# Using toroidal transformers

Discussion in 'General Electronics Chat' started by MAS62, Jun 29, 2009.

1. ### MAS62 Thread Starter New Member

Jun 29, 2009
6
0
I have a circuit running off a LM7815 voltage regulator. On the Input side I have a bridge rectifier because my supply is from a 24VAC toroidal transformer.
The input voltage to the regulator is therefore 24V-1.4V = 22.6VAC...so far so good.
However the output voltage is 12.6 volts but I don't understand why.
If I connect a DC bench power supply to the bridge rectifier AC side the regulator gives 15VDC output as expected....so what is the toroid doing to my regulator?
For reference the output of my regulator is driving a 115mA load.

Apr 5, 2008
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3. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
No, you do not see the diode drop through a bridge. However, you need a filter capacitor to eliminate the ripple voltage. Doing so will raise the 7815 input voltage to something like 33.6 volts (1.4 X the AC voltage). Your 7815 will have the proper output (78xx regulators run a bit low, so look for about 14.85 volts out), and the regulator may get warm because of dissipation - 246 mw.

Note - used wrong calculation for power, which is I * E, or 2.14 watts.

Last edited: Jun 29, 2009
4. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,785
771
@MAS62
I really donno what the 1.4V is.

It seems that you are not using a filter capacitor after the bridge.

Still there is a flaw in your circuit. Let me explain.
First of all with a 24VAC input to a bridge with the proper filter capacitor will give you around 33.9V DC.
You must use decoupling caps at the input and output of the 7815, in order to stabilize the output. Around 10uF at the proper voltage will do fine. A filter cap for your stated load can be a 1000uf.

But keep in mind that at a load of 115mA with an input of 33.9V, the regulator will dissipate around 2 watts. You must provide heat sink for the reg.

But there is a little problem that bugs me.
The differential voltage will be at 18V DC. I donno what type of regulator u have but I don't think a standard regulator can handle that much voltage.
So to say the input must be at minimum 3V more than the regulator rated voltage and no more than say 5V. These are the values I will use if I am using 78XX series regs.
in your case reg input can be 18V to 25V DC. These values will not cost u too much heat and space

Rifaa