In a message dated 17/11/2007, gpayton880@....... writes:

I failed to mention that my goal is for three instrument covers.
 
IDEA:   How about an inexpensive 12v battery charger, with  a sufficient max 
current rating, wired to three separate 15 ohm 25w  resistors mounted on an 
aluminum plate for proper heat dissipation?   

If my math is right, that would be about 10 watts to each cover and a  total 
of about 2.4 amps current demand on the  charger



Hi Jerry.
 
    You will likely find that the nominal rating that  you get from battery 
chargers is NOT continuous. If you try to draw the maximum  amount of current 
continuously, I doubt if it will last long and is very likely  to overheat.
 
    I use resistors across the Utility Supply quite  happily and I have 240 V 
AC. You just screw in light bulbs don't you? What are  you worried about? Use 
a fused plug?
 
    If you want to use low voltage, buy a toroidal  transformer.
 
    You need to use an Al sheet and bolt on resistors  anyway. Earth the Al 
sheet. If you try to use ordinary high temperature wire  wound resistors, the 
hot thermal plumes will stir up the air in the top of the  box. 
    There are plenty of cheap ways of getting this  wrong.
 
    The object is to heat the air in the TOP of the box  evenly so that there 
is a stable positive temperature gradient right down to  ground level. 
 
    Regards,
 
    Chris Chapman




   





 =20

In a message dated 17/11/2007, gpayton880@....... writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
  
I failed to mention that my goal is for three instrument covers.
  
 
  
IDEA:   How about an inexpensive 12v battery charger, =
with=20
  a sufficient max current rating, wired to three separate 15 ohm 25w=20
  resistors mounted on an aluminum plate for proper heat dissipation? =20
  
  
 
  
If my math is right, that would be about 10 watts to each cover and a=
=20
  total of about 2.4 amps current demand on the=20
charger

Hi Jerry.
 
    You will likely find that the nominal rating th=
at=20
you get from battery chargers is NOT continuous. If you try to draw the maxi=
mum=20
amount of current continuously, I doubt if it will last long and is very lik=
ely=20
to overheat.
 
    I use resistors across the Utility Supply quite=
=20
happily and I have 240 V AC. You just screw in light bulbs don't you? What a=
re=20
you worried about? Use a fused plug?
 
    If you want to use low voltage, buy a toroid=
al=20
transformer.
 
    You need to use an Al sheet and bolt on resisto=
rs=20
anyway. Earth the Al sheet. If you try to use ordinary high temperature wire=
=20
wound resistors, the hot thermal plumes will stir up the air in the top of t=
he=20
box. 
    There are plenty of cheap ways of getting this=20
wrong.
 
    The object is to heat the air in the TOP of the=
 box=20
evenly so that there is a stable positive temperature gradient right down to=
=20
ground level. 
 
    Regards,
 
    Chris Chapman