Transistor latching circuit problem

Thread Starter

TheNight Silence

Joined Feb 4, 2016
13
Hi all
I've made a latching switch using a transistors but it's not working as it supposed, and i don't know why
When pressing the push button the led is lit but it's not latched since it turns off when depressing the switch
pls help why it's not latching the output voltage?
translatch.png
 

#12

Joined Nov 30, 2010
18,173
Add a diode from the top of R2 to the left of R4.
When the push button applies Vcc to R2, Q2 has no voltage across it and thus doesn't get involved in the current flow.
 

crutschow

Joined Mar 14, 2008
24,099
To avoid any possibility ambiguity, the diode #12 suggested you use should be in series with the wire going from output to input and the diode anode should be towards R2.
 

cmartinez

Joined Jan 17, 2007
6,667
Couldn't make it latch either :oops:

Maybe I did something wrong with the sim. V2 delivers a short pulse through D3, thus simulating a switch, IMHO. But current flows through the LED at all times, 5.774mA when V2=0 and 5.775mA when V2=5

Capture01.JPG



However, if I simply cut out D3, and leave V2's positive side floating, the circuit does latch... what's going on????

Capture02.JPG

What am I doing wrong?
 

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crutschow

Joined Mar 14, 2008
24,099
Couldn't make it latch either :oops:
.....................
What am I doing wrong?
You are doing nothing wrong. It's just that, for whatever reason, the circuit goes into the latch mode when LTspice is doing its initial DC bias calculation before doing the transient analysis
If you start the supply voltage at 0V, it works.

Edit: Jony beat me to it.

upload_2016-4-3_11-32-16.png
 

Thread Starter

TheNight Silence

Joined Feb 4, 2016
13
Hi friends
I've put a diode as your suggestion,but sorry it doesn't work,even the led didn't lit when i press the switch
when releasing the button
The voltage across the diode= -5V,Vc(Vout Q2)=-.094 V(with respect to ground),Vbe(Q1)=4.98V
when pressing the switch
The voltage across the diode= -4..65V ,Vc(Vout Q2)=-0.91 V(with respect to ground),Vbe(Q1)=4.98V
 

AnalogKid

Joined Aug 1, 2013
8,252
I also use a voltage source and a series diode to simulate a switched input, mainly because I've never figured out the LTS switch component. But I make the voltage source a pulse with a visible delay and relatively slow rise and fall times so I can see how the transition affects each circuit element. For this circuit, a 1 second delay and 1 second rise time makes the circuit latch every time, with or without D2.

ak
 

crutschow

Joined Mar 14, 2008
24,099
Hi friends
I've put a diode as your suggestion,but sorry it doesn't work,even the led didn't lit when i press the switch
when releasing the button
The voltage across the diode= -5V,Vc(Vout Q2)=-.094 V(with respect to ground),Vbe(Q1)=4.98V
when pressing the switch
The voltage across the diode= -4..65V ,Vc(Vout Q2)=-0.91 V(with respect to ground),Vbe(Q1)=4.98V
Vbe(Q1) cannot be 4.98V if it's properly connected. It should be no more that 0.8V.

Below is the simulation with the LTspice switch model. I added a resistor between the base and emitter of Q1 to take care of any leakage currents and help insure that it powers up in the OFF state.

upload_2016-4-3_15-48-33.png
 

GopherT

Joined Nov 23, 2012
8,012
It does indeed... I bet (not my life though ;)) that there's an equivalent circuit for a grounded LED
Probably, I just didn't bother designing one. The one in above post was my best memory of an SCR from discrete transsotres. PNPN.

In simulation, swapping the current limiting resistor and LED cause the circuit not to latch. In both cases below. They work fine as shown.

image.jpg
 
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Thread Starter

TheNight Silence

Joined Feb 4, 2016
13
really i need the circuit to be controlled by single pulse from my PC serial port which is generated by a peace of software,
so i need the circuit work from single push button, since i will remove it and apply the control signal in stead;
i hope you could help
 

AnalogKid

Joined Aug 1, 2013
8,252
If the PC output pin is making a positive pulse, you will need to connect it to the left side of R4 through a diode (1N914, etc.) to prevent a transient conflict between the PC output and Q2 collector.

ak
 
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