LM1875 and DIY Transformer

Thread Starter

bumba000

Joined Oct 7, 2015
97
Hi All,
I have built up a few circuits using the LM1875. First worked great, next two not so much and now I'm onto the third. This is what I've done


I'm sure you've all seen these boards


I made my own boards and sourced all individual components.

So now I need to power this little guy. I guess I didn't realize the special type of power. I just assumed V+ and V- and I neverminded that G and figured I'd figure it out later. Well now I have a few of these little circuits built and have no way of powering them. ha ha

I'm going to build a transformer *today*. I have found a so called "constant" number of 42 which is used in calculating the number of turns on the primary winding. What is 42? what does that number represent in the world of transformers?

Please and Thank you,
John
 

Audioguru

Joined Dec 20, 2007
11,248
A power supply that has V+ and V- also has 0V. The Grounds in your schematic connect to the 0V of the power supply.
Instead or trying to make a 3-wires transformer with a 0V center-tap why not buy one instead?
 

Reloadron

Joined Jan 15, 2015
7,546
So now I need to power this little guy. I guess I didn't realize the special type of power.
Special type of power?
LM1875 20W Audio Power Amplifier can use a pretty wide supply range. Why wind when all you need is a simple center tapped transformer to make a dual supply? A 36 Volt center tapped transformer, a full wave bridge rectifier and some filter caps is all you need. For an audio amp you don't even really need a regulated supply and it can be a little above or under +/- 25 volts.

I see AG was here as I typed this so there you have it.

Ron
 

bertus

Joined Apr 5, 2008
22,290
Hello,

As AG said, buy a centre tap transformer.
A 2 X 18 Volts , 4 Ampere would be working fine fo this amplifier.

Bertus
 

Thread Starter

bumba000

Joined Oct 7, 2015
97
I don't have a 36 volt center tapped transformer on hand which is why I'm going to have to make one. purchasing a transformer would be the easy way out and it would mean waiting until at least the middle of next week before it would arrive. What does the 42 refer to when calculating windings?
 

Reloadron

Joined Jan 15, 2015
7,546
What does the 42 refer to when calculating windings?
I have no idea and have never heard of it as a constant for anything transformer related. Where did you see it referenced? While rolling your own is noble making a transformer is not easy starting with the core. Not as simple as winding some wire on a core anyway.

Ron
 

Thread Starter

bumba000

Joined Oct 7, 2015
97
Well I've watched a lot of youtube and done a little reading. here is what I've been looking at
and then their website (using google translate)
http://www.construyasuvideorockola.com/transformador.php

one part in the video uses 42 in some math and then it's also mentioned on the above referenced website page ...
Calculating the number of turns of copper wire
There is a constant that is the number 42, we will not go into details about the origin of this number, since the idea is not to delve into mathematics
Thanks, John
 

Reloadron

Joined Jan 15, 2015
7,546
Well from the translated web page I get:
"Calculation of the number of turns of the copper wire
There is a constant that is the number 42, we will not go into details about the origin of this number, since the idea is not to delve into mathematics, if not to get people with little knowledge to make transformers.
To calculate the number of turns or turns of copper wire, in our example, 42 is divided by 12.8 cm2, which is the core area of 3.2 x 4.
Number of turns = 42 / 12.8 Cm2
42
divided 12.8 = 3.28 turns or turns of wire per volt .
This means, that for the primary winding , it is 120 volts from the outlet, multiplied by 3.28 , it is equal to: 393 turns or turns of copper wire. If in your country the voltage of the public network is 220V , multiply, 220 volts by 3.28 = 721 turns in the primary winding".

I still have no idea where they get 42 as a constant. Maybe another member better versed in transformer theory than myself knows. While they use it in the video which is based on the web page they also do not mention why 42? They are dividing it by the square root of the core. Years ago, more than I care to remember I recall a constant of 50 having to do with that but I haven't a clue why? This goes back over 40 years, maybe it was in school.

Hopefully someone with good strong transformer theory will come along. Good luck with this as hand winding and making the core will be a heck of a project.

Ron
 

Thread Starter

bumba000

Joined Oct 7, 2015
97
hey Ron,
Yeah there's a formula for range finding using a mil-dot reticle too ( Size (inches) ÷ Size (MILs) x 27.77 = Range ). There's a constant there too ( 27.77 ), but it can be adjusted slightly for longer or shorter range range finding to be even more precise. I think it has to do with the curve of the earth, but I'm not sure. I know that 99% of the time I just use the commonly accepted constant though. maybe that's what I should do here.

lol

Thanks, John
 

Reloadron

Joined Jan 15, 2015
7,546
Oh, something I can better relate to, MIL verse MOA in range finding. :) Target Height in inches * 27.77 / Height in Mils = Range in Yards. There are other formulas if you want to work in yards. My big downside was learning most of this stuff in inches, feet and yards. I remember that when we figure target size in inches, Mil relation formula numerator is the Target height in Yards times 1000. Divide 1000 by 36 (inches in a yard) and you get 27.77. Don't hold me to any of this as all my scopes are MOA. :)

As to the transformer? I haven't a clue where the constant comes from. I would try it as you mention as apparently it works. I hope this works out for you because despite being a pile of work it is challenging but will be cool if it all comes together.

Ron
 

Thread Starter

bumba000

Joined Oct 7, 2015
97
LOL Ron that's great. Yeah I never realized the power of 42 I guess. I'll have to read more into this once my transformer project for powering the audio in my briefcase laptop lcd with raspberry pi project comes to a close. and to think... all of this started because I wanted to use a raspberry pi and python scripts to launch firework shells. Holy crap! Just light the fricken fuse and run away! LOL

Thanks guys,
John
 

Audioguru

Joined Dec 20, 2007
11,248
when I was 42 I sure as heck felt better than now at 68. My vision, hearing and everything worked much better including my then young mind.
You neglected routine maintenance.
I am almost 73 and I am better than when I was 42 since I got the lenses in my eyes replaced (due to cataracts) and now my vision is perfect.
I got digital hearing aids and now my hearing is better than normal (due to the perfect EQ and the selectable features).
I even got the arteries in my heart fixed (due to a heart attack) and now it is better than most younger people. I have good strength and stamina now.
My teeth are also better. Retirement is wonderful since every day is a Saturday, I do whatever I want whenever I want.
I don't pay income tax, the government pays me (a pension).
I get senior's discounts on many things I buy. I pay nothing for medications and medical care.
 
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