SCR RELAYS SELECTOR
A simple but expandable and customizable microcontroller-free relays selector and driver circuit based on SCRs.
I needed to switch some loads, one at a time, in a simple and cheap way.
Thus this circuit that uses SCRs (Silicon Controlled Rectifier) as its main active components was born.
The relays are actuated by buttons but microcontrollers compatible inputs and a 5 V output to power external circuits are provided as well.
The whole circuit is powered by a small switching module HLK-PM01.
There is a power indicator red LED and a status blue LED for each relay and protection fuses.
If you need more relays just add more sections like the dashed line highlighted one visible in the schematic (you can also remove sections of course! Let at least one plus the Off button). The power relays can be replaced with signal relays, optocouplers, phototriacs or solid-state relays with minimal variations to the circuit.
The operation of the circuit is simple but not trivial. A SCR is normally triggered by a small gate current pulse, after which it can maintain the on state until the current flow passing through it is interrupted.
When you press a button the current required to trigger the SCR can reach the gate through its resistor but the charge variation of the capacitor C3 causes a short positive pulse on the base of the PNP transistor Q1 (normally kept in saturation state by resistor R3) which resets its collector current and the current through any other SCR which may be active as well. So only the SCR corresponding to the pressed button can be activated and switches the relay to the on state. The same happens if instead of pressing a button you provide an adequate voltage via an external input.
The SCRs used are of the kind "Sensitive Gate Trigger Current" and the transistor Q1 has been selected for the low drop of voltage Vce.
Looking for a free simulation software to better illustrate the operation of the circuit I found "Circuit Simulator" which allowed me to perfectly and easily verify the operation of this circuit. So I asked the author for permission to mention his very interesting and excellent work and put a link to its associated Web page. I think it's really worth to give it a go! http://www.falstad.com/circuit/circuitjs.html
Here's the link to a short video I made with the simulation of this circuit https://youtu.be/OC335jgW-YI (choose HD quality) and attached you find the schematic and some pictures of the prototype.
Best regards,
Anto
Thus this circuit that uses SCRs (Silicon Controlled Rectifier) as its main active components was born.
The relays are actuated by buttons but microcontrollers compatible inputs and a 5 V output to power external circuits are provided as well.
The whole circuit is powered by a small switching module HLK-PM01.
There is a power indicator red LED and a status blue LED for each relay and protection fuses.
If you need more relays just add more sections like the dashed line highlighted one visible in the schematic (you can also remove sections of course! Let at least one plus the Off button). The power relays can be replaced with signal relays, optocouplers, phototriacs or solid-state relays with minimal variations to the circuit.
The operation of the circuit is simple but not trivial. A SCR is normally triggered by a small gate current pulse, after which it can maintain the on state until the current flow passing through it is interrupted.
When you press a button the current required to trigger the SCR can reach the gate through its resistor but the charge variation of the capacitor C3 causes a short positive pulse on the base of the PNP transistor Q1 (normally kept in saturation state by resistor R3) which resets its collector current and the current through any other SCR which may be active as well. So only the SCR corresponding to the pressed button can be activated and switches the relay to the on state. The same happens if instead of pressing a button you provide an adequate voltage via an external input.
The SCRs used are of the kind "Sensitive Gate Trigger Current" and the transistor Q1 has been selected for the low drop of voltage Vce.
Looking for a free simulation software to better illustrate the operation of the circuit I found "Circuit Simulator" which allowed me to perfectly and easily verify the operation of this circuit. So I asked the author for permission to mention his very interesting and excellent work and put a link to its associated Web page. I think it's really worth to give it a go! http://www.falstad.com/circuit/circuitjs.html
Here's the link to a short video I made with the simulation of this circuit https://youtu.be/OC335jgW-YI (choose HD quality) and attached you find the schematic and some pictures of the prototype.
Best regards,
Anto
Discussion (2 comments)
Abdullah Eyles 7 years ago
A very interesting project but I would like to comment on the introduction written in the Elektor email message which states:
"Very common on old equipment, today they seem to have disappeared completely. I am talking about those pushbutton banks of which only one button could be activated at a time, like cassette player’s piano keys or channel selectors on radios."
I would like to point out that piano keys do not have such a mechanism; if so, how would one play a chord (more than one note at a time)? Or do you mean piano (type) keys on cassette players?
One of my favourite 'pastimes' when i was young was pressing fast forward and rewind at the same time to see what happened... That's how Engineers begin learning, isn't it? :-))
With regards,
Stephen
anto 7 years ago
thanks for appreciating the project! The Elektor introduction was written by Clemens Valens, who, I guess, with "cassette player's piano keys" intended just the player's keyboard, which has nothing to do with the keyboard of a piano(musical instrument).
By the way, you reminded me of some childhood memories: your favourite "pastime" was also one of my favourite ones, and probably the beginning of my passion for electronics! :-))
Best regards,
Anto
Dirk Hansen 7 years ago
Zwar ein recht einfaches Design, aber auf eine Verriegelung der Relais oder Tasten würde ich auf keinen Fall verzichten.
So ist es möglich, zwei Aushänge zu aktivieren wenn man zwei Tasten exakt gleichzeitig drückt. Zugegeben ist das bei der sehr kurzen Zeitkonstante unwahrscheinlich, aber theoretisch möglich.
Und Vorsicht bei höheren Betriebsspannungen, es wird nicht erwähnt, dass diese Schaltung nur für 5V geeignet ist.
Durch das Differnzier Glied an der Basis von Q1 entsteht dort ein Puls mit fast der doppelten Betriebsspannung. das ergibt eine negative Ube die sich wie folgt berechnet :
(Ub-Ube) + (Ub-Uf (von Diode V6...)) - Ub, gekürzt : Ub-Ube-Uf
Bei 5V bekommt Q1 eine negative Ube Spannung von etwa 3,6V ab, das kann ein normaler Bipolar Transistor noch verkraften. Laut Datenblatt des BC327 sind max 5V -Ube zulässig.
Bei 12V wären es aber schon 10,6 V, da bricht die Ube Diode durch und da kein Strombegrenzungswiderstand in Reihe vorhanden ist, wird der Transistor wahrscheinlich zerstört werden.
anto 7 years ago
Anto