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Old 19th January 2013, 03:08 PM   #21
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And yes, the ground connection goes to the centre tap on the power transformer

The only difference in setup between the working setup and this, is that the working setup is using a ATX power supply (PC/computer power supply) that has +12V and -12V outputs and Ground. If I connect that to the input and ground of the 7805 and 7905 respectively it all works fine. I have +5V and -5V on the outputs of the 7805/7905 but when I connect this power transformer and rectifier I have +5V and -8V

after the rectifier I have +11.97V and -11.97V
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Old 19th January 2013, 03:22 PM   #22
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You've got something wrong somewhere - check your connections very carefully.

4K7 and an LED. Please do some homework !!!!

An average Red LED needs Vf of about 1.2V and most guys will run them at about 5-10mA. As the 7805 likes a 10mA load we'll assume 10mA.

5-1.2 = 3.8V that is the voltage to be dropped across the series resistor.

R=E/I ... 3.8V/10mA = 380 Ohms. This is not a standard resistor so we compromise and 390R would be OK.

Your 4.7K is doing nothing.

Last edited by KatieandDad; 19th January 2013 at 03:26 PM.
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Old 19th January 2013, 04:07 PM   #23
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Did you try the diodes ?
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Old 19th January 2013, 04:22 PM   #24
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Quote:
Originally Posted by KatieandDad View Post
You've got something wrong somewhere - check your connections very carefully.

4K7 and an LED. Please do some homework !!!!

An average Red LED needs Vf of about 1.2V and most guys will run them at about 5-10mA. As the 7805 likes a 10mA load we'll assume 10mA.

5-1.2 = 3.8V that is the voltage to be dropped across the series resistor.

R=E/I ... 3.8V/10mA = 380 Ohms. This is not a standard resistor so we compromise and 390R would be OK.

Your 4.7K is doing nothing.

Thanks for that. Like I said in the first post, I am new at this. And since it did work with another power source I didn't know what could be wrong.

It did work fine when I changed the resistor to 330 Ohms. I got -5.12V now.

Thanks to you and all others who helped. I do wish to make a suggestion, and that is to not get frustrated with us beginners. It really isn't much fun to ask for help and get a reply that basically says "Get out of here and do your homework, idiot!" Anyway, that is how I read it.

Thanks again, I don't think I will be posting anything on this forum again. It's not the first time I or another beginner get an ear full for not knowing something most of you might consider über basic.
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Old 19th January 2013, 04:40 PM   #25
Mooly is online now Mooly  United Kingdom
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That's interesting (and its great you have got it working ) because the data sheets I looked at don't specifically mention a minimum current and in fact show application and test circuits without. There are so many manufacturers of these "generic" parts and they are all different (take two data sheets, and one quotes 1amp max output current and the other 1.5amp). It wasn't until Andrew mentioned the minimum current issue that I remembered.

The forums are a pretty good mix generally... don't take it too much to heart and keep on asking
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Old 20th January 2013, 11:15 AM   #26
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Quote:
Originally Posted by hyperion007 View Post
Thanks for that. Like I said in the first post, I am new at this. ..

Thanks to you and all others who helped. I do wish to make a suggestion, and that is to not get frustrated with us beginners. It really isn't much fun to ask for help and get a reply that basically says "Get out of here and do your homework, idiot!" Anyway, that is how I read it.

Thanks again, I don't think I will be posting anything on this forum again. It's not the first time I or another beginner get an ear full for not knowing something most of you might consider über basic.
You had a problem and posted a question.
On the same day you received a number of answers,
some of them more useful than others. These people don't
know how far your knowledge goes and what your equipment
looks like but try to help anyway.

It may also be possible that you don't understand everything
that is suggested, for instance the term "oscillation". This is
where Jan drops in telling you to use your brain and "ask
yourself" (or Miss Google).

An advice like this is always true and should not be taken
personally, because we can not do your fix but only give
some suggestions. The rest of the work is up to you. I think
with these answers you got an idea how to proceed and you
should not be prevented from asking again and learn from others.
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Old 20th January 2013, 11:31 AM   #27
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Let me clarify then, The only reply I had a problem with was the one containing "4K7 and an LED. Please do some homework !!!!"

It's not constructive nor is it very nice.
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Old 20th January 2013, 11:34 AM   #28
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Sorry if I caused offense, it wasn't meant.

Using an LED is one of the more fundamental aspects of electronics.
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