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Transformers revisited.

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HerbertMunch

New Member
Hi all,

Im trying to work out if my circuit will work and is valid, could someone have a glance at it please?

Im trying to use a 240v- 9v transformer. The transformer has two secondary coils, both 9 volt (thanks Brian for the info:) )

Would it be possible to use both coils simaltaneously to step down 240v to both 9v and 18v?

I need to power a fan at 18v and power a 7805 with 9v, to produce 5v at the same time.

Many thanks.
 

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picbits

Well-Known Member
If you keep the two grounds seperate then it should work fine

The moment you link the two grounds from the 18v and 9v outputs you're going to have problems
 

ronsimpson

Well-Known Member
Most Helpful Member
I do not know where RelayIN comes from (a negative source) but check if D1 is pointing in the right direction.
 

ronsimpson

Well-Known Member
Most Helpful Member
Can the fan run from +/- 9 volts?
You could make +9 which drives the 5 volt regulator, and -9 volts. The fan is connected from +9 to -9.
Do you know how?
 

mneary

New Member
Your diagram uses the same name, GND, for the anodes of both bridges. These two nodes cannot be connected together.
 

rjvh

New Member
why you use a full bridge for the fan ?
a fan is not that critical on the power
i would use 2 diodes on from the coil 1 and one in opposite direction from coil2
for the fan and see it as one circuit
don't conect the negitive to the ground
the 5V circuit i would conect the same as you sugest

Robert-Jan
 

HerbertMunch

New Member
Hi, thanks all for your help.

Ron: I think i know how, but what would this acheieve?
The fan is a 12v case fan (computer).
I want to pwm the fan at 18v because it is too much of a wimp for the job at hand:) Would what you suggested, make the fan run quicker?
Using a bigger fan would have been the ideal option, but I have allready foolishly spent along time on designing my case in autocad, and now have manufactured it!

Mneary:
Ok thanks, just out of interest, what would be wrong with that:eek: ?

Robert-Jan: Im using the full bridge just for board simplicity (im very lazy!), also see above.

I have included a new schem, is this right now?

Many thanks,
chris.
 

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jdahl

New Member
Just a quick question, Whats the reasion for using the polarized
capacitor, why not just a bigger c2 and c4? Does a polariezed cap decrease Vripple better than a ordanary?
 

mneary

New Member
If you had connected the two GNDs together, you would have had only 9V.

Now you're disconnected the windings from one another. They need to be connected, and it's good to call them "common". you can connect this point to GND if you like.

A notation convention: +18V and -18V would normally be called +9V and -9V (0V halfway between them).

For further simplification, you only need one bridge. The 'common' of the transformer would be connected to GND, you can eliminate BR1 and use +9V created by BR2. C1 and C2 stay the same, but C3 and C4 are connected between -9V and GND.
 

mneary

New Member
Please excuse the PAINT cut and paste; here's what I have in mind.
 

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Hero999

Banned
Just connect up one bridge rectifier with a capacitor between the - rail and the centre tap and another capacitor betweent the + rail and the centre tap.

Also not that an 18V transformer will give 18*1.414-1.4 = 24V not 18V.
 

HerbertMunch

New Member
mneary said:
Please excuse the PAINT cut and paste; here's what I have in mind.

Not at all, thanks for taking the time to mod it, a picture speaks a thousand words.


hero999 said:
Just connect up one bridge rectifier with a capacitor between the - rail and the centre tap and another capacitor betweent the + rail and the centre tap.

Also not that an 18V transformer will give 18*1.414-1.4 = 24V not 18V.

I thought that the transformer would produce less than 18v?


Thanks alot both of you.
Chris.
 

HerbertMunch

New Member
Also, how do you think a case fan would hold up to double the rated voltage?
what sort of duty cycle would you recommend?
 

Hero999

Banned
HerbertMunch said:
I thought that the transformer would produce less than 18v?
No, because the output is given in RMS and when you add the bridge rectifier you get the peak voltage.

Also, transformers are rated at full load and produce even higher voltages off load. If your transformer has a regulation of 20%, it will give 21.6VAC which is 21.6*1.414-1.4 = 29V. This is important to consider when looking at the voltage rating of the regulator and input capacitors.
 
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