Thread: An efficient 100Amp low voltage cut out?

Not entirely sure but likely to be found behind the instrument cluster mounted on the bulkhead in a 90/110 with heated rear window on the old style door (pre TD5).

Just tested mine to see if works, it puts out a negative trigger when fed >=13.7v (adjustable), another relay handles the load.

Originally Posted by manic

Ha, I knew I was missing something!
The one you linked looks like it takes a 3amp pulse to switch the contact. All other times 10mA. That would work!
Seeing as I already have low voltage detection at 4mA , looks like I only need a simple latching relay. Cheers!

Excellent solution.

I always use heavy mechanical relays for charging dual batteries etc as they have virtually nil voltage drop across the mechanical contacts, and the coil consumption doesn’t matter when the engine’s running. I use latching relays in 240vac work, but it never occurred to me to use them in 12vdc auto work. Wotta silly snag.



CAUTION THOUGH:
Coil operated mechanical relays produce a huge high current, high voltage spike across their coils in opposite direction to the supply when they’re turned off, which will let the smoke out of amps, radios, and other solid state devices. This spike is generated by the collapsing magnetic field cutting across the coil wires, just like any other electrical generator. This highly destructive ‘back EMF’ (ElectroMotive Force) is a basic consideration in relay circuits and must be suppressed at the relay (with a diode connected in reverse to the coil operating voltage). General grade relays will have this diode suppression built in, but low quality or special purpose ones often don’t. I often connect one if I’m not sure, or if I want to ensure protection of horribly expensive devices.

You can easily fit a standard 1/2/6 Amp rectifier diode across the terminals, thus:
(Edit2: ignore dots, the app thingy doesn’t like blank spaces)



..................<<<<<<< direction of back EMF <<<<<<

.......................................rectifier diode
......................................———|<â €”——-
.....................................| ...................|
.....................................| + relaycoil — |
Coil (+ve) >>>>>>>———UUUUUUUU————————†” >>>>>>> (batt/chassis -ve)

Supply (+) ———————— contacts (NO) ———————————————— > Load (+ve)

(Commonly available: IN4007, IN5408, 6A10 etc)



Another thought though:
Solid State (SS) relays are getting better, with the development and use of MOSFETs rather than older bipolar type transistors. I prefer and often use MOSFET switches for controlling low or high current devices, or when the original relays fail. Easy to build yourself if you’re handy with circuits and a soldering iron (about $5 instead of maybe $45 OEM relay).

But good quality commercial solid state dropout relays use MOSFETs instead of the older Bipolar Transistors. Last time I looked, I think Australia’s own Redarc and GSL (& others) sell MOSFET low voltage dropout units. They don’t mention MOSFETs but you can tell by the absence of huge heatsinks, and the low current/power loss in the specs.

MOSFETs just need a voltage present to hold them on, with virtually no current needed (like a max of maybe 100 microamperes, or 0.0001A or 0.1mA).
Bipolars need a significant current to hold them on (let’s say about 1/10th the load at high amperage).

So older bipolar transistors in older or cheaper SS relays might burn something like (1 Volt x 25 Amps =) 25 watts across their switch terminals. That’s gunna need a biiig heatsink to dissipate all that. Even at parked Audio and Comms current, like say 2 Amps, they’ll burn a minimum of about (0.6v x 2A =) 1.2 watts (or about 1.2/12 =) 0.1A or 100mA plus their operating current. Use your UHF or run your Audio, then you might increase that to about 5Amps, so 5A x *0.8V = 4W [4/12] = 0.33A) or 330mA (336mA incl op current) just to keep the circuit turned on. *bipolar transistors drop more voltage as their load current increases.


So good quality solid state switches now usually use MOSFETs, which lose minuscule power.
A durable, proven, purposed automotive MOSFET will drop very little voltage (maybe 0.01V) across them at high amps (like 50A), losing something like (0.01 x 50 = 0.5 watts). That’s why they don’t need or use large heatsinks. 5A @ 12V will burn around (5 x 0.01 =) 0.05Watts or (.05W/12V =) 4.2mA

(and I wonder if the latching relays use a mosfet circuit and driver or output switch, which could explain the low 4mA draw ?)

Anyway, hope that helps.


Edit: ps, I think the high power (50A) bus to the front and rear outlets is a very sensible idea even for quite low power devices; it makes for much lower voltage drops, so your gear works properly. Thin wires have significantly higher resistance than thick wires, under load.

cheers

I know I'm late to the party but if you need something a bit bigger than the ZEVA unit you've ordered you could try this:

Victron BatteryProtect 12/24V-100A

According to their specifications the unit has an active consumption of 1.5mA and a draw of 0.6mA when in shutdown/low mode.

Originally Posted by manic

Not entirely sure but likely to be found behind the instrument cluster mounted on the bulkhead in a 90/110 with heated rear window on the old style door (pre TD5).

Just tested mine to see if works, it puts out a negative trigger when fed >=13.7v (adjustable), another relay handles the load.

If it puts out a negative trigger (ie, switches to ground), then it’s virtually certainly a MOSFET device, as the most simple and reliable ones need the switch or ‘gate’ terminal to be made more +ve than the ‘source’ terminal (-ve terminal, where all the little electrons are waiting madly to rush up to meet the positive side) to switch on.

For the OP, just in case (I think I’m probably teaching you how to suck eggs though): when using a ‘negatively switched’ relay output device like these later model Landy ones, you connect the negative wire of your (relay or other) load to the ‘in’ or ‘sw’ terminal of the relay, and the ‘out’ or ‘load’ terminal to ground.

ps. In hindsight, these SS relays are in quite a few, if not all later models. Certainly my D1 and beyond. And if they can switch the currents in a window heating circuit, they can happily handle maybe 30A or so. You might be able to use two for a 60A nominal circuit supply - just connect the input/output of the first one (? via relay) to the sense input of the second one. You can’t connect them in parallel, as they’ll have slightly different trigger voltages (no matter how carefully you calibrate), so the first one to switch would carry or drop all the load, and change the supply voltage, which would effect the sensing (and is at this very moment confusing my brain) ... blah blah blah ...

Conclusion: for 30A (or as rated) max loads, use the existing unused seat heater, aircon fan or window heater solid state relay, as the relay, wires, fuses are already connected and make for a sturdy & very neat job. For higher currents or lower voltage drops, using one of the units mentioned by the other guys would be simpler and less messy.

Cheers

Originally Posted by DrDeath

CAUTION THOUGH:
Coil operated mechanical relays produce a huge high current, high voltage spike across their coils in opposite direction to the supply when they’re turned off, which will let the smoke out of amps, radios, and other solid state devices. This spike is generated by the collapsing magnetic field cutting across the coil wires, just like any other electrical generator. This highly destructive ‘back EMF’ (ElectroMotive Force) is a basic consideration in relay circuits and must be suppressed at the relay (with a diode connected in reverse to the coil operating voltage). General grade relays will have this diode suppression built in, but low quality or special purpose ones often don’t. I often connect one if I’m not sure, or if I want to ensure protection of horribly expensive devices.

This only applies if you are adding some of your own unprotected electronics, or if the final product is going to be sold in the EU.

Every single electronic device, not just in vehicles but in commercial and domestic use, be it 12v or 240VAC, already have spike protection built in.

And a perfect example is the fact that most relays do not have any form of spike protection. They did for a short time in the mid 80s and early 90s but no one bothers now.

Originally Posted by DrDeath

ps. In hindsight, these SS relays are in quite a few, if not all later models. Certainly my D1 and beyond. And if they can switch the currents in a window heating circuit, they can happily handle maybe 30A or so.

Did you check out the link I posted that shows the internals of the Land Rover 'VSR' Im refering to?

Heres an image
https://fcache1.pakwheels.com/origin...d60ecf8a3d.jpg

When I tested it, 13.7v went in , ~12.5v came out!

I think if it was used to supply 30A it would have been binned a long time ago. Thankfully LR did not use it to directly supply the current to the rear demister which is likely just under 20A. The one found in your 300tdi, whats the part number, does it supply the amps?

Originally Posted by DrDeath

You can easily fit a standard 1/2/6 Amp rectifier diode across the terminals, thus:
(Edit2: ignore dots, the app thingy doesn’t like blank spaces)

(Commonly available: IN4007, IN5408, 6A10 etc)

Diodes - good point! My rear demister circuit has a diode between the VS relay and the load switching relay, which suggests the VS relay does not have inbuilt protection from fly-back voltage spikes.

I have now found a separate unprotected output in the wiring loom for the VS relay which I will use to reset my Low Voltage Disconnect AND toggle a 100amp SPDT relay. I'm trying to figure out what I need to suppress flyback on a 100amp SPDT. Would a single IN4007/IN5408/6A10 across the coil be sufficient?

For those who might be interested to know: The spare output from the VS trigger was found on a three pronged plug located on the LHS of the bulkhead (near heater), the wire is black with a silver/grey/slate stripe. I believe it's intended use was to supply a factory split charge system on pre TD5 defender/county

Reading some of the BS and caustic responses above reminds me of why I don't frequent this forum as often as I used to.....