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How to use the same RELAY on 12V & 24V vehicles
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floobydust:

--- Quote from: JDW on August 09, 2019, 01:13:36 am ---... Again, the reason I even need a dual-switch relay is to break 2 signal wires coming from the PUSH Start button (switch) of a vehicle, which will result in starter immobilization....

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Have you considered that during cranking (12V) drops quite low and your relay might cut out or chatter. I will have to dig and get the exact info about the push-to-start, I thought it's just a digital input on an ECU.

A reminder 24V vehicle electrical systems, especially trucks, are brutal.  For 24V, the standard test is -600V, +200V spikes, so no off-the shelf Vreg can take it.
I no longer design for load dump as it's one specific failure mode (loose battery connection) and modern alternators use zener diodes for the rectifiers. It's also expensive additional circuitry, so I tell sales/marketing the product will not cover that beyond blowing a fuse.

I did use one of the National Semi (TI) SMPS IC's and they died and blew up in automotive use despite being designed for it. The problem was Vin dV/dt would make the buck converter latch up and short. Very few SMPS IC's now have this spec on their datasheet. It is a trap to avoid.
soldar:

--- Quote from: floobydust on August 09, 2019, 07:23:31 pm ---OT I don't think so, constant current does not give constant speed. Low cost cassette player motors have centrifugal switches for constant speed or a tachogenerator.
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I have never seen centrifugal switches and I don't even know how they could work because flutter would be terrible. The old, cheap cassette players that I ever saw just had a simple circuit like that.


--- Quote from: floobydust on August 09, 2019, 07:23:31 pm ---That circuit you post is a disaster - from 24-12V with a 0.5A coil?! 12W of heat total? It's a coffee warmer.
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It "burns" 6 W which is the con side. On the plus side it is extremely simple. I already said as much when I posted it so I don't understand your negative attitude. You are repeating what I said.
floobydust:
OT Cassette-player brushed DC motors use either internal centrifugal switches, voltage regulator, or a tachogenerator+transistor/IC for speed control. I worked in a repair shop and have seen 100's of them. The capstan flywheel deal's with flutter. The point is constant current control is not used in them.

6W is an awful lot of heat, just pick out a heatsink and you will see it needs to be huge. I haven't seen a relay with 24R coil and 0.5A coil current- for another 6W, it's pretty big too. So I thought it's a circuit that could work but the example's details are coffee warmer kind of hot and made me cringe. You could do it with one transistor as the MCU's output voltage is stable at 3.3V for CC control.

The car's pushbutton start somehow is tied into the keyfob RFID, so I still think it's a low current 12V digital signal. You would have to guess the switch's current rating and match that with a relay. Some people use a 4N35.

weird video cassette motor internals:
JDW:

--- Quote from: floobydust on August 09, 2019, 07:42:58 pm ---Have you considered that during cranking (12V) drops quite low and your relay might cut out or chatter.

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Yes, I have.  There is a very quick digital message that is sent across one or both of those wires  when you have the break held down and then you push that Push Start button, such that it would only take milliseconds to transfer. That data enables starting. In other words, that data will be passed completely before cranking even begins.  The purpose of the relay is to sever those lines (at times I specified with my MCU) to ensure that the starter enabler data is never passed and therefore no one could crank the engine.

 In other words, by the time the data is passed and you start cranking, it’s OK if the relay cuts out because the data has already been passed to enable cranking in the first place.
NiHaoMike:
Since it's a small signal, have you considered an opto MOS solid state relay?

--- Quote from: floobydust on August 09, 2019, 07:42:58 pm ---A reminder 24V vehicle electrical systems, especially trucks, are brutal.  For 24V, the standard test is -600V, +200V spikes, so no off-the shelf Vreg can take it.

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It's not that expensive to clamp those just like how kV level spikes are clamped on AC mains - use a MOV.
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