Silicon Controlled Rectifier

[teh_pwnerer795]

Originally Posted by Kevin Sevcik

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.

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

[Kevin Sevcik]

Originally Posted by teh_pwnerer795

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.

[teh_pwnerer795]

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



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. http://www.surplussales.com/Semicond.../Diodes-1.html

[Scott L.]

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.

[teh_pwnerer795]

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.

[EricVanWyk]

I've attached a picture of what I think the schematic ought to look like. I didn't include a transformer because I don't have a symbol for it handy.

The SCR will trigger when the gate to cathode voltage (the two wires sticking up) exceeds the threshold.

For the love of cheeseburgers with mustard, please don't plug anything into this that you hold dear to your heart. At least not on your first attempt.

Check the capacitor you are using to see how much reverse voltage it can handle. It might be a good idea to add a second snubber diode hooked right across the cap's leads. This is not necessary in a perfect world, but parasitics can really get you in this high di/dt environment.

I might have read your link wrong, but I think that SCR is only rated for 16A.

Tape your experiments!

[teh_pwnerer795]

Ye i just noticed that the SCR is only 16 amps .. i was looking at the package surge rating. which is 400 amps.

[Al Skierkiewicz]

Mitch,
I am just getting around to rereading some of this post and your other question about bleeder reisistors. Bleeders are most often used to "bleed" off the charge on a high voltage cap when the power is removed. High quality and computer grade caps can store lethal voltages for quite a while. The bleeder then slowly discharges the cap to prevent these voltages from causing other problems in your design or hurting someone. The value is chosen so that the cap is discharged in a reasonable length of time but does not provide too great a load on the power supply. My old ham power supply, HP23B, uses 2 100k 2 watt resistors across a 125 microfarad cap in the 850 volt supply. In that configuration the resistors also help stabilize the power supply by providing a permanent load. Since the supply is used in the transmitter, load current doesn't reeally appear until you transmit without the resistors. (BTW the 850V supply is a voltage doubler)
In one of your schematics, there is a resistor in series with the cap. That resistor limits the "inrush" current to the cap. A capacitor that is discharged appears as if it is shorted when voltage is first applied. By using a series resistor you limit the current to a point that will not blow the primary fuse but still charge the cap. The digital TV transmitter I work on uses a bank of such resistors on a three phase high voltage power supply. There is a timed delay relay that shorts out the resistors after the caps in supply are partially charged. This transmitter uses a 35,000 volt power supply at 2 amps and the two main filter caps (in parallel) are 15" x 6" x 30" each and live inside an explosion proof enclosure with the transformer and rectifiers.

[EricVanWyk]

Originally Posted by teh_pwnerer795

Ye i just noticed that the SCR is only 16 amps .. i was looking at the package surge rating. which is 400 amps.

Oh, you should be fine then. My bad...

If you are up to it, it would be a good exercise to guesstimate and then calculate what the surge current will be.

[teh_pwnerer795]

so if i add another SCR that is parallel with the capacitor and have a high resistor in series with the SCR. When the gate is activated, i can discharge the Cap saftey and slowly without sparks flying? this is similar to the idea of a bleeder resistor

[Al Skierkiewicz]

Mitch,
I don't know if who you are asking the question of here. An SCR across the cap is not needed if just the bleeder is used. If you plan on using an SCR to connect the bleeder, then you never really know if the SCR has fired and the charge has been depleted. Don't forget that a short across a charged capacitor will draw immense currents for a short period of time. This could have devastating effects on both the SCR and the cap when done repeatedly. The bleeder draws much lower current over a longer period of time. Of course you don't know if the bleeder resistor has gone open circuit without your knowledge and that is why you always want to make sure your caps have discharged before working on the equipment. Check twice, then check again before going to work inside the covers.

[teh_pwnerer795]

K i see where your coming from about the SCR being fired or not. I just wanted a safe discharge if the robot was about to be turned off. I was thinking of using a bleeding resistor to do that ... or just a HV Load, maybe a halogen bulb light bulb.

[Al Skierkiewicz]

Originally Posted by teh_pwnerer795

maybe a halogen bulb light bulb.

That would produce a significant load on the power supply.

[teh_pwnerer795]

Originally Posted by Al Skierkiewicz

That would produce a significant load on the power supply.

im sort of confused when you mean power supply. If the power supply is off. ie. not charging. and the capacitor is somewhat charged... how would putting a load across the cap effect the power supply?

.... or.. i must of forgot to say that i would put a SCR or transistor in series to set off the HV load.

[Al Skierkiewicz]

Mitch,
My fault, as often when someone talks about putting a lamp across a source, it is implied to be permantly wired without a switch. BTW discharging a high voltage cap into a lamp shocks the filament due to the pulse like current during discharge. A lamp is a very low impedance when cold.