- #1
GirishC
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Hi
I am currently using LCM555 timer for generating variable frequency based on relative Humidity (RH) of surrounding. The LCM555 is configured as astable multivibrator and centered at 20KHz. RA = 24K, RB = 200K and C is capacitive sensor having value of 160pF at 30C, 0%RH. The C is given by formula
C = C0*[1 + HC0*RH]
C0 = 160pF at 30C, 30%
HC0 = 3420ppm/%C
RH = % relative humidity
temperature derating factor is
dC= - 0.0019*(T-30) pF
T = temperature in C
When I sweep the RH from 0% to somewhere 45% my 555 timer stops working. I have to replace the LCM555 timer and keep RH below 45%. I have similar circuit with RA = 40.2K, RB = 360K and C is 105pF and centered at frequency 18KHz and it works very fine till 98%.
When I talk to National about this problem, I received a feedback saying I can not use this chip beyond 5.6KHz (max), according to datasheet. I have queries based on my experiment
1. How can upper limit being 5.6KHz is possible. We have many application where 555 used goes much above 100KHz.
2. What causes failure of LCM555 and why it works in other configuration? I have verified the current requirement of discharge pin and trigger pin. What am I missing?
3. What is the solution to my problem.
Could anyone through light on this?
Girish
I am currently using LCM555 timer for generating variable frequency based on relative Humidity (RH) of surrounding. The LCM555 is configured as astable multivibrator and centered at 20KHz. RA = 24K, RB = 200K and C is capacitive sensor having value of 160pF at 30C, 0%RH. The C is given by formula
C = C0*[1 + HC0*RH]
C0 = 160pF at 30C, 30%
HC0 = 3420ppm/%C
RH = % relative humidity
temperature derating factor is
dC= - 0.0019*(T-30) pF
T = temperature in C
When I sweep the RH from 0% to somewhere 45% my 555 timer stops working. I have to replace the LCM555 timer and keep RH below 45%. I have similar circuit with RA = 40.2K, RB = 360K and C is 105pF and centered at frequency 18KHz and it works very fine till 98%.
When I talk to National about this problem, I received a feedback saying I can not use this chip beyond 5.6KHz (max), according to datasheet. I have queries based on my experiment
1. How can upper limit being 5.6KHz is possible. We have many application where 555 used goes much above 100KHz.
2. What causes failure of LCM555 and why it works in other configuration? I have verified the current requirement of discharge pin and trigger pin. What am I missing?
3. What is the solution to my problem.
Could anyone through light on this?
Girish