008
23,543
What I wanted to do was measure the different currents put out by the individual supply voltages. If I understand correctly I can only measure the total current from all 3 supply voltages ?
No.
You just need to wire the three switch banks as below so that one goes through the meter and the other two are grounded:

1574299891443.png
 

crutschow

Joined Mar 14, 2008
23,543
@crutschow - Isn't that what I have in the schematic ?
Not quite.
Look at the switch in you schematic.
The current you are trying measure is being shorted to ground by the other switch banks.
(I'm assuming those are ganged switches).
Another thing, it seems that a 4 pole, 3 position switches are not cheap. Is it possible to use mosfets as
switches if I can not find a economically priced mechanical switch ?
Yes.
Below is an example circuit using N-MOSFETs:
(Note that circuit assumes the voltage drop across the current meter at maximum current is less than 0.5V)

1574308275684.png
 

Thread Starter

noweare

Joined Jun 30, 2017
109
@crutschow I'm not a circuits guy, I was assuming the common for each source can't be kept isolated from each other in real world circuits but i guess it must be a fairly common thing.

I liked how you used the diodes to decide what fets to turn on based on what current is being measured.
 

crutschow

Joined Mar 14, 2008
23,543
I was assuming the common for each source can't be kept isolated from each other in real world circuits but i guess it must be a fairly common thing.
Are you referring to each voltage source?
It depends upon how each voltage is generated as to whether they can be isolated.

But in this case they aren't since they all go to the same ground point through the switches.
I liked how you used the diodes to decide what fets to turn on based on what current is being measured.
It's an old logic technique called "diode OR".
 
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