Thermocouple based temperature controller - Value of Diodes

[garg29]

Value of Diodes

hi,

I was going through a readymade Thermocouple based temperature controller. The controller is having the following shown circuit diagram for "Cold Junction". There are two diodes(in SMD package) which are not having value printed on them, the junction is giving Fix 2.56Volts(as shown) & the output is coming around 1.90volts at room temperature of around 20 deg. Celcius and rises with rise in temperature. I think They must be Zener diodes but what value?, can someone please give some idea as which are those diodes? & what's the use of these diodes?

Thanks a lot,

With best regards,
garg

 

[elgiovo]

Re: Value of Diodes

What do you mean by "diode value"? Diodes are characterized by threshold voltage \[V_\gamma\] and, if Zener, by Zener voltage \[V_z\]. I suppose the value \[3.9V\] stands for the Zener diode \[V_z\]. You can see it is Zener because of the z-shaped symbol after the triangle. Therefore, diodes \[D_2\] and \[D_3\] are not Zener. Value \[V_\gamma\] changes depending on technology (I'd like to point out it's only a conventional value given to simplify the diode I-V characteristic and to make it piecewise linear).

 

[garg29]

Value of Diodes

Thanks elgiovo. From the value i meant the part number & if D2 & D3 are not zeners how can they give a voltage at the junction(2.56volts as shown) in the cofiguration shown.

And one thing more I have drawn the circuit diagram from PCB, I know z shaped is a zener , (first try to see what i have shown in pictures, & what i have said) D2 & D3 are unknown to me so i have not drwan them as zeners. They are glass type red coloured diodes, I'm not sure if there are diodes other than ZENERs in Glass (red colored) packages.( You can see in the pic)

THANKS A LOT.