My Official OM617 Toyota Pickup Repower Thread

[funola]

Looks good.

Originally Posted by mach4

Ok, like so?

[OM617YOTA]

Mach4-

PM me your address, I'll drop a few diodes in an envelope and send them over.

Funola-

Thank you for all your help! I'll be adding diodes as soon as the new relays get here.

[Cadin]

Looks like I'll be going this path as well. Thanks for the research!

[Stretch]

Originally Posted by funola

You want diodes across the relay and vac pump motor to protect the vac switch and relay switching contacts.

Even this simple electrickery has me confused.

To me a diode is a non return valve - how can a non return valve in parallel help with anything? Assuming the flow is from positive to negative - what would come up from the earth that would need to flow the opposite way?

[mach4]

Originally Posted by Stretch

Even this simple electrickery has me confused.

I had the same feeling. I did a quick Google search and found this that helps make sense...sort of.

Here is a more accurate description. When the +5V is removed, the coil's current cannot change instantaneously (since it is highly inductive). The collapsing magnetic field around the coil continues to force a current the slowly decays towards 0.

So if the coil current was 1A when +5V is applied, it will continue to be 1A immediately after +5V is removed (and decaying to 0A eventually).

The question is, with the +5V removed, where is that current going? If there is no "easy" path for the current to flow (i.e., terminal B as before) then it will "find a path" which can involve burning through the insulation on the coil's wiring, etc. and this is accompanied by a high voltage because the current decay is very fast (from 1A to 0A as fast as possible). The high voltage is a byproduct of the nearly-instantaneous change in current (towards 0A) because there is just no easy path for the current to follow at this point.

Now with the diode there, we are providing an "easy" path for the current to follow that does not involve burning the coil wire insulation. So current flows from one terminal of the coil (terminal A), through the diode, then back to the coil at terminal B. The voltage at terminal A is therefore limited to ~0.7V higher than terminal B and high voltages are avoided.

From this site - Why do I need a diode over the coil contacts on a relay ? - Arduino Forum

[funola]

Originally Posted by Stretch

Even this simple electrickery has me confused.

To me a diode is a non return valve - how can a non return valve in parallel help with anything? Assuming the flow is from positive to negative - what would come up from the earth that would need to flow the opposite way?

Here's a good paper on the subject. It shows a suppressing diode across the relay or the switch. It is more common to have it across the relay.

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCsQFjAA&url=http%3A%2F%2Fwww.te.com%2Fcommerce%2FDocumentDelivery%2FDDEController%3FAction%3Dsrchrtrv%26DocN m%3D13C3311_AppNote%26DocType%3DCS%26DocLang%3DEN&ei=RkCrUuKZGMipsQTb6YHIDw&usg=AFQjCNFh9HS5tLUuL8CFri4nlwb6sksRlg&bvm=bv.57967247,d.cWc&cad=rja

[funola]

Just noticed an important item missing. A FUSE! Put one as close to the battery as possible.

Originally Posted by mach4

Ok, like so?

[funola]

Originally Posted by funola

Cool! Can you help me find a data sheet for an IC in an old VW diesel cluster if i send you a p/n? I've come up empty

edit: iircc 1N4001 through1N4004 all have the same amp rating of 1 A. difference is in the prv (peak reverse) voltage. 4001 has prv of 1000 V, 4004 is 4000 V. Since we are working with 12 V, any will do.

ps. you'll know when the diode is hooked up wrong, it may blow up like a fire cracker. Get a few in case that happens. no harm done other than some excitement.

I take the "Since we are working with 12 V, any will do" statement back. The kickback voltage get go above 1000 V depending on the number of turns of the coil. So it wouldn't hurt to use the higher voltage diodes.

[Stretch]

Thanks for the explanation guys - and thank you google!

[funola]

Come to think of it, automotive systems do not generally use clamping diodes on relays and rely on switches to absorb the kickback voltage. Clamping diodes are always used in relay circuits with solid state switches which are more fragile. So my recommendation for a diode in this instance is unwarranted. Sorry for misleading.

[mach4]

Originally Posted by funola

Come to think of it, automotive systems do not generally use clamping diodes on relays and rely on switches to absorb the kickback voltage. Clamping diodes are always used in relay circuits with solid state switches which are more fragile. So my recommendation for a diode in this instance is unwarranted. Sorry for misleading.

Ok, so here is our final configuration...

[funola]

Originally Posted by mach4

Ok, so here is our final configuration...

Why not fuse the vacuum switch also (same fuse)?

[mach4]

Originally Posted by funola

Why not fuse the vacuum switch also (same fuse)?

I was assuming that that would come off a fused source from the car and not need a supplemental fuse. But, the point is well taken that that source should be fused one way or the other.

[OM617YOTA]

Either way, clamping diodes are very little trouble and very little cost and they won't hurt anything at all.

[OM617YOTA]

Got the upgraded relay in place for the vacuum pump. Functions exactly like the previous one - I definitely need an adjustable vacuum switch.

Had an odd encounter at the steel yard yesterday. Dude at the steel yard asked "Why don't you get a real truck?"

I asked if he insults all his customers, then got in and drove away without buying anything.