240volts to 220volts

[Kevin Hoyland]

Hello can someone put me on the right track . I have a Saba radio thats work on 220 volts. How do i drop 240 volts to 220 volts and make sure it is fail safe? Please can the reply be in laymans terms.Ps the mains in the house are reading 239to 243volts. Thank you.

Kev.

[Ed_Dinning]

Hi Kev, probably the simplest answer would be to use a power resistor, but to calculate this we need to know the current the radio draws, or at least its rated wattage.

It may well work OK on 240+ volts, but will certaily run hotter.
The power resistor will also run hot by the way.

Ed

[Herald1360]

Almost as simple, less variable in output and nothing running hot- find a 240:20 volt transformer (240:24 volt not so accurate but probably easier to find) with a SECONDARY current rating greater than the radio AC input current at 220V. Connect the primary to the mains and connect one side of the secondary to the line side of the mains. Measure the voltage between the other end of the secondary and neutral. If it's 220V (216V) you've got it right first time, if it's 260V (264V) swap the secondary round. Connect radio line side to TX secondary and neutral to mains neutral. Voila!

Chris

[Boom]

Find out how much current the set draws and make up a set top period lamp with a suitably rated bulb wired in series with the set to get rid of the extra power. It will look lovely and recycled light must be better than recycled heat.

[stevekendal]

I have a 220 volt Dutch 100 watt guitar amp that was given to me because on 240 volts, it kept detroying EL34s. Technically, a bit suss, but I just mounted a suitably beefy dropper resistor in a well-ventillated position. Chose the value by trial and error and the resulting voltage doesnt vary enough under load to cause any problems. Its had 3 hours use every week for the last three years now and has only fried one resistor. Much cheaper than output bottles or transformers Steve.

[daviddeakin]

Originally Posted by Herald1360

Almost as simple, less variable in output and nothing running hot- find a 240:20 volt transformer (240:24 volt not so accurate but probably easier to find) with a SECONDARY current rating greater than the radio AC input current at 220V. Connect the primary to the mains and connect one side of the secondary to the line side of the mains. Measure the voltage between the other end of the secondary and neutral. If it's 220V (216V) you've got it right first time, if it's 260V (264V) swap the secondary round. Connect radio line side to TX secondary and neutral to mains neutral. Voila!

Chris

And because a picture speaks a thousand words:
http://img.photobucket.com/albums/v3...g?t=1249338435

[Alf]

Thanks very much! I'd forgotten all about that method of voltage reduction.

[Kevin Hoyland]

Hello and Thank you all for all your Help.

I have fitted a 10 Watt 100 ohm wirewound Resistor in the live side of the 240 volt feed. The power to the radio is now 215 volts. The Resistor is running hot but worked fine for 2 hours to day. I do have a spare transformer with 220 volts output and i think this a good idea thank you.

Regards Kev.

[Herald1360]

With 240 in 215 out and 100R that's 6.25W so your 10W resistor should be fine, as long as it's mounted as the manufacturer recommends.

(Up to about 31.5V across the resistor stays under 10W)

Chris

[Kevin Hoyland]

Thanks for working it out Chris.

Kev.

[dseymo1]

A slight variation would be to use any suitably-rated mains transformer with 220 and 240V primary taps as an autotransformer.

[Leon Crampin]

Absolutely not. The secondary has to occupy approximately half the winding space of the transformer in order to provide the bucking voltage.

Putting it simply, if you need 220V at 5A from a 240V supply, the transformer must provide 20V at 5A (antiphase) and therefore be rated at 100VA.

Using any "ordinary" transformer in this way would severely overheat the primary section between 220V and 240V - this would be wound with light gauge wire to take the primary current and would occupy only a fraction of the winding space.

Leon.

[Ray Cooper]

Originally Posted by Leon Crampin

Absolutely not. ...
Using any "ordinary" transformer in this way would severely overheat the primary section between 220V and 240V...

I think that the get-out here is that dseymo1 said:

...use any suitably-rated mains transformer...

If the load (at 220V) is, say, 100VA and you use a 100VA rated transformer, then the primary windings will be well within their rating. It's not immediately intuitive, but the current flowing in the 220-240V section of the winding HAS to be less than the current supplied to the load from the 220V tap: the remainder of the current is made up by current flowing 'backwards' out of the 0 - 220V section of the winding. So for a small autotransformation ratio like this one, the currents in all parts of the primary windings will be less than the load current: hence for a 100VA load (say) a transformer rated at 100VA should find life easy.

[Herald1360]

Yes....

if you neglect any magnetising current determined by the primary inductance (which will be constant anyway), the current in the 240-220 portion of the winding will be that required for 100VA at 240V. The current in the 220-0 portion of the winding will be the current required for 100VA @ 240V - current taken for 100VA @ 220V, which will actually be a small reversed current (relative to the 240-220 current direction) of about 10% of the magnitude of the 240-220 current. So the primary of the 100VA transformer will be dissipating LESS than it would if the transformer was delivering 100VA from its secondary.

If you consider a 1:1 autotransformer (admittedly a trivial case) there would be NO current in the winding due to the load, ONLY the magnetising current determined by the primary inductance.

Of course if you use the bucking transformer approach, for 100VA load at 220V, the transformer only needs to be rated for (100/220)*20 = 9.09VA which is much cheaper if you have to buy it!



Chris

[fiesata]

I think the UK specifies 230 V +10% or -6% so theoretically you may be getting around the 220 V at times

Our supply at home is usually is between 235-242 V when I have tested it so we must be getting our money's worth

[Leon Crampin]

It goes against the grain of good engineering to "mis-apply" a transformer in this way. Who on earth would use a 100 VA component when, correctly applied one of about 10 VA rating would do?

As a comment to the post above, the widening of the mains voltage tolerance is a mean trick. You buy energy (power x time) and for a fixed load, power varies as the square of voltage. By allowing the supply to rise to 253V, the providers sell more energy and shorten the life of your consumers.

The network was designed to provide a nominal 240V (+- 6%) so voltages are now nearly always towards the upper limit of the new specification. At times of crisis, the voltage can now sink to almost 216V - progress?

Leon.

[Hans_SA7AUY]

The mains voltage will most likely not be of any problem for you. You will not need any drop resistor.
Many years ago (I think it was in the late 70's or early 80's) the European countries harmonized their mains voltages to 230V. Here in Sweden we used to have 220V and in the UK it used to be 240V. In most places in Sweden we have increased the voltage to 230V but mostly you can measure the voltage in the outlet to anything from 220V to 230V. In the UK the voltage should have been decreased to 230V, and officially it is decreased but I know that in practice it is not. The reason is that the tolerance for the 230V is +/- 7% (or something like it) and both 220V and 240V is within that.

/ Hans

[Ray Cooper]

Originally Posted by Leon Crampin

...Who on earth would use a 100 VA component when, correctly applied one of about 10 VA rating would do?...

Anyone who had a suitable available transformer doing nothing else useful might care to try it: I know, I would. I certainly wouldn't recommend that they go out and purchase such an item specially for this purpose. In any case, auto-transformers custom designed for the job would be better. But all this involves an outlay in cash, which any respectable experimenter will try to avoid. After the concept is proved is the time to open the wallet.

[Kevin Hoyland]

Hello Gents.

Thank you for all the replies. I have fitted a different make of resistor to see if it runs cooler than the other one. It is still too hot for my likeing, for the resistor to be inside the Radio. This resistor is the same rateing (10w/100ohm) but the voltage is now 218 volts constant. The lower voltage on the radio is 125 V. and as i say above i am not happy with the heat from the resistor. So, what do you think to me running the radio from a 110 volts transformer? Obviously this is 15 volts below 125v setting.
Ps voltage settings are (125-150/220)

Regards Kev.

[JimMac53]

Hi Kev,

I've done the same thing (100 ohm, 10 watt resistor) for a couple of German Grundigs I've restored which didn't have a 240 vac setting. The real reason I dropped the input voltage had more to do with the heater voltage than the HT. Where I live the mains runs about 245 vac (actually I've never measured it lower), which had the heater voltage over 7 vac, too high for my liking. After completing the restoration I checked the cabinet temperature with the back on and I seem to recall it ran about 50 C, which I felt wasn't too bad. I did mount the 100 ohm resistor on the top side of the chassis and kept it separated from the other components.

The 110 v transformer should be fine also and does keep the added heat out of the radio.