7812 and 7805 for powering a stamp and 2 IC''s
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Posts: 46,084
Hi
Sound like thermal overload and is shutting down the regulators.
Quick calculation, from 20V to 7812 dissipate 0.8 Watt and then 7805
will dissipate 0.7 Watt with a load of 100ma.
In my opinion both regulators should be on a heatsink.
And also check the data sheet in adding diodes to prevent any damage
to the regualor if you are using say a 100uF cap on the output.
Regards
--- In basicstamps@y..., timevex@o... wrote:
>
>
>
> Hi,
>
> 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?
>
> Thanks for you help
>
> Tim
Sound like thermal overload and is shutting down the regulators.
Quick calculation, from 20V to 7812 dissipate 0.8 Watt and then 7805
will dissipate 0.7 Watt with a load of 100ma.
In my opinion both regulators should be on a heatsink.
And also check the data sheet in adding diodes to prevent any damage
to the regualor if you are using say a 100uF cap on the output.
Regards
--- In basicstamps@y..., timevex@o... wrote:
>
>
>
> Hi,
>
> 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?
>
> Thanks for you help
>
> Tim