There is a problem with this circuit. The original document specified 3V for power supply. However, the capacitor 13 is only charges to around 0.6v and then stops when I used 3V, and it doesn't trigger SCR 7. I tried 9v and it works sometimes with this voltage. When I disconnect the gate of SCR 7, it seems to be charging fully. I am using TICP106D SCR. In the original document the SCR was BRY55. Both are sensitive gate SCR as far as know, so they should be interchangeable. Both have max Igt 200uA. I don't understand why the current is leaking this much through the gate. Do you have any suggestion? Disregard the rest of the circuit as they have no effect on capacitor 13.

 

Do you have any suggestion?

To begin with - Please try another SCR... If that's not an option please measure thyristor7's (i.e. the SCR in question's) gate current (while C13 is charging) and post your findings here

Best regards and good luck!
HP

 

Thank you HP for the reply. I disconnected gate of SCR from RC node and probe the two point using fluke 87V on uA range. To my surprise there was no current. I also have to mention that 14a is also disconnected. I don't understand if there is no leakage, why it is not charging while gate of SCR is connected. Might be because I am using breadboard? considering the resistor is 4.7 mega ohm.

 

Thank you HP for the reply. I disconnected gate of SCR from RC node and probe the two point using fluke 87V on uA range. To my surprise there was no current. I also have to mention that 14a is also disconnected. I don't understand if there is no leakage, why it is not charging while gate of SCR is connected. Might be because I am using breadboard? considering the resistor is 4.7 mega ohm.[/QUOTE]

Please consider the following...

However, the capacitor 13 is only charges to around 0.6v and then stops

Interestingly, the TICP106D's typical Vgt (i.e. 'Trigger EMF') is specified at 600mV as well...

I disconnected gate of SCR from RC node and probe the two point using fluke 87V on uA range. To my surprise there was no current.

Thus it appears gate leakage is negligible at gate-cathode EMFs much less than Vgt -- to which (it seems) E(C13) approaches only to reach 'equilibrium' prior to attainment of Igt...

So... Next step
Please connect the current indicator (as described in your post above) but leave it connected such that you might observe the gate current when E(C13) 'plateaus'...

Best regards
HP

 

Do you mean between RC node and the gate? would you like to see more information about how this circuit works? please see the attached file.

Kindest regards

 

@Baphomet12

ADDENDUM to post #4:

Please note that, dependent upon values, the circuit of Q19 may greatly influence triggering behaviour! -- Moreover, with regard to R8 alone - attainment of Igt requires a minimum E(C13) of 4.4V (To wit: Igt{200uA} * R8{22KΩ} = 4.4V)

Best regards
HP

 

Do you mean between RC node and the gate?

Correct!

Best regards
HP

 

The measurement showed no current to the gate. The referred 200uA for Igt is the maximum value, according to the data sheet. The typical Igt is 5uA.
Kind regards

 

The measurement showed no current to the gate.

Am I correct in my understanding that:
1) There is no measurable gate current when E(C13) plateaus at ≈ 600mV?
2) The sole connections to the gate are R12 and C13? -- As per:

I also have to mention that 14a is also disconnected.

Might be because I am using breadboard? considering the resistor is 4.7 mega ohm.

Assuming 'YES' to 1 & 2 above -- leakage via surface conduction is a distinct possibility -- Still, It'd be a first in my experience

You might try joining (soldering) the gate, R12 and C13 in 'mid air' above the breadboard...

The referred 200uA for Igt is the maximum value, according to the data sheet. The typical Igt is 5uA.

Indeed -- However good design will 'embrace the spread', as it were

Best regards and please keep us posted...
HP

 

@Baphomet12

It occurs to me that (Re: the circuit under analysis) a 600mV 'drop' across the gate-cathode junction corresponds to a leakage current of ≈ 510nA - which being rather unlikely observable via a DMM's uA range...
Which brings us back to square 1 -- To wit: Please try another SCR

Although I am unable to locate 'Gate Leakage' characteristics for the cited thyristors, suspicion of disparity in the former is not unreasonable...

Best regards - and again - please keep us posted!
HP

 

charging a 47uF capacitor thru' a 4.7 M ohm resistor........

 

charging a 47uF capacitor thru' a 4.7 M ohm resistor........

Does seem a bit 'iffy' considering I{limit}≈638nA --- Howbeit the TS reports expected charge conditions (gate disconnected)...

My guess is that the circuit was 'scaled' from a higher supply EMF design during which R12 was somehow 'neglected'...

Best regards
HP

 

Ramussons, do you think the capacitor is too large or the resistor? They actually work when the gate of SCR is not connected (capacitor charges). They are functioning to give 3 minute arming delay. I tried 1M resistor with 220uF cap but the ground path, which is a fail safe feature, through 14 to 27 to ground, didn't work as intended.

 

Ramussons, do you think the capacitor is too large or the resistor? They are functioning to give 3 minute arming delay.

While I cannot speak for @Ramussons I feel they're both too large -- Good design practice will first select the resistor for appropriate current conditions then select the capacitor for appropriate timing... -- That said: 3 minutes via a discrete RC network is a 'tall order' where any amount of current is required...

Best regards
HP

 

Dear HP, I completely agree with you. The delay around 1 min is more feasible. If you find spare time in the future, could you please have a look at the attached file in above posts. It explains the function of the device in details so maybe we could tune it for 9v. Thank you

 

It explains the function of the device in details so maybe we could tune it for 9v. Thank you

Inasmuch as 'reassignment' of the supply EMF would require redesign of the the entire circuit - Let's continue troubleshooting the circuit of thyristor 7 for the nonce

Please try the following:

For test purposes:
1) Substitute R12 with a 470KΩ resistor.
2) NOTE: Do not change C13 at this time!

Note: The above suggested test arrangement will establish an I{limit} ≈ 6.4uA and a TC ≈ 22s (and, hence, a trigger delay of ≈ 5s (To wit: T[0V to Vgt{600mV}] ≈ 0.224 'time constants').

Assuming C13 charges and triggers the SCR as expected following the suggested substitution of R12, the desired delay is most practically achieved via empirical adjustment of C13 following re-connection of R14A (I strongly advise use of a low leakage electrolytic [often identified via an orange 'jacket''] or, better still, a tantalum electrolytic capacitor...

Please be advised that incorporation of a current limiting resistor (in series with the gate) is highly advisable!

Best regards
HP

 

Hi HP, I change the resistor as you requested and also tried 1M ohm resistor for R12. Capacitor charges (I used 9v) with no problem. But the issue is the fail safe is not working. The function of fail safe is if the wire 10 is not properly connected during the charging of capacitor, the base of transistor 22 no longer grounded ( point 9 is disconnected), thus it conducts, which provide a path for current from 14a, 14, 20 and 21 to ground which prevents capacitor to charge. This also prevents light 18 from turning on which alert the user that wire 10 is not connected properly. When I used 470K and 1M ohm, although the wire 10 was disconnected, the capacitor kept charging and and after a few second turned both SCRs on. Resistor 20,21 are 1M ohm. Do you think if I use lower values resistors for 20,21, it would provide better current path to the ground? The capacitor must not charge if the wire 10 is broken or faulty.
Kindest regards

 

Also, when I used 3V, the capacitor (tantalum 47uF) also charged. After which, by disconnecting wire 10, the SCR 3 didn't turn on. I don't know because SCR 7 didn't turn on or it did turn on but the voltage or current at the gate SCR 3 was low.

 

Hi HP, I change the resistor as you requested and also tried 1M ohm resistor for R12. Capacitor charges (I used 9v) with no problem. But the issue is the fail safe is not working. The function of fail safe is if the wire 10 is not properly connected during the charging of capacitor, the base of transistor 22 no longer grounded ( point 9 is disconnected), thus it conducts, which provide a path for current from 14a, 14, 20 and 21 to ground which prevents capacitor to charge. This also prevents light 18 from turning on which alert the user that wire 10 is not connected properly. When I used 470K and 1M ohm, although the wire 10 was disconnected, the capacitor kept charging and and after a few second turned both SCRs on. Resistor 20,21 are 1M ohm. Do you think if I use lower values resistors for 20,21, it would provide better current path to the ground? The capacitor must not charge if the wire 10 is broken or faulty.
Kindest regards

Also, when I used 3V, the capacitor (tantalum 47uF) also charged. After which, by disconnecting wire 10, the SCR 3 didn't turn on. I don't know because SCR 7 didn't turn on or it did turn on but the voltage or current at the gate SCR 3 was low.

At this point I'm inclined to the notion that the circuit (as a whole) is essentially misdesigned --OR-- was incompletely/imperfectly adapted from another circuit featuring a significantly greater supply EMF requirement...
Please advise me as to the design/circuit 'background' such that I may better assist you...

Best regards
HP

 

At this point I'm inclined to the notion that the circuit (as a whole) is essentially misdesigned

Yes I agree. No way it works with 3V. I tried other voltages, it works most of the time with 6v and above. Regarding the capacitor not charging sometimes, it is all SCR's fault. I thought to replace TICP106D with something new so I purchased new SCRs, 2N5061, which to my surprise they don't work at all. Not even with 9V supply and changing the resistor 12 to 1m ohm, whereas TICP106D works most of the time if supply voltage is 9v. I don't understand what property makes 2N5061 ( and some other ) so bad for this application. So, yes it all boils down to SCR apparently.

Regarding the background of circuit, it was a patent for some other application but I am going to use it as a permitter defence i.e security alarm.
I need something compact for tent security when I go camping. This has all the features I need, break wire, low power consumption, LED to show the progress of exit delay and fail safe that alerts me if break wire is faulty prior to activation, so no false alarms.

Kindest regards