Are self clearing PNP-NPN latches possible? (discharging a capacitor 20V to 3V) - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: April 21, 2025, 06:10:54 pm
content: I've got an application where I want to discharge a 1uF capacitor charged to 24V via a 1MOhm resistor down to <3V using a latching circuit.I know how to build a robust latching discharge with comparators and etc. but an academic question came up.Can you build a self clearing latch (that discharges a capacitor from ~24V down to ~2V) with just 1 NPN and 1 PNP and unlimited diodes, resistor, and capacitors? (Or if that is not possible, 'several' BJTs)Playing around in a simulator it seemed possible, but somewhat random whether or not the latch self cleared after the capacitor discharged most of the way.It seems to me that a latch (and really most NPN/PNPs used as switches) are probably 'far too linear' to be able to do so reliably. And it would probably take a substantial amount of additional complexity/'reinventing the wheel' to make something so 'simple' to work.

author: mtwieg
author_rank: Frequent Contributor
timestamp: April 21, 2025, 06:39:53 pm
content: Sure, what you're drawn is basically an SCR.

author: ledtester
author_rank: Super Contributor
timestamp: April 21, 2025, 06:41:14 pm
content: Here's a stackexchange analysis on making a SCR with two transistors:https://electronics.stackexchange.com/questions/344321/can-a-thyristor-be-built-from-two-transistors

author: incf
author_rank: Frequent Contributor
timestamp: April 21, 2025, 06:48:47 pm
content: What are the criteria for the circuit being "self clearing" vs sitting partially-on in an equilibrium state? (when used to discharge a capacitor fed from a 1M resistor like in the example)

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 21, 2025, 06:53:07 pm
content: Your trigger is not current-limited and would destroy the bottom NPN B-E junction upon first trigger, otherwise would work fine.What your circuit still must do without a comparator is precisely determine when the timing cap has reached the desired level to trigger this SCR.Your cap discharge would automatically clear because the discharge of this BJT SCR would fall below the BJT SCR trigger voltage..but this means the SCR discharge of the cap would not stay latched without additional circuitry and automatically reset.Your circuit thus would require another latching SCR BJT pair to keep the SCR triggered to then keep the timing capacitor from recharging again instantly after discharge and there would be a need for circuitry to reset the latch once triggered.A very simple circuit to accomplish what you want would be a very cheap 8-pin MCU to do everything perfectly without any timing resistors or caps, just a few lines of code.

author: incf
author_rank: Frequent Contributor
timestamp: April 21, 2025, 07:13:25 pm
content: It can fail to self clear, see attachedThere seem to be certain criteria for self-clearing vs not self-clearing in a two transistor latch. Although, I am not sure what they are. (HFe, forward voltage, leakage, etc.)

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 21, 2025, 07:20:04 pm
content: r6 would be better 100-times smaller = 100-ohms.What keeps the BJT SCR from remaining latched is the holding current of this makeshift SCR. The ~20 uA of the timing resistor would likely keep the latch latched and it would stay latched and not self clear.Holding Current is the critical value to determine and saturated beta would vary too much with different transistors, even of the same typemeaning the transistors would have to be selected each build.

author: incf
author_rank: Frequent Contributor
timestamp: April 21, 2025, 07:28:04 pm
content: It looks to me like that the ratio of current going into the NPN vs the PNP could be one factor that determines it's behavior. (self clearing vs getting stuck)Having a smaller proportion of current going into maintaining the latch causes it to stay latched longer/all the way to zero? Seems counterintuitive to me.One version 'works' the other fails, and I am not confident that I fully understand why it is the case.I feel like the nuances of latch design have been figured out before by someone else - mathematical criteria for latching and unlatching. But I don't know where to find it.

author: mtwieg
author_rank: Frequent Contributor
timestamp: April 21, 2025, 07:39:33 pm
content: The magic criteria for this circuit to continue conducting is when the product of the betas for the two BJTs is greater than one.The gate-emitter resistors (R4 and R5 in your schematic) cause the effective betas to decrease rapidly as current decreases. For example, for 100K that means beta will drop off when current through each BJT is less than 0.6V/100K = 6uA. So your SCR will have a holding current of just 12uA. But V3 and R3 will provide 20uA, which is higher than the hold current, so the SCR will never turn off.Try decreasing R4 and R5 to 10K or lower and it should turn itself off. But of course if you increase V3 or decrease R3, then this might not be the case anymore.

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 21, 2025, 07:41:23 pm
content: The BJT SCR transistors have higher current gain at higher currents and that why the saturated turned-on voltage value is lower.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 22, 2025, 07:15:54 am
content: Quote from: incf on April 21, 2025, 06:48:47 pmThere are two numbers to worry about, for net operation :1) Trigger current - the current needed to start regeneration.If you have 1Meg feeder, that will be quite low. 1uA means the cap will charge to 19V instead of 20V2) Holding current - the current needed to stay latched.If you have 1Meg from 20V, that will be just under 20uA MAX for your holding current.If you have an external trigger pulse, life is much simpler as only 2) needs targeting.Your initial 100k RBE will shunt 5uA at 500mV (per leg, if using equal values), far too low to unlatch when driven from the 20uA source.Around 20-25k will be the ballpark to have holding current > 20uA, which will of course be thermally dependent with VBE.

author: Zero999
author_rank: Super Contributor
timestamp: April 22, 2025, 07:39:43 am
content: Why not simply use an SCR, rather than trying to make one? As long as the holding current is significantly above 24µA, then it will turn off, when the capacitor has discharged.

author: incf
author_rank: Frequent Contributor
timestamp: April 22, 2025, 05:05:22 pm
content: Does a suitable one exist with a sub 100nA leakage current at room temperature? (or do whatever a '3904/'3906 can do vs temperature)None of them seem to have published specs or typical graphs at low voltage.

author: Zero999
author_rank: Super Contributor
timestamp: April 23, 2025, 08:23:44 pm
content: Quote from: incf on April 22, 2025, 05:05:22 pmLeakage is typically specified at the full voltage rating, which is typically an order of magnitude above what you're using. It will be much lower in your application. Use a small SCR, to keep the leakage low. If in doubt, buy some devices and test them.

author: incf
author_rank: Frequent Contributor
timestamp: April 23, 2025, 08:32:48 pm
content: Would the leakage at low voltage really be much lower that the leakage at high voltage?For things like a 1N4148 diode leakage is nearly constant with respect to voltage.

author: tggzzz
author_rank: Super Contributor
timestamp: April 23, 2025, 08:37:33 pm
content: Quote from: Zero999 on April 23, 2025, 08:23:44 pmWith many semiconductor devices the leakage/bias/etc current is small. Measuring small currents takes time, which translates to expense. For production tests of "unimportant" parameters, that might be prohibitive.Hence it is common to specify a current that can economically be pass/fail tested and state that is the maximum. Actual currents can bemuchsmaller.

author: incf
author_rank: Frequent Contributor
timestamp: April 23, 2025, 09:01:45 pm
content: Quote from: tggzzz on April 23, 2025, 08:37:33 pmThis is true. But can you rely on it for a decade or more without running into trouble? I feel like if something changed that affected typical leakage current, of all of the customer designs using a particular model of SCR, my design would be the only one affected.I've spent years straightening out/dealing with other people's designs where they did stuff like that. Eventually a batch occurs where the tolerances stack the wrong way (invariably it goes to the customer/location that causes the greatest trouble possible).

author: tggzzz
author_rank: Super Contributor
timestamp: April 23, 2025, 09:13:54 pm
content: Quote from: incf on April 23, 2025, 09:01:45 pmOh, there are many examples of that. For example, old 2N3055 circuits started oscillating, because modern 2N3055s had fTthat was much higher than original 2N3055s, due to process improvements.Not sure whether your decade means 10 years production, 10 years in service lifetime, or just a factor of 10.But the main point is that if a parameter is particularly important to your application, then you have to do your own specific production test - and potentially component selection.