I need help,AC/AC power transformer,please take time and read...

[Masimo]

Hello,
I need some help,and i hope you will understand what is bothering me.I apologize up front for my English,and for dumb questions i will probably ask you here.So,i built Objective 2 by NwAvGuy headphone amplifier from scratch(more info here http://nwavguy.blogspot.hr/2011/07/o2-headphone-amp.html) and this amplifier requires to be powered from AC source,it will not work if power it with DC supply.I can't find where to buy AC/AC wall transformer with EU plug(or it is too expensive).On datasheet for this amplifier it says AC power supply can be used if it is in range 14VAC-20VAC and minimum current should be 200mA(info from website

"The Objective2 accepts the following power adapter characteristics:

Type: AC-to-AC Transformer (AC/AC, not AC/DC!)
Voltage: 14VAC minimum, 20VAC maximum
Current: 200mA minimum, no maximum
Plug Size: 2.1 x 5.5mm (Inner Diameter x Outer Diameter) )

Since i can't find where to buy it I was planing to use things i already have.I have 120VA toroidal transformer,Primary winding requires 230V AC(from wall outlet) and secondary winding outputs 20V AC with maximum current of 6A,(6Ax20V=120VA(W) transformer).

NOW my question is can use this transformer to power up my amplifier.I was thinking to connect power cord to primary winding(and to wall outlet) and to connect secondary winding to AC input on amplifier.I don't want to burn my AMP because i am dumb.

I am electrotechnician,finished 4 years of school and i still don't know will transformer send 6A to my AMP and smoke it,or will AC devices take only the current it requires.
I believe this will work,but i want someone to confirm it.Common sense tells me if for example i connect light bulb to 200V,and it is 100W bulb it will draw 0.5A of current(W=U*I) because that is preset with bulbs input resistance(lets say 100 Ohm)...so following Ohm's law I=U/R i concluded input voltage and input resistance determine what Amperage will device draw from source.So if i am right input voltage and input resistance of my amplifier will control what current will amp draw in order to operate.

Edit (mod) fixed link

 

[(*steve*)]

Yes, the 20V 6A transformer should be fine. The specs go as far as to tell you there is no maximum current rating.

It may be wise to check that the secondary voltage is not too high given the low load on the transformer.

 

[Masimo]

Thank you for confirming my concerns,and thank you for advice to check voltage.If voltage is to high perhaps i could add resistor in series to secondary in order to lower voltage 1-2V

 

[(*steve*)]

Adding a resistor is not a good option.

Measure it and get back to us. Even better might be to contact the designer of this device and ask what a safe maximum AC input voltage is. I assume that their recommendation of 15 to 20 via takes into account factors like line voltage variation and load regulation (this is your issue).

If they allowed for 15% load regulation, then you would be safe up to 23VAC unloaded.

 

[Masimo]

You are right,for this amp max. voltage is 22VAC,and considering high power of my transformer it will give about 30-32V because of low load.I think i will buy one small transformer from ebay.Found 230/15V 10W transformer for 10$.If i am right 15V 10W transformer should output max. current 0,67A...so that should do it

 

[(*steve*)]

Yep that sounds much better.

 

[Masimo]

Thank god for this forum,otherwise i would kill my amp with that 2kg transformer hahahahahahaha.4 years of school and I am still noob for electronics.Thank you sir for teaching me usefull thing ...i didn't knew low loaded transformer outputs higher voltage.I wish you a good day,greetings from Croatia

 

[elebish]

Thank god for this forum,otherwise i would kill my amp with that 2kg transformer hahahahahahaha.4 years of school and I am still noob for electronics.Thank you sir for teaching me usefull thing ...i didn't knew low loaded transformer outputs higher voltage.I wish you a good day,greetings from Croatia

Wow, Croatia! Been there a number of years ago. Great people!

 

[davenn]

i didn't knew low loaded transformer outputs higher voltage

that should be unloaded transformer outputs higher voltage

 

[Masimo]

that should be unloaded transformer outputs higher voltage

I am sorry,English is not my native language so i make mistake or two .It is important that you understand the essence of what am i trying to say.So "low loaded" and "unloaded" is close enough.But i think "unloaded" is wrong...it means there is no load at all."low loaded" means there is very low load compared to what is my transformer capable of(my amplifier needs 200-500mA from transformer that can output 6A).

 

[Masimo]

Wow, Croatia! Been there a number of years ago. Great people!

I'm glad you like my country

 

[Audioguru]

I have a box full of AC-DC wall plug power supplies. I am looking for 5VDC at 350mA and found a few that are labelled 5V at 500mA, 1A and 2A. Most have a voltage that is about 6V to 9V at 350mA. I found a 5V one but its cord is too stiff. I used one that is 5.2V and it has a long flexible cord.

 

[Masimo]

I have a box full of AC-DC wall plug power supplies. I am looking for 5VDC at 350mA and found a few that are labelled 5V at 500mA, 1A and 2A. Most have a voltage that is about 6V to 9V at 350mA. I found a 5V one but its cord is too stiff. I used one that is 5.2V and it has a long flexible cord.

I am sorry but i haven't fully understod what do you need...what is the problem?

 

[Audioguru]

The problem is that some power supplies are labelled 5V but produce up to 9V which is too high for my circuit.
I have a cheap power supply rated at 9V that produces 19V into a low current load. It gets hot even if it has no load.

 

[Masimo]

The problem is that some power supplies are labelled 5V but produce up to 9V which is too high for my circuit.
I have a cheap power supply rated at 9V that produces 19V into a low current load. It gets hot even if it has no load.

If you need DC power to go to your circuit then you can use voltage regulator...LM7805 should do it.If the voltage is really high then use a few regulators in a row(for example if you have 20V and you need 5V then use 15V regulator first,and after that 12V regulator....then 8V and finaly 5V.).Just step down voltage slowly in way i described so every regulator lower voltage 3-5V.More voltage that regulator needs to dissipate more hot it will get.If you input 20V directly on LM7805 it will get really hot and probably get damaged.I have done that in few cases when i had no other options(and i don't recommend that unless that is only option you have),but you will need a heatsink on every regulator.If you buy TO220 package regulators just buy a pack of TO220 heatsinks.

 

[Audioguru]

Heatsinks are made in many sizes.