The Shop > Electronics & IC Programing
Help With Schematic Diagram
PekkaNF:
When it comes to car diagrams and componenets, I would not trust them. Numerous times the execution is fine but documents are inaccurate. Typically manufactures use 2-3 suppliers for each componenet or system.
One somewhat similar case I had with Citroen. Diagram did not show the diode, but the original relay had internal one. Another relay was externally similar to standard relay, but it was rated 40A. Maintenanace found it intermitent, replaced it with lower rating standard item. and it not only burned the contats, but it also melted the socket - nice when you have about two dozen relays on one backplate. Great - took pigtais to control a small relay that activated two stadard relays (because the circuit was a bit unusual).
Another case was door electric lock mechanism on Seat Cordoba. Original door locking mechanism had different microswitch and motor wiring than "original spare". Different manufacturer and the other one had a back lock that was designed to pretty much prevent removal of the door if the lock was not opened electronically - good deterrent to prevent resale of stolen car parts, bad if the door lock happend to jam locked - as the failure mode often is.
BaronJ:
--- Quote from: mexican jon on March 04, 2018, 03:25:34 PM ---On a relay with such a small coil I wouldn't worry about adding diodes :scratch: that is unless you are using something very delicate to fire the relay :thumbup:
--- End quote ---
This is bad advice ! You can easily generate hundreds of volts back emf from any inductor, whether it is a relay coil or a transformer. These voltages can cause irreparable damage to any electronic components that may be connected to the power rail.
Think "Kettering Ignition" ! OK the coil for those is wound to produce high voltages, but exactly the same principle applies.
Just as an aside for those that may be looking for a cheap method of making an ignition coil, a few dozen turns of wire round an existing relay coil winding, can be made to produce enough volts to fire a spark plug.
mexican jon:
--- Quote from: Baron on March 05, 2018, 07:42:56 AM ---
--- Quote from: mexican jon on March 04, 2018, 03:25:34 PM ---On a relay with such a small coil I wouldn't worry about adding diodes :scratch: that is unless you are using something very delicate to fire the relay :thumbup:
--- End quote ---
This is bad advice ! You can easily generate hundreds of volts back emf from any inductor, whether it is a relay coil or a transformer. These voltages can cause irreparable damage to any electronic components that may be connected to the power rail.
Think "Kettering Ignition" ! OK the coil for those is wound to produce high voltages, but exactly the same principle applies.
Just as an aside for those that may be looking for a cheap method of making an ignition coil, a few dozen turns of wire round an existing relay coil winding, can be made to produce enough volts to fire a spark plug.
--- End quote ---
I would agree when you are talking about process control, PLC control etc. But look at nearly all conventional car type relays / solenoids :scratch: hardly any have anti spike diodes installed. :thumbup:
BaronJ:
--- Quote from: mexican jon on March 04, 2018, 03:25:34 PM ---I would agree when you are talking about process control, PLC control etc. But look at nearly all conventional car type relays / solenoids :scratch: hardly any have anti spike diodes installed. :thumbup:
--- End quote ---
I wouldn't disagree that few automotive relays have protection diodes fitted ! However all car manufacturers go to great lengths to protect sensitive components from this sort of damage. Whilst adding expensive multiple diodes all over the place is not always done, or cost effective, a few TVR diodes are commonly used to suppress these voltage spikes, usually placed at points in the circuits that could be damaged.
Look up "Load Dump Protection". See what car manufacturers have to protect electronics from.
With regard to PLC's, I'm sure that you will find that their circuits are well protected from spike voltages, though their relay output contacts may not be. Its been a long time since I did anything with them, I don’t find programming much fun.
awemawson:
Knowing the price of replacement ECU's and the like in vehicles, and the totally irrelevant cost of a diode in my view it's a no brainer - put in the back emf diodes and sleep easy :scratch:
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