BJT monostable switch.

[Lightium]

I am frustrated, no matter how I try to balance this circuit to get the expected output I get unexpected results. Can someone walk me through to a predictable and functional circuit?

 

[schmitt trigger]

“Unexpected results”.
Hmmmm
Hmmmmmm

Can you describe what you expected and what actually happened ?

 

[Lightium]

I expect 1.5 - 2 volts 10mA on the gate of the triac and that the triac turns on then off due to C1 and R4 time constant.

 

[schmitt trigger]

The Triac can ONLY be turned ON by the gate and then it will latch up.

Turn off ONLY happens when the main terminal current drops below the holding current. Read the data sheet to obtain this value.

But in your circuit as you are essentially shorting a battery, this will NEVER happen. The Triac will remain on until the battery is depleted. Which in a SPICE software, only happens when you close the simulation.

 

[rjenkinsgb]

You could use a PUJT such as a 2N6027 as a pulse generator, but not a triac. The circuit arrangement is different.

https://www.electroschematics.com/programmable-unijunction-transistor-put-flasher-circuit/

 

[Tony Stewart]

I wonder what the purpose of your circuit might be.

 

[AnalogKid]

Since you are switching a DC source, a TRIAC is almost certainly the wrong (or less optimal) part for the job.

What are you building? Tell us in detail what it is you are trying to achieve. Voltages, currents, timing relationships, external conditions - all of it.

ak

 

[Lightium]

I am just working on a monostable oscillator as a switch . Couldn't think of any application so i tossed something together.

 

[Lightium]

I don't think the topology i am using is a good one.

 

[AnalogKid]

It has been three days and this thread has gone nowhere. That is because your question and circuit requirements are not at all clear.

For something like this, it always is a good idea to construct a truth table or spec list relating all possible operating conditions with all possible outputs. For example:

1. What do you expect to happen when power is first applied to the circuit (SW1 open)?

2. What do you want to happen when you press the button?

3. What do you want to happen if you hold the button for 2 continuous seconds? Note that the C1-R4 time constant is 47 milliseconds.

4. What do you want to happen when you release the button?

ak

 

[AnalogKid]

It looks like you want to push a 50 millisecond, high-current pulse through an inductor when the beam is broken. What are you building?

ak

 

[Lightium]

Hello and thanks for your time. I am making a switch that I can produce a voltage and current for a determanned amount of time to trigger and untrigger an SCR or triac.

I am trying to simulate a button press of 20ms to activate the monostable and trigger the triac for say 100us.

 

[JimB]

to trigger and untrigger an SCR or triac.

You need to study thyristors and triacs.

When a thyristor (SCR) is turned on, it stays turned on.
It cannot be turned off by changing the gate voltage.

A thyristor will only turn off when the current flowing from anode to cathode drops below some low value, the Holding Current.

When a thyristor (or a triac) is used in an AC circuit the current goes to zero every half cycle, and so the thyristor stops conducting.
But in a DC circuit, then the thyristor is conducting, you have to stop the current by some other means.

JimB

 

[Lightium]

Thanks JimB.

I am silly, I do have a good handbook on diodes and derivatives like triac's. I should read it.

 

[JimB]

I do have a good handbook ... I should read it.

Yes, RTFM!

JimB