Insulated 230 VAC Power Supply + Variac

[frantal]

I have built a project of an insulated Power Supply 230V AC followed by a Variac.
I would add a circuit to obtain DC output.
The voltmeter I bought has only 3 poles: one + = power supply, one = to test the voltage and common negative.
Could I connect all as I show in the schematic attached?
Many thank for the help and suggestions.

[bob91343]

It's not clear what you have and what you are asking.  What voltmeter has three connections?

[Gyro]

What voltmeter has three connections?

A digital one that reads down to 0V.

[bob91343]

The diagram must not be complete.  It's not well drawn, as it's hard to see where things go.  I was looking for the 5V terminal on the voltmeter but the diagram doesn't show it.

[rcjoy]

Assuming that panel meter works off 12 VDC, it seems it should work.

I like the soft start feature.

[ledtester]

I recognize the guy's handwriting... it's from the Post Apocalyptic Inventor youtube channel -- this video:

https://youtu.be/_pEmpvcNmXg

Schematic first appears at around 2:16.

I find it interesting that you want to use a DC meter on the output of this AC power supply.

[frantal]

Yes panel meter works with 12V DC that I would obtain from the delay circuit, considering that it only powers the relay. My doubt was if the 2 negative poles, if connected as on the schematic, should make problems. Just this. Thanks for the help!

[Damianos]

Yes panel meter works with 12V DC that I would obtain from the delay circuit, considering that it only powers the relay. My doubt was if the 2 negative poles, if connected as on the schematic, should make problems. Just this. Thanks for the help!

If the entire auxiliary circuit, plus the voltmeter itself, is well insulated (can operate floating) for the relative voltages, then it will do the job.
There are other points of the diagram that are questionable. For example:
- the two 15Ω/10W resistors are thermally protected? For the case that something goes wrong in the auxiliary circuit.
- a 1KVA transformer that supplies an unfused output plus a 2KW variac, seems a little out of the "laws"!
- what is the maximum current of the variac? If it is a usual 120% maximum output voltage, the 10A fuse means about 2.75KW!
- what will happen if there is something connected to any of the two AC outputs and something else on the DC output, simultaneously?
- are the two small transformers self protected (impedance/inductance or thermally)?
- what current and voltage ripple is expected from the DC output?
Maybe there are more points than need consideration.

[frantal]

Thanks for the origin of the schematic. It was interesting to see it! I obtain that schematic from another video, maybe a man from India, so it was not so complete as the explanations I found here.

[frantal]

- What does it mean "Can operate floating"?
- I don't know how to protect the two 15Ω/10W resistors.
- I used a 10 A magnetothermic switch for switching on the transformer and another 16 A for the Variac. Are they not enough?
- As for the DC output I thought of using it for example to test old capacitors or little else, so I would use it exclusively without having other users connected.
- The two transformers (one of 12V/1A for delayed ignition + the voltmeter for DC output, and the other of 24V/1A for the two instruments, ammeter and voltmeter), have no
  protection other than the thermal fuse inside it.
Thanks for help

[Damianos]

- What does it mean "Can operate floating"?

This means that the entire circuit must be considered to operate at the peak voltages that may be present at any two points of it. So, forget any wire/cable that is rated for doorbell or automotive use, they have not any useful insulation for this project. Also the smaller (auxiliary) transformers must have enough insulation between primary and secondary windings and the chassis. For example: if the positive DC terminal is connected to ground or chassis (intentionally or from any leakage), the negative side (the common of the auxiliary circuit) will have a peak voltage difference with the mains' live conductor more than 700V. This may create some bad results.

- I don't know how to protect the two 15Ω/10W resistors.

I think that instead of these two resistors is better to use a proper "inrush current limiter" (an NTC resistor for this use), keeping the the relay circuit, that reduces a little the power losses but, mainly, it gives to the NTC the opportunity to cool down for the next start. Another way is to attach proper thermal protection switches to the existing resistors, connecting them in series with the resistors and moving the transformer primary after them.

- I used a 10 A magnetothermic switch for switching on the transformer and another 16 A for the Variac. Are they not enough?

I think that 10A for a 1KVA transformer on 230V is a ... little high. Same for the variac.
Also the 1KVA transformer to feed the 2KW variac is at least problematic...

- As for the DC output I thought of using it for example to test old capacitors or little else, so I would use it exclusively without having other users connected.

Testing old capacitors at 400V with a current of 16A, I think is not recommended.
How it is sure that anything is disconnected from the other outputs?

- The two transformers (one of 12V/1A for delayed ignition + the voltmeter for DC output, and the other of 24V/1A for the two instruments, ammeter and voltmeter), have no
  protection other than the thermal fuse inside it.

This may be enough but it is better to check their specifications if they need protection or can be used without it.

Thanks for help

You are welcome!

PS: I am writing this after midnight so don't take it as strict instructions!

[frantal]

I post some images of the project in progress. I can't take great changes, but where I can make it better I will try.
Maybe that with the images the project appears clearer.
In any case there are a fuse for insulating transformer (I thought 5A) and for the Variac. Moreover also the CC line is protected by a fuse (5A).
The transformer is 1 kW and the variac is 2 kW: this, in my opinion, could be considered a better functioning of the variac because it will not receive more than 1 kW (~5A) and I don't claim to get a higher current... I wrongly wrote that the main was a 10A magneto-thermic switch and Variac 16A. In reality it is the opposite as you can see from the front panel photo.

[Damianos]

I am not sure if I understand well what is showed in the pictures! So, I have to say: at least keep the different sections (input, outputs AC and DC) separated. For example place the wires/cables in different routes, in a way that they don't mix/touch each other.
Usually we use a more powerful source to feed a weaker load, not the opposite. In the latter case the source will be overloaded...
A 16A protection at 230V means more than 3.5KW, so it protects nothing!
Also 10A at 276V (assuming a "typical" variac) means ... calculate it!

[trobbins]

frantal, was the 1kVA isolation Tx tripping a mains feed cb when directly energised?

Why do you need an isolated variable AC output?

Can you describe what capacitor testing you have in mind, and how you think this equipment will assist you?

[frantal]

The use I would like to make of it is to be able to attempt repairs to appliances, power supplies, etc. with the protection of not causing short circuits to the home system. Furthermore, it would allow me to use an oscilloscope for example, without the fear of burning it for the Earth and Neutral connection when I perform some measurements on users connected to the home network.

[sam_sam_sam]

If you are measuring ac voltage and you are using a ac volt meter module then yes depending on the type of meter that you are using the meter input voltage that powers the meter is hooked to main input after the on/off switch the other input which is meter sensing input terminal would go on the output side of the transformer output terminal to the output outlet plug

Now you would be able to monitor the output voltage if this is what you are after but make sure which side of meter is common to the input and output do not get them mix up because you could have some real issues if you do pay close attention to the diagram that is on the meter module that you have and take a multi meter and check if the diagram is correct before you start wiring it up and before you power it on and it is very important if the meter module you have also measures current then you really need to be careful doing this

[trobbins]

frantal does that mean you haven't tested if the isolation transformer inrush causes upstream protective device tripping, and are really just preparing the soft-start on a whim?

[Damianos]

The use I would like to make of it is to be able to attempt repairs to appliances, power supplies, etc. with the protection of not causing short circuits to the home system. Furthermore, it would allow me to use an oscilloscope for example, without the fear of burning it for the Earth and Neutral connection when I perform some measurements on users connected to the home network.

I saw this just now!
To avoid tripping the fuses/breakers of the main electric panel, use local lower value ones. For example I have made a small panel with 1A, 2A and 6A thermal breakers and respective sockets.
If I understand well what you mean for the oscilloscope use, then it is a little dangerous. Normally we do the opposite; we isolate the DUT of the power grid and not the instrument. Even in the latter case it is needed a study of what we can do and what not.