Controling a main transformer

[thimios]

Are you looking for an easy way to control the primary side of a main transformer?
Do you need a safe way for your diy amplifier first test?
You haven't a variac?
This circuit is for you.
This was publised in Elektor many years ago.
Using this circuit it is possible to use a fixed voltage power supply as a variable one.
Testing this i'm getting 2x3-28v A.C from a standard 2x30V transformer.
To keep core of transformer absolutely silent, it is necessary to connect a lamp in parallel with the primary side of the main transformer..(this is from mine experience)
Keep in mind that glow lamp rheostat can't be used here.
This circuit ISN'T ISOLATED,RISK OF ELECTRIC SHOCK!!!
Use potentiometer with plastic axis only!
Place all the circuit in a plastic box.
Use inlet and outlet specific for 220v A.C

 

[trobbins]

Why would you want to use this phase regulated technique? For a first timer, it would be a lot safer and simpler to just power equipment through any step down transformer (if no Variac is available), such as a 230:115 or 230:30 or 230:12.

 

[thimios]

Why would you want to use this phase regulated technique? For a first timer, it would be a lot safer and simpler to just power equipment through any step down transformer (if no Variac is available), such as a 230:115 or 230:30 or 230:12.

If you have 5-6 step down transformers or a multi tap one you can use those ,if not, controlling an existing transformer is a much cheaper solution.

 

[trobbins]

For the application where I want to use a Variac when testing an amp, I wouldn't want additional phase regulated noise coupling in via HT, heater, earth loop. So I'm trying to appreciate what application would suit, compared to a simpler setup where just a fixed low Vac is used for initial checkout.

 

[nigelwright7557]

If you need to use a lamp anyway why not just put it in series with the mains ?
That will limit the current.

For my new amp builds I just use a 2 amp 30VAC transformer into a bridge and smoothing capacitors. If the transformer hums loudly on power up then I know there is a fault. Given it is 2 amps then its not enough to blow an output transistor providing I turn it off soon enough if a fault occurs.

 

[AndrewT]

That uses a Triac to "switch" the output repeatedly.
The AVERAGE power is 100% if the switch remains ON all the time.
The AVERAGE power is 0% if the switch is OFF all the time.
The power levels in between those two limits is a duty cycle regulated average power.

The system output is still the high level input. i.e. put in 240Vac and potentially the output is as lethal as the Live input.

 

[rikkitikkitavi]

I have built circuit similar to this ( but with beefier components) and it is a good way to annhilate good silicon proper , phase control of primary side on a xformer.

Phase comtrol of transformers with capacitive load is hard. Load is of course reflected through transformer.

Use atleast a 25A rated SCR/triac.

The output AC voltage is obviously not a pure sine but that is in most cases ok.

 

[thimios]

If you need to use a lamp anyway why not just put it in series with the mains ?
That will limit the current.

For my new amp builds I just use a 2 amp 30VAC transformer into a bridge and smoothing capacitors. If the transformer hums loudly on power up then I know there is a fault. Given it is 2 amps then its not enough to blow an output transistor providing I turn it off soon enough if a fault occurs.

Lamp in series isn't a regulated power supply. Using this circuit you have the benefit to regulate the secondary .