7805 regulator problems?

[MrDEB]

Using the attached schematic, I adjust the tempctr pot (5k) and the LCD display dims.
I measured the voltage on the 7805 output as well as other points but read mv not 5 as it should.
What is wrong with my power supply design? the 7805 bad or ?
sorry for the negative power supply image
wondering if the two 300 ohm resistors are to big? not enought current but the circuit works ? it just goes dim while adjusting either the tempctr pot or the sensor value resistor (the pic is set up to read ADC and compare the two. I added more resistance to the voltage dividers in the ADC section. seemed to help a little until I added the opti-coupler to the mix.
I get the feeling I amd drawing too much current but the 7805 is not getting warm. MAYBE BAD?

 

[JimB]

Have you tried measuring the input voltage to the 7805?

The 300 ohm resistors on the input to the 7805 are a poor idea.
They will reduce the 7805 input voltage very quickly as the supply current increases.

Your main circuit diagram is nearly unreadable, where is signal R01 going to?
What is the 7805 output when tempctr is turned to give maximim resistance?

JimB

 

[MrDEB]

ran in LTSPICE

found using the present power supply circuit I can only draw 300ma max.
I needed 12v or so for the 7805 input, resone for resistor voltage divider. Never happy with the resistors anyway. The transformer puts out 24V ac.
Going to remove the resistors and 7805 and insert a LM317 voltage regulator circuit.
will post revised schematic later.

 

[davenn]

The transformer puts out 24V ac.
Going to remove the resistors and 7805 and insert a LM317 voltage regulator circuit.
will post revised schematic later.

Yes thats a much better idea The LM317 can handle a much higher input (~38-40 V) so will easily handle the ~ 32V DC out of the bridge rectifier/ smoothing cap
Im having deja vue with this problem of yours, was it elsewhere in this forum or another forum that I first saw this problem ~ 4-5 days ago ? lol


cheers
Dave

PS just for a bit of info.... its the usual practice in circuit diagrams to have power supply feeding in from the left side of the diagram ( usually top left). No big deal, its just where people, looking at a cct diag, expect to find it lol