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Can u pls explain a threshold volt for a ujt

Youngfella

Feb 1, 2013
8
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Feb 1, 2013
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I came accros a post talking abt ujt relaxation oscillators(led flasher),but what i dont undestand is that they say the ujt emmiter needs a threshold voltage to switch on so the capacitor can discharge through the emmitter and open it,similarly i saw a circuit online the battery was producing a voltage of 6volts the battery was then connected to a resistor and then to a capacitor to reduce the charging time of d capacitor d positive of the capacitor was then connected to the emmiter of d ujt d designer explained that when the capacitor charges to a threshold voltage the emmiter opens up,(question 1:-is the suppy 6v not enough to open d emmitter why does it need a special threshold what is a threshold,how is d threshold different 4rm d supply 6v),i dont undestand isn't the supply voltage 6v enough to open d emmiter,(question 2:-or does a capacitor store more volt than its supplied volt,what is d difference btw the battery volt already connected to the capacitor and through the emmiter and d threshold that it cant open d emmiter,that it is until a threshold volt is reached,and what is this threshold?pls help
 
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Laplace

Apr 4, 2010
1,252
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Apr 4, 2010
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1,252
UJT threshold voltage is the peak emitter voltage (with respect to B1) Vp at which point the operating curve enters its negative resistance region, leading to a sharp rise in the emitter current and a lowering of the emitter voltage to that of one forward biased PN junction, Vf. The UJT datasheet should give a value for the Intrinsic Standoff Ratio = (Vp-Vf)/V_B1B2 from which you can calculate the UJT threshold voltage.
 
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