Bob - 95 993/C2

<a href="http://www.miataforum.com/bobh/993_dme_relay_dissected.htm" target="_blank">Link to the Pictures</a>

Randall G.

Thanks Bob ... interesting photos. I'll be saving them to my archives.

FWIW, the 944.* relay in my '91 never failed in 112k miles. I recently replaced it with the updated 993 unit, keeping the old 944 part as an emergency spare.

Robert Henriksen

I have to confess here... what the heck IS a relay, anyway, and what's the purpose?

I know 'wire'... 'fuse'... 'battery'... 'shocking the @#%@#%@#% out of myself on 120v'... but don't for the life of me know what a relay does. Pass the baton?

Jeff 993TT

I believe what a relay does is pretty much like a switch.

For example, take your ignition and starter. If all the current to the starter was passing through your ignition switch, then it would have to be have to be a pretty beefy ignition switch, able to handle several hundered AMPS. The key would probably get pretty hot while you were cranking the car. Not to mention, if there was a short of some type, you might be SOL.

What a relay does is take a smaller current and operates a switch for a (usually) larger current. So the current thorugh your ignition switch would be as small as necessary to operate the starter relay. The starter relay in turn would then allow the big juice through to the starter.

When driving on the freeway at speed, I've seen cars with their brakelights in the bright position ( as if the brakes were being depressed ) and I've speculated it was probably a stuck relay somewhere.

Hopefully, some others with more electronics experiece can also expand on this?

As far as being electically shocked, I've been pretty fortunate not to have it happen to me yet. However, I remember a physics friend tell me that if you just use one hand, you'll be ok. As long as you don't complete a circuit through your heart ( ie while using two hands. )


Jeff

Randall G.

&gt;As long as you don't complete a circuit through your heart ( ie while using two hands).

Actually, this is true--to a point, I suppose. Keeping one hand in (for example) a pocket means there's less chance of current flowing across your chest and heart. That said, working at a utility, I've seen photos of lineman injured while working on high voltage lines. Pretty gorey stuff .... big entry and exit wounds, internal injuries, etc. But, again, we're talking lots of volts here.

Your sensivity to shock is directly proportional to your resistance. So ... if you've just jumped out of the shower (or are in the shower!), or are very sweaty, or are standing in a puddle of water, you're much more suceptible to electrical shock. It's the current ... lower resistance = greater current. I once chased my girlfriend around right after she got out of the shower, DMM in hand, just to prove the point.

It's actually amazing how little current it takes to kill ... in the milliamps range. Fortunately, your (dry) skin typcally has a resistance of ~1 Mohm, so if you shock yourself with 12V, you're only talking 12V/1e6 ohms = 0.012 milliamps.

A.J. - 95 993

I have been in the process of finishing my basement, and did everything myself. All for the first time, so I was learning as I went (unfortunately, to the detriment of my 993 projects). I was rightfully very respectful of the 110V in the wires that I was running, especially when installing the sockets/light switches. I have accidentally touched hot to ground, and sparks WILL fly, and locally, metal will melt VERY quickly. But as mentioned, that's because metal is a very good conductor. I have also shocked myself and gotten zapped. <img src="graemlins/c.gif" border="0" alt="[ouch]" /> Let me qualify this to say that this not to be taken lightly, and is terribly dangerous. But under the conditions, the buzz was far less than I expected. And wasn't painful, just startling. But I was on a wooden ladder which was on carpet, or just standing on carpet, with rubber flip-flops on. It seems like I have gotten shocked across my body, too. I didn't die, no burns. That goes back to the resistance that Randall mentioned. Your body is an electrical device, is well insulated, and can be recorded with a volt meter. If I had an open cut on my finger, that resistance would have been significantly reduced. If I was on my aluminum ladder, worse yet. Throw in a little water, and you are in a very dangerous situation. You can lick the terminals on a 9V battery, but you wouldn't dream about licking a 12V car battery. It's not the voltage that's deadly. To end my off topic diversion, let me say that I still go far out of my way and take lots of extra time to make sure the power is turned off at the electrical panel or by a switch somewhere before I will do simple electrical work. Big risks usually start off as little risks.

Randall G.

As long as we're on the subject, suppose I'll add a little more .... <img src="graemlins/yltype.gif" border="0" alt="[typing]" />

While it's the current that kills, high voltage is actually more dangerous than low voltage:

I = V/R

So, either increasing the voltage (V) or lowering the resistance (R) will increase the current (I).

Given household 110VAC with dry skin, you'll only shock yourself with 0.11 milliamps. Not dangerous, as A.J.'s experience will attest. Now, drop your body's resistance to about 1000 ohms, and the current rises to 110 milliamps--enough to kill. In Europe, with their 230VAC, they only need to drop their body's resistance to about 2000 ohms to kill. So, one could say their household electricity is twice as dangerous as that in the good old U.S. of A.

E. J. - 993 Alumni

I knew I should have stayed awake in physics class. It is physics right <img src="graemlins/c.gif" border="0" alt="[ouch]" />

E. J.

Robert Henriksen

Well, it sure sounds like I'm a lucky son-of-a-gun. I was moving a light fixture in the shop/factory where I was working at age ~22. I THOUGHT I'd turned off the right circuit at the breaker box <img src="graemlins/oops.gif" border="0" alt="[oops]" />

I was standing on a workbench, about 3.5' above a bare concrete floor. Was pulling the Romex wiring out of the metal light fixture, and took 120v from one hand to the other.

I 'danced' on that for a second or so, and managed to wrench away & went pitching over backwards, head-first for the concrete floor a LOONG way away. (3.5' plus another 6' of my own height) I managed to put a hand out above my head, and took my entire body weight on that one hand breaking my fall. It's amazing I didn't break my wrist! As it was, it was pretty sore for a couple of days.

One of those things I look back on, and shake my head <img src="graemlins/c.gif" border="0" alt="[ouch]" /> <img src="graemlins/oops.gif" border="0" alt="[oops]" />

superpat

<a href="http://www.howstuffworks.com/relay.htm" target="_blank">Here's how a relay works.</a>

As a teen, I once hooked up an electric fire in the garage. on of the ones with a wire wrapped round the 'bar'. I wanted to see if it was heating, so I put my hand through the guard (duh! <img src="graemlins/c.gif" border="0" alt="[ouch]" /> ) and touched the bar...

POW!!!

240v (I'm in the UK) through my hand - luckily it was earthed on the fire guard... No damage, but I certainly jumped!!!

Pat