How to lower mains voltage using resistors

[johnm]

Hi folks!

I want to drop the level of mains voltage supplied to a pre-amp from our measured 247V 50Hz AC wall outlet, to 220V, which the pre-amp is spec'd for.

I know this can be done by inserting resistors before the mains primary. My question is what is the formula for working this out please?

I have read it's also best to use four x 10W wire-wound resistors when the value of resistance is known, to increase power dissipation.

Any help would be much appreciated!

Thank you,

- John

[leadbelly]

Originally posted by johnm
I want to drop the level of mains voltage supplied to a pre-amp from our measured 247V 50Hz AC wall outlet, to 220V, which the pre-amp is spec'd for.

Are you sure you need to do this? Are you actually getting some adverse effect, or is it just a concern about specs?

[cerrem]

Keep in mind that the source impedance, MAINS OUTLET, will no longer be low.... Assuming your load draw is CONSTANT or else the added resistance will create more problems...
Lets say for example you have a CONSTANT 400mA draw of current from your mains.... You need 27V drop....That would be roughly 67 ohms in resisitance on the HOT LEAD.... Nuetral and Ground should be left alone....
This 67 Ohms would produce roughly 10W of heat.... So use atleast x4 in power ratting.... Mount in box that is properly GROUNDED with outlet and plug mounted to box...
My recomendation is to use an auto transformer or VARIAC of some type to do this properly....
E= Voltage
E= IxR
W=(I^2) x R or W= ExI



Chris

[ak_47_boy]

Voltage divider. Extremely inefficient.

[Boris_The_Blade]

Get a Variac

[HollowState]

You could try bucking the primary with a 24 volt transformer, but sometimes going more then about 5% gets hairy and may pull a lot of current. You just have to try it and see.

In case you aren't sure what bucking is, it's putting the secondary of a low voltage transformer (of suitable current) in series with the main transformer's primary. Then connecting the LV transformer's primary to the line out of phase. (Or connect the secondary out of phase, doesn't matter) There's a 50% chance of getting it right the first time. One way will buck, the other will boost.

Victor

[johnm]

Thanks for all the replies guys

Unfortunately a variac is out of the question at present - I don't have the funds available for now.

The transformer(s) in question are from a Ming Da MC-7R pre-amp which I imported three years back. It was a European model and contains the 220V transformer, rather than the 240V item which the proper UK version comes with. I am now using the transformers and chassis for a new project.

If poss. I'd like to go the resistor route at present unless I'm OK using a 220V tranny on a 247V line? Is it OK (safe!) to connect the primaries to the 247V mains, and then using a meter connect the probs to the 300V secondary, and CT to measure the secondary voltage?

Thanks,

- John

[Wavebourn]

It is Ok if it does not hum more than on 220V and is not too hot.

[nigelwright7557]

Originally posted by johnm
Hi folks!

I want to drop the level of mains voltage supplied to a pre-amp from our measured 247V 50Hz AC wall outlet, to 220V, which the pre-amp is spec'd for.

I know this can be done by inserting resistors before the mains primary. My question is what is the formula for working this out please?

I have read it's also best to use four x 10W wire-wound resistors when the value of resistance is known, to increase power dissipation.

Any help would be much appreciated!

Thank you,

- John

In my valve pre amp I used a transformer to drop mains to 12VAC. This was used for the heater cct but also went into a reversed transformer of 18VAC to mains which gave about 180VAC for the HT for the valves.

This is certainly safer than messing directly with mains voltages.
It also isolates the cct from the mains.

[wakibaki]

Mains voltage varies anyway.

Well designed equipment will operate over a range of voltages.

The difference between the nominal voltages is ~10%.

This probably falls within the range which the equipment will tolerate, but when the voltage is at its highest the stress is greatest and the temperature highest. This can result in accelerated failure.

If you are drawing close to the rated current then naturally the rated power may be exceeded, but lower currents will keep this and the temperature within limits, in which case breakdown of the insulation is unlikely.

Obviously you are operating the equipment out of spec, and this constitutes an unquantified risk, for which you are responsible. The risk of catastrophic failure is increased although probably remains very small. It is up to you to decide whether the risk is acceptable. Such a risk would be considered unacceptable in a professional context.

w