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Thread: Solid state AUX relay

  1. #11
    Senior Member Bitsyncmaster's Avatar
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    Location:  Leonardtown, MD

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    Just for your info. I'm measuring the conduction resistance so you can calculate the heating produced by any load current. I pump 5 amps through the load and measure the voltage drop in millivolts from the power pin to the load pin.

    Well it looks like those 85 amp lugs I used on the AUX relay really make a big difference in resistance (voltage drop). I tested three pined relays with the same setup and got the following.

    My solid state headlight relay resulted in 2.02 milli-ohms.

    A standard TYCO 30 amp relay resulted in 13.4 milli-ohms.

    A BEULER 40/60 amp relay resulted in 5.6 milli-ohms.

    My AUX relay resulted in 0.50 milli-ohms.

    So lets plug a 30 amp load onto each relay and compute the watts of heat it produces. This is not adding the heat produced by the coil power in standard relays which runs about 1 watt.

    Headlight relay = 1.818 watts
    TYCO relay = 12.06 watts
    BEULER relay = 5.04 watts
    My AUX relay = 0.45 watts
    Dave M vin 03572
    http://dm-eng.weebly.com/

  2. #12
    Senior Member Bitsyncmaster's Avatar
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    I got my 60 amp supply repaired and just ran the resistance check at 40 amps just to verify my old 5 amp check was correct. This also gives me more resolution.

    My AUX relay resulted in 0.4935 milli-ohms.

    So the resistance is resulting the same testing at 5 amps or higher amps.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  3. #13
    Senior Member mhanch's Avatar
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    So dave - this makes me think - is there any reason to proactively replace the current AUX relays with something more modern in general? Would that perform any better in the system or is it not really needed?

  4. #14
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by mhanch View Post
    So dave - this makes me think - is there any reason to proactively replace the current AUX relays with something more modern in general? Would that perform any better in the system or is it not really needed?
    This is just to reduce some heat generated in the relay compartment. The OEM relays don't fail much.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  5. #15
    Senior Member Bitsyncmaster's Avatar
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    I was doing some measurements of resistance on some circuit boards and I found the plug in relay contacts to be the most culprit to voltage drop (resistance). I was doing these test to decide how much a 4 oz copper saves compared to a 2 oz copper on my circuit board.

    So my previous posted results were not very accurate with the standard relay values. The AUX relay was accurate since I measured the voltage drop right on the bolt terminals. These are the correct values with a 20.0 amp load. These voltage drops were measured right at the base of the relays terminal, not on the mating connector terminal. The resulting resistance is just the voltage drop divided by the current.

    My solid state headlight relay resulted in 0.985 milli-ohms. My two MOSFETs = 0.50 milli-ohms so my circuit board is 0.485 milli-ohm.

    A standard TYCO 30 amp relay resulted in 2.55 milli-ohms.

    A BEULER 40/60 amp relay resulted in 2.54 milli-ohms.

    This also shows why your relay pins can overheat. Make sure they are clean and tight.
    Last edited by Bitsyncmaster; 12-09-2021 at 05:27 PM.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  6. #16
    Senior Member Bitsyncmaster's Avatar
    Join Date:  May 2011

    Location:  Leonardtown, MD

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    My VIN:    03572

    I get bored building my products so to take a break, I ran a contact resistance test on a standard 70 amp aux relay. This is a new SONG CHUAN part number 897-1AH-D1 70 amp rated at 14 volts.

    Interesting the cold start contacts dropped 0.0520 volts at 20 amps and the voltage dropped continually to 0.0293 volts where it settled at 12 minuets time. Since copper resistance increases when heated, I'm guessing the contacts are some alloy that resistance goes down when heated to cause that resistance drop.

    So anyway that relays best contact resistance is 1.465 milli/ohm. So my solid state AUX is three times better.

    Another interesting fact is that standard relay has bare copper battery and load pins but the control pins are normal tin plated. Maybe they found copper better for connector contacts.

    I turned the control voltage off and on a few times after that test and voltage drop ran around 0.04 volts so it does look like the resistance drops due to heating.

    Voltage was measured on the relay pins at the base of the relay so no connector drops affect my readings.
    Last edited by Bitsyncmaster; 01-03-2022 at 07:55 AM.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  7. #17
    Junior Member DeloreanJapan's Avatar
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    Location:  Japan . Tokyo

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    Red face

    Hi David Mckeen . I have some question about the Heavy duty relay

    Recently I've been have a issue with stock Hella Heavy Duty Accessory Relay , I bought the modern Heavy duty relay from Delorean Go.

    That I have never been use it until 2 years later now, recently I have a unknow issue of Car wont start . after checking the current and connection , I think its the

    stock Silver Hella relay have acting abit wired . So naturally I just change 2 relay, but the car just wont start . with ignition key turn for 1 clicked , I can here the magnetic coil been pulled and making that Clicking sound from relay , but no radio and center console power . turn the ignition key for 2 clicked, relay will make will make sound ,But starter wont crank .

    Maybe the relay I bought from Dgo was defective or the Spec of the relay was wrong ?
    its a 14VDC 70AMP 4 pin relay . both relay internal Ω was around 50~60 Ω and seems normal .

    In the end I just bang on the relay and fix the problem , in the end I switch both relay make the priority First for the ignition relay . since old one acting abit wired, so a dead radio can still make me drive to home .

    Sorry its a Long artical . But Im very hoping looking forward when you have resourse to make the Solid state relay when you ready .

    Best regards Peter

    PS: Dont forget I have a RPM relay returned to your hand, ill order something in future , so you can return it with the new bought items.

  8. #18
    Senior Member Bitsyncmaster's Avatar
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    The standard AUX relays are 70 amp rated units so I would guess you just got a defective replacement unit. Yes swapping the smaller control wires will swap what functions turn on with each key position. Good idea.
    Dave M vin 03572
    http://dm-eng.weebly.com/

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