Regulator not regulating

[CoTang]

Hiya!

I have a quick question, I'm using a 78L05 national 5+ regulator. I hooked it up as the datasheet says and I connected to my little circuit that draws about 10mA. When increase the input voltage to 6 voltage the regulator maintains a nice 5.0V voltage. but when I increase the input voltage theto say about about 6.5 to 7.2V, the regulator jumps up in output voltage to like about 5.8V. Has anyone come across something like this?

P.S.

I replaced the regulator twice. Same thing.

 

[audioguru]

A 78L05 needs an input voltage of 7.0V minimum to have its output regulated at 5.0V at 10mA. A few might work with an input voltage as low as 6.5V. If its input voltage is less then its output voltage will not be regulated and will be less.

Since your regulators create an output voltage higher than 5.0V plus 8% then maybe you have the pins connected incorrectly.

 

[CoTang]

Yeah, I checked the connection and its all good.... triple checked....when I crank the voltage pass 7.5V it gives me an output voltage of 6.2V..... what can cause the voltage to increase...?

 

[kchriste]

I suspect you have your input and output pins swapped. So check it a 4th time. Maybe post a picture?
That said, you do need to have bypass capacitors connected, as close as possible, to the input and output terminals. 0.1:mu:F should be OK for this.

 

[audioguru]

The load on the 78L05 regulator is supposed to be connected to its ground like a series regulator circuit, not to a positive supply voltage like a shunt regulator circuit. The 78L05 is a series regulator, a zener diode is a shunt regulator.

 

[Nigel Goodwin]

I suspect you have your input and output pins swapped. So check it a 4th time. Maybe post a picture?
That said, you do need to have bypass capacitors connected, as close as possible, to the input and output terminals. 0.1:mu:F should be OK for this.

I agree, almost certainly wired the wrong way round!.

 

[CanonMan75]

I agree, almost certainly wired the wrong way round!.

You were lucky if that's all it's doing while wired the wrong way round... I've seen quite a few go bang when the power is applied - even with only a couple of volts difference between input and output

 

[grim]

got the filter capacitors fitted too?

 

[audioguru]

Is the ground pin connected to ground?

 

[CoTang]

Acutally its not the connection on the regulator itself.... the circuit I have it attached to seemd to be what causing it. Maybe nigel can explain it to me a lot better. I have a pic16f877a where pin 3 (AN0) is connected to the power supply itself to monitor the voltage that is supplying the regulator. I'm using it as a cheapy battery monitor. When I have that line disconnected the regulator seems to work fine but when I connect it back the symptom reappears.

P.S.

I havn't even program the darn thing yet. Just checking to see if I get all my voltages in the right place.... I think I'm going to program it as an analog input and see what happens...

Well...just got back from programming it and still no luck. Hmmm..... maybe I'm forgetting a few components....I have that line hooked up directly to pin 3 (AN0). Am I suppose to have other stuff hooked up to it as well?

 

[Nigel Goodwin]

Acutally its not the connection on the regulator itself.... the circuit I have it attached to seemd to be what causing it. Maybe nigel can explain it to me a lot better. I have a pic16f877a where pin 3 (AN0) is connected to the power supply itself to monitor the voltage that is supplying the regulator. I'm using it as a cheapy battery monitor. When I have that line disconnected the regulator seems to work fine but when I connect it back the symptom reappears.

P.S.

I havn't even program the darn thing yet. Just checking to see if I get all my voltages in the right place.... I think I'm going to program it as an analog input and see what happens...

DON'T!!!

You MUST NOT connect the input pin directly to the incoming supply, it MUST NEVER go higher than the Vdd supply to the PIC. To do what you want use two resistors as a potential divider to feed the PIC pin, assuming your supply can never go higher than 10V you can use two equal values to divide it by two.

PIC inputs pins almost all have protection diodes, you are feeding power through the protection diode to Vdd and bypassing the regulator - and may already have killed your PIC, although they are pretty sturdy devices!.

 

[audioguru]

On Toshiba's datasheet for a 7905 they show a protection diode connected backwards from the input to the output. Then the regulator is useless.
Just like the protection diode of the PIC in this circuit.

 

[CoTang]

DON'T!!!

You MUST NOT connect the input pin directly to the incoming supply, it MUST NEVER go higher than the Vdd supply to the PIC. To do what you want use two resistors as a potential divider to feed the PIC pin, assuming your supply can never go higher than 10V you can use two equal values to divide it by two.

PIC inputs pins almost all have protection diodes, you are feeding power through the protection diode to Vdd and bypassing the regulator - and may already have killed your PIC, although they are pretty sturdy devices!.

Huh....never knew that. It still works believe it or not, currently I'm still programming away. Thanks for the info....

P.s.

by chance where about in the manual did you find that info?

 

[JimB]

There are some lessons to be learned here.

Several guys have been scratching their heads, all coming up with quite possible solutions to the problem as it was originally presented - a problem with a 78L05 regulator.

When the big picture is on view (the PIC with an input pin connected to the input of the regulator), the problem becomes obvious.

Lesson 1, describe the whole problem, not just the little bit which APPEARS to be causing the fault.


The OP eventually gained insight into (but not the reason for) the problem on his own, by disconnecting the rest of the circuit from the regulator. Well done.

Lesson 2, when nothing makes sense, break the problem down into little bits, find out what works on its own, then start connecting the bits together one at a time until it stops working. Then you have most likely found the problem.

JimB

 

[Nigel Goodwin]

Huh....never knew that. It still works believe it or not, currently I'm still programming away. Thanks for the info....

PIC's are amazingly difficult to damage - I still use one with my fingerprint burnt in the top!

by chance where about in the manual did you find that info?

Look at the electrical spec at the back - maximum input voltages on I/O pins. Also how did you expect to read 9V with a 5V A2D?.

 

[CoTang]

There are some lessons to be learned here.

Several guys have been scratching their heads, all coming up with quite possible solutions to the problem as it was originally presented - a problem with a 78L05 regulator.

When the big picture is on view (the PIC with an input pin connected to the input of the regulator), the problem becomes obvious.

Lesson 1, describe the whole problem, not just the little bit which APPEARS to be causing the fault.


The OP eventually gained insight into (but not the reason for) the problem on his own, by disconnecting the rest of the circuit from the regulator. Well done.

Lesson 2, when nothing makes sense, break the problem down into little bits, find out what works on its own, then start connecting the bits together one at a time until it stops working. Then you have most likely found the problem.

JimB

I agree with what you are saying, but in "Lesson 1" the whole problem was with the regulator at that time. If I knew the whole picture then I wouldn't be asking about the regulator itself but more of the reason why that Pin is casuing the fault when connected etc.
"Lesson 2" again I agree with you. I eventually turned off the television and ripped the entire circuit apart into small chunks. A lot harder when everything soldered....sheesh...took me a while. Aside from all this, the post with solutions for the regulator problem did indeed help me gain insight into the related problem.

 

[CoTang]

PIC's are amazingly difficult to damage - I still use one with my fingerprint burnt in the top!



Look at the electrical spec at the back - maximum input voltages on I/O pins. Also how did you expect to read 9V with a 5V A2D?.

Hmmm... I always thought that 5V meant max TTL input and with an analog input it can go above 5V like all other A2D I used in the past... honestly, this is my first time using the PIC analog inputs. I decided to use it in a last minute thing after reading your tutorial on Analog inputs and testing it out on my little labx board. Although my labx board has a pot and max input is 5V.... Lack of knowldge on my part.

 

[evandude]

Hmmm... I always thought that 5V meant max TTL input and with an analog input it can go above 5V like all other A2D I used in the past... honestly, this is my first time using the PIC analog inputs. I decided to use it in a last minute thing after reading your tutorial on Analog inputs and testing it out on my little labx board. Although my labx board has a pot and max input is 5V.... Lack of knowldge on my part.

if you look in the datasheet for information about the A2D, you'll see that it normally uses Vdd and gnd as + and - voltage references, and thus that's the entire range of input voltages over which it can give you a reading - an input of Vdd will give you the maximum possible reading (1023) and an input of gnd will give you the minimum possible reading (0). As nigel said, a resistor divider is all you need to get your input voltage into the right range.