I'm working on creating a PSU which is to supply +/-15v and 5v on one board and +/-12v and +/-6v on another board. The problem is that it keeps blowing my 3 amp slow blow fuse. I'm pretty new to all this and realise I'm stuck. Obviously maybe up the fuse. However, it's not supposed to be creating a load ( I don't think) .

 

The circuit looks correct but suggest breaking it down in sections to troubleshoot.
Start with the bridge rectifer circuit without the regulators connected.
If the fuse doesn't blow verify the plus and negative voltages across C1 and C2

 

and where is this fuse? it does not seem to appear on schematics... is it on the primary side? what is the supply voltage? 230VAC?

 

Welcome to AAC!

The problem is that it keeps blowing my 3 amp slow blow fuse.

I don't see a fuse in the schematic.

What is the thermistor across the primaries for?

Too much whitespace in the schematic. And text over components and useless colors.

 

You have two nodes labelled PWR_FLAG. That puts a rectifier diode across the secondary winding creating a short.

Edit: You also have two nodes labelled PWR_FLAG that create a short across the output of the bridge rectifier.

 

To trouble shoot the circuit, disconnect the input all three regulators, then see if the fuse blows. If it doesn't blow, the fault is in one of them, so connect the input to one of them, and power it up again. If the fuse blows, that is where the fault is. If it doesn't, blow connect a second one. If the fuse blows, that is where the fault is. If it doesn't, blow the fault is in the other regulator.
If the fus blows with all the regulators disconnected, check for a bad rectifier or capacitor, or one connected the wrong way round. If everything checks out OK, then you have a bad transformer or a wiring error.

 

You have two nodes labelled PWR_FLAG. That puts a rectifier diode across the secondary winding creating a short.

Edit: You also have two nodes labelled PWR_FLAG that create a short across the output of the bridge rectifier.

They're not like net names and won't create a short.
https://docs.kicad.org/9.0/en/getti..._started_in_kicad.html#electrical_rules_check

 

hrs is correct...
still don't see main side circuit. also it would be nice to see actual construction. there may be an obvious wiring mistake or part mismatch. schematics (the part shown) is fine. my guess is that transformer is not wired correctly, possibly swapped primary and secondary sides. primary may have taps for different mains voltage. so i would say disconnect the transformer and test it separately. mark all terminals, check continuities between all of them. highest resistance winding is the primary in this case. then power it from some low voltage AC source (like another transformer, like 9-12VAC or whatever) and then measure voltages across all transformer wires. this will allow pretty safe measurements - the values will simply be scaled down but the voltage ratios will match transformer design. another option is to insert something into primary circuit to limit voltage/current (and allow measurements). variac (if you have one) or an incadescant light bulb.

 

I'm working on creating a PSU which is to supply +/-15v and 5v on one board and +/-12v and +/-6v on another board.

±15V and 5V on one board.
±12V and ±6V on another.
I see 5V at J3. +12V at J1. -12V at J2.
Where's the ±15V? Where's the ±6V?
What's missing? Where's the fuse?

 

Check the pin out of the L7912 it should be the same as the L7812.

 

Check the pin out of the L7912 it should be the same as the L7812.

They have different pinouts:

 

Sorry I stand corrected

 

The circuit looks correct but suggest breaking it down in sections to troubleshoot.
Start with the bridge rectifer circuit without the regulators connected.
If the fuse doesn't blow verify the plus and negative voltages across C1 and C2

Well the rectifier test !! blew the fuse. I went back to just the transformer and well the fuse did blow but not quite so dramatically also I noticed a spark on the switch terminal. I had used bootlace ferrules but now I.ve removed those and put them directly to the terminals.
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Are you saying the fuse blew with just the transformer connected?

 

What is that link at the bottom of the transformer (green line) in the schematic? That is tying secondary to primary.

 

and where is this fuse? it does not seem to appear on schematics... is it on the primary side? what is the supply voltage? 230VAC?

Its not on the CD but I put one in for safety sake. 230v 80 va. its mains . live wire to fuse-switch the onto a terminal "Chocolate" block with a 390v varistor.

 

Are you saying the fuse blew with just the transformer connected?
View attachment 356285

Yes but the switch arches / sparked cause oof a crimped connector I swapped that out but was knackered by the time I ha finished so Tomorrow. we shall see.

 

Yes but the switch arches / sparked cause oof a crimped connector I swapped that out but was knackered by the time I ha finished so Tomorrow. we shall see.

My assumption is a defective transformer if nothing was connected to the secondary.
Are you sure the 230vac is connected to the primary side?

 

I do see a critical error!! The 0.1 mfd capacitors right close to the regulators, as described in the applications literature!! They really do matter. It is possible for an oscillation in the regulator to cause problems, although probably not fuse popping. BUT a wrong connection ona regulator might cause that.
One other thing: It is a good idea to check the input circuit resistance with an ohm meter prior to applying mains power.Use a meter with a low-Ohms scale.

 

How do I wire this as it looks like I've got it wrong