Re: Could someone please explain how this works?
- From: John Popelish <jpopelish@xxxxxxxx>
- Date: Sun, 24 Jun 2007 19:19:21 -0400
Eric R Snow wrote:
Greetings All,
This link:
http://www.vellemanusa.com/downloads/0/manual_mk138.pdf
is the schematic for a thermostat kit I built. Even though it works I
don't understand how it works. The high temperature limit is too low
for what I want to use it for. But I think that using less resistance
for R5 will raise the upper temperature limit. Am I right? And if I
am, why? Thank You,
This circuit compares the voltages from two resistive voltage dividers, one made with 3 fixed resistors, one of which has a variable tap, and one made with a fixed resistor and a temperature dependent resistor.
As temperature falls, the resistance of the thermistor rises, lowering the voltage from its divider. When that voltage falls below that from the adjustable tap, the comparator output swings positive, turning on the relay driver transistor.
A limitation of the circuit is that the comparator does not work when its two inputs are closer than 1.5 volts to its positive supply, but since the dividers are driven by only a fraction of that supply (regulated by a zener diode) I doubt this limitation will be a factor. But rather than change R5, I would lower R4 to raise the temperature range where the thermistor divider passes through the tap choices. This will also raise the lower end of the adjustment range, though, keeping the total temperature span about the same.
.
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