UJT Oscillator current analysis

[kingh12]

Hi,

I am designing sawtooth generator circuit by using UJT. The circuit is shown below. When I look throught ossiloscope the output signals are suitable. But I don't know how to calculate the current on R1 and R2..So I cant compare the output voltages between measured and calculated. Can anyone help me about how to calculate these currents. What can be the formula? This will help me for testing the voltages on ossiloscope.

Thanks..

 

[D.A.(Tony)Stewart]

R1 is used only for applying external bias voltage

Terminals E and B are the active terminals between C1 and R2

When the base trigger level is reached, R2 is switched to C1 with some internal resistance to discharge the capacitor.

the emitter voltage will result from C1 discharge current into R2 therefore

Ic=Vg/R2=C1 dV/dt

R3C1 is most of the rise time ( some from gate leakage)
R2C1 is most of the decay time ( some gate-emitter ESR and capacitor ESR not included)

 

[kingh12]

Actually the voltage on the base2 (channel A connected) doesn't have to be =Vbb-Ib2R1 ? And the voltage on base1=Ib1R2 ?
I found that capacitor charges until 4V...there isnt any problem with the C1 voltage, but I couldn't figure out what must be the next step to find the currents. The emitter current equals what? How can I pass through base current by using emitter current?

 

[D.A.(Tony)Stewart]

sorry its been almost 40 yrs since I played with UJT's.. one base-emitter junction goes thru a negative resistance transition
https://upload.wikimedia.org/wikipedia/commons/thumb/8/8a/UJT_caratteristica.png/220px-UJT_caratteristica.png

like SCR's and gas-tubes , in this case the BE junction discharges the capacitor.

 

[goldsmith]

esigning sawtooth generator circuit by using UJT. The circuit is shown below. When I look throught ossiloscope the output signals are suitable. But I don't know how to calculate the current on R1 and R2..So I cant compare the output voltages between measured and calculated. Can anyone help me about how to calculate these currents. What can be the formula? This will help me for testing the voltages on ossiloscope.

Hi
A question : what is the benefit of knowing the current through R1 and R2 ? is that important in design process ? when you want design a UJT based oscillator , you will never need to consider currents of R1 and R2 and everywhere else .
isn't it ?
Best Wishes
Goldsmith