AC voltage drop

[skal1]

OK lads


i have a transformer the  output  is 9vac , 5.5Amps , i need to drop 2.7volts with a resistor so i am left with 6.3volts .

How do i calculate this drop.


cheers

skal1

 

[dissonantstring]

hi skal1,
not to give you a difficult time, but you've been on the forum for how long? ;D
this might help: http://www.sengpielaudio.com/calculator-ohmslaw.htm
BTW, all i did was google ohm's law calculator resistor drop and this page came up first in the list.
the current you list is the max current rating of the transformer correct?
you'll need to know the current draw of the circuit to calculate your drop resistor value.
kind regards,
grant

 

[ruffrecords]

i have a transformer the  output  is 9vac , 5.5Amps , i need to drop 2.7volts with a resistor so i am left with 6.3volts .

How do i calculate this drop.

Assuming the load is 5.5 amps then the resistor required is V/I = 2.7/5.5 = 0.49 ohms. The power dissipated in it is I*I*R = 14.8 watts!!

You will need  really big one.

Cheers

Ian

 

[gemini86]

maybe you could mount that bug 20 watt resistor upright like another tube... it's gonna be as big as one.

 

[zebra50]

Might be a job for a lightbulb. Or buy another transformer!

Are you really pulling 5.5A?

 

[JayDubrek]

That does sound like a lot of current! Assuming this is for valve heaters...what tubes are you using and how many?

 

[Nora]

I think his transformer has max 5.5A output.

In order to calculate the resistance required for the voltage drop, we need to know your current draw of the load (heaters?)

Transformer VA output is just a maximum rating.

 

[JayDubrek]

Exactly Arno!

 

[skal1]

ok lads thanks for all your responses.

so i am powering 1 poor man module with ac heater , according to the poorthread 1 board eats 2.6A so now i can do the maths


the  load is 2.6 amps then the resistor required is V/I = 2.7/2.6 = 1.038  ohms, am i correct?


cheers

skla1

 

[ruckus328]

FYI, you're still gonna be burning alot of watts.  I would look into (at least) a 10W power resistor that can be heatsunk.

 

[tv]

You could be fancy and do it with some RF filtering..
.. and using cheaper 5W resistors.

 

[elektroivo]

i would be best to buy new transformer.... when You pull out one tube, on other tubes there will be higher voltage. Have You considered DC heating? Just put simple rectifier and 3-terminal regulator (LM350 or similar). It would be quieter (hum, noise), aldough it would shorten life span of tubes.

best,
Ivica

 

[skal1]

Thanks lads.


@tv  why did you put a resistor on both legs , is this because it is AC ?

cheers

skal1

 

[tv]

.. if you look at it that way..

it's mainly for convenience - if you look at it from that perspective. Try to imagine how it would fit together when assembled.

The side effect would be better symmetry and rf-rejection. If any of this matters..

 

[skal1]

i get you now


cheers
skal