Problems with LM78XX regulators???

Minion · 11th May 2007, 05:46 PM

Hi, I have built a PSU useing LM7XXX Pos and neg regulators but they don"t seem to be working......

I am useing the 15v regulators and I am useing a 18v 0v 18v Torroid and the PSU is supposed to Provide 15v regulated for Preamps and Unregulated 25v for the Power amp stage.....

When I measure the Voltages I get -10v on the Neg regulator and 0 v on the Positive regulator but the unregulated power is Fine at 25v......

Here is a Pic of the Schematic and my PCB layout......

The only change from the Schematic is that I am useing 10,000uF caps as the filter caps as opposed to 1000uF used in the schematic.......

Can someone please point out were my Problem is???

Thanx a Lot.....

Christer · 11th May 2007, 05:57 PM

Check again that you didn't rotate the regulators 180 degrees.

Minion · 11th May 2007, 06:13 PM

That was the First thing I checked, and the regs are in correctly

Wait!!!! I think I have got it,....OK....I think I have found the Problem , I think I might have the Pos and neg regulators switched......

I"ll check later to make sure.....Thanx

p27182 · 11th May 2007, 08:25 PM

the 7900 series has pin1 as the ground...

i think...

-p

p27182 · 11th May 2007, 08:25 PM

nevermind. im an idiot and didnt look at your schematic carefully enough.
my bust.
-p

Christer · 11th May 2007, 08:27 PM

I also thought he had got the pins wrong at first, but it turned out the pins are numbered in the order 1,3,2, not 1,2,3 for some obscure reason.

Edit: Or maybe not so obscure. It is probably so they can have the same pin number for the same function for all package types, but is confusing.

Minion · 11th May 2007, 10:29 PM

Thanx Guys....I figured it out....It was that I accidentaly put the Pos reg on the Neg rail and the Neg reg on the Pos rail....

After changeing the regs the Amp is working correctly, it still has a Pretty Bad Hum problem even though I star grounded everything to the same Chassis ground point.....

It was a simple 18w Guitar amp useing a couple BC550/560 and an OPA627 and a couple Tip41/42 Transistors for the Power amp stage and an all J-fet preamp stage.......

Cheers

Nordic · 12th May 2007, 07:49 AM

minion , the fixed voltage regs tend to be more noisy than a properly used variable one.... you may want to increase the caps on output side to about 100uf... not more, as you will start the reg. ringing...

Just after the bridge, I would (if I desgned it) probably have used 2 sets of filtercaps (and probably only 2200uf each) seperated by a 1 or 2 Watt 10R resistor...

Minion · 12th May 2007, 04:38 PM

Hi, I actually Have some 100uF caps on the Preamp and Power amp PCB"s close to the Opamp pins and close to the Output transistors......

I was going to use the LM317/337 regs but It turned out that i ran out of the Positive 317 regulators so I use the 7XXX regs as that was all I had on hand.....

Cheers

11th May 2007, 05:46 PM	#1
Minion diyAudio Member Join Date: May 2006	Problems with LM78XX regulators??? Hi, I have built a PSU useing LM7XXX Pos and neg regulators but they don"t seem to be working...... I am useing the 15v regulators and I am useing a 18v 0v 18v Torroid and the PSU is supposed to Provide 15v regulated for Preamps and Unregulated 25v for the Power amp stage..... When I measure the Voltages I get -10v on the Neg regulator and 0 v on the Positive regulator but the unregulated power is Fine at 25v...... Here is a Pic of the Schematic and my PCB layout...... The only change from the Schematic is that I am useing 10,000uF caps as the filter caps as opposed to 1000uF used in the schematic....... Can someone please point out were my Problem is??? Thanx a Lot.....

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11th May 2007, 05:57 PM	#2
Christer diyAudio Member Join Date: Sep 2002 Location: Sweden	Check again that you didn't rotate the regulators 180 degrees.

11th May 2007, 06:13 PM	#3
Minion diyAudio Member Join Date: May 2006	That was the First thing I checked, and the regs are in correctly Wait!!!! I think I have got it,....OK....I think I have found the Problem , I think I might have the Pos and neg regulators switched...... I"ll check later to make sure.....Thanx

11th May 2007, 08:25 PM	#4
p27182 diyAudio Member Join Date: May 2007	the 7900 series has pin1 as the ground... i think... -p

11th May 2007, 08:27 PM	#6
Christer diyAudio Member Join Date: Sep 2002 Location: Sweden	I also thought he had got the pins wrong at first, but it turned out the pins are numbered in the order 1,3,2, not 1,2,3 for some obscure reason. Edit: Or maybe not so obscure. It is probably so they can have the same pin number for the same function for all package types, but is confusing.

11th May 2007, 10:29 PM	#7
Minion diyAudio Member Join Date: May 2006	Thanx Guys....I figured it out....It was that I accidentaly put the Pos reg on the Neg rail and the Neg reg on the Pos rail.... After changeing the regs the Amp is working correctly, it still has a Pretty Bad Hum problem even though I star grounded everything to the same Chassis ground point..... It was a simple 18w Guitar amp useing a couple BC550/560 and an OPA627 and a couple Tip41/42 Transistors for the Power amp stage and an all J-fet preamp stage....... Cheers

12th May 2007, 07:49 AM	#8
Nordic diyAudio Member Join Date: Sep 2005	minion , the fixed voltage regs tend to be more noisy than a properly used variable one.... you may want to increase the caps on output side to about 100uf... not more, as you will start the reg. ringing... Just after the bridge, I would (if I desgned it) probably have used 2 sets of filtercaps (and probably only 2200uf each) seperated by a 1 or 2 Watt 10R resistor...

12th May 2007, 04:38 PM	#9
Minion diyAudio Member Join Date: May 2006	Hi, I actually Have some 100uF caps on the Preamp and Power amp PCB"s close to the Opamp pins and close to the Output transistors...... I was going to use the LM317/337 regs but It turned out that i ran out of the Positive 317 regulators so I use the 7XXX regs as that was all I had on hand..... Cheers