u/Least-Anything2804

I want to add an analog ammeter so I can watch current draw in real time as I slowly increase the input voltage.

For those of you who do this regularly:

• What current range do you find most useful?
• I can get either a 1/5A meter or a 5/10A meter. Which would give the most useful indication when powering up unknown or faulty SMPS units?
• Is a low range meter generally better for spotting abnormal current draw early? Or would that sudden current draw damage my meter if it exceeds it's range?

Also, what would you typically expect the current to look like while slowly increasing voltage on a variac?

For example:

• Healthy SMPS: does the current stay very low until the startup threshold is reached and then increase smoothly?
• Faulty SMPS: what current patterns usually indicate trouble (shorted primary MOSFET, bad cap, shorted secondary, etc)?

I'm mainly interested in what the ammeter needle does as voltage is increased from 0V upward and how you distinguish a normal startup profile from a suspicious one.

Any examples from your own repair bench would be appreciated.

I’m trying to decide whether to buy an isolation transformer and could use some input.

Option 1: standalone toroidal transformer

• 500 VA rating
• ~125€
• No enclosure, fuse, or sockets (just the transformer)

Option 2: prebuilt unit (PeakTech)

• 500 VA rating
• ~241€
• Comes with casing, fuse, switch, and sockets ready to use

The prebuilt one is basically double the price, and it seems like I’m mostly paying for the enclosure and safety components.

So I’m wondering: wouldn’t it make more sense to just buy a standalone toroidal transformer and, for the same money, even step up to something like a 1 kVA unit? Or is there a good reason to go with the prebuilt option (safety, shielding, grounding, etc.)?

Additional concern: MCB tripping / inrush current
If I go with a 1 kVA toroidal transformer, is there a realistic chance it will trip my household MCB on startup due to inrush current? If yes, what’s the best way to mitigate that in practice?

My current idea is:

• Use a series incandescent bulb as a current limiter on first power-up
• Once the transformer is energized , bypass the bulb

Additional concern 2:
I would feed my variac from that isolation transformer (variac also rated at 1kVA).

• Would the variac also require the same procedure to start up? After the isolation transformer has been powered on?
• Or is the main inrush problem only at the toroidal transformer stage, making the variac effectively “safe” once the isolation transformer is upTrying to understand the correct safe startup sequence and whether a permanent soft-start circuit would be better than the light bulb method.

Any practical experience or corrections appreciated.

standalone 500VA toroidal transformer

prebuilt 500VA isolation transformer