Silicon Controlled Rectifier

Hey i am currently building a circuit and several websites have suggested to use an SLR. My question to you guys is, What is a Silicon Controlled Rectifier? What is it’s sole purpose within a circuit?


An SCR is kind of similar to a transistor, yet significantly different. An SCR acts like an electronic switch, that once a current is applied to the gate, current can flow through anode and cathode until the current is removed from the entire device, compared to a transistor where the current has to only be removed from the gate.

What are you making that uses an SCR? Something high(ish) voltage?

Think of an SCR as a transistor that feeds itself. This allows a tiny control signal to activate it, but it means that you can’t turn it off at the gate - something else needs to cut the current off.

This means that it has a lot of transistor like properties. Most importantly, its dissipation mode is a voltage drop - this makes it good for high voltage applications.

Please let us know what you are up to, it sounds interesting.

This year our robotics team could not afford to take part in FIRST Robotics. So we divide our team into two. Our group is building a EM Gun. Its going to be a 3 stage coil gun using #23awg copper wire. Our barrel is a aluminum hallow shaft measured at 3/8 of an inch. Im still trying to work out the numbers and schematics but everything takes time and research:) I am going to be using voltages around 350-400 and SLR’s are apparently really good at controling and switching these high voltages and ampere’s.

An SCR is a silicon controled rectifier. This particular type of device can act as a switch and can be turned on with a very low control voltage but as it’s name implies (and it’s schematic symbol shows) it can only control current in one direction. It’s other failure is once turned on, it cannot be turned off without removing the power that it turned on. This charachteristic while it sounds bad works very well in controlling pulsed DC power or in acting as a phase controled switch on AC power. It actually makes an interesting power supply when used in a bridge configuration. It’s AC version is a triac and those are sued in virtually every light dimmer in use today.

I thought it might be something like that.

A word of warning - be careful when paralleling these devices. Add a small resistor to the gate of each SCR. This will prevent the gates from “ringing”. Gate ringing destroyed my friend’s coil gun that used SCRs. To over simplify, gate ringing causes the SCRs to turn on and off very very rapidly.

Also, I’m not sure if you need to add a ballast resistance to spread the current over multiple devices. Can someone verify this?

While SCRs can work for you application, I’d only use them in a single coil coilgun design. MOSFETs or IGBTs are probably a better choice since you can turn them off. Also, don’t forget to put a diode across each coil so that you don’t fry your electronics from the current generated when the field collapses.

What type of resistor would i use>? 10 ohms? or like 10 million ohms?

As far as your barrel material, you should remember that metals conduct and thus will generate eddy current opposing the magnetic field of your coil and robbing you of some power. There a few ways around this, but I’ll leave that for you guys to discover.

You could use SCRs if you powered each coil separately, which I think you’d be doing anyways. I think starting with SCRs would be a safer bet than IGBTs since they’ll stay on until they’ve dissipated all the energy in the caps. This would reduce your rate of fire since you’d have to recharge from zero every shot, but you wouldn’t have to worry about a charged cap wanting to kill you after a firing.

Also, it’s highly important to understand the purpose and placement of the extra resistor and diode you’ll be adding. The coil of the gun is basically an inductor, and will be trying to keep a constant current going through it. Once your cap is spent, the constant current from the coil will act to charge it in the negative direction, which is baaad for the polarized caps you’ll be using. So the idea is to re-route this current through a diode and resistor to dissapate the energy stored in the coil. The circuit would look something like:

 + --------|>-------
               |   |
      Resistor Z   3
Cap            |   3 Coil
         Diode ^   3
               |   |
 - -----------------

The size of the resistor depends on how quickly you want to dissapate the energy, and how large a voltage differential your circuit can handle. The maximum current going through it is determined entirely by your coil’s inductance and cap’s size. After that, Ohm’s law will tell you what the voltage drop across it will be and how many watts it will dissipate at this peak current.

This site may help with your project.

Awesome… im gonna wrap my mind around this for a couple of days. Im not that logical so it will take me a while. I will be posting my circuits within a fews days. It would be wonderful if any of you could lend suggestions.


keep posting if you theres an easier way to explain either the SCR or bleeder resistor. :slight_smile:

Hey! actaully… i think i somewhat understand this stuff guys :smiley: :smiley: below are two photos that i scanned just recently… let me noe if this makes any sense before i blow myself up :yikes:

NOTE* You will have to save the pictures and rotate/resize them. Srry any inconvenience.

Charging Circuit: Circuit2.JPG

Fire Mech: Mech.JPG

C1 and C2 are basically linkers between the two separate circuits.

My power supply, Transformers, and capacitors are a big unknown question mark…??? so again … as mentioned before im still working out the numbers :slight_smile:

NOTE* ifthe images are really unreadable… then let me know i’ll rewrite and scan them with a better dpi

  1. The cap goes between C1 and C2. When you’re firing, you want high current provided by a high voltage through a low resistance. Your HV power supply couldn’t supply the current on its own, so you charge up a cap and use it as a source of high current (for a very short time). Ideally, you disconnect the HV power supply from the cap just before you fire so the power supply doesn’t melt trying to supply that kind of current.

  2. You can trigger the SCR directly from the anode. That is, if you had a switch (with a resistor in series) connecting the anode to the gate, and you closed the switch, your coil would fire. Of course if you’re serious about a multi-stage coil gun, you’d be triggering the SCR with yet other solid state devices like some transistors, etc.

Can you please post the part numbers you are proposing? I couldn’t read them very well in the jpg.

I’m not sure what all the diodes next to the coil are for. You just need a single diode in anti parallel to the coil.

If you are using a single SCR, you can ignore what I said about ballast resistance. Just the gate resistance is necessary.

The diodes in series with the coil are redundant and everything looks to be in the wrong polarity to allow current flow. The capacitor that is wired in series with the coil will form an LC circuit which will modify the resultant current in the coil. Also be advised, that fire switch and battery will be elevated to the supply voltage if and when the SCR fires.

Im planning to have a spike set off the gate. The multistage will be set off by the program.

Note that they have a definite response time for the mechanical switching, plus I don’t know that it would be easy to direct hook it up to the gate. Honestly, you’d probably have a lot better results if you used a solid state device like a few high voltage transistors connected directly to digital outputs on the RC. The response time would be loads better and, hey, you’d get to learn about transistors too!

Also, if you care at all about precise timing, you get to learn about working in the ProcessLocalIO() loop and working with interrupts and timers.

Cool. So i’ve made a small list of what i have to change.

  1. reverse diodes
  2. move capacitor
  3. remove the bleeder diode and diode just after the anode of the coil.

We are using these pneumatics sensors that detect a metal object. I will be placing these over the aluminum to detect the projectile passing through. The program will then sorta calculate when to shot it off or if the projectile even moved. -.-’

The digital outputs and transistor idea sounds really good and i’ll be looking into it. As well with the ProcessLocalIO() loop and interrupts :slight_smile:

The only part that i have a model # for is the SCR… The model # is (M25/16-1k). This part can be found on the following website. *at the very bottom.

Not sure what the timing is on the robot controller, but you will need sensors and actuators with millisecond and microsecond response time and program proccessing. all solid state components(no mechanical relays). Also I’d recommend using optical isolators between any controls and components that may inadvertantly have HV. Another thing to consider is that HV will arc through are if both potentials are near each other, so use wires rated for HV/HC, if current is high enough you also may risk crushing the barrel of the coil gun, and keep computer equipment at a safe range as a EM Pulse may be generated at the time of firing. also when working with high voltage, have someone else around that can get medical help if you inadvertantly get a severe shock.

ahaha ye so if my radio stops working i’ll know whY:O lmao ye im sort of worrying about that now. The EMP thats gonna be created. I’ll just try and move the components are far away as possible.