Relay Launch Controller

A standard automotive relay is a good choice, cheap and easy to find. However, they might require a minimum of 8 or 9V to kick in reliably. You could look for a relay for a 6V system (lawn tractor, etc.). Another option is to buy an industrial relay from a place like digikey.com etc. They usually have links to the manufacturers spec sheets, so you can check the min coil voltage.

I'd recommend going to 12V on each end and use the 12V automotive relay. At least 20 or 30 Amps for the rating, which gives you better contacts that will last longer even though the typical igniter uses a peak current of a few amps for a couple hundred milliseconds. Cheap relays will carbonize over time or even weld together (nobody wants a stuck relay!).

Here's some info on a launch control system I designed:

-John DeMar

There are several high quality automotive relays that would work well. Make sure the contacts are rated for at least 30 amps dc at 12 volts. Ac contact relays should not be used. The coil ohms are important. To keep the cable bulk down you will probable use small gauge wire between the 2 units. Several hundred feet of cable can lead to substantial voltage drop. Dc relays should not be used with reduced voltage as they need to smack closed fast to prevent excessive arcing. A coil of

80 ohms or greater should be used. All electr> >

I couldn't find a 356 on the AllElectronics web page. The 630/631 are SPDT rated 30/40amps NO/NC, 81 ohm coil with internal clamping diode. Somewhere I ran into plans that indicated the relay must have the clamping diode. What does that do? You could use the second contact to indicate (led, buzzer,...) the relay isn't stuck closed?

Joel. phx

And nobody would hook the battery up wron,....zizzt... auhhh crap. ;)

Sorry, Old catalog. Haven't found details of the listed relays. I Know the P and B relay at hosfelt is well made. The Siemens Pand B web site has a white paper on dc relays that will tell you more than you want to know. Clamping diode? Some relays limit in rush current, some limit the coil collapse when de- energized. Magnetic blow out is nice. Don't think it's necessary for this application I would worry about the contact material, Inrush rating, and coil power requirements Some of the P and B PRD series relays are tough. Gary Deaver

A clamping diode protects the circuit energizing the relay coil from the voltage spike of the coil's collapsing magnetic field when it's current is cut off. The spike may, for instance, cause arcing across the contacts of a control relay or switch, or put an overvoltage across a semiconductor driver. Its like those voltage spike protectors that come with power strips for computers, only different. :) If the relay doesn't come with one and you can't live without it, chose a power rectifier diode with a high peak current rating. Make sure its installed correctly; polarity is everything to a diode.

Be careful using a bridge rectifier for polarity protection... the two diode drops will give you ~1.5V less and might not be enough drive voltage.

-John

A "fast recovery" type diode reversed biased across the relay coil is usually recommended if there isn't one internal to the relay. Something like a 1N4934 or eq. (Hosfelt has these).

Adding to the previous message about polarity protection, the back EMF clamping diode will be forward biased if the coil voltage is reversed. It could drain the battery pretty fast and/or burn out the clamping diode. In my relayer pad box design, I have a diode in series with the input to protect against polarity reversals. I use a Schottky diode (150mA or more) because it has a lower forward diode drop than a standard silicon diode. This allows the max drive voltage to make it to the relay coil. There's already quite a bit of loss in the cable out to the pad, so it usually can't afford much more voltage drop. BTW, good old 14/3 orange power cords are cheap and have nice connectors already on them. :)

See:

-John

Okay, Professor, I defer. I just find, er, found the power diodes to be bulletproof, available, and cheap, if not exceptionally fast. :)

I guess it depends on the particular diode bridge. Typically power diodes have a slightly higher forward voltage drop, but you're correct that it's more pronounced at higher current. Even at relatively low current, a bridge always has two diodes conducting, so you'll get about

1.5 volts lost. A single diode would be better, but of course it wouldn't work if the polarity was reversed.

I've had some relays from the same batch kick in reliably and others not, using the same length of cable. As you say, it's best to have a solid voltage as close to the rating as possible when driving the relay. Using the rated coil resistance and the resistance per foot of the cable, it's pretty easy to get an idea of the loss. My solution used by MARS(NY) and WOOSH(WI) is to have a Schottky in series and a fast recovery in parallel across the coil. And use 22 or 24 AWG signal cable for the long drops, and orange power cords for shorter distribution (with my 3-wire scheme).

-John (hey, we're k>

Another tip is to use a multi-pole relay, with the poles in parallel. This effectively increases the current rating of the relay.

I prefer a double throw relay. I connect my continuity check to the other pole. Since I don't have a continuity button, having it operate all the time, touching the clips together is a good test for a shorted relay. Beep and it's OK. Spark and it's *NOT*!

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

You could also use a piezo buzzer across the battery input to alert you to reverse polarity. I don't think piezos are harmed by reverse polarity.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

Not true. The piezo's themselves aren't harmed, but all of the little piezo buzzers have a little 'drive circuit' in them that is DEFINITELY polarity-dependent.

David Erbas-White

For sensitive circuitry it will be too late by the time you hear the buzzer. A couple diodes are cheaper anyhow.

-John

I use the other pole, and also double-throw, to determine three states: test, armed, and stuck relay. A $2 PIC looks at these levels and produces unique tones (such as a nice "wooop! wooop!" alarm sound ;). I also input the level of the continuity and produce a test tone whose pitch is proportional to the continuity current... high pitch=good igniter. All this with no extra parts beyond the relay, the PIC and a handful of cheap parts. The WOOSH section just added a bunch of these pad boxes to their system based on my original analog design. Plug&Play replaceable. MARS has only one of them I left there at NYPOWER as a test case.

That link again:

A side affect of the 8-bit sampling and my crude firmware is that the darn thing sounds like on old Atari video game sometimes. ;) Get dirty clips or a flakey igniter and it sounds like Galaxian or Space Invaders. We'll call it a feature.

-John

I know I've reversed them with no harm. What I don't know is if they can take that for long term and still work.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

kaplow snipped-for-privacy@encompasserve.org.TRABoD (Bob Kaplow) wrote in news: snipped-for-privacy@eisner.encompasserve.org:

You could always use a small diode to protect the piezo from reversed polarity.