AVR-Chat

At 04:43 PM 9/23/04 -0700, you wrote:
>attached here is the datasheet of the relay i used (6V). i don't think its 
>possible to supply the coil with greater than 6V. or maybe i just 
>misunderstood the datasheet.

OK, a pull-in voltage of 4.5V so if you are having trouble pulling the 
relay on then your voltage is dropping below that point.  A max coil 
voltage of approx. 6.8V.

With an 18V input to your power supply and a 5V out you are dropping 13V 
across the 7805 and with a 50mA draw for the relay you are dissipating over 
2W just to pull in the relay.  I think your best option for driving this 
would be a small pwm drive.  The inductance should be high enough that even 
a low frequency would work, you could drive a small FET (with proper 
flyback suppression diode, but you should have that already) either from 
the micro or gate a 555 or similar frequency generator.  The 4.5V-6.8V 
range on the coil corresponds nicely to your 12-18V input voltage range 
with a fixed 37% PWM.  If you could know the input voltage and adjust the 
PWM accordingly you could be even better suited (hmm, should even be able 
to do that with an analog circuit).

>i can't find a 5V relay in our local store so i have to live with that 6V 
>relay. also, maybe the voltage regulator is alright. i'll try to reduce 
>the input voltage.

Two questions, What is the thermal impedance of the 7805 case and heatsink 
you are using?  And what is the current draw of the rest of your 
circuit?  Getting rid of that 2W should help at least a little.

To give an example: National quotes  5 C/W  for a TO220 case
Digikey has a 6W heatsink that is rated for ~16 C/W (they have heatsinks 
covering at least 3W to 10W listed for TO220)

That combination gives ~21 C/W which for a 2W reduction in power would 
reduce your junction temperature by 42C.  The interface between the to-220 
and the heatsink will introduce additional thermal resistance so this is a 
conservative estimate.


Robert

" 'Freedom' has no meaning of itself.  There are always restrictions,
be they legal, genetic, or physical.  If you don't believe me, try to
chew a radio signal. "

                         Kelvin Throop, III