How to convert a 120v circuit to 220v?

[Ultra Spidey]

Hey,
I want to make the Real McTube, a la http://www.dogstar.dantimax.dk/tubestuf/mctube.htm . Alas, in my country we receive 220v, not 120v. What changes do I need to make so the circuit will work on 220v? Would changing the transformators, T1 to 220>12 and T2 12>120 would be enough?
Thanks

[R.G.]

The only change needed is to make T1 (the one the AC power line comes in on) be a 220V primary, not a 120V primary.

What this means to you is that you use a garden variety 220V to 12Vac 0.5A wall wart for T1, and you'll then have to go find a 120Vac to 12Vac 0.5A wall wart for T2. That is likely to be the hardest thing to do, as the wall warts in your country will all have 220V inputs.

[Ultra Spidey]

Thanks alot, RG

Also, what does the circle sign near the input and the output mean? Logic says shielding, am I right?

[axg20202]

You would also have to use higher rated components to take the higher voltages.

Yes, those circles are the shield connections.

[Ultra Spidey]

I'll only need a higher rated lamp, no? Since i'm pumping the voltage to 120v, like in the original circuit.

[oskar]

I'll only need a higher rated lamp, no? Since i'm pumping the voltage to 120v, like in the original circuit.

correct!

[Paul Perry (Frostwave)]

You could make T2 a 230 to 24V transformer. These are quite common (12v and 24V plugpacks are often seen.)

Now, T1 has an output of 12V.. when you run that to the 24 V side of the second transformer, you will get the 115 that you want.

***DISCLAIMER: I havn't done this myself.

[R.G.]

You're right, Paul - using a 220 to 24Vac transformer for T2 works fine.

[andronico]

Paul is right  .  In my country Argentina we have 220V. too. I´ve used a 220 to 12V for T1 and a 220 to 24V for T2 and works really well in my Real Mc Tube.  My implementation uses for T2 a transformer with a middle point in the secondary (220 to 12+12V) in this way I can select with a toggle switch between 115 and 220v output.  You need proper values in the capacitors to use this variation.

[rogeryu_ph]

Guys,
I'm also interested on Real Mctube. Here in our country uses also 220v so kindly guide me. If i'm going to use T1 220v - 12v = 24v - 220V for T2 still my rating capacitor voltage is 33uf 160V? or 33uf 220? What would be the out ac Volt  of T2 ?

Thanks,
Roger

[Paul Perry (Frostwave)]

I'm also interested on Real Mctube. Here in our country uses also 220v so kindly guide me. If i'm going to use T1 220v - 12v = 24v - 220V for T2 still my rating capacitor voltage is 33uf 160V? or 33uf 220? What would be the out ac Volt  of T2 ?

Since the T2 is designed to go between 24V and 220v, but is being fed only 12V, it will output 110V.

So the circuit after T2 is identical in all ways to the original.

[R.G.]

Remember - transformers don't have voltages or impedances. They have RATIOS.

The voltage number is just an assurance that the transformer will work at any voltage UP TO the stated voltage and not be damaged.

A 220->12V transformer is a transformer that causes a secondary voltage to be 12/220= 0.05454... times the voltage put on the "220" side to appear on the "12" side, and works with any voltage up to 220 on the "220" side.

Notice here that I fudged it a little. Low power transformers that are said by their makers to be 220->12 (for example) at some stated current will actually be 220 to some slightly higher voltage to make up for the resistor losses inside the transformer because small transformers have big resistive losses. I'm just making up numbers similar to what I have seen before for small transformers.

So a transformer rated at "220->12Vac at 250ma"  will typically really be "220->14Vac at 0ma, with an 8.5 ohm equivalent internal resistance on the secondary".

T2 will then be a 220->24Vac transformer at some current, higher voltage at zero current. So it's really a 220->29Vac (guessing!) transformer with a 16 ohm internal resistance on the 29V side. To use it backwards, we work the ratios. The "29V" secondary gets supplied 14V through an 8.5 ohm resistor, and itself has a 16 ohm resistance. Then it steps up whatever voltage is left by the ratio 220->29.

In this case, if you just hooked up the transformers and didn't put any load on them at all, you would have T1 making 220Vac input times 14/220 ratio or 14Vac on its secondary. T2 would be 14Vac times maybe 220/29 = 106Vac on the T2 output side. You would then rectify and filter this to 1.6*1.414 = 150Vdc, so you would only need caps that would work at 150Vac. In fact, it will be even lower.

If you are using 150ma of the nominal 12V side to run the heaters and the rest to run T2, you get 14V through 8.5ohms. For T2, you get whatever voltage T1 makes after the 8.5ohm losses, through the 16ohm T2 losses, then that voltage times 220/29.

As an example, if you use 2ma of DC on T2's output:
The 2ma looks like 4ma RMS to the transformer for heating purposes, but because it causes the rectifers to conduct in big pulses, for voltage purposes, it looks like about 16ma for voltage loss. That's on the 220 side.

The current is reflected to the 29V side of T2 by the inverse of the voltage ratio, so the current in the29V side is 16ma * 220/29 = 121ma, which passes through the 16 ohm resistance of the 29V side. The "29V" side is only being fed what's left over after the "12V" side of T1 is being fed the voltage left over after you get 150ma for the heaters and 121ma for the high voltage through an 8.5ohm resistor on the "12V" winding. So you get a voltage of 220(primary) times 14/220 (ratio) minus 0.150 plus 0.121 milliamperes times 8.5 ohms or 11.7Vac at the "12V" winding. OK so far.

Then this 11.7V is applied through the 16 ohms of the  29V side of T2 and multiplied by 220/29 to get the real voltage available at the rectifiers after T2. That number is 11.7V from T1 minus 0.121*16ohms times 220/29, which works out to 74Vac on T2's secondary and an output voltage of 74*1.414 = 104.7Vdc after the rectifiers.

You can't use 160V caps because the voltage if you pull out the tube bounces up to the no-load condition of 150Vdc, and if you happen to have a high-line condition then, the voltage will be as much as 15% higher and pop your filter caps.

All this accounting gets to be dismal work, eh?

[rogeryu_ph]

So, I could use my 33uf 250V cap for T2 which is 24v-220v ratio and it's safe.

Roger