Schematic explanation

[bryan]

ALl:

Wondering what the purpose of R6, D4, C2, ZD1 is. I assume a simple regulator to keep the voltage to the circuit below the zener voltage. What is strange is ZD1 is 13v and the supply is 12v?

http://www.escol.com.my/Projects/Project-03(Thermostat-1)/Thermostat1-sch.jpg

 

[kubeek]

Assuming a car use, the input voltage could go well above 14.4V in a 12V system, and with even larger peaks when high current devices are switched off.

 

[bryan]

So with a regulated supply of 12v then the components are unneccesary

 

[jpanhalt]

D4 is for reverse polarity protection. You may want to retain it.

John

 

[MikeMl]

D4 is for reverse polarity protection. You may want to retain it.

John

D4 could also be there to keep the circuit alive while the car is being started. During cranking, the car's voltage can sag to < 8V, so the diode would prevent the capacitor from discharging back into the starter motor.

 

[bryan]

Strange, I don't think the circuit is for automotive though. It is a temperature controller. But yes, probably a good idea to retain D4, can't hurt anything.

 

[MikeMl]

What are D1 and D2 doing there? Not needed if using opamps that have common-mode input range down to their Vss pin.

 

[bryan]

Here is a link to the project. IC3 is a LM358

https://electronics-diy.com/electronic_schematic.php?id=1043

 

[MikeMl]

LM358s do not need to have their inputs offset by two diode drops. Their input common-mode range allows them to do differencing with both inputs up to -0.5V below their Vss pin. The author of the original article is a newbie...

Here is a link to the project. IC3 is a LM358

https://electronics-diy.com/electronic_schematic.php?id=1043

 

[bryan]

Any other issues you can see with the schematic. Looks like I can drop the ZD1, R6, C2, D1 and D2.

 

[jpanhalt]

What temperature range do you need to control/monitor? The device can output down to -1.0 Volt at temperatures <<0°C (See: Figure 7 of the datasheet).

John

 

[bryan]

Around 40 Celsius and not critical. Just a controller for some fans in a aquarium hood. Turn the fans on at around 35-40 Celsius. Hysteresis not critical

 

[MikeMl]

Any other issues you can see with the schematic. Looks like I can drop the ZD1, R6, C2, D1 and D2.

No hysteresis. I predict that the relay will chatter as the LM35 heats and cools.

How stable is your 12V supply. If stable, you can delete the LM431.

I needed a simple thermostat to switch 120Vac recently, and I **broken link removed**. Works just fine, and it was cheaper than buying the parts to build one...

 

[bryan]

According to the authors note's on the web link R2 sets the hystersis

 

[audioguru]

The author of the original article is a newbie...

The project is at electronics-diy which is from a 3rd-world country. Most of their projects use obsolete parts, have serious errors and DO NOT WORK.

 

[jpanhalt]

The web site is hosted from Provo, Utah, USA

http://electronics-diy.com.ip4.bz/

I realize that is a long way from Montreal, but Third World? Really.

John

 

[audioguru]

The web site is hosted from Provo, Utah, USA

http://electronics-diy.com.ip4.bz/

I realize that is a long way from Montreal, but Third World? Really.

John

I am not in French Montreal which is another 3rd-word country. 90% of the people in Canada speak English.
I thought Electronics-DIY is from India but it can be hosted from anywhere.
Their pcbs are nice but most of their parts are obsolete (the BA1404 on the home page has not been made for almost 10 years).
The website with a similar name in India also uses old obsolete parts.

 

[jpanhalt]

Actually, I thought we were discussing a specific circuit, not a hobbyist's web site. The only changes suggested so far were for parts that were not necessary for the envisioned use. None, so far as I can tell, were actually "wrong," given the wide range of temperatures over which the LM35 will work.

Your views about Quebec are indeed interesting. Except for an unfortunate period in the early 19th century, Canadians have been friends of America -- never Third World. When you leave, please turn out the lights.

John

 

[audioguru]

Your views about Quebec are indeed interesting. Except for an unfortunate period in the early 19th century, Canadians have been friends of America -- never Third World.

Many Quebecers are poor (like people in the 3rd-world) because they throw away their small income. Most of them do not speak English which is used in most of the world.

 

[MrAl]

Any other issues you can see with the schematic. Looks like I can drop the ZD1, R6, C2, D1 and D2.

Hello there bryan,

ZD1 is most likely a zener diode, voltage unspecified, but probably 6 to 10 volts. R6 limits the current through that diode.
C2 is a filter capacitor to help keep transients from the input from affecting the circuit too much.
The action of ZD1, R6, and C2, is to keep transients that are often found in an automobile off of the supply for the circuit. Without them it may act a little erratic depending on how bad the 12v supply (from the car) really is.

D1 and D2 act to create a minus supply voltage at the cathode of D2. So the cathode of D2 is negative 1.4v with respect to ground. This does a couple of things...
First, it helps A1 to cope with voltages near ground even though it may do well without it.
Second, it helps the output of A1 to go all the way to zero or below.
Third and more importantly, in this circuit the LM35 can work from about +2 to +150 degrees C, but with the addition of the two diodes D1 and D2 it is possible to extend that range from -55 to +150 degrees C. What is missing however is an 18k resistor from the output of the LM35 to ground (GND) which is also needed to get that extended range. So to get that range you need to add an 18k resistor. You'd also have to connect the voltage reference to the minus supply voltage though, and then the two diodes might have to be connected through one or two wires to bring them out to the same location as the LM35 so their temperature changes along with the LM35 instead of with the other circuit elements.
Now the other components would also have be checked to see if they can stand +150 degrees C too, so maybe the top end will be more limited than that, but to get the negative temperature range down to -55 deg C you need the two diodes and add an 18k resistor.