Help Thyristor circuit

[AndyHardwick]

Hi,

I wanted to build what I thought was going to be a quick circuit for my kids using thyristors and transducers. The project idea was to create a target for them to fire their Nerf bullets at, the transducers would be part of the target acting as a pressure switch and the thyristor would be a simple latch to light an led to indicate that the target was hit.

Example circuit here : http://tinyurl.com/ya42uad2

Testing a single thyristor / transducer it worked great, so I put in parallel multiple and I assumed I would be done. The issue is that the thyristors either trigger randomly or don't come on at all. I assume I got my resistor calculations wrong but I've tried several now I've lost sight of or missing something.

Thyristor = P0102AA
Igt = 200microA

Any advice would be most appreciated.

Thanks in advance
Andy

 

[Minder]

Your single gate resistor could be the issue.
M.

 

[AndyHardwick]

Your single gate resistor could be the issue.
M.

Thanks Minder for the response. I’m using 5 Thyristors, one per transducer. I don’t fully understand what your suggesting.

 

[Minder]

Independent gate resistors, one for each SCR (5).
M.

 

[Harald Kapp]

I suggest you tie down each thyristor gate to 0 V via a high resistance resistor, e.g. 475 kΩ, one for each gate. A floating gate can easily catch noise and trigger the thyristor. These resistors will ensure the gate voltage is 0 V even with (moderate) noise present.

What @Minder imho suggests is using a separate 330 Ω resistor for the trigger connection to each thyristor to minimize the risk of mutual influence.

 

[AndyHardwick]

I suggest you tie down each thyristor gate to 0 V via a high resistance resistor, e.g. 475 kΩ, one for each gate. A floating gate can easily catch noise and trigger the thyristor. These resistors will ensure the gate voltage is 0 V even with (moderate) noise present.

What @Minder imho suggests is using a separate 330 Ω resistor for the trigger connection to each thyristor to minimize the risk of mutual influence.

Thanks. I’ll give these suggestions a try.

Andy

 

[AndyHardwick]

I have tried the suggestions. Thanks. The results are very similar to before. However now, once the first thyristor latches, the second thyristor would latch but the first would switch off. I'll keep trying with the resistor values.

 

[Minder]

What is the actual value of the gate resistor, normally it would be closer to 100Ω.
Is the 9v source capable of supporting the full load, without a dip?
M.

 

[AndyHardwick]

@Minder The gate resistor started at 330Ω but dropped it to 130Ω. I'm tying down the gate to 0v with 630kΩ and the led has 330Ω

Currently I only have two thyristors in an attempt to understand this better and eliminate other issues.

 

[AndyHardwick]

@Minder The gate resistor started at 330Ω but dropped it to 130Ω. I'm tying down the gate to 0v with 630kΩ and the led has 330Ω

Currently I only have two thyristors in an attempt to understand this better and eliminate other issues.

Ok, Update. I focused too much on the resistor values. It appears my problem now is the piezo transducer. With out the transducer the circuit now works fine. It seems I'm getting a very small (9mV) voltage but this is enough to trigger the gate.

 

[PAFMElb]

Hi,

I wanted to build what I thought was going to be a quick circuit for my kids using thyristors and transducers. The project idea was to create a target for them to fire their Nerf bullets at, the transducers would be part of the target acting as a pressure switch and the thyristor would be a simple latch to light an led to indicate that the target was hit.
Example circuit here : http://tinyurl.com/ya42uad2
Thyristor = P0102AA
Igt = 200microA
Andy

Thyristors and triacs do not like having their gate lead left open circuit. Needs to be tied down with a resistor. Totally agree with comment by Harold Kapp. This reduces false triggering by everything from radiation into the gate (not not the nuke kind - the EM kind to false triggering by dV/dt effects which are more likely with sensitive gate types. Big thryristors seem to manage better but it is dubious practice to leave any of them open gate. High temperatures also contribute to false triggering. Again, not an issue if you're only driving an LED.

Your 330R resistor is going to pull around 25mA gate current. Way too much, and likely to dip the battery supply voltage as the battery ages. Why not use a resistance that delivers around 500uA to the gate?

Without looking up the specs of P0102AA try 10K gate to cathode, and around 10K to 22K for the common trigger supply resistor. It also helps noise immunity to have a small cap from gate to cathode but try the resistor alone for a start.

What did you use for the transducer/switch?