Russian voltage and rheostat dimmers

[Fishn sparky]

Good evening sirs,

I have a student in class who is Russian and recently went back home. He tried an experiment that failed and asked me to explain why. I can't because I'm not familiar with European voltages. Apparently, Russian voltage operates at 220v. My student tried to use a rheostat dimmer and turned down the voltage to 120v. He then plugged in a wall charger that solely operates at 120v and it didn't work. Why?

I do know that the frequency is 50hz instead of 60hz and that rheostat dimmers are just a variable resistor inside but I can't explain why this doesn't work. Any explanations? Thoughts are always welcome and if this is a simple answer, I apologize!

 

[electrofelon]

Good evening sirs,

I have a student in class who is Russian and recently went back home. He tried an experiment that failed and asked me to explain why. I can't because I'm not familiar with European voltages. Apparently, Russian voltage operates at 220v. My student tried to use a rheostat dimmer and turned down the voltage to 120v. He then plugged in a wall charger that solely operates at 120v and it didn't work. Why?

I do know that the frequency is 50hz instead of 60hz and that rheostat dimmers are just a variable resistor inside but I can't explain why this doesn't work. Any explanations? Thoughts are always welcome and if this is a simple answer, I apologize!

I believe that rheostat based dimmers havent been used in a long time. Modern dimmers use what is called a triac which clips off part of the wave form. He may have fried the dimmer instantly if he gave it 220.

 

[Jraef]

I believe that rheostat based dimmers havent been used in a long time. Modern dimmers use what is called a triac which clips off part of the wave form. He may have fried the dimmer instantly if he gave it 220.

And/or the power supply cannot handle being fed with the chopped voltage from that triac dimmer and simply shut itself off to avoid damage... if he's lucky.

 

[GoldDigger]

If he used a Russian dimmer, it would have been OK being fed 220V, but the output which averaged out to 120V on a meter could have had moments of much higher voltage than 120, as noted in Jraef's post.

 

[drcampbell]

Most small chargers use a simple switch-mode power supply which tolerates a range of different voltages & frequencies. It's doubtful that either 220 sinusoidal volts or a triac-clipped waveform (same or lower peak voltage) would damage anything. The rheostat probably isn't necessary or desirable.

It's likely that it has nothing to do with the different voltage and there's something simple wrong, such as a wiring error.

 

[kwired]

If it is a true variable resistor there is no voltage drop across it until there is current flow, which leaves one asking how did he measure 120 volts before throwing the other load into the circuit?

If it is a solid state dimmer - you still need a load connected and current flowing before any measured output voltage will mean anything.

If anything the load used to stabilize the voltage reading was the impedance of the meter used to take the reading, now throw the charger in the circuit and you have changed the load as well as the voltage across that load - for either type of dimmer.

 

[steve66]

If he used a Russian dimmer, it would have been OK being fed 220V, but the output which averaged out to 120V on a meter could have had moments of much higher voltage than 120, as noted in Jraef's post.

How would it have been OK?

Dimmers are made for dimming lights. They were never intended to be used as a voltage regulator to supply 120 volt chargers or any other loads rated for 120 volts from a 220 volt source.

 

[dkidd]

Doesn't any believe that maybe OP was right and it was a rheostat?

 

[GoldDigger]

How would it have been OK?

Dimmers are made for dimming lights. They were never intended to be used as a voltage regulator to supply 120 volt chargers or any other loads rated for 120 volts from a 220 volt source.

All I meant was that the dimmer would not be damaged.

Sent from my XT1585 using Tapatalk

 

[gar]

170112-0910 EST

Fishn sparky:

Your definition of a rheostat is correct, an adjustable resistance.

A rheostat's ability to perform a dimming function is greatly dependent on the resistance range of the rheostat, its electrical current rating, and the nature of the load.

Since the word dimmer was used assume the load is an incandescent bulb.

Some values for a Phillips 75 W 120 V bulb are:

Volts - Amps - Bulb resistance
----------------------- Rheostat resistance to get dimming voltage
000 -- 0.00 -- 021 -- infinite
030 -- 0.27 -- 111 -- 333
060 -- 0.41 -- 146 -- 292
120 -- 0.60 -- 200 -- 0

Really need to know if his experiment was an incandescent bulb and a rheostat.

To scale the above bulb to 220 V multiply volts by 1.83, divide current by 1.83, and multiply resistance by 1.83*1.83 = 3.35 .

A suitable rheostat for the 75 W bulb at 120 V would be 500 ohms at a current rating of at least 0.6 A, and that means the rheostat requires a power rating of at least 180 W for a linear wound resistance.

I don't have time to check my values and calculations.

.

 

[gar]

170112-1254 EST

Fishn sparky:

My student tried to use a rheostat dimmer and turned down the voltage to 120v. He then plugged in a wall charger that solely operates at 120v and it didn't work.

If one really has a rheostat dimmer, then what was the load on this dimmer to produce 120 V? What is the size of this load in relation to the wall charger? With the charger connected what was the voltage measurement on the charger input terminals?

Others have pointed out a number of points. Greater clarity on the experiment is needed.

Could it have been a Variac (variable auto-transformer), and not a rheostat? A Variac could provide a fairly good stepped down voltage.

.

 

[Electric-Light]

Those dual mode 50W/1kW travel power adapter things have a 25VA 0-115-230 auto transformer (which transforms 1/2 of VA supplied) on low setting for things that require clean power and high setting with a preset dimmer adjusted to deliver 50% Vrms for hair dryers, desk lamp and such.

 

[GoldDigger]

Those dual mode 50W/1kW travel power adapter things have a 25VA 0-115-230 auto transformer (which transforms 1/2 of VA supplied) on low setting for things that require clean power and high setting with a preset dimmer adjusted to deliver 50% Vrms for hair dryers, desk lamp and such.

The "preset dimmer" is usually just a single diode to produce half wave output. Really awful for anything that cannot tolerate a DC component.

 

[Electric-Light]

The "preset dimmer" is usually just a single diode to produce half wave output. Really awful for anything that cannot tolerate a DC component.

The one I have seen is not. What you're describing makes 170v rms, not 120v from 240v.

 

[GoldDigger]

The one I have seen is not. What you're describing makes 170v rms, not 120v from 240v.

Thank you for reminding me of that!
A single diode will indeed reduce the power by a factor of two, and that in turn will only reduce the RMS voltage equivalent by root two.