Connecting variac

[anders.a]

I’m about my variac for the first time. Just to not make anything stupid. How do I connect it?
My guess is, connected in series:

Power supply – variac - light bulbs (protection against power surge) – fuses – transformer.

[AndrewT]

Hi,
I don't think so.

The Variac input is connected across the mains. The output is connected to the load (transformer) mains input.

Is your's a packaged mains plug input and socket outlet to the load? or an uncased DIY job that needs to be wired up?

The Variac is a transformer with variable tapping point on the secondary. Most, maybe all, are autotransformers which means the secondary is directly connected to the mains primary winding.

Take great care to properly insulate the Variac from accidental touching with tools and/or hands. Only use the supplied insulated handle for adjusting the output voltage.
The light bulb voltage dropper is normally used as an alternative to using the Variac but there would be nothing wrong with using the bulb and Variac primary in series at the mains input. You may be able to use the bulb at the output in series with the secondary and the load (transformer) but since at start up the Variac output is only a few volts the benefit may be lost. I cannot think this alternative through logically.

I don't know the capacity of your Variac but they are usually only used for low power set up and testing to get the circuit working. As you turn up the wick (bias) the Variac starts working quite hard and reaches it's limits, particularly at the extremes of it's adjustment range. For full power testing, do not use the light bulb nor the Variac. Connect the amp directly to the mains supply.

[mzzj]

Originally posted by anders.a
I’m about my variac for the first time. Just to not make anything stupid. How do I connect it?
My guess is, connected in series:

Power supply – variac - light bulbs (protection against power surge) – fuses – transformer.

You mean "Variable auto transformer" or something else?

I wouldnt call it series connection...
Variacs are wired like normal auto-transformer, possibly three connectors: in-out-common, wired as live-out-neutral. output terminal is the one connected to wiper-arm.

[anders.a]

AndrewT
Let me see if I managed to digest what you wrote.
This is a DIY Aleph-X. The variac is used for first start up as advised by Nelson Pass. When everything is set (DC absolute and relative bias supply and AC current gain) I’m not going to use it anymore.
The variac is properly isolated. The light bulb is necessary because I fried the wire the first time I tested the power supply. Here is a link to the power supply http://www.kk-pcb.com/power-x.html
What I’m unsure of as I really don’t know what I’m doing where to connect the variac.
The variac is rated input 220V output max 260V 9A.

[AndrewT]

Hi,
Plug the Variac into the mains outlet.
Measure the voltage at the outlet socket. Turn the adjustment knob down to minimum. The measured voltage should fall to nearly zero. Plug in the Aleph to the Variac outlet socket and switch on. Slowly adjust the variac voltage up, monitoring the Aleph PSU voltage and any others you want to keep an eye on. This slow turn on will gradually charge up the capacitor bank without a high currrent surge and all should be safe.
Your 9A 220V supply is equivalent to 2kVA and should easily cope with all testing of the Aleph and should also allow setting the amp to full bias.
At low output voltage (<50Vac) and high output voltage (>160Vac) the Variac may need to be derated to 75% of max current but this still equals 1.5kVA at 220Vac.

I suggest you add a soft start to allow the transformer to start up without blowing the supply fuse. A 600VA would normally need about T6A or T8A to switch on reliably. With all that capacitance you may need to go one size higher. A big fuse like that allows enormous power through on a fault condition and is not very effective protection. With soft start, the fuse can be reduced to T2A to T3A and provides much closer protection for improved safety.

[Tweeker]

Most of my variacs have no plug. Connections are line, brush, and common. A nine amp variac is typically safe to provide at least 9 amps regardless of voltage. Line is input, brush is output, and common is Neutral.

[Tweeker]

Variable Transformer Wiring Schematics.

[BeachAssault]

The above mentioned link has apparently moved. I found it on this page:
Variac Variable Transformers