PSN-L Email List Message

Subject: Re: Thermal Control for Sensors
From: Stephen & Kathy skmort@............
Date: Sun, 18 Nov 2007 20:12:51 -0800


Hi Chris,
Actually as I understand it, it is the switch off temp that is fixed!   
In other words, once the temp has reach its operational point, the 
control starts skipping cycles to maintain a max set temp???  I think we 
are talking about micro volts between on and off, which could be micro 
degrees, or perhaps milli degrees diff???   At any rate I'll let you and 
the other experts figure it out,,   in the mean time, except for the two 
bulbs over the last 11 years and a couple of temp change bumps 5 or 6 
years ago, it has been rock solid for me here in Northern Calif with 
winter temps to about 13 degrees F.   There are a few afternoons in the 
summer when the temp hits 105 or so, when my system loses temp control!  
I could fix it by raising the temp, but it isn't critical for my 
interest!   Others may have a more critical need!   That is where you 
and other come in.

The main reason I like this circuit is that it doesn't chop the cycle 
like dimmers do, so there is very little if any noise generated!

Thanks for your comments, others may want to incorporate them!  It is 
kind of a trick to discuss a circuit that hasn't been seen, ha!   The 
circuit has a transistor controlled by the NTC that drives a low current 
SCR which drives a high current SCR.  They get their operating voltages 
directly from the 117, with a 10 volt zener,,  so no power supply,,  simple!
  Stephen

ChrisAtUpw@....... wrote:
> In a message dated 18/11/2007, skmort@............ writes:
>
>     Small Space Heater Control.
>
> Hi Stephen,
>  
>     The problem with this is that the switch on temperature is fixed. 
> You would be OK if you used a differential thermometer, one sensor on 
> the ground and the other in top of the box. These could be two diodes, 
> two Pt elements or two Si thermometer micorcircuits with an amplifier 
> and offset control.
>     The difference between not supplying enough heat and getting 
> reverse convection which upsets the seismometer and providing too much 
> heat which stirs up the air in the whole case is not that great. It 
> depends on the ground and outside air temperatures. The easiest method 
> is to provide a constant power to a heated Al panel and vary this 
> until the convection just stops in your particular box.
>     If you use the Al cased resistors bolted to an Al sheet, solder 
> on insulated wires and cover the joints with adhesive heatshrink tube, 
> you can get a completely safe system. It will tend to track the 
> environmental changes fairly well and keep the top of the box just 
> warm enough, but setting it up in the first place may take a few tests 
> with different heater powers.
>     Another alternative is to use a commercial energy control module, 
> such as is provided by a light dimmer control and use this to drive 
> your heater resistors. But these do tend to generate RFI and you may 
> need some protection capacitors.
>  
>     Regards,
>  
>     Chris Chapman



  


Hi Chris,
Actually as I understand it, it is the switch off temp that is fixed!   In other words, once the temp has reach its operational point, the control starts skipping cycles to maintain a max set temp???  I think we are talking about micro volts between on and off, which could be micro degrees, or perhaps milli degrees diff???   At any rate I'll let you and the other experts figure it out,,   in the mean time, except for the two bulbs over the last 11 years and a couple of temp change bumps 5 or 6 years ago, it has been rock solid for me here in Northern Calif with winter temps to about 13 degrees F.   There are a few afternoons in the summer when the temp hits 105 or so, when my system loses temp control!  I could fix it by raising the temp, but it isn't critical for my interest!   Others may have a more critical need!   That is where you and other come in.

The main reason I like this circuit is that it doesn't chop the cycle like dimmers do, so there is very little if any noise generated!

Thanks for your comments, others may want to incorporate them!  It is kind of a trick to discuss a circuit that hasn't been seen, ha!   The circuit has a transistor controlled by the NTC that drives a low current SCR which drives a high current SCR.  They get their operating voltages directly from the 117, with a 10 volt zener,,  so no power supply,,  simple!
  Stephen


ChrisAtUpw@....... wrote:
In a message dated 18/11/2007, skmort@............ writes:
Small Space Heater Control.
Hi Stephen,
 
    The problem with this is that the switch on temperature is fixed. You would be OK if you used a differential thermometer, one sensor on the ground and the other in top of the box. These could be two diodes, two Pt elements or two Si thermometer micorcircuits with an amplifier and offset control.
    The difference between not supplying enough heat and getting reverse convection which upsets the seismometer and providing too much heat which stirs up the air in the whole case is not that great. It depends on the ground and outside air temperatures. The easiest method is to provide a constant power to a heated Al panel and vary this until the convection just stops in your particular box.
    If you use the Al cased resistors bolted to an Al sheet, solder on insulated wires and cover the joints with adhesive heatshrink tube, you can get a completely safe system. It will tend to track the environmental changes fairly well and keep the top of the box just warm enough, but setting it up in the first place may take a few tests with different heater powers.
    Another alternative is to use a commercial energy control module, such as is provided by a light dimmer control and use this to drive your heater resistors. But these do tend to generate RFI and you may need some protection capacitors.
 
    Regards,
 
    Chris Chapman

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