SCR Gate Voltage - Page 1
Forum section: electronics / beginners

author: hgg
author_rank: Frequent Contributor
timestamp: November 07, 2014, 05:58:10 pm
content: Hi everyone,I am trying to build a simple circuit that will switch on a light using the 3.3V signal from a motion sensor.This is the motion detector which works pretty well. Very sensitive and with a wide angle of detection:The idea is to use one these in each room and when motion is detected it will switch on a light andswitch off all other lights that are connected on the same 12V solar battery supply so as you movearound the house you will have a decent emergency lighting with the minimum requirements from thesolar panel and battery. Only one bulb will light up at a time.Anyway, I was trying to use an SCR as a latching switch with the following circuit:The LED is lit when motion is been detected, but after that, the detector circuit gets blocked.The problem is that even with the detector disconnected, the gate signal is always at -4.5VWhat am I doing wrong...?Something else. Can you help me 'read' from the datasheet what is the maximum gate voltage that the SCR can handle?http://docs-europe.electrocomponents.com/webdocs/0db0/0900766b80db02ec.pdfThank you.George.

author: Simon
author_rank: Unknown
timestamp: November 07, 2014, 06:30:51 pm
content: Err I'm no SCR expert but you need two wires from your control circuit as you need a ground and a signal, you can't just sent a signal.

author: hgg
author_rank: Frequent Contributor
timestamp: November 07, 2014, 06:33:59 pm
content: Sorry....I forgot to draw the common ground.They are both powered from the 5V source and share a common ground.(I did not find the ground symbol in the digikey online schematic software)

author: Simon
author_rank: Unknown
timestamp: November 07, 2014, 07:22:19 pm
content: Well if the cathode is the common reference for the SCR you have put the LED and resistor in series with the SCR gate voltage so now you need 2.5-4V + SCR trigger voltage that makes well over the 1V you have. You need to draw a ground wire to make clearer what you have.

author: hgg
author_rank: Frequent Contributor
timestamp: November 07, 2014, 07:49:35 pm
content: Here is the output of hggCAD.PIR output voltage is 3.2V when triggered and 0V when not, but even if I have the PIR circuitand resistor divider disconnected I get -4.5V at the SCR gate.

author: SeanB
author_rank: Super Contributor
timestamp: November 07, 2014, 08:01:47 pm
content: Put the load in the anode side. SCR circuits are not happy with a floating cathode, they will rise up to the anode voltage on the gate. Plus the resistors are wrong, have the 10k as pull down and the 4k7 as current source.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 07, 2014, 08:07:20 pm
content: The SCR won't turn off again. You need a digital logic circuit to implement the "one-hot" flip-flop. MOSFETs will suffice for the power switching.Tim

author: hgg
author_rank: Frequent Contributor
timestamp: November 07, 2014, 08:23:55 pm
content: QuoteI didn't know. I'll keep that in mind, thanks!Tried that as well and I am getting again -4.5VI am taking the 3.2V and using the resistor divider I give 1V to the SCR gate because I stillcannot figure out from the datasheet its maximum gate voltage...QuoteIndeed, I am aware of that. If you can disconnect its power for a brief time though, it will reset itselfbut I don't know if this can be done easily. Flip flops would be my last resort if I cannotfind an easy 'analog' way to do it.

author: wraper
author_rank: Unknown
timestamp: November 07, 2014, 08:27:05 pm
content: As T3sl4co1l said, SCR will only turn on. You need to completely remove voltage from it to turn it off, it is how this device works. Moreover at 12V SCR is not good because of the voltage drop, logic level mosfet would be much better.

author: hgg
author_rank: Frequent Contributor
timestamp: November 07, 2014, 08:31:34 pm
content: I've never worked with logic level mosfets.Mosfets act as switches but is it easy to make it a latching switch?Do you have to use two of them like in the flip flop configuration?Thanks.

author: wraper
author_rank: Unknown
timestamp: November 07, 2014, 08:33:37 pm
content: Quote from: hgg on November 07, 2014, 08:23:55 pmThere is no maximum gate voltage, it is current driven. And your circuit supplies too little current for most of the SCRs. Voltage drop on the gate will be below 1V, it is only PN junction, so will act similar to transistor base if thinking about voltage drop on it.

author: wraper
author_rank: Unknown
timestamp: November 07, 2014, 08:35:46 pm
content: BTW, just noticed. You are using SCR completely wrong. You must apply gate voltage between gate and cathode. Load must be connected on the anode side.

author: hgg
author_rank: Frequent Contributor
timestamp: November 07, 2014, 08:44:44 pm
content: I see. Thanks.I did it just to be on the safe side.It works but the problem is still the -4.5V at the gate which I cannot explain.When I power it with 12V, the gate voltage is at -11.5VI've read that you can check an SCR with a multimeter using its diode function at theAnode and Cathode terminals. When I test between anode and cathode I get nothing.I only get the diode behaviour between the cathode and the gate!QuoteI did it. Same thing.

author: wraper
author_rank: Unknown
timestamp: November 07, 2014, 08:54:45 pm
content: It cannot be same thing by definition. When you are using load on the cathode side, of course when SCR opens, gate voltage will shoot up to almost anode voltage.Here is how you should drive it, of course you don't need potentiometer, just current limiting resistor.

author: wraper
author_rank: Unknown
timestamp: November 07, 2014, 09:06:20 pm
content: Quote from: hgg on November 07, 2014, 08:44:44 pmBut what you was expecting to see? It is at it should be. How can you expect go get conduction between anode and cathode when SCR is closed?

author: hgg
author_rank: Frequent Contributor
timestamp: November 07, 2014, 09:26:38 pm
content: Please don't shoot me...The circuit was ok, I was not measuring the gate voltage correctly... Too late and I am tired.It works. As a bonus, if I short the gate to ground it resets itself.Is this normal?Only with the LED. If I use a lamp with a lot of current it does not reset. I have to removethe power.

author: hgg
author_rank: Frequent Contributor
timestamp: November 07, 2014, 09:48:35 pm
content: T3sl4co1llooks like I would have to follow your suggestion for the one-hot flip flops.Can you suggest a reference that would help?Any rough draft schematic?What I need is when one light gets lit, all the others turn off.Maybe all the lights would have to share a signal line as well as the 12VDC supply.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 08, 2014, 02:46:25 am
content: You could, perhaps, use an R-S flip-flop for each output channel. A CD4043 might be handy (quad FF), or several.You would need to wire it so that each motion detector fires the reset for all other FFs, and sets its own. You might use diode-OR logic to implement this.Tim

author: wraper
author_rank: Unknown
timestamp: November 08, 2014, 03:27:24 am
content: Quote from: hgg on November 07, 2014, 09:26:38 pmLikely current it below hold threshold when using led.

author: SeanB
author_rank: Super Contributor
timestamp: November 08, 2014, 05:24:29 am
content: It is an expected behaviour, as you are starving the lower NPN transistor of base current. It then turns off the upper PNP transistor and turns off the SCR. At higher current the resistance of the base layer means part of the die does not turn off, so it stays on. If you apply a negative voltage to the gate, and draw a pulse of about half the load current, then the SCR will turn off.Sony used to make Gate Controlled Switch thyristors, which they used as high voltage high current switches in TV set line output stages, before transistors that would survive line output stage use reliably were developed ( line output is a harsh use, you have high voltages, fast edges and high circulating currents, along with the need to both turn on fast without saturating the transistor and turn off very fast with a very fast rising collector voltage doing it's best to keep the transistor turned on via Miller capacitance. You have to drive the base with 3-5A of current, and it has to ramp up linearly to just keep the device conducting with the instantaneous collector current, and has to sink considerable current during turn off as well) to what they are today.No longer made though, but most SCR devices will operate in limited current as GCS units, though they are not designed with that in mind, as you need a very low resistance of the gate on the lower NPN to enable the gate to draw current to turn the entire device off. Otherwise only part turns off, and the remainder stays on, and with enough current hot spots form and kill the device.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 08, 2014, 06:19:08 am
content: AFAIK, all SCRs are GTO to some extent, it's just that GTOs are optimized to have a somewhat useful turn-off "hFE" (meaning, a gate current of 50-200% of anode current -- between "almost" and "more than enough" to shut it off, brute force!). Whereas conventional SCRs may have enough series resistance in the gate connections that the entirety of the SCR structure cannot be turned off by any safe current level.At low currents (near the holding current), gain drops and conduction becomes much more sensitive to gate current, junction temperature, load current and all that. SCRs are a "runaway" device, meaning, below some magnitude of operation, they will be unconditionally stable; it's only when you operate them at higher levels that they start behaving weird (like, latching on). Obviously, SCRs are intended to harness that behavior.This is also an important lesson for digital circuitry, because CMOS processes (97% of all chips?) always have, and it seems always will, use a P-N-P-N structure in their construction. The diodes in that structure are forward biased (and therefore, partly turn on the parasitic SCR structure) during input or output pin overdrive; modern e.g. HC CMOS processes are claimed to withstand up to 100mA without latchup, but when they say "freedom from latchup", it's not that they literally cannot: it's just that they're better than previous generations. There is no "never" around an SCR structure, only a "try harder"!Back to SCRs, it's generally good design practice to drive them continuously, anyway. The drive doesn't have to be DC, which is helpful: you can use a transformer and diode to drive an SCR, simply driving the transformer from a gated oscillator. The diode is used to prevent reverse gate voltage, which won't necessarily turn it off (as I said above), but won't help turning it on either. The advantage is, if the SCR is reverse biased at the moment you start driving it, if you're only driving it with a single pulse, it won't fire, and you skip a cycle. Repeatedly banging on the gate is almost as good as driving it with DC, and a lot simpler to arrange (you need the transformer anyway for a DC-DC isolator, but using it for pulses rather than DC means saving all that extra circuitry).TRIACs can also be driven from AC, with the difference that the gate conducts in both directions, so you don't need (or want) the extra diodes. You probably do want a coupling capacitor though. But because TRIACs are almost exclusively used for AC SSR applications, it's more common to drive them from a photo-TRIAC, which is also much cheaper and more compact than building a gated oscillator plus transformer.Tim

author: hgg
author_rank: Frequent Contributor
timestamp: November 08, 2014, 08:27:12 am
content: Thank you!Very useful and interesting information.Apparently there is the SCS Thyristor, Silicon Controlled Switch, which has both an Anode and a Cathode Gatewhich you can use them to turn them on and off but I cannot source any. Maybe this would have been aneasy solution for my project.Anyway, I think I will have to do some reading on flip flops and its transistor implementations.Thank you guys.George.

author: hgg
author_rank: Frequent Contributor
timestamp: November 08, 2014, 10:25:35 am
content: T3sl4co1lone question.Diode OR logic works from many signals to one receiver, but in this case we have one signal tomany receivers.The first detector has to set its own flip flop and to reset all the others, except itself.How can I prevent the flip flop resetting itself but at the same time allow to be reset from other flip flops?Will this be possible with only one reset line?Thank you.George.p.s. Except if I can find the following logic gate:S R Q=====0 0 00 1 01 0 11 1 1

author: atferrari
author_rank: Frequent Contributor
timestamp: November 08, 2014, 01:28:59 pm
content: In a power supply I built, an SCR acted as kind of crowbar protection. To reset it, there was a NO pushbutton in parallel. Pushing it briefly diverted the current to ground thus bringing the SCR to off.

author: hgg
author_rank: Frequent Contributor
timestamp: November 10, 2014, 03:26:07 pm
content: atferrariyou are right.If I short the anode and the cathode of the thyristor, it turns off. The only problem is that this type of resetwill light up all the lamps at the same time while being reset.Anyway I think I will use a relay and will disconnect the flip flop's own reset line during the reset pulse.I am still wondering though if you can build a logic gate with the above truth table from other standardlogic gates.Thank you.

author: SeanB
author_rank: Super Contributor
timestamp: November 10, 2014, 05:02:00 pm
content: You can use a set of film capacitors to interconnect all the anodes together. This way when the first one turns on it charges all the capacitors up, and if another anode turns on the current pulse across the capacitor as it charges will turn the first SCR off, leaving only the second one on. Third will turn off the second and so on.Look up thyristor quiz circuits for a circuit.http://www.allaboutcircuits.com/worksheets/thyrcir.htmlQuestion 15 is a good start.

author: hgg
author_rank: Frequent Contributor
timestamp: November 10, 2014, 08:17:22 pm
content: Hello SeanB,Nice find. The webpage has a lot of interesting thyristor applications.I am not sure I understand 100% what will happen when you have more than two lampslike in the following photo.For example lets say Lamp1 is lit, all the other lamps are off and Lamp4 switches on.Will the parallel capacitor effect propagate from SCR4 to SCR1?One negative is that with this configuration you need to cut the 'reset' line at eachlight fixture and connect it with the capacitor to the anode of the SCR, but if it works, itsa very nice solution.

author: atferrari
author_rank: Frequent Contributor
timestamp: November 11, 2014, 12:35:33 am
content: Quote from: hgg on November 10, 2014, 03:26:07 pmI do not see how that could happen at all.Rather busy for more two days. I will read carefully the whole thread to see what makes sense to me.You are divertin the current of THAT SCR in particular (and only THAT one). How the load of the others will be ON during the closure? One pushbutton per SCR...how comes?Give me time to understand.

author: hgg
author_rank: Frequent Contributor
timestamp: November 11, 2014, 06:23:41 am
content: QuoteIts because I will use a single reset line and all the lights that will see the line high, willtry to reset themselves with the same method. In this case by shorting anode to cathodefor a short time. (There would be no push buttons. The anode/cathode bypass will beinitiated by the motion sensor signal)

author: atferrari
author_rank: Frequent Contributor
timestamp: November 11, 2014, 08:35:20 am
content: Quote from: hgg on November 11, 2014, 06:23:41 amYou could go away simply with this: for every SCR, put one NC pushbutton in series,between the load and the anode. Just push the button of the SCR that needs to be turned off. No logic and away you go.

author: hgg
author_rank: Frequent Contributor
timestamp: November 11, 2014, 02:48:43 pm
content: atferrari, I will not use any switches. The lights will turn on and off from the motion sensor signals.SeanB, great solution! It works!I've just tried it on the breadboard with 3 sensor circuits and it works.Very simple and efficient with almost zero current consumption from the thyristor circuits.The strange thing though is that I did not have any non polar capacitors and I used somepolarized electrolytics. I don't know why... but it still works perfectly.Thanks,George.

author: atferrari
author_rank: Frequent Contributor
timestamp: November 12, 2014, 11:45:08 am
content: Quote from: hgg on November 11, 2014, 02:48:43 pmOK OK sorry for insisting on the wrong solution.All pushbuttons back in the drawer now.

author: hgg
author_rank: Frequent Contributor
timestamp: November 12, 2014, 04:30:09 pm
content: Ok,Two solutions for the self reseting solar lighting project.One is the 'digital' one which I have not tried yet but I think it will work.T3sl4co1lI think you were right about the Diode OR Logic...and the analog with the little capacitor trick suggested by SeanB, which I have tried.Below is a single SCR triggering light circuit without the capacitor connection to the other light circuits.I think that I will use the analog version but I have another problem...The little motion sensors although they work perfectly, when you first power them up,they send a high signal just as if motion has been detected. This makes of course allthe lights to switch on at the same time, needing a lot of current from the solar batterywhich is a small 12V/7Ah.Is there an easy way to delay or block the sensor signal for 3 seconds until it goes low?Any ideas are appreciated.Thank you!George.