Transformer problem

[xXAxHillbillyXx]

where in the heck can I find a 220V Primary, 36V Center-tapped Secondary transformer?.. I searched all over mouser.com and could not find it at all... not even close I need it for a power supply I am building.

thanx,
Nick

 

[ericgibbs]

where in the heck can I find a 220V Primary, 36V Center-tapped Secondary transformer?.. I searched all over mouser.com and could not find it at all... not even close I need it for a power supply I am building.

thanx,
Nick

hi,
Do you mean a 18V 0v 18V type, if yes ,look here:

**broken link removed**

 

[Dvinchy]

The same thing that you need can be created from a couple of the same transformers like 220v / 18v or even from some three transformers like 220v / 12v.

 

[Nigel Goodwin]

The same thing that you need can be created from a couple of the same transformers like 220v / 18v or even from some three transformers like 220v / 12v.

How would you create 18-0-18 with three 12V transformers?

Like Eric says, it's a standard transformer.

 

[Hero999]

Three 12V transformers won't work because there won't be a centre tap.

 

[tcmtech]

How big of one do you need?
Digikey, Newark, Allied Electronics, and many other online suppliers carry many sizes and styles of transformers.

 

[Dvinchy]

How would you create 18-0-18 with three 12V transformers?

That right there is no any neutral with three.

 

[xXAxHillbillyXx]

this is the psu i am building i dont know anything about theses transformers if one of you can tell me where i can can get one that will work that would be fantastic.
Variable DC Power Supply - RED - Page36https://www.redcircuits.com/page36.htm

 

[Hero999]

You should be able to use a 30V transformer for that circuit but the value of C1 might need increasing.

Unless it's important for it to work down to 0.7V, an LM317 based circuit would be better.

EDIT:
I've just noticed it used a biphase rectifier which means the peak DC voltage will be about 25V which is far too low.

I haven't looked at the rest of the circuit in detail but it wouldn't surprise me if there are other errors.

Maybe an LM317 is the best solution?

 

[tcmtech]

I would just use a off the shelf 24 VAC furnace control transformer and switch to a full wave bridge rectifier set up and skip the dual half wave and center tap approach.

 

[roltex_rohit123]

from the diagram you need a 36- 0 -36 transformer... ask for 36 o 36 transformer 2 amp. you would get it

 

[Hero999]

Sorry but the diagram is wrong, as mentioned above, a 36V centre tapped transformer won't work.

It's a rubbish circuit anyway, the voltage is highly dependant on Q1's Vbe which will vary depending on the current and the ambient temperature.

I'd recommend building an LM317 based regulator. Adjustable current limiting can be added with a couple of transistors and an op-amp can be added if operation down to 0V is a requirement.

 

[schmitt trigger]

Sorry but the diagram is wrong, .................
It's a rubbish circuit anyway, .......................
...................

I 100% agree with Hero's assesment of the circuit.
The unit does not even have a zener or voltage reference!!
Plus other mistakes....

 

[Nigel Goodwin]

Sorry but the diagram is wrong, as mentioned above, a 36V centre tapped transformer won't work.

Of courcse it will - that's what it's designed for - a 36-0-36 certainly wouldn't work

It's a rubbish circuit anyway, the voltage is highly dependant on Q1's Vbe which will vary depending on the current and the ambient temperature.

It's certainly a rubbish circuit though, but bear in mind it never suggests it's a regulated supply, just a variable one.

 

[Hero999]

Of courcse it will - that's what it's designed for -

That's what I thought until I looked more carefully at the diagram.

**broken link removed**

Notice it uses a biphase rectifier?

The peak voltage across C1 is 18√2 minus any rectifier losses, let's say 24.5V at full load.

Q2 and Q4 are configured as a Darlington pair, with a saturation voltage of about 2V at full load so in order for the output voltage to be 24V the input needs to be at least 26V.

It won't work up to 24V, need I say any more?

 

[xXAxHillbillyXx]

Thanx for all the replies if you guys say it will not work then I am not going to build it, HERO999 recommended a lm317 based psu. That would be fine all I am wanting is a stable dc power supply that I can adjust the voltage and current from something like 0-20V and 0-2A. those numbers are not set in stone I just want around that and have pretty fine control of the current. If any one can supply a schematic for this that would be VERY appreciated.

Thanx,
Nick

 

[ericgibbs]

That's what I thought until I looked more carefully at the diagram.

Notice it uses a biphase rectifier?

The peak voltage across C1 is 18√2 minus any rectifier losses, let's say 24.5V at full load.

Q2 and Q4 are configured as a Darlington pair, with a saturation voltage of about 2V at full load so in order for the output voltage to be 24V the input needs to be at least 26V.

It won't work up to 24V, need I say any more?

hi hero,
As you say not enough overhead from the bridge to give a +24Vout.
Ignore the image section regarding the regulator data.

 

[Reloadron]

Just a little footnote but whatever is built it should be built around existing mains power. A transformer for 220 V mains would not fare to well here in the US. This works better when people include their locations in post. I guess we can assume the poster has 220 Volt mains power?

Ron

 

[Nigel Goodwin]

That's what I thought until I looked more carefully at the diagram.

**broken link removed**

Notice it uses a biphase rectifier?

The peak voltage across C1 is 18√2 minus any rectifier losses, let's say 24.5V at full load.

Q2 and Q4 are configured as a Darlington pair, with a saturation voltage of about 2V at full load so in order for the output voltage to be 24V the input needs to be at least 26V.

It won't work up to 24V, need I say any more?

Yes - you need!

The voltage on the input is probably going to be higher than 24.5V - and in any case, it's NOT a regulated supply, so if it drops a bit under full load at full voltage, it doesn't matter. Suggesting you move from an 18V to a 36V transformer is ludicrous, will blow the capacitors up, and massively overheat the regulator.

While you were 'looking carefully' did you not read the discussion of the transformer?.

 

[Hero999]

The voltage on the input is probably going to be higher than 24.5V - and in any case, it's NOT a regulated supply, so if it drops a bit under full load at full voltage, it doesn't matter

Funny, I thought it was supposed to be a regulated supply. Even if the voltage is higher than 24.5V, say 30V off load, it certainly will drop below 26V when a load is connected and the ripple valley will be even lower, probably about 18V.

Suggesting you move from an 18V to a 36V transformer is ludicrous, will blow the capacitors up, and massively overheat the regulator.

I didn't suggest that.

I'd suggest using a 30V untapped transformer with a bridge rectifier and 50V capacitors.

While you were 'looking carefully' did you not read the discussion of the transformer?.

I admit, I didn't read it before but now I have, I can say it's inadequate because it doesn't discuss peak voltage, dropout, ripple, current rating, and filter capacitor sizing; all important things to consider when selecting a transformer and filter capacitors.

Thanx for all the replies if you guys say it will not work then I am not going to build it, HERO999 recommended a lm317 based psu. That would be fine all I am wanting is a stable dc power supply that I can adjust the voltage and current from something like 0-20V and 0-2A. those numbers are not set in stone I just want around that and have pretty fine control of the current. If any one can supply a schematic for this that would be VERY appreciated.

The LM317 will nearly do that.

The maximum current rating is 1.5A but I've had no problems drawing 2A for extended periods of time. the datasheet says the the current limit varies between 1.5A and 3.4A, 2.2A is typical but don't bet on it. The current can also be boosted by adding another transistor or more.

The current limit is fixed between 1.5A and 2.2A but can be made adjustable by adding a couple transistors.

The minimum voltage setting is 1.25V but it can be reduced to zero using a band gap reference or an op-amp.

https://www.electro-tech-online.com/custompdfs/2010/03/LM117.pdf

It depends on what you want.

How important is:

Operating down to 0V?
Having an adjustable current limit?
Being able to supply 2A rather than 1.5A?

As previously mentioned, it's possible to do all of the above, with additional components.