# Can u pls explain a threshold volt for a ujt

Discussion in 'General Electronics Discussion' started by Youngfella, Feb 4, 2013.

1. ### Youngfella

8
0
Feb 1, 2013
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

Last edited: Feb 4, 2013
2. ### Laplace

1,252
185
Apr 4, 2010
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.