PSN-L Email List Message

Subject: Re: Thermal Control for Sensors
From: "Jerry Payton" gpayton880@.......
Date: Sat, 17 Nov 2007 11:38:41 -0600


Thanks Keith,

Yep, you are right.  I had forgotten those facts in my calculations. 
However, the absolute wattage is not critical.  If I can just maintain an 
average within "normal" operating range, I'll be happy.

Thanks for reminding me.  Matter of fact, I closed the Harbor fright website 
to read this message.
Jerry


----- Original Message ----- 
From: Keith Payea
To: psn-l@..............
Sent: Saturday, November 17, 2007 11:24 AM
Subject: RE: Thermal Control for Sensors


Jerry:

Those chargers ARE pretty cheap power supplies.  Their only downfall is 
voltage regulation.  The output, when lightly loaded, will be a lot higher 
than 12 volts.  Mine reads about 15.4 volts with no load, dropping to about 
14 near full load.  I have one of the Harbor Freight units with a couple of 
switches to select voltage and mode.

You might need to experiment with resistor values to get what you want.  The 
wattage changes with the square of the voltage for a given resistor.

Good luck,

    Keith



--------------------------------------------------------------------------------
From: psn-l-request@.............. [mailto:psn-l-request@............... On 
Behalf Of Jerry Payton
Sent: Saturday, November 17, 2007 8:16 AM
To: psn-l@..............
Subject: Re: Thermal Control for Sensors


Hi Pete, Chris and Others,

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

Jerry.


----- Original Message ----- 
From: Pete Rowe
To: psn-l@..............
Sent: Friday, November 16, 2007 1:53 PM
Subject: Re: Thermal Control for Sensors


Hi Jerry
Since we don't know how well your insulation works, it
is hard to define how many watts you need to dissipate
to keep the inside of the box at a certain
temperature. I doubt that you need even 10 watts if
your insulation is good. I would start with some
simple experiments. You'll need a thermometer inside
the box that you can read from the outside without
opening the box. Most any inexpensive indoor/outdoor
thermometers will be fine. Be aware that most of those
thermometers are only accurate to +/- 2 degrees C. For
this measurement that is ok.
Then get a 1 amp 12 volt DC wall wart and a 150 ohm 5
or 10 watt resistor. Put the resistor in your box and
hook up the 12 volts to it. The resistor will
dissipate about 1 watt (P= Esquared/R). Let it run for
a day and see if the temperature in the box goes up
and levels off. If this temp is ok, then you're done.
If you need more watts, calculate different resistors
for 2  or 5 watts and repeat the experiment. You
should find that you don't need a thermostat if you
sneak up on a fixed power dissipation that keeps the
temperature stable. If your insulation is very poor or
the outside temperature has large swings, then you'll
need more watts and a thermostat to cycle the heater.
It is best to avoid thermostats because they always
make electrical noise.
I hope this helps.
Pete Rowe
--- Jerry Payton  wrote:

> I am searching for wiring details for using Aluminum
> Housed Wirewound Power
> Resistors used for thermal control inside an
> insulated housing for the
> seismic sensors.  It has been suggested wiring
> several such resistors in
> series to accomplish a 10-30 watts from the 110v AC
> house supply source.
> I'm a little leery about the house current for
> safety reasons and possible
> AC hum.
>
> In studying the Mouser Electronics catalog, I can
> find several possible
> configurations using one to several resistors in
> series.  The resistors
> specify their wattage capability, but have different
> resistance values to
> choose from.
>
> If anyone has done this and has specific values
> etc., I'd appreciate hearing
> from you.
>
> Light bulbs attract bugs and other unwanted
> critters!
>
> Thank you,
> Jerry
>



      ____________________________________________________________________________________
Be a better sports nut!  Let your teams follow you
with Yahoo Mobile. Try it now. 
http://mobile.yahoo.com/sports;_ylt=At9_qDKvtAbMuh1G1SQtBI7ntAcJ
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Thanks Keith,
 
Yep, you are right.  I had forgotten those facts in my=20 calculations.  However, the absolute wattage is not critical.  = If I=20 can just maintain an average within "normal" operating range, I'll be=20 happy.
 
Thanks for reminding me.  Matter of fact, I closed the Harbor = fright=20 website to read this message.
Jerry
 
 
----- Original Message -----=20
From: Keith Payea=20
To: psn-l@..............
Sent: Saturday, November 17, 2007 11:24 AM
Subject: RE: Thermal Control for Sensors

Jerry:
 
Those chargers ARE pretty cheap power = supplies. =20 Their only downfall is voltage regulation.  The output, when = lightly=20 loaded, will be a lot higher than 12 volts.  Mine reads about 15.4 = volts=20 with no load, dropping to about 14 near full load.  I have one of = the=20 Harbor Freight units with a couple of switches to select voltage and=20 mode.
 
You might need to experiment with resistor = values to get=20 what you want.  The wattage changes with the square of the voltage = for a=20 given resistor.
 
Good luck,
 
    Keith


From: psn-l-request@................. [mailto:psn-l-request@............... On Behalf Of Jerry=20 Payton
Sent: Saturday, November 17, 2007 8:16 AM
To: = psn-l@..............
Subject: Re: Thermal Control for=20 Sensors

Hi Pete, Chris and Others,
 
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 = 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=20 of about 2.4 amps current demand on the charger
 
Jerry.
 
 
----- Original Message -----=20
From: Pete Rowe =
To: psn-l@..............
Sent: Friday, November 16, 2007 1:53 PM
Subject: Re: Thermal Control for Sensors

Hi Jerry
Since we don't know how well your insulation = works,=20 it
is hard to define how many watts you need to dissipate
to keep = the=20 inside of the box at a certain
temperature. I doubt that you need = even 10=20 watts if
your insulation is good. I would start with some
simple=20 experiments. You'll need a thermometer inside
the box that you can = read from=20 the outside without
opening the box. Most any inexpensive=20 indoor/outdoor
thermometers will be fine. Be aware that most of=20 those
thermometers are only accurate to +/- 2 degrees C. For
this=20 measurement that is ok.
Then get a 1 amp 12 volt DC wall wart and a = 150 ohm=20 5
or 10 watt resistor. Put the resistor in your box and
hook up = the 12=20 volts to it. The resistor will
dissipate about 1 watt (P=3D = Esquared/R). Let it=20 run for
a day and see if the temperature in the box goes up
and = levels=20 off. If this temp is ok, then you're done.
If you need more watts, = calculate=20 different resistors
for 2  or 5 watts and repeat the experiment. = You
should find that you don't need a thermostat if you
sneak up = on a=20 fixed power dissipation that keeps the
temperature stable. If your = insulation=20 is very poor or
the outside temperature has large swings, then = you'll
need=20 more watts and a thermostat to cycle the heater.
It is best to avoid=20 thermostats because they always
make electrical noise.
I hope this = helps.
Pete Rowe
--- Jerry Payton <gpayton880@.......> = wrote:

> I=20 am searching for wiring details for using Aluminum
> Housed = Wirewound=20 Power
> Resistors used for thermal control inside an
> = insulated=20 housing for the
> seismic sensors.  It has been suggested=20 wiring
> several such resistors in
> series to accomplish a = 10-30=20 watts from the 110v AC
> house supply source.
> I'm a = little leery=20 about the house current for
> safety reasons and possible
> = AC=20 hum.
>
> In studying the Mouser Electronics catalog, I = can
>=20 find several possible
> configurations using one to several = resistors=20 in
> series.  The resistors
> specify their wattage=20 capability, but have different
> resistance values to
> = choose=20 from.
>
> If anyone has done this and has specific = values
>=20 etc., I'd appreciate hearing
> from you.
>
> Light = bulbs=20 attract bugs and other unwanted
> critters!
>
> Thank = you,
> Jerry
> =



     =20 _________________________________________________________________________= ___________
Be=20 a better sports nut!  Let your teams follow you
with Yahoo = Mobile. Try=20 it now.  http://mobile.yahoo.com/sports;_ylt=3DAt9_qDKvtAbMuh1G1SQtBI7ntAcJ=
__________________________________________________________

Pub= lic=20 Seismic Network Mailing List (PSN-L)

To leave this list email PSN-L-REQUEST@............... with=20
the body of the message (first line only): unsubscribe
See http://www.seismicnet.co= m/maillist.html=20 for more information.

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