Help With Schematic Diagram

[snub]

In the schematics below, there appears to be 2 diodes wired in parallel with the coil on this relay. I would have thought they would be wired in series. Just checking as I don't want to fry it.

[BaronJ]

Hi Snub,

The circuit is fine !  As shown the relays appear to have built in diodes, if you are using the ones specified then they must be wired with the correct polarity to the coil terminals.  Otherwise using relays without diodes, the coil polarity doesn't matter but any diode added must be the right way round.  All the diodes are doing is acting as spike suppressors.
 

[philf]

In the schematics below, there appears to be 2 diodes wired in parallel with the coil on this relay. I would have thought they would be wired in series. Just checking as I don't want to fry it.

Snub,

You mention "2 diodes wired in parallel with the coil on this relay".

There are two relays each with one freewheel diode! As Baron says these are there to suppress spikes thus protecting the contacts in the switches SW A & SW B.

Phil.

[snub]

Thanks for the replies. This is actually only 1 relay, it has dual coils. It is also very tiny so I don't know how they could find room for diodes inside. 17 mm x 14 mm x 13 mm. Actually, I can't figure how they got anything inside that tiny cube, yet it's rated at 20 amps. It.s the one on the right in the 3rd picture.

[snub]

Well, I screwed that up. I'll try again.
Here's a link to the relay information.  https://www.panasonic-electric-works.com/pew/eu/downloads/ds_61205_en_ct.pdf

[PK]

Whilst some relays do have diodes inside, most do not.

As Phil said the schematic shows external diodes fitted across the coils to prevent them generating large reverse voltages when the switches are opened and the magnetic field in the relay collapses.
Without the diodes these large voltages would generate arcs across the switch contacts and wear them out.
PK

[philf]

Hi Snub,

Just had a look at the data sheet. Even though they are in the same housing there are two independent relays. There doesn't look to be any diodes internally so you'd have to add them.

Phil.

[BaronJ]

Hi Snub,

Some one has incorrectly used the symbol for a relay with built in diodes on the circuit diagram that you posted. After looking at the data sheet there are no diodes built into that relay, though they could have been.  You will have to add diodes yourself, any 1N4007 will work.  The 1N4007 is rated at 1000 volts, 1 amp.

You mention how they could incorporate diodes into a housing of that size along with all the other bits, well the die size for common diodes are only a couple of mm in area, and they wouldn't have any packaging around them to increase the size either.

[snub]

Thanks for your help, all. Much appreciated.

[mexican jon]

On a relay with such a small coil I wouldn't worry about adding diodes  that is unless you are using something very delicate to fire the relay 

[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]

On a relay with such a small coil I wouldn't worry about adding diodes  that is unless you are using something very delicate to fire the relay 

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]

On a relay with such a small coil I wouldn't worry about adding diodes  that is unless you are using something very delicate to fire the relay 

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.

I would agree when you are talking about process control, PLC control etc. But look at nearly all conventional car type relays / solenoids  hardly any have anti spike diodes installed.

[BaronJ]

I would agree when you are talking about process control, PLC control etc. But look at nearly all conventional car type relays / solenoids  hardly any have anti spike diodes installed.

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   

[BaronJ]

Hi Andrew,  I 100% agree.

A couple of years ago I was asked if I could repair some Mercedes ECU/EMC boxes. Real swine to get into them, with pressed steel plate box construction with a roll sealed steel lid.  It took an age just to get the lid off.  Problem was that even after you got in there, the PCB was coated with a thick transparent epoxy.  Horrible thing !  You could measure a very low resistance across the power rails on the socket pins, you could even see the burnt shorted device through the coating.  I eventually rejected the units and gave them back.  At the time I think the cost of an exchange one was in the £1200 + region.  Heaven knows what they did with them when they went back.

[AdeV]

As others have mentioned, the diodes are generally known as "snubber" diodes, and they're used to prevent a spike of reverse-polarity power from being put onto the power rails by any kind of inductive load (typically, a relay coil, or motor winding). The reversed spike can cause significant damage to electronics, such as microcontrollers or the MOSFETs which are often used to switch things on & off under MCU control.