Transformer ratings?

[Kasamiko]

I got this audio amp without its power transformer. Based on the rectifier and capacitor connection it uses a CENTER-TAPPED transformer of about 3.5 amp.
Now my problem is, what might be the approximate secondary AC output (unloaded) of the transformer? The secondary filter capacitors are rated at 4,700uF/35VDC. Is it safe to assume an 18-0-18 volts secondary? ???
The output transistor used are TIP41 and TIP42..
Any help will be appreciated. ;D

rhonn

 

[ante1]

Hi Rhonn,

I think it will work fine with this transformer if it

 

[Kasamiko]

Hi Rhonn,

I think it will work fine with this transformer if it

 

[Kevin Weddle]

For a power transformer, you will always get the 120VAC on the input. This means you will have whatever voltage on the ouput that it is stepped down to. Loaded or unloaded. So, yes you can assume a maximum of 50VAC according to the capacitors.

 

[audioguru2]

Hi Kevin,
We don't always get 120VAC on the mains. At home and at work, the voltage varies from 115V to 125V.

You also don't get a voltage out of a transformer that is the same as its rating. Since a transformer's windings have resistance, its output voltage changes with loading, so a 50VAC rated transformer could have an output of 55 or 60V with a light load.

You can't judge the voltage requirement of a circuit by the voltage rating of its capacitors. The voltage rating of the capacitors is frequently doubled, for longer life. You must also consider the voltage, current and power dissipation ratings of its transistors and resistors.

If the transformer was rated at 50VAC at full load with a 120V input, the rectified DC output could be as high as 43.5V, which would blow-up the capacitors and transistors, and maybe burn the resistors.
The recommended transformer is 18-0-18 with a light load that would be about 15V per side (30VAC) at its rated current.

 

[Kevin Weddle]

I think you have misjudged the situation. This is a center tapped transformer. 50 VAC turns into 25VAC applied to each diode and 35 volt capacitor. 25 VAc is 35 volts peak.

 

[audioguru2]

Hi Kevin,
Nope. You forgot about times when the mains is 125V and the transformer is lightly loaded. The capacitors and transistors will go:
[move]Ka-Boom![/move]

8) 8) ;D

 

[ante1]

Hi Audioguru,

Sorry to step in like this but I think Rhonn has 230VAC in his outlets! So 125V is no problemo . ;D ;D ;D

 

[audioguru2]

Hi Ante,
I know. ;D

Kevin started talking about 120VAC and I should have corrected him but I just continued his discussion.

He, he. Take an amplifier with Kevin's 120V/50VCT transformer to The Philippines and plug it into 230VAC. Even in far away Sweden you'll hear the big
[move]Ka-Boom![/move]

8) ;D

 

[Kasamiko]

Hi Ante,
I know. ;D

Kevin started talking about 120VAC and I should have corrected him but I just continued his discussion.

He, he. Take an amplifier with Kevin's 120V/50VCT transformer to The Philippines and plug it into 230VAC. Even in far away Sweden you'll hear the big
[move]Ka-Boom![/move]

8) ;D

Heres the results..

Line voltage input 220VAC
Transformer secondary 18-0-18 VAC @ 5 Amp.
Output with FULL load is 25-0-25 VDC

With a 25-0-25 transformer @ 5 Amp I got this result:
Secondary output is 32-0-32 VDC!!

Almost 35 Volts near the maximum voltage of the filter capacitor.. ;D

I used the 18-0-18 transformer to play it safe..
or I can use the 25-0-25 and replacing the 35 volts cap with 50 volts


[move]NO! Ka-Boom![/move]


rhonn

 

[ante1]

Hi Rhonn,

I don

 

[audioguru2]

Hi Rhonn,
Yeah, it's best to play it safe.
If the mains voltage rose to 240VAC and the amp was lightly loaded or had no load but still had a high output signal, the output transistors would be swinging much more than their rated maximum voltage if you used a 220/25-0-25 transformer.

You don't want the transistors to release any of the important compressed smoke that they have inside. ;D

 

[steven2]

audio guru how about telling me how you get these words move across the screen like you did before when you replied to another topic and you got these smilies doing the same thing, id really like to lern that, this may be the begining of a new trend.

 

[surajbarkale]

[move]Use this syntax[/move]

[move]Text you want to scroll[/move]

Or you can click the button with scrolling M & type between the tags.

 

[steven2]

[move][/move] is this how you do it

 

[steven2]

[move][/move]i cant get it right still[move][/move]

 

[audioguru2]

Hi Steven,
1) Click the moving "<M<" button that gives this:
Bracket move bracket, bracket /move bracket.

2) Then type your words in between the center brackets.

3) You can add colour and size the same way, type your words in between the center brackets.
[move]G'day mate![/move]

4) You can put a smile in between the center brackets too.
[move] ;D ;D ;D ;D ;D ;D ;D ;D[/move]

5) You can type bold characters in between the center brackets too.

 

[Kasamiko]

Hi Rhonn,

I don

 

[ante1]

Good choice Rhonn, now I am calm!

[move]Member of Audiogurus html school! cool ;D ;D ;D[/move]