7812 and 7805 for powering a stamp and 2 IC''s
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One thing I have seen is regulation problems if you only use the tab to
ground the 7812 / 7805 and the connection is flaky. Current draw should not
factor in unless you are pulling too much. Might try some additional
filtering on the input and output of the regulator per the diagram on the
spec sheets.
Original Message
> I'm using a 7812 regulator to power a stamp from a 20v source. I'm
> also using a 7805 to power to IC's from the 12V from the 7812.
>
> My problem is that my 5v from the 7805 keeps drifting low, it starts
> at around 4.5 and will drift slowly down to 3.0v . I can't figure out
> why this is happening, I've tried other 7805's and I get the same
> result. The circuit works fine off my bench 5v supply. I'm thinking
> that this could be happening because I'm not drawing enough current
> that it can't keep a stable voltage with a real low power draw.
>
> The only thing the 7805 is powering is 2 IC's (7 seg driver, RS232
> level converter) 2 digit 7 seg display and one bi-color LED. I'm
> guessing my total current load is less than 100ma perhaps more like
> 50ma.
>
> Is their a minimum current load for the 7805 or 7812 regulators?
> Should I just hang a shunt resistor on the 5v to load it down?
ground the 7812 / 7805 and the connection is flaky. Current draw should not
factor in unless you are pulling too much. Might try some additional
filtering on the input and output of the regulator per the diagram on the
spec sheets.
Original Message
> I'm using a 7812 regulator to power a stamp from a 20v source. I'm
> also using a 7805 to power to IC's from the 12V from the 7812.
>
> My problem is that my 5v from the 7805 keeps drifting low, it starts
> at around 4.5 and will drift slowly down to 3.0v . I can't figure out
> why this is happening, I've tried other 7805's and I get the same
> result. The circuit works fine off my bench 5v supply. I'm thinking
> that this could be happening because I'm not drawing enough current
> that it can't keep a stable voltage with a real low power draw.
>
> The only thing the 7805 is powering is 2 IC's (7 seg driver, RS232
> level converter) 2 digit 7 seg display and one bi-color LED. I'm
> guessing my total current load is less than 100ma perhaps more like
> 50ma.
>
> Is their a minimum current load for the 7805 or 7812 regulators?
> Should I just hang a shunt resistor on the 5v to load it down?