Silicone Controlled Rectifier

RoninLB · 06-30-2003, 09:22 PM

I'm doing an electronic CHT/EGT aircraft meter..

I want the unit to turn off when I turn the key off/no problem.. I don't want the unit on when I'm starting the 911.. and I don't want a simple on-off switch because sooner or later I will not turn it off before I start the engine/problem. this may damage the unit. I checked the ignition switch, and no circuits are off when starting, then turn on when in the run position, I think.

so a friend suggested a "silicone controlled rectifier". which is supposed to allow current flow with a momentary on switch.. then it allows current to flow until I turn engine off. so if I start the engine the electrionc unit is always off till I hit the momentary on switch to the "silicone controlled rectifier"/idiot proof..

does anyone know anything about this ?
does anyone have another idea ?
does anyone know what part I need ?
does anyone know where I can buy it ?

I got big bucks invested in this and can't use it safely.. so I haven't fired it up yet ...............Ron

BennyBroadcast · 06-30-2003, 11:07 PM

An alternative to an SCR would be a latching relay with a momentary pushbutton switch for activaton. The operation would be exactly the same. Start motor let it stabilise then push the button. The relay will pull in and latch until the circuit is unpowered, the relay will rlax and stay that way until you push the button again. The trick would be to power the relay from the ignition so that there was no danger of powering the circuit up until the engine was running. You can make a latching relay out of any relay with two or more poles. If you're interested I could sketch it for you and e-mail it.

Cheers

Ben

Doug Zielke · 07-01-2003, 06:08 AM

Or....
An adjustable, time-delayed latching relay.
Look at industrial electrical supply houses, and ask to see the Omron catalog.

targa80 · targa80's Garage

I did a search in google and came up with two web sites one describes the function of the basic SCR and the second provides data on NTE part info. I have an old college book that describe using the SCR as a switch.

SCR theory

NTE part info

silverc4s · silverc4s's Garage

Ron,
The SCR is what is commonly used in CD ignition systems to fire the main HV (300-500v) pulse (as stored in a capacitor) through the ignition coil, resulting in a Very HV (>20KV) pulse to your spark plug. Because this circuit recovers very fast relative to engine RPMs, the voltage is flat across all speeds, unlike the normal coil ignition in which voltage drops off as RPM increases, the exact opposite of your engines needs.

IN your application, with much lower voltage and low current as well, the latching relay would be as good a solution, and probably cheaper and less complex to implement. The SCR has some ON resistance, generally higher than that of a closed relay, causing some voltage loss in the circuit, depending on the current flow. Assuming that this metering circuit is ~12volts and very low current, the difference in voltage at the sensor would likely be less than 0.2 volt, no big deal, I guess. The SCR would likely last forever if it was sized correctly for the application and mounted properly, while relays do wear out, as we DME owners all know...

Other than that the SCR circuit would have a better cool factor, and you would gain knowledge to apply to future problems, perhaps. Either one will work for your needs here...

Quicksilver · 07-01-2003, 07:55 AM

Put a T on the oil pressure fitting and use an oil pressure switch to turn it on and off. Simple, low tech, and cheap.

Wayne

kstylianos · 07-01-2003, 08:05 AM

Quote:

Originally posted by Quicksilver
Put a T on the oil pressure fitting and use an oil pressure switch to turn it on and off. Simple, low tech, and cheap.

Wayne

Ohhh....I like that idea much better. Or tap into the current oil pressure switch, although the circuit will be open when pressure is up.

Quicksilver · 07-01-2003, 10:12 AM

With the am oiling restrictor update the idle presure should be fine or you can get the 4# idiot light sender and that would be should work fine.

(After spinning a rod bearing I like the idea of the cam oiling restrictor!)

Wayne

alan911 · 07-01-2003, 11:35 AM

The other device that triggers on start up but not suppose to when you first turn the key is the air meter circuit. Or the fuel pump circuit itself. And it has a relay.

As you know, this fires the fuel pump only when enough air flows through the CIS system just prior to full run. But this may not be the proper timing you're looking for. This won't work if you can hear your fuel pump when you first turn on the key.

Seems that Quicksilver has the best ,cheapest, lowest tech idea.

RoninLB · 07-01-2003, 12:29 PM

Thanks Gentlemen for the brain power...

I've already blown 2 Jacobs Accuvolt units because of using the battery cable chassis ground lug, instead of a chassis ground further away.. on the 2nd "warrenty return" the tech said I should be at least 2 feet away from the cable chassis ground lug.. so the CHT/EGT electronic gauge system instructions stressed using the engine ground and having the unit off till engine is started..

the oil psi T is sometimes used to shut down some fuel systems in marine applications.. and I start a rebuilt engine by disconnecting the ignition and build oil psi first.. so I figure the same oil psi may happen before the engine starts.. especially when carbs are installed. I'm being over cautious on this one.

the engine air meter would be a good shut down. but not an on switch because the engine would be still cranking when the meter moves.

The SCR info and latching relay info is precious.. and I have to hit the suggested sites.. the book suggestion on the SCR is definately great.. but the Radio Shack small cheap book "Getting Started in Electronics" has a simple laymen's explanation of the SCR.

the time delay sw. is sometimes used in audio systems. but afraid to take a chance on that.. the speaker amp "boom" is prevented. but I'm not convinced that it will prevent a "boom" on this crazy expensive toy I'm installing..

So, someone suggested I pull the ignition switch and check all the pins that way.. because there should be an unused pin that will be off in the start position, and be powered in the on position.. I only checked the ignition switch by checking the attached hanging wires.. if that is NG. then I'll go for the SCR.. I'll track down the proper unit using the info provided here..

thanks guys for keeping me rolling...............Ron

Quicksilver · 07-01-2003, 01:30 PM

If it is that sensitive then your probably right about the oil pressure. You might think about an oil pressure switch in series with a manual on/off switch in the cockpit. Another idea would to have a "normally on" relay in series with the oil pressure switch and use the starter output of the seleniod to turn it off.

A good source a generic electronic curcuits can be the local library or of course the internet. I have been looking for some things in this vein for a while and I had some good luck searching for "solid state relay", "transistor relay" together with "latching". From what little I know, an SCR seems to be more commonly used as a switch on AC curcuits. Most of the curcuits that I have seen for DC switching use a transistor for the switch

RoninLB · 07-01-2003, 02:11 PM

Quote:

Originally posted by Quicksilver
[B an oil pressure switch in series with a manual on/off. Another idea would to have a "normally on" relay in series with the oil pressure switch and use the starter output of the seleniod to turn it off.

I have been looking for some things in this vein for a while and I had some good luck searching for "solid state relay", "transistor relay" together with "latching".[/B]

I want to avoid the manual on/off, because when I'm not focused I'll do the routine start/stop engine routine.. and it'll cost be bigbig bucks.. it's specifically written how to wire this unit up.. the Jacobs Accuvolt was not that specific in the instructions.. so they gave me the 3rd unit.. after blowing two of them.

and my friend does the solid state relay act on marine diesels.. let me know your specific situation and I'll tell my "old" friend, like me, who is only doing electric marine work.. do a direct e-mail, and put something in the subject header I'll recognize.. RoninLB@optonline.net

RoninLB · 07-01-2003, 02:21 PM

Bingo...

when I tested the ignition switch yesterday I used an ohm meter.. and when I had resistance, not an open, I assumed it was closed on the "start" position.. WRONG.

so I rechecked the ign. sw. under power after a suggestion from Austrialia.. the pelicanhead was correct.. the X ignition wire.. which is Red/yellow is off in the start sw. position.. so I soldered 2 14ga. wires to the tapped line.. one will go to this new CHT/EGT aircraft unit.. the other will go as a backup to the Accuvolt on/off trigger.. I must have 8 hours in brain fuel into getting this correct.