how do you drop ac voltage

[skal1]

Hi


i have a high secondary winding that needs to be droped about 200vac , cand this be done at the secondary before rectification, or should it be done after  rectification.


primary 240vac uk


thanks

skal1

 

[Mark Burnley]

Do you need 200v at the secondary, or do you want to drop 200v?

What's the current draw?

It's difficult to drop 200V resistively unless you're looking at a very low constant current draw (due to I^2*R losses!) And a regulator has to "lose" the voltage through heat dissipation (again, proportional to I drawn)

A new transformer is easiest if you're looking at dropping 200v.

...or, use a 240V:"x"V second transformer fed from this secondary winding, taking into account the lower input voltage to its primary, or a second transformer directly off the mains supply.

What are you trying to do? It'd be useful to know the context- too many variables!!

Mark

 

[shabtek]

can be done with a cap too if load is constant...
what's the formula?

 

[Mark Burnley]

Yea,

Using the capacitative reactance trick is good- (using the impedance of the capacitor as a resistance, worked out at a particular spot
frequency- Xc= 1/2*pi*f*c)but you have to protect against the inrush spike at switch on- also as you mention, load is particularly important, and current available is low.

I use this method to power the LEDs straight off the mains in my homebrew solder station...

Wouldn't trust it much further though!!

Mark

 

[sahib]

Yes if the application was mentioned it would have been helpful. But if you have a transformer that you want to utilise, then take the windings off the secondary by checking every so often until it is down to the voltage that you require.

 

[jdbakker]

And be sure that you're not measuring an unloaded voltage while comparing it to a loaded spec.

Not that that - by itself - would soon account for 200V, but still.

JDB.
[and yeah, it helps to know what you're trying to do]

 

[MartyMart]

It's for his PM670.
Skal you said that you "dropped" the transformer, would had to have been a pretty hard drop to
damage it .... what happened ?

If the voltage is that far off, it would seem to be damaged though.

I used an "in-line" with secondary resistor on my D_AOC to drop 50v using a 25watt 1k1 metal resistor
but that can't be done with such a high voltage drop of 200.

That was due to the German traffo wanting to see 220v AC from the wall and I have around 247v at my place.
I wanted 250v on the secondary and had 300v.

Regards,
Marty.

 

[skal1]

It's for his PM670.
Skal you said that you "dropped" the transformer, would had to have been a pretty hard drop to
damage it .... what happened ?

If the voltage is that far off, it would seem to be damaged though.

I used an "in-line" with secondary resistor on my D_AOC to drop 50v using a 25watt 1k1 metal resistor
but that can't be done with such a high voltage drop of 200.

That was due to the German traffo wanting to see 220v AC from the wall and I have around 247v at my place.
I wanted 250v on the secondary and had 300v.

Regards,
Marty.

yes it is the PM670 tx  i was  just thinking i could drop the secondary ac , but from the answers i am getting it seem impossible , so that tx is shoot, will ave to wait till after xmas to find a replacement .

what do you think made the 250vac winding go so high .

 

[keefaz]

what do you think made the 250vac winding go so high .

Just a thought, maybe primaries wrong connected, like 120VAC in parallel, so the transformer acts as step up 1:2 ?

 

[jdbakker]

Let's try this one last time.

what do you think made the 250vac winding go so high.

Measure it with a load.

If you measure it unloaded, and compare it to loaded measurements, the numbers you get will be meaningless. This is particularly true for a CRC-filtered supply such as used in the PM670.

JDB.
[It's very hard to imagine a failure mode where a dropped transformer suddenly gives a higher output voltage, unless as said you've rewired the primaries]

 

[skal1]

ok my friends secondary 250vac tap is 350vac all other taps are ok , with the 250vac connected to the pm psu voltage after rectifier is near 500v dc mark.


transformer was OK until i drop the psu which shorted out all the transistors on the 250, 130 volt supply, i then replace all transistors diode to the psu and now i can not get the 250v 130v supply to work  all other supply's are working fine 17v and the -17v.

I  think because there is  500vdc at the m** 92 and  irf840 this  is putting the transistors in to thermal protect that is way the circuit loads down when i connect the tube boards, this what i think but i might be wrong.


skal1