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Old 8th Dec 2019, 7:16 pm   #1
Lef de art
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Default AC 250v to 220v with resistor in series ? Can happen ?

Hello forum .
I have a Geloso tube tape recorder but it works with 220v. My home ac is 250v and some times 260v and after some hours of use it gets really hot.
I have adjusted a 150ohm 5watt resistor in ac and the voltage has dropped to 220V but the resistor in 10 minutes has burn.
Im was thinking with a 150ohm 100watt resistor can this work correctly ?
Thanks in advance guys.
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Old 8th Dec 2019, 8:35 pm   #2
kalee20
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Default Re: AC 250v to 220v with resistor in series ? Can happen ?

Yes, it can work.

Measure the voltage with it switched on and working.

Very basically, if your 150 ohm resistor reduces voltage by 30V, it must be passing 0.2A (which is quite reasonable for a tape recorder). Now 30V and 0.2A in a resistor means the resistor is running at 6W. This is too much for a 5W component, though not greatly so.

Your 100W resistor should be fine. But make sure it is kept cool.
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Old 8th Dec 2019, 8:50 pm   #3
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Default Re: AC 250v to 220v with resistor in series ? Can happen ?

An alternative is to use a 30V transformer to subtract 30V from the input. If you do this, make sure you subtract, not add.
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Old 8th Dec 2019, 9:57 pm   #4
David G4EBT
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Default Re: AC 250v to 220v with resistor in series ? Can happen ?

Quote:
Originally Posted by TrevorG3VLF View Post
An alternative is to use a 30V transformer to subtract 30V from the input. If you do this, make sure you subtract, not add.
Known as a 'bucking' transformer.

This 35 min tutorial goes into a lot of detail, but to 'cut to the chase' see about 18 minutes in:

https://www.youtube.com/watch?v=iVyJ4ELidLU

Hope that's of interest.
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Old 8th Dec 2019, 10:35 pm   #5
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Default Re: AC 250v to 220v with resistor in series ? Can happen ?

It is a real problem when the mains voltages climb to the 250 -260 region. We have a lot of it here with solar grid tied inverters, our official value is 230v.

This was causing negative effects for all my workshop equipment/appliances so in the end I had a 15kW rated auto-transformer installed on the line input feed to my house, it now sits at around 228-232v now most of the time. It was a very expensive item cost about $1500. I call it a "Solar defense" transformer.

A bucking transformer is a much better option than a resistor for a single appliance.

Along the way trying to deal with this problem I found another solution that can work to drop voltage to an appliance with an "off the shelf item".

I had an industrial 240/240V isolation transformer, I think it was rated around 500 to 700W. I noticed that they had arranged it so that on full load it was 1:1 ratio. Which meant on lower loads the output voltage (and the transformer turn's ratio) was about 10 to 15% higher on the secondary than the primary voltage. So I simply reversed the primary and secondary winding connections and it gives about a 10V drop for most loads with insignificant heating.
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Old 8th Dec 2019, 11:07 pm   #6
Lef de art
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Default Re: AC 250v to 220v with resistor in series ? Can happen ?

Thanks for advices guys
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Old 9th Dec 2019, 9:04 am   #7
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Default Re: AC 250v to 220v with resistor in series ? Can happen ?

You could also try this explanation.

https://sound-au.com/articles/buck-xfmr.htm

It gives the alternative form in figure 4 by using the transformer as an auto transformer.
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