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Old 23rd Feb 2009, 6:21 pm   #1
acorn
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Smile Choke use as dropper

Hi all

I have used a 25R large wattage resistor in the past to drop the UK mains voltage to accommadate a chinese transformer rated at 220 volts ac , my question is would it be possible to use a choke which would give you that same resistance so it would do the job much better without the excessive heat generated by a dropper resistor.

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Old 23rd Feb 2009, 8:49 pm   #2
Studio263
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Default Re: Choke use as dropper

I'd be a bit careful, if the choke saturates through too much current it will revert to being a low value resistor...
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Old 23rd Feb 2009, 9:12 pm   #3
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Default Re: Choke use as dropper

Best method by far is a suitably rated auto transformer, mains to 0-240V winding and amp to 0-220V Tapping. Though my Chinese amp was labelled for 220V I found the heater voltage well down on the auto transformer but came up to 6.3V with direct mains input. It has been running fine for the last two years, I think I was lucky in having an amp with a suitable UK rated transformer. I have however seen on the internet some sorry tales about other makes of Chinese amplifiers getting fried on our mains supply through having no correction applied. The heater voltage measurement seems to be a good indicator.
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Old 23rd Feb 2009, 11:19 pm   #4
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Default Re: Choke use as dropper

Hi

You can use a 240 - 20V transformer.

Connect the mains side to mains, and the 20V side in series with the 240V i/p to your amp. You have to make sure you get the phasing the right way around, one way it will add 20V and the other way subtract 20V.

I have seen this system used in 220V refrigeration gear sold in the UK.

The transformer 20V needs to be rated at the same current as the amp is going to draw + a bit.
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Old 24th Feb 2009, 12:32 am   #5
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Default Re: Choke use as dropper

An alternative to a resistor is to use a capacitor in series. This gives "wattless" voltage dropping so no heat. You will however have to experiment with the value but it will be of the order of a (few?) mfd I would guess. It must have good insulation and must not be an electrolytic. There is an article somewhere a few years ago about how to go about calculating the value required but I cannot recall where. I have used this technique for a number of items, probably the most common being to reduce the speed of cooling fans to cut down the noise.
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Old 24th Feb 2009, 12:38 am   #6
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Default Re: Choke use as dropper

A 100W bulb will typically drop about 20V. You can put it in an external box with a European socket for the '220V' output.

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Old 24th Feb 2009, 3:14 pm   #7
GeorgSc
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Default Re: Choke use as dropper

Hello acorn,
what does that "chinese transformer" drive?
Is the load constant or variable?
Is this a transformer for use in China or is
it a transformer for export to Europe?
Without this data one can only guess what
to do.
Regards
Georg
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Old 24th Feb 2009, 6:19 pm   #8
acorn
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Smile Re: Choke use as dropper

Thanks lads,

Plenty food for thought I like the capacitor idea but what kind of cap and what value to use I expect it would have to be a 5uF solid state poly type rated at 300 volts ac at least to take care of any spikes in the AC supply.

Hi George , it would be for a Chinese import to the UK rated with a 220 ac input.

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Old 25th Feb 2009, 3:20 pm   #9
GeorgSc
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Default Re: Choke use as dropper

Quote:
what does that "chinese transformer" drive?
As You did not answer the question above, I suppose
You do not know the importance of it.
One needs the power rating of the transformer
and the actual power load from the transformer.
Without that any fantasy like capacitors or chokes
is possible.
If there is a label on the transformer, tell us
what is written on, or, better post a picture.
In general, either nothing is needed, or a resistor,
or the 20 Volts transformer.
Capacitors or chokes are not the usual solution exept
in cases of very small power.
Georg
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Old 25th Feb 2009, 6:35 pm   #10
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Default Re: Choke use as dropper

Use a say a 240 V to12.6 volt filament transformer and wire the 12.6 volt winding in series with the '240 VOLT" input of the chinese transformer but in anti phase to change the input to the chinese transformer to be your mains less 12.6 volts. Make sure that the filament transformer is antiphase and not aiding!
John.
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