capacitor calculation

Thread Starter

mahmut_kelesoglu

Joined Aug 12, 2022
67
It could if there was a fault on the board.
okey you are right. it could
but ı have a question. if regulator drops short circuit level, could ı be able to still measure 3.2 volts at the output of the regulator when lcd board is connected. ı think regulator gives zero volt in that situation.
just curious.
 

WBahn

Joined Mar 31, 2012
29,978
ı share my circuit right below. apologize for being late...
Thank you for finally sharing your circuit. The problem is that you have a floating ground. There is no DC path from your loads back to the power source.

If you want to have bipolar supply outputs, then you need to either use a center-tapped transformer, or you need to use one side of the transformer output as your ground and use the other to drive a rectifier to pump the two sides on alternative halves of the cycle.
 

WBahn

Joined Mar 31, 2012
29,978
okey you are right. it could
but ı have a question. if regulator drops short circuit level, could ı be able to still measure 3.2 volts at the output of the regulator when lcd board is connected. ı think regulator gives zero volt in that situation.
just curious.
Your supply is all messed up. As you try to power something with the 5 V output the return current has no where to go. That messes up the balance between your capacitors and will pull your supplies in all kinds of strange directions.
 

WBahn

Joined Mar 31, 2012
29,978
in our main regulator board, we have transformer that drops 220v ac to 17 volts ac. then, bridge rectifier converts that 17 v ac to dc. then the regulator use it.
Do you need a -5 V output? If you do, then you really should use a center-tapped transformer. If not, then you need to use one side of the transformer as your ground.
 

Thread Starter

mahmut_kelesoglu

Joined Aug 12, 2022
67
Thank you for finally sharing your circuit. The problem is that you have a floating ground. There is no DC path from your loads back to the power source.

If you want to have bipolar supply outputs, then you need to either use a center-tapped transformer, or you need to use one side of the transformer output as your ground and use the other to drive a rectifier to pump the two sides on alternative halves of the cycle.
we used center tapped transformer with center tap connected to gnd. we have two +15V output and common gnd.
 

Thread Starter

mahmut_kelesoglu

Joined Aug 12, 2022
67
Do you need a -5 V output? If you do, then you really should use a center-tapped transformer. If not, then you need to use one side of the transformer as your ground.
we have center tapped transformer with center tap connected to gnd. we want to get +5 and -5 volts output.
 

Audioguru again

Joined Oct 21, 2019
6,673
Your original schematic is missing the center-tapped transformer.
The rectified transformer produces almost plus and minus 20VDC which overheats the voltage regulators that need no more than
7VDC.
 

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Thread Starter

mahmut_kelesoglu

Joined Aug 12, 2022
67
Your original schematic is missing the center-tapped transformer.
The rectified transformer produces almost plus and minus 20VDC which overheats the voltage regulators that need no more than
7VDC.
Actually ı know that and fixed it with connecting a resistor series in the input of the regulator. So, we have 17V at the input of the regulator .
 

Thread Starter

mahmut_kelesoglu

Joined Aug 12, 2022
67
How did you calculate 1k?
I think that might be the problem. I note that it is conspicuously absent from your circuit diagram. You have limited your output current to about 4mA (the resistor limits it to 10mA and the 7805 has 6mA quiescent current)
you might be right but it is not my problem. my problem is about voltage drops when connecting the lcd board. if the current is below the quiescent value, then my regulator would not give 5 volts when nothing connected. but it gives 5 volts. so, ı think the problem is not about the current value.
 
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