Component Identification

OK, fuse may have been a broad brush, PTC is more correct, hence why it is not "blown".

19th down in the list on the first page.
 

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I replaced the blown Zener with a 1N4747 & the circuit powers up as it should.
Nigel mentioned that this Zener was for over voltage protection.

Am I correct in thinking that the only way the an over voltage can occur to blow the original Zener is from inductive spikes from the relay coils.
I see no other way that an over voltage situation can occur?

I don't want to connect this circuit board back into the machine if this can happen again without attempting to fix the cause of the issue.
Maybe I am looking at it the wrong way as well?

Like I mentioned in a previous post that other similar boards suffer from the same problem as pictured before.
 

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There's no back emf protection shown in your diagram, so if that is the case, it could have been the cause although I would have thought it more likely to take out what ever is driving the relays.
 
To be honest I am not entirely sure of what I should do & require some advice, I don't want to create issues for the receiver circuits in any way.
All I know is that if an over voltage Zener diode gets damaged then obviously there was something that caused it, inductive spikes are all I can think of.
 
OK.

First up, can you trace what drives pin 85 of the relays and draw itout?

Reason - there may be some protection for spikes at that point, but you need to check before adding anything else.
 
Isn't this an in car board?, you get massive spikes on the power rails in a car, hence the protective measures.
 

augustinetez​

Terry,

I thought about your suggestion that the driving end of the relay would be more likely to have the transient spikes & in reality the receiver circuits open collector output would be the first things to suffer damage which is not the case.

Does this make any sense if not don't hesitate to say so, I am trying to learn more?

Today I refitted the circuit board to the machine & it is operating just fine with no issues on both channels.

I then realized that the relays are switching individual inductive loads.
This being the case maybe the fast switch off time of the relay would cause a very large transient spike back from the inductive load through the relay contacts or arc across them which would send the high voltage spike directly back to the 12v rail on the circuit board & onto the 20V Zener diode & it would blow out.

There appears to be no suppression method used for the load.
The switch off time for the relay is 10 milliseconds, I am unsure of the switch off time of the load & I neglected to measure the Resistance & Inductance to try to work it out, which I will have to do.

Could this possibly be the cause of the issue?
 
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I don't think inductive spikes from the inductance of the relay coil would cause an over voltage at the zener diode. When the current trough the coil is broken pin 85 will try to go positive and pin 86 will try to go negative. The energy stored in the relay coil would not be enough to produce a significant change in the voltage across C1 and it would try to reduce the voltage across C1. The inductive spike would be more likely to damage the driver device connected to pin 85 of the relay coil.

Les.
 
Thanks Les, could you have a look at post #28 & see what you think,
I posted it just before your reply.
 
I wouldn't worry about it, it could have been a mains transient or anything, and replacing the zener with a 1W one will mean it's less likely to fail for a similar reason in the future.

I presumed it was a car device, because of the relay numbers (85 etc.)
 
Re post #28 IF the relays are suitably rated for the motors that they are driving then there should be enough isolation between the contacts and the coil. The way the contacts are numbered suggects that they are automotive type relays. (I suspect the unit may be designed to remotly control a winch on a truck.) What is the voltage and current rating of the motors that the unit is driving ?

Les.
 
No mains involved, just a 12v car battery with no battery charging system attached.
I understand that it doesn't appear to be much of a problem but when you have about 300mtrs of hose out down shear drops in extreme heat & you need the spool to retract under power to aid people scrambling back up with the hose & it doesn't work because of a 10 cent diode, boy it's a real issue then & can be life threatening.
Hence the persistence to make sure it never happens again.
 
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I should have mentioned that the relays are driving magnetic clutches very similar to car aircon clutches, so hold down remote button switch on clutch engaged, release remote button switch off disengage. They look exactly like car aircon clutches with mounting brackets attached.
12v & approx 3 to 4 Amps each at a guess, I haven't measured the current draw as yet.
I haven't measured Resistance or Inductance of them as yet.
The clutches engage to allow drive of two large hose reels when required, individually one per channel, two channels, two remotes & two receivers.
The clutches can be switched on & off many times per minute in some situations, on the datasheet for the relays it mentions that the maximum switching rate is 6 times per minute, in real use these can be switched more than double that amount depending on the terrain they are used in.
 
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Page 5 of the MCR datasheet shows what is missing - the diode across the inductive load - in this case across terminals 85 & 86 of the relays.

I would be adding those but use 1N4004 or better diodes instead of 1N4148's.

As per Nigel's suggestion, a 1W zener instead of the original would be a good idea.

They sound like hefty magnetic clutches, so possible they can be putting some big spikes on the supply line.

Rigorous testing would be a good idea after repairing this one with a load 2-3 times what is normally expected.
 

Then leave the zener out, it's not doing anything if there's no mains or charging involved, the board is presumably intended to be mounted on a vehicle?.

Or just fit a 1W zener?.
 
You could add diodes in parallel with the magnetic clutches. They need to be rated to be able to handle the current taken by the clutch for a few hundred miliseconds. This will make the relay contacts last longer and get rid of any spikes.

Les.
 
Yes, page 5 of MCR, I found that a little confusing when I first read the datasheet because the same company who supply the MCR are the same company who designed & manufactured this circuit board & they did not add those diodes in the design?
It made me question why & made me unsure?

I will do some testing over the next week or so & give some feedback or ask for more help.
I was unable to use my oscilloscope where the truck was located with no mains connection but I will have it here in a few days & I'll check it out with the scope.
 
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dogman said:
they did not add those diodes in the design?
It made me question why & made me unsure?
Click to expand...
Possibly because of the many different uses the control pcb might be put to and they just forgot their own advice, wouldn't be the first time.

I'm assuming from your comments re hoses these are for hose control reels on a fire truck.
 
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