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[SOLVED] Problems with 7912 regulator

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rdiaz

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Hi, I'm building a power supply with +24V, +5V and +/-12V. I already have a 3 secondary transformer with 22V, 7V and 14-0-14V (peak voltage in these 3 cases). I'm using 7805, 7824, 7812 and 7912 for DC regulation. I was using 47uF in 7824's input, the other regulators's input I've used 10uF capacitors. I've checked also the GND-I-O configuration from the 7912, and used 104 ceramic capacitors besides the input capacitors, and also in the output. 7824 and 7805 are working properly, but I have 0.2V aproximately from 7812 and 1912. Also I've tried adding 1K resistance in the output of each regulator but still doesn't changes. I don't know what else to try! please any suggestions?
 

shahbaz.ele

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dear try 7812 without 7912 first.
then only 7912.
if they worked properly in individual steps.
then add the other one.
may be transformer problem.
 

kamranravian

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check out your regulators separatly
also check out transformer to 7805 with its positive side.
individual testing may be helpful as suggested by shahbaz.ele
 

rdiaz

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I've tested the 7812 and it worked without problems shahbaz.ele, but the 7912 doesn't work , instead it gets really hot so I had to turn the supply off. I checked each secondary transformer, and everything seems OK. I think if they give me the voltage I want there wouldn't be a problem right? or there is something else I should notice? .... All the regulators except the 7912 work without a problem kamranravian, I've tested them all. Is there any other option I should try?
 

rdiaz

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Yes, I've checked that 7912 pin configuration alexan_e. Also I've tried the 7912 separately but it gets really hot, I don't understand why because I didn't add any load in the output. Also I tried using a ceramic 100nF capacitor in the input and another one in the output, besides the 10uF electrolytic capacitor in the input. This is driving me crazy, because I ran out of ideas!
 

alexan_e

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can you provide a connection scheme?

---------- Post added at 16:17 ---------- Previous post was at 16:11 ----------

Does it look like

dualvreg.gif
 

dselec

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scematic first
maybe u have ripple at input of neg reg or have short at reg out.
what reg u using metal or plastic ?
 

rdiaz

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Well, as you can see I didn't connect the 28V transformer tap centre, I think it's not usable so I didn't even draw it. All the circuit in the scheme is wired in a breadboard, and the ceramic capacitors are 0.1uF. Fuente.png
 

rdiaz

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Now I'm just trying with both 7812 and 7912, I'm using TO-220 plastic package dselec. Also I should have said that is a L7912CV. I forgot to change in the scheme I provide that is 7812 and 7912 instead of 7815 and 7915.

---------- Post added at 10:32 ---------- Previous post was at 10:29 ----------

the scheme looks like alexan_e 's figure, although I didn't draw the tap centre, because I think is not useful in this case. What do you think?
 

alexan_e

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The x1-2 diode (D2) in inverted and gives a positive voltage in the output
 

dselec

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like alex said ..d2 invert and join the grd to center-tap immediate
and more why dont u using bridge rectifier full wave ?
1=grn
2=in
3=out
 

alexan_e

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the scheme looks like alexan_e 's figure, although I didn't draw the tap centre, because I think is not useful in this case. What do you think?

What did you mean by that?
I hope the center tap is connected to the ground, right?
 

rdiaz

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So alexan_e, you're telling me that I should invert the X1-2 diode to make the 7912 work? ... I'm gonna try that if so!
 

alexan_e

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Yes, the diode connected that way provides a positive voltage to the input or 7915, you have to invert it so that it outputs a negative voltage
 

rdiaz

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Thanks for your reply dselec, I didn't use a bridge rectifier because I'm using low currents (max = 100 mA). If I connect the tap centre is not gonna heat the circuit? for an overcurrent maybe? I hope you can clear me that up please, I't the first time I'm doing a power supply.
 

dselec

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Alex
What wrong with this guy ? is he reading all the post why is he ignoring the center tap.
 

alexan_e

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The center tap is the common reference point and will be used as ground.
If it is the first time you do a power supply you should at least follow a schematic like the one I posted in post #7 that works perfectly.
The diode direction is shown there but in your case you are not using D2, D3 so ignore them (although you will get cleaner supply if you use them) and see the direction of D1, D4.
 

rdiaz

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Thank you guys for your reccomendations. alexan_e, I inverted the diode you told me and dselec I connected the center tap, now from the 7812 I get +12V and from the 7912 I get -4.6V. Is there any way I can get -12V from the 7912?
 

alexan_e

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I'm not sure if there is still something wrong or if you simply have damaged the 7912 with the wrong input.
The output of the regulators is protected but the input is another issue.
Do you have another 7912?
 

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