Split

[Electronman]

Hi all.

A question regarding to make a split (dual) power suply out of 78xx and 79xx.

I have a 7808 and a 7908 and want to get 14 to 15V max (+-7 to +-7.5) out of them.
Is the silicone rectifier diode the BEST why to do so? Any problem when performing? I want use this power supply with an op-amp as an inverting/noninverting amplifier. (a CMOS chip is presented in the said circuit).

Thanks

 

[ericgibbs]

Hi all.

A question regarding to make a split (dual) power suply out of 78xx and 79xx.

I have a 7808 and a 7908 and want to get 14 to 15V max (+-7 to +-7.5) out of them.
Is the silicone rectifier diode the BEST why to do so? Any problem when performing? I want use this power supply with an op-amp as an inverting/noninverting amplifier. (a CMOS chip is presented in the said circuit).

Thanks

hi,
The 7808 is a fixed +8V reg and the 7908 is a fixed -8V reg.

If you have a mains transformer with a dual 12V or a centre tapped winding of about 12V/0v/12V you would require a full wave bridge rectifier and smoothing caps. This would give you +8V/0V/-8V with these two regulators.

 

[Hero999]

It is possible to use a voltage doubler circuit and a non-centre tapped transformer but it means that the transformer can be destroyed if there's a large difference between the positive and negative currents.

If you're still interested I'll post a schematic.

 

[Electronman]

Thanks.

Please post the said schematic.

I Have a 2x12 transformer (center taped), But my CMOS chip does not Allow more than 15volts, So I have to down the volatge comming through those regulators to 15v or a bit less. +8 0 -8 is not suitable because It will give me 16V not MAX 15V, So what about using 2 rectifier diodes at the output of each regulator (ie 7808 and 7908) to down the output voltage of the circuit to safely handel the CMOS chip?

 

[ericgibbs]

Thanks.

Please post the said schematic.

I Have a 2x12 transformer (center taped), But my CMOS chip does not Allow more than 15volts, So I have to down the volatge comming through those regulators to 15v or a bit less. +8 0 -8 is not suitable because It will give me 16V not MAX 15V, So what about using 2 rectifier diodes at the output of each regulator (ie 7808 and 7908) to down the output voltage of the circuit to safely handel the CMOS chip?

hi, You dont need the voltage doubler circuit with a centre tapped transformer, a full wave bridge will be OK, with cap filtering.

Whats the current rating of the 12V/0/12V transformer.?

 

[audioguru]

Silicone is used for fake women's breasts.
Silicon is what a diode is made from.
A huge difference.

 

[Electronman]

Can you let me know your mean by an circuit? Do you know that I need a center taped suply for my circuit? Are you telling that I am able to reach to that goal without a center taped transformer (while I have one woth current rating at 2A)? I do not like any unstbility.

My main question which caused me to start this thread was about how to create a good and stable +15 0 -15 from two 7808 & 7908???

 

[Nigel Goodwin]

My main question which caused me to start this thread was about how to create a good and stable +15 0 -15 from two 7808 & 7908???

For a start you need 7815 and 7915, those you mentioned are only 8V versions.

 

[ericgibbs]

Can you let me know your mean by an circuit? Do you know that I need a center taped suply for my circuit? Are you telling that I am able to reach to that goal without a center taped transformer (while I have one woth current rating at 2A)? I do not like any unstbility.

My main question which caused me to start this thread was about how to create a good and stable +15 0 -15 from two 7808 & 7908???

hi,

I think you are getting confused, you said initially.

I Have a 2x12 transformer (center taped), But my CMOS chip does not Allow more than 15volts, So I have to down the volatge comming through those regulators to 15v or a bit less. +8 0 -8 is not suitable because It will give me 16V not MAX 15V, So what about using 2 rectifier diodes at the output of each regulator (ie 7808 and 7908) to down the output voltage of the circuit to safely handel the CMOS chip?

Tell us what you are powering with this proposed power supply.

 

[Electronman]

oK here's waht I mean.
Sorry for any confusing. of course I made my self confused by the voltages too, but I guess this schematic will help you to understand me.

 

[ericgibbs]

oK here's waht I mean.
Sorry for any confusing. of course I made my self confused by the voltages too, but I guess this schematic will help you to understand me.

hi,
Have a look at this option.
I understand that the dual psu is going to be used for low current outputs.???
The 330R resistors on the outputs are important, they will pass about 20mA and keep the serial diodes active and hold the outputs low by ~0.7V when there is no load connected to the psu outputs. OK.?

If you require higher currents and the transformer can supply the required current, it may be necessary to use higher capacity electrolytic capacitors also heat sinks for the regulators.