Soldering Iron Timer2 by P-MOS FET

189

03:05:58

Time=RC*ln(Vcc/VTO)
at Vcc=5V, VTO=2V
2Mohm*1000uF==>29min20sec
Timer time can be measured by current integrator system. Volt-meter show the time directory,as 10V=>10s.(Integrator Capacitor must be reset before measurement.)

published 10 years ago

hurz

10 years ago

I calculated something above 30minutes33sec, you have measured it 29min20? If yes its probably the leakage current which speeds it up a little. What is the partnr of this PMOS?

hurz

10 years ago

With a nasty trick you can simulated with your original RC values http://everycircuit.com/circuit/5544406556344320

ono

10 years ago

Thank you for appropriate advice. Your calculation is right. I obtained same calculated result. 29'20" is the value indicated by this time measuring system. I set VTO of this model P-MOS as 2V. But this timer will be off when gate voltage is slightly high than real VTO. VTO in EverCircuit(EC) is defined by the gate voltage at Id=0. The relay in EC has 250mV as "must release voltage".This means that the Id is 250mv/100ohm=2.5mA at time of relay off. Maybe the actual VTO of this FET will be 2.0~2.1V. In VTO=2.05V and Vcc=5V, I can calculate the timer time=1783s=29min43s.

hurz

10 years ago

A few percent error is what "applied engineering" is all about ;-) How much do you expect VTO does change over temperature?

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