more slitcharge relay diode?

Following Oldsparks suggestions I have been planning to use the charge light wire of the (now working) alternator on my landrover discovery 300tdi to trigger a relay that will power a supplementary fusebox giving me a selection of charge switched positive supplies that can then go on to switch other relays that carry my split charges that are for the towed caravan trailer or the second accessories leisure battery in the truck when not towing. This came up when I expressed worry that the charge lamp wire would possibly struggle with triggering a bunch of relays,so using it to trigger just one would take care of that. The worry I have now,is that someone has put forward that I need to use a diode on that first relay to protect the alternator and I dont know what diode to use or where to attach it across the terminals. I have looked through the relays pages and diagrams and am none the wiser....mainly due to my own stupidity no doubt,but any ideas would go down fine.

Check the Diodes page? Diodes It has a list of commonly used diodes. If you look through the starter kill relay diagrams, you will see the diode attached across terminals 85 and 86 with the band near the positive (86) terminal. Starter Kill

Glad you figured that the first relay can trigger all the others. It is so obvious - in RETROSPECT! That is the way to extend any isolator for higher load current rating, or to trigger more relays (ie, one bigger or several other isolator circuits). (And of course each circuit has a fuse or beaker at EACH end - ie, next to EACH battery.)    FYI: The master isolator-relay (powered by the alternator - actually its [internal or external] voltage regulator) should be as small as possible. How small? Small enough so the power its coil takes doesn't overload or blow the regulator (alternator D+ or L output). How small is that? Old mechanical (old external) regulators could handle a few Amps, but modern solid-state (electronic) regulators may only be able to supply some mA though many can handle typical 60Ohm or higher (250mA or lower) coil currents. Nearly all can sink (accept/ground) from at least one if not several 3W/250mA dash lights.    PS - a spike protection diode may not be required - especially in systems that use relays to control fuel pumps, fuel cutouts, and other engine controls (mainly fuel on carby vehicles; some auto-chokes etc) as they usually power relays too. [ Yes - the UIBI aka charge-controlled battery isolator is an additional relay load to such OEM relays. The OEM(s) can be "slaved" off the UIBI, but with diode isolation and a 3rd diode to isolate other OEM signals - like +12V cranking (to close relay to pump or pass fuel) to allow starting/priming before the alternator charges. ] HOWEVER - there is no harm including a spike protection.      In fact my L-circuit has blown (but the alt still charges fine) and I cannot yet say is was not damaged by a spike - I have no protection (please don't judge me Howard - I am more responsible elsewhere!!). (I did assume the relay-load was big, else coincidental failure - but now YOU have me wondering... Another damned 12volt Rookies - a mere 26 posts and already reminding me how I once used to think! (Thanks for the reminder!))

Thanks guys,all handy to know. re: Rookie Status - after 30 plus years in the heavy end of vehicle work and growing up on a farm with a trucking business,having worked as fleet maintainance on vehicles from car based van all the way up to maximum gross,I have finally learned that I know very little for certain and that it is always best to risk the scorn and ask.....you may only get told what you already know,viz: (in my case at least)that one is an idiot), but at least you get a second opinion ! So...just on the offchance,for safeties sake, a diode across 85 and 86 with anode to 86 on a minature mA relay (what can I rob one of those out of ?) to run a fusebox of charge triggered supplies. The diode to be calculated from the link given. Yes? And thanks for the reply,awdeclipse....yes,I had checked the diode page and done several searches first. I am no stranger to forum etiquette so should maybe have mentioned that.And the fact that I didnt find any definites and wanted some practical experience answers to back up theory lead to me asking for advice.The rights and wrongs of that,well,see above

KATHODE to 86 - that's the line-end and assuming 86 os more +ve than 85 as is the convention. It's not the anode! That will connect (short) 86 to 85 and blow the control signal (alternator, alarm) or the diode. Remember - Cathode = Kathode where K is the line end as per its symbol --K-- or --|<--. Farm AND trucking - yes, the makings of a thinker.

...and thats why I need to check !

oldspark wrote: Glad you figured that the first relay can trigger all the others. It is so obvious - in RETROSPECT! That is the way to extend any isolator for higher load current rating, or to trigger more relays (ie, one bigger or several other isolator circuits). (And of course each circuit has a fuse or beaker at EACH end - ie, next to EACH battery.)    FYI: The master isolator-relay (powered by the alternator - actually its [internal or external] voltage regulator) should be as small as possible. How small? Small enough so the power its coil takes doesn't overload or blow the regulator (alternator D+ or L output). How small is that? Old mechanical (old external) regulators could handle a few Amps, but modern solid-state (electronic) regulators may only be able to supply some mA though many can handle typical 60Ohm or higher (250mA or lower) coil currents. Nearly all can sink (accept/ground) from at least one if not several 3W/250mA dash lights.    PS - a spike protection diode may not be required - especially in systems that use relays to control fuel pumps, fuel cutouts, and other engine controls (mainly fuel on carby vehicles; some auto-chokes etc) as they usually power relays too. [ Yes - the UIBI aka charge-controlled battery isolator is an additional relay load to such OEM relays. The OEM(s) can be "slaved" off the UIBI, but with diode isolation and a 3rd diode to isolate other OEM signals - like +12V cranking (to close relay to pump or pass fuel) to allow starting/priming before the alternator charges. ] HOWEVER - there is no harm including a spike protection.      In fact my L-circuit has blown (but the alt still charges fine) and I cannot yet say is was not damaged by a spike - I have no protection (please don't judge me Howard - I am more responsible elsewhere!!). (I did assume the relay-load was big, else coincidental failure - but now YOU have me wondering... Another damned 12volt Rookies - a mere 26 posts and already reminding me how I once used to think! (Thanks for the reminder!))

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