Power Transformator Problem

[Agung Kurniawan]

Hi everyone...
I have an anoying problem here. My pedal power supply have 7809 and the common filtering inside it.
The transformator was the common tipe one.
After turn the ps for about 5-10minute the transformator becaming heat. Even sometime there's smoke out of it.
Did there is any problem that causing the heat on tranformator?
Thanks..

[pdavis68]

If there's smoke coming out of it, it's done. You'll need to replace it. The insulation in the windings must have failed.

[Agung Kurniawan]

If there's smoke coming out of it, it's done. You'll need to replace it. The insulation in the windings must have failed.

Why did this problem came out?

[R.G.]

I think there is some problem with the way the transformer is connected. Can you supply a schematic diagram of the circuit you are using?

Transformers are very tough, and can supply power to circuits that have errors. But they can only supply this extra power for a short time before overheating.

The problem is usually in what the transformer drives.

[pdavis68]

As R.G. said, it's usually what it's driving.

What's the transformer rating and do you have a schematic?

Is there a fuse? If so, what's its rating and type (slo/fast)?

[Agung Kurniawan]

my schematic just look like that but with this rating:
primary    : 220V (I live in Indonesia)
secondary : 20V
Diode rect : 3A
Capacitor  : 4,700uF and 100nF

Im using center taped transformer with 1A rating. I ommit the LED indicator and the diode on 1st and 3rd lug of the 7809.
I didnt use any fuse. is that any problem with mine?

[Rob Strand]

Im using center taped transformer with 1A rating.

What are you doing with the centre-tap?
Are you using a bridge rectifier (with 4 diodes) or only two diodes?

20V

So is that 10V-0V-10V, or 20V-0V-20V?

Capacitor  : 4,700uF and 100nF

Have you loaded all 4 caps shown in your schematic (C1, C2, C3, C4)?
If you leave off any caps you can have trouble with the regulator oscillating (and maybe overheating).
This is especially a problem if you the regulator is too far from the caps.  C2 and C4 need to be right on the
regulator pins.


Also, check you have the electrlytic caps and all diodes around the right way.

[Agung Kurniawan]

What are you doing with the centre-tap?
Are you using a bridge rectifier (with 4 diodes) or only two diodes?

I just connect the CT directly to ground and I'm using 4 diode for the rectifier

So is that 10V-0V-10V, or 20V-0V-20V?

The 20V come after the diode rectifier, it's 18-0-18

Have you loaded all 4 caps shown in your schematic (C1, C2, C3, C4)?

yep, 4,700uF on C1 and C3 while the 100 on C2 and C4
I place all filter capacitor near the 7809
my pcb board is about 6cmx9cm (I thing)

I just don't understand why my transformer can be into very heat

[idiot savant]

I just connect the CT directly to ground and I'm using 4 diode for the rectifier

There's your problem right there. In a bridge rectifier setup you do not use the CT. You're shorting half of the secondary.

[Rob Strand]

I just connect the CT directly to ground a

That's the problem!

See this page:
http://www.hammondmfg.com/pdf/5c007.pdf

Option 1: (recommended)
Since your transformer is 18-0-18.   I recommend using the "Full wave" connection not the "full wave bridge". For this connection it is correct to connect the centre-tap (0V) to ground.  You only need two diodes. 

This will give you about 25V DC at the input of the regulator - you don't want more than that.

Option 2:
Connect the AC input of the 4-diode bridge to 18V and 0V and do not connect anything to the other 18V connection.
This will also give you about 25V DC at the input of the regulator.

The benefit of Option 1 is it fully utilizes the power of the transformer.

[Agung Kurniawan]

Ok thanks everyone. that was really helping
I was miss about the CT and Diode rectifier

[Agung Kurniawan]

One more question:
Would it be better to feed the regulator with bigger current like 2A or 3A?

[Rob Strand]

Would it be better to feed the regulator with bigger current like 2A or 3A?

The 1A transformer with a full-wave rectifier is correct for a 1A regulator.

However because the DC input voltage (25V) is much higher than 9V the regulator will get hot and you will not be able to get 1A output at 9V.

You need to put the largest heatsink you can find on the regulator (and use heatsink compound when you mount it).

When you buy a heatsink it has a rating called "thermal resistance" measured in "deg C/W"  the smaller the number the bigger the heatsink.

Heatsink Rth (deg C / W)        Iout max (A)
No heatsink                            80 mA
20 deg C/W                           210mA
10 deg C/W                           340mA
5  deg C/W                            500mA        <----- probably not much point using larger than this
2.5  deg C/W                         630mA         large heatsink but very small increase in output current

As you can see, you cannot reach 1A before the regulator gets too hot.
If you want 1A  the transformer voltage must be lower.

[Rob Strand]

Heatsink Rth (deg C / W)        Iout max (A)
No heatsink                            80 mA
20 deg C/W                           210mA
10 deg C/W                           340mA
5  deg C/W                            500mA        <----- probably not much point using larger than this
2.5  deg C/W                         630mA         large heatsink but very small increase in output current

You might get 10% more current than what I said here.
It depends on the regulation of the transformer.

Also if you use a smaller filter capacitor, say 1000uF,  you can get more output current.  You will get close to 1A with a 2.5 deg C/W heatsink.  However you will also get more ripple so you get a little more humm on the output of the regulator.     

[Agung Kurniawan]

Thanks, I get it now

[anotherjim]

If you have a 12-0-12 transformer secondary, why not to do it this way? There would be about 16v into your regulator.

[Rob Strand]

If you have a 12-0-12 transformer secondary, why not to do it this way?

With a 12-0-12 tx and at 1A DC out, the design is a little close to dropout at the bottom of the ripple with a 2200uF cap but a 4700uF cap has enough safety margin.   Full-wave rect, Tx  24 to 30VA and Heatsink <= 12 degC/W.

[Agung Kurniawan]

If you have a 12-0-12 transformer secondary, why not to do it this way?

yep, last day I just cut the -ve terminal like Rob said. and its working, my transformer run with better tenperature. but now Im going to use the 2 diode rectifier.

[Rob Strand]

In your schematic, keep C2 and C4 at 100nF.

[Agung Kurniawan]

In your schematic, keep C2 and C4 at 100nF.

Yep I keep anything except the diode