ATX Power Supply - Rotary Switch for voltmeter/ammeter display - Page 1
Forum section: electronics / beginners

author: quadrant
author_rank: Contributor
timestamp: April 23, 2020, 09:37:46 pm
content: HelloI'm currently attempting to make a bench power supply from an ATX power supply. In several tutorials on Youtube (such as this one), a "dual voltmeter/ammeter" is used to show the current and voltage delivered by the power supply. A non-shorting rotary switch is used to choose which rail (3.3V, 5V, or 12V) is displayed by the meter. For example, here is the schematic from the above tutorial:I am building the power supply with the goal of safely supporting up to5 amps. The most affordable rotary switches I can find (less than $10 USD), are rated for low DC currents. When I look for something that can handle 12VDC and 5A, the prices go above $20 USD. Some examples from Digi-Key (one,two):For this application (switching the rail displayed by the meter), is itsafe to use the lower ratedand more affordable rotary switches? If not, what is a wallet friendly alternative to using a rotary switch?

author: themadhippy
author_rank: Super Contributor
timestamp: April 23, 2020, 11:01:51 pm
content: Quoteyes,the only current going through the switch is whatever current the meter requires and that aint much.

author: Whales
author_rank: Super Contributor
timestamp: April 23, 2020, 11:08:49 pm
content: Indeed this design only uses the switch to select which voltage rail is being measured. Few uA or mA will travel through the switch at most.Interesting idea to use the common ground through the ammeter; but I don't like it.Note 1: This means that the black terminal of this PSU will not be at ground potential, instead it will a few hundred mV to a few V above ground (depending on the current shunt used by the little current meter + the amount of current you are drawing). If you hook up the ground of an oscilloscope, signal generator or any other grounded equipment then you will getunexpectedresults.Note 2: Check the current specs of your panel meter, make sure it can do what you want "continuously". Those tiny little connectors don't look 5A rated.

author: quadrant
author_rank: Contributor
timestamp: April 24, 2020, 01:23:16 am
content: Thank you all for the replies! This is good to hear.@WhalesThank you for the additional notes!QuoteIf I remove the connectors, and instead solder directly onto the PCB (as mentioned in the video), is this problem bypassed? Are the relevant traces in the PCB thick enough to safely handle continuous 5A?

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 24, 2020, 01:25:41 am
content: Quote from: Whales on April 23, 2020, 11:08:49 pmThat current circuit connector can handle 5 A.

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 24, 2020, 01:44:35 am
content: Quote from: quadrant on April 24, 2020, 01:23:16 amNo, that will not fix the problem. In addition to the ground voltage offset problem, the ammeter will display the total current in all supplies which is really quite useless. The rotary switches can be had on eBay for a $1. There is no point in wasting $10 on decent switch to use on what is a throw away computer power supply.You would need separate ammeters, one for each supply with an isolated power supply for each one for those cheap digital meters to function correctly. You cannot switch the ammeter like you can with the voltage, even if the switch could handle the current. Think about it and if you cannot figure out why you cannot do it, ask.In the end, you’ll just have a noisy power supply with crappy regulation and no current limiting. Converting an ATX supply into a “bench supply” is an exercise in futility.

author: Whales
author_rank: Super Contributor
timestamp: April 24, 2020, 04:53:45 am
content: I used an ATX power supply as my bench supply for years, I couldn't afford a better one. Still now I use a PSU I've made rather than a pre-built.quadrant: I would personally skip the in-built meter and just use cheap DMMs, that way I'm in control of exactly how things are measured. May or may not suit you.EDIT: those panel meters are low-side-measure only, so you can't easily convert them to avoid this problem unless you use separate panel meters for the A and the V.

author: Gyro
author_rank: Super Contributor
timestamp: April 24, 2020, 08:31:03 am
content: The one important parameter that you need to check. You must ensure that the switch is NOT a make-before-break type. if you used one of these it would short the rails together while changing position.P.S. After looking them up, the two Digikey ones you listed appear to be break-before-make, so are ok.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 24, 2020, 01:26:23 pm
content: Quote from: WattsThat on April 24, 2020, 01:44:35 am- I think it is absolutely not a problem because one will usually use only one voltage at time. Even if two voltages simultaneously - it is ok.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 24, 2020, 01:29:56 pm
content: Quote from: Gyro on April 24, 2020, 08:31:03 am- That is very useful note. There is a hint how to use any type rotary switch - just put some ballast resistors in series with every voltage. For example, >=470 Ohm, or even 1-2 k. such a value won't change readings (or accuracy).

author: quadrant
author_rank: Contributor
timestamp: April 24, 2020, 09:48:29 pm
content: Is my following understanding of why the ground potential difference is a problem correct?The ground potential difference arises because the current shunt in the ammeter is a low value resistor. As such the path to ground via the ammeter is not direct, but instead must pass through the shunt (a very small, but non-zero load). As such, a circuit that is supplied with this PSU does not have a direct path to ground - the current must pass through the shunt before reaching earth ground. If you connect measurement equipment (such as an oscilloscope) that has a direct path to earth ground (there are no obstacles in the way), the current from the PSU will opt to go through the measurement equipment's path to ground (since lower resistance than going through the ammeter's path). This might be a problem if the circuit is using more current than the measurement equipment can safely handle?

author: quadrant
author_rank: Contributor
timestamp: April 24, 2020, 09:57:33 pm
content: Sorin made a followup video on the PSU. Among the issues he addressed was specifying the need for a non-shorting (break-before-make) rotary switch. Apparently the arc caused when switching between poles when using a make-before-break rotary switch, makes some PSUs turn off (as a safety feature).

author: Whales
author_rank: Super Contributor
timestamp: April 24, 2020, 10:27:44 pm
content: Quote from: quadrant on April 24, 2020, 09:48:29 pmYes. Additionally:- Voltages will droop under load (eg 0.2 ohm shunt @ 3A = 0.6V) and then this drop will vary depending on what test lead/equipment gnds you have attached.- Ammetter will show incorrect numbers (current will be taking multiple paths to ground)- EDIT: some test equipment (eg oscilloscopes) will read less accurately because of the larger current going through the test leadsHigh-side current measurement (where your ammetter connects between your load and the + of the power supply) is generally preferred. You still have a small voltage drop, but it's more practical to share grounds with your other equipment.The way it's done in better equipment is to regulate the voltage after the current meter. This isn't easily possible here without extensive modification and testing.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 25, 2020, 06:00:21 am
content: There is no need to change anithyng. That Chinese VA-meters have 0.01-0.05 Ohm shunt - it is ok.