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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
Perfboard wall wart supply draws a lot of current!
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sebsongs



Joined: Feb 09, 2013
Posts: 40
Location: Kristianstad, Sweden

PostPosted: Sat Feb 23, 2013 3:52 am    Post subject: Perfboard wall wart supply draws a lot of current!
Subject description: What's wrong with my supply?
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Hi.

I built the MFOS wall wart supply and it doesn't seem to wanna work.
I got only one large filter capacitor on each rail, 2200uF 63V.
I didn't have the exact right capacitors for the 7815s and 7915s, so there is 10uF before and 47uF after.
And I am using 1N5400s instead of 4004s.

So there's some things not quite up to spec, but I think this should work anyway.

I get 7V on the positive rail and someting like -22 on the negative.
On the input of the 7815 there is 15V and input of the 7915 has -22V, same as output. I thought it was shorted but I cant see or measure a short circuit anywhere on the board.

And it draws 650mA measuring on the input AC.

My thoughts are that the chips have been damaged in some way.
I did have the chips in another PSU that blew up, but it was the diode bridge that blew, so i figured that the chips didn't even notice that current.

Oh, and the 7815 gets hot fast, the 7915 does not.

Any thoughts or help? I'm lost and thinking of ordering a set of components now that is up to spec. This build was for testing only, as I had the stuff at home. I'm anxious to build my yusunth VCOs but I want a PSU first so I know I can test the VCOs when they're built.

Thanks

Oh, and here's my layout for the perfboard:

Posted Image, might have been reduced in size. Click Image to view fullscreen.
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PHOBoS



Joined: Jan 14, 2010
Posts: 1527
Location: Moon Base
Audio files: 319

PostPosted: Sat Feb 23, 2013 5:19 am    Post subject: Reply with quote  Mark this post and the followings unread

well your design looks ok,. allthough I would personally mirror the bottom side and have the GND rail going straight in the middle of the
PCB (migth require a jumper wire or two). But it should be working. The changes you made in component values shouldn't matter, there might
be some ideal values but I've build them with all kinds of capacitor values and it works. At this point I would say faulty regulators is
most suspect unless there is still a short or bad solder joint somewhere (always check over and over and don't forget to meaure).

Quote:
On the input of the 7815 there is 15V and input of the 7915 has -22V

Can you try without the regulators installed ? (I know desoldering perf can be a PITA). you should have equal voltages at both inputs.

_________________
"My perf, it's full of holes!"
http://phobos.000space.com/ http://www.acidtrash.com/ Stickney Synthyards
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sebsongs



Joined: Feb 09, 2013
Posts: 40
Location: Kristianstad, Sweden

PostPosted: Sat Feb 23, 2013 6:36 am    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
well your design looks ok,. allthough I would personally mirror the bottom side and have the GND rail going straight in the middle of the
PCB (migth require a jumper wire or two). But it should be working. The changes you made in component values shouldn't matter, there might
be some ideal values but I've build them with all kinds of capacitor values and it works. At this point I would say faulty regulators is
most suspect unless there is still a short or bad solder joint somewhere (always check over and over and don't forget to meaure).

Quote:
On the input of the 7815 there is 15V and input of the 7915 has -22V

Can you try without the regulators installed ? (I know desoldering perf can be a PITA). you should have equal voltages at both inputs.


Great tips!

I desoldered the regulators and now the circuit is drawing 0A and there's 23V DC on each input for the regulators.

So, they were shot. Need to get some new ones then!

Thanks!

I've had so much trouble with this one, so it's good to finally find the faulty components. I don't know why I didn't suspect the regulators earlier. Well well...
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sebsongs



Joined: Feb 09, 2013
Posts: 40
Location: Kristianstad, Sweden

PostPosted: Tue Feb 26, 2013 1:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

So I ordered all new components for the WWPSU.

It's now alive and delivers nice +-15VDC.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

One question: I incorporated a fuse on the board. Put a 500mA in there but it blew the moment I put voltage on the input. I guess it was the startup current that exceeded 500mA. The regulators go to 1A so what should the fuse be? 1.5A? 1A?
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PHOBoS



Joined: Jan 14, 2010
Posts: 1527
Location: Moon Base
Audio files: 319

PostPosted: Tue Feb 26, 2013 2:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

great to hear you got it working,. looks very neat too Very Happy

as for the fuse,. you only have 1 in there and if the max current for your regulators is 1A then your fuse should be the same.
if you use a 'bigger' fuse the worst case scenario could be that you're only drawing current from either positive of negative power line
and exceed the max regulator current but not that of the fuse. However those regulators are allready protected against too much
current draw in case of a short but a fuse is always a good option, so you can try a bigger one to see if that works.

btw did you use a fast or slowblow fuse,. 1A slow should work I think (those caps are not THAT big).

Another thing is that you probably want to put some heatsinks on the regulators. Depending on how much current you want to draw
from them they can get pretty hot.

_________________
"My perf, it's full of holes!"
http://phobos.000space.com/ http://www.acidtrash.com/ Stickney Synthyards
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sebsongs



Joined: Feb 09, 2013
Posts: 40
Location: Kristianstad, Sweden

PostPosted: Tue Feb 26, 2013 2:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
great to hear you got it working,. looks very neat too Very Happy

as for the fuse,. you only have 1 in there and if the max current for your regulators is 1A then your fuse should be the same.
if you use a 'biggger' fuse the worst case scenario could be that you're only drawing current from either positive of negative power line
and exceed the max regulator current but not that of the fuse. However those regulators are allready protected against shortcircuit,. but a fuse is always a good option, so you can try a bigger one to see if that works.

btw did you use a fast or slowblow fuse,. 1A slow should work I think (those caps are not THAT big).

Another thing is that you probably want to put some heatsinks on the regulators. Depending on how much current you want to draw
from them they can get pretty hot.


Thanks! I think it came out pretty great too. Smile

It was a 500mA slow fuse. I'll try 1A slow and see where it takes me.
I'll keep an eye on current draw when I start to gather some modules.
If it gets hot I'll put on some heatsinks!

I see this as a one step into a more powerful supply. This will be great for testing and getting started though!
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