Testing my thinking about single supply output

Thread Starter

be80be

Joined Jul 5, 2008
1,928
Ok I made this to test out how to make a single supply that has vertical ground.

This is what I have the transformer with to caps and a small load.
the output is 19 volts across the caps. From the negative side to ground I get little over 11 volts.

The positive side to ground I get 8 volts I guess the caps are not that close in value.
I then added 2 regulators 7905 and 7805 the 7805 has no output. Im thinking the caps
not good. R1 is not hooked too the ground it's 10 k
Screenshot from 2017-11-09 22-43-41.png
 

dl324

Joined Mar 30, 2015
8,869
If you're trying to use a full wave bridge rectifier to get positive and negative supplies, it won't work unless you have a center tapped secondary.

To get a negative supply with a transformer that doesn't have a center tap, you need to use half wave rectifiers.

Your schematic has so many unnecessary wire jogs and scenic routes that it's tedious to read. It's more typical to draw this way:
upload_2017-11-9_20-3-23.png
 

Thread Starter

be80be

Joined Jul 5, 2008
1,928
I seen one posted on here by wendy but I can't find it now I was just playing with this.
The caps should have half the supply voltage if there the same mine I guess are not

you need to use half wave rectifier
That's where I went wrong so it is the bridge that I used that causing the problem.
 

DickCappels

Joined Aug 21, 2008
5,876
Remember, The LM317 can only source current and the LM337 can only sink current. If, for example, you put a heavy load on one output what will keep the output of the bridge & capacitor from shifting with respect to ground and causing one of the regulators to drop out of regulation?

You need a center tap on the transformer or a power supply splitter such as that mentioned by MrChips in post #4.
 

ScottWang

Joined Aug 23, 2012
6,819
If the current you need is not that much then you can use the circuit below, otherwise you need to use as bertus shown.

Screenshot from 2017-11-09 22-43-41_be80be.png
 

Thread Starter

be80be

Joined Jul 5, 2008
1,928
I didn't have the hang of qucs when i made the circuit.
I didn't have a short the wire router just placed the resistor so it was on top of the ground.
I mainly just playing with virtual grounds not making a supply.
I found a good center tap for my supply right now I'm just messing around with the virtual ground
thought i maybe nice to be able to have offsets to see what happens.
Thanks for the info.
I figured why I wasn't getting what I was thinking I hooked the 7905 up using the 7805 pinout.
well that wasn't the regulator I killed the 7805 was the 7905 was still working the 7805 is shot
Funny thing is the 7905 got hot it still works the 7805 is dead and didn't even get warm.
 
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