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7805 12 volt power supply

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npn

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Hi

I saw a circuit somewhere, in which a 7805 was being used to make a 12 volt supply. The circuit used two resistors-one across the output and common terminals and the other across the common-ground terminals.
My question is: there must be a limit to how much voltage the supply can provide, right? Is the transformer secondary peak voltage that upper limit?
For example, if I use a 0-9 transformer, does that mean I can vary the voltage from 5 volts to (9xsqrt2) volts?
 

npn said:
Hi

I saw a circuit somewhere, in which a 7805 was being used to make a 12 volt supply. The circuit used two resistors-one across the output and common terminals and the other across the common-ground terminals.
My question is: there must be a limit to how much voltage the supply can provide, right? Is the transformer secondary peak voltage that upper limit?
For example, if I use a 0-9 transformer, does that mean I can vary the voltage from 5 volts to (9xsqrt2) volts?
Click to expand...

input voltage conditions for a voltage regulator '7805' is that the input voltage should at least be greater by 2 volts than the output.. or it can be in between 5-18 volts.

yes you can vary the voltage in the range of 5 volts to 18 volts.

by the way, is this circuit a dc-dc boost converter ? i am asking because the circuit supplies 12 volts from 5volts.
 
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    npn

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Yes, 9x1.414=12.7 Volts. 9 volts is the RMS value and when you rectify it and treat smoothing with a smoothing capacitor in the circuit you get Peak voltage, which is 12.7 volts per your question. Now regulating it for 5 volts fixed or using a variable regulator you can get as much as you want within the range minus regulator drop volts.
Here is a link for your reference on same:
**broken link removed**
 
He is asking to get voltage varied from 5~12 volts. And wants to use LM7805 which can do the job.
 

Hussainb said:
input voltage conditions for a voltage regulator '7805' is that the input voltage should at least be greater by 2 volts than the output.. or it can be in between 5-18 volts.

yes you can vary the voltage in the range of 5 volts to 18 volts.

by the way, is this circuit a dc-dc boost converter ? i am asking because the circuit supplies 12 volts from 5volts.
Click to expand...
Nope, the circuit would be supplying 12V using a 5V regulator, thus needing at least 14V input to maintain 12V out. So there's no voltage boosting involved (and never is for linear regulators like 7805, the VIn-Vout difference is wasted as heat).

To do this would require keeping the 7805's GND pin at 7V, and since a 7805 sends a not-so-small current to its GND pin in order to maintain regulation (up to 6 mA according to datasheet I have here), keeping that accurate would require very low-value resistors. Or perhaps a zener diode between 7805's GND pin and actual ground (with resistor between Vout & GND pin) ?

FWIW: I can't possibly imagine why you'd want to use an 7805 to make fixed 12V out - grab an 7812 instead.

If variable Vout is what you want, get LM317 or so. Much better suited to that job & output goes down to 1.25V. Both 7812 and LM317 cost peanuts.
 

If odd components are supposed to be used for a straight job, I assume it as if some one wants to carry out some experiment or the th3 market is of far access and the Junk Box contains some parts be used.
by the way, is this circuit a dc-dc boost converter ? i am asking because the circuit supplies 12 volts from 5volts.
Click to expand...
This is not a boost converter but he is using a 9 volts transformer to get 12 volts output.
For example, if I use a 0-9 transformer, does that mean I can vary the voltage from 5 volts to (9xsqrt2) volts?
Click to expand...
 

You can use some switch mode regulator, eg. from National Semi Simple Switcher family. Or just good old MC34063.
 

Raza said:
If odd components are supposed to be used for a straight job, I assume it as if some one wants to carry out some experiment or the th3 market is of far access and the Junk Box contains some parts be used.

This is not a boost converter but he is using a 9 volts transformer to get 12 volts output.
Click to expand...

yep it always happens with me... i am lazy enough to go to the market which is far and i have E-waste lying around always.
yes i understood he is using 9 vlts ....
the pdf you posted, i am going to try making it soon...

Question
I have a nokia charger which has an output of 5v dc with 890mA
as you already know, chargers are light weight,maybe transformerless? I dont know.
circuits using capacitors to limit current cant provide upto 890 mA... so whats inside this charger which makes it light weight with so much current??
I have a single working charger and i do not have special screwdrivers to open it.
 

Husainb,
If you are crazy enough :)-D) to explore what is inside just break the cove and see what is inside, otherwise here is complete project for your reference even if you want to assemble one yourself.
http://www.powerint.com/sites/default/files/PDFFiles/der267.pdf
This small Ferrite cored transformer is almost weightless.
Enjoy
 
Raza said:
Husainb,
If you are crazy enough :)-D) to explore what is inside just break the cove and see what is inside, otherwise here is complete project for your reference even if you want to assemble one yourself.
http://www.powerint.com/sites/default/files/PDFFiles/der267.pdf
This small Ferrite cored transformer is almost weightless.
Enjoy
Click to expand...

yes many times i have felt the urge to open it but due the special screws and the very tight threading i never was able to open it...I dont even have a spare to break open it...those cheap duplicates are available in market but i never felt the urge to break open a original one and use a duplicate one.
 

Raza said:
Yes, 9x1.414=12.7 Volts. 9 volts is the RMS value and when you rectify it and treat smoothing with a smoothing capacitor in the circuit you get Peak voltage, which is 12.7 volts per your question. Now regulating it for 5 volts fixed or using a variable regulator you can get as much as you want within the range minus regulator drop volts.
Here is a link for your reference on same:
**broken link removed**
Click to expand...

hi npn
i think raza give you good answer
 

Thanks everyone!

Yes I wanted to make a variable supply with a minimum voltage of 5 V and had no shortage of 7805s :-D
I guess i'd be better off using a 317. (A 7812 won't be able to give a voltage below 12 V, right?).
I saw a similar circuit to the one which Raza posted earlier (using a 7805), and I wanted to know how far the voltage could be increased.
 

saw a similar circuit to the one which Raza posted earlier (using a 7805), and I wanted to know how far the voltage could be increased.
Click to expand...
What you mean by of increasing voltage? Do you mean using 7805 and increasing voltage beyond 12 volts or what? I would suggest you to use LM317 as in normal design you can get from 1.2 to 37 volts in variable range and 1.5 ampere current. Just needs only two external resistors for complete circuit in minimum.
 

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