Why can't this voltage regulator maintain the voltage?

chenbingjy

Why can't this voltage regulator maintain the voltage? [Copy link]

 

The circuit diagram is as shown above. After power on, the voltage across C81 is -12.09V, no problem. The original intention of designing the circuit was to make the voltage across the Zener diode about -10V. But the actual voltage across the Zener diode is about -1.7V. The voltage across R58 is about 10.4V. Is there anything wrong with this circuit? Where is the problem? Thank you!

maychang

It's strange, how can +24V become -12V after passing through a linear three-terminal voltage regulator chip? 78L12 is not a switching power supply chip.

maychang

After power on, the voltage across C81 is -12.09V, which is OK. Are you sure it is a negative value? Is the measurement correct?

bigbat

I don't understand. Is the 78L12 chip a negative voltage output chip? I remember that the 79 series should have a negative power output? How can this power supply input be +24V and output a negative power supply? I don't quite understand.

仙猫

The red and black test leads of the multimeter are pinched in reverse.

chenbingjy

maychang posted on 2019-3-8 19:47 After power on, the voltage across C81 is -12.09V, no problem. Are you sure it is a negative value? Is there any mistake in the measurement?

There is really no mistake in the measurement. This circuit is not designed by me.

chenbingjy

Xianmao posted on 2019-3-9 00:36 The red and black test leads of the multimeter are pinched upside down.

They are not pinched upside down.

673285421

Don't laugh at the OP, we are all newbies here , it is obvious that I copied the picture wrong.

maychang

chenbingjy posted on 2019-3-9 08:46 There is really no mistake in the measurement. This circuit is not designed by me.

"This circuit is not designed by me." That is the original designer's problem. The -12V marked on this circuit should be +12V, and it is impossible to measure -12.09V. You should measure a relatively small positive voltage to the ground at the -10.4V mark.

chenbingjy

is K78L12-500R3. I will check the information again.

maychang

chenbingjy posted on 2019-3-9 11:18 It is K78L12-500R3. I will check the information again

No need to check. First, 78L12 is positive input and positive output, there is no doubt about it. Second, linear regulators cannot change the voltage polarity, and can only reduce the voltage, not increase the voltage.

chenbingjy

Look, everyone, I am not lying.

maychang

chenbingjy posted on 2019-3-9 11:23 Dear experts, please take a look. I am not lying, am I?

This module series does have products that can output negative pressure. A netizen named "Sirigu" in this forum has used it. However, this module is not a linear voltage regulator, but a switching power supply. This can easily cause confusion.

maychang

chenbingjy posted on 2019-3-9 11:23 Dear experts, please take a look. I am not lying, am I?

If this kind of module is used, then the design of the electrical schematic in the first post is correct, and your measurement is also correct.

captzs

chenbingjy posted on 2019-3-9 11:23 Dear experts, please take a look. I am not lying, am I?

However, there are also problems with the 4.3V voltage regulator tube outputting a 10V voltage.

maychang

chenbingjy posted on 2019-3-9 11:23 Dear experts, please take a look. I am not lying, am I?

As for the question mentioned in the first post, "the original intention was to have a voltage regulator of about -10V at both ends, but the actual voltage regulator was about -1.7V at both ends", I will find a curve chart. Note: The nominal voltage of the voltage regulator you are using is 4.3V.

chenbingjy

captzs posted on 2019-3-9 12:45 However, there is also a problem with the 4.3V voltage regulator tube outputting 10V voltage.

That part was drawn wrong and was not corrected. The actual 10V voltage regulator tube used is 4740

captzs

chenbingjy posted on 2019-3-9 13:03 That place was drawn wrong and it was not corrected. The actual 10V voltage regulator tube 4740 is used

R58 is 4K, only 0.5mA. Generally, the working current of the voltage regulator tube is several mA. Try changing it to a resistor less than 1K. But according to your description, it seems that there is another abnormal reason.

zhuyebb

The host is awesome, I strongly support...

maychang

chenbingjy posted on 2019-3-9 13:03 That place was drawn wrong and it was not corrected. The 10V Zener diode 4740 was actually used

This is the voltage-current characteristic curve of a certain type of Zener diode. As can be seen from the figure, the stable voltage of the Zener diode changes with the current. The test current specified for each model in the figure is 5mA. If the current is not 5mA, then the stable voltage of the Zener diode is not the nominal value. The voltage regulator you are using has a specified test current of 58mA for 4.3V and 25mA for 10V. The resistor in the figure is more than 4k ohms. If it is a 10V voltage regulator, the current is only 0.5mA, which is obviously far from 25mA. The tube works at the lowest bending part of the characteristic curve.