Replacing 1930's relays

Discussion in 'The Projects Forum' started by offthegridshed, Aug 24, 2015.

  1. offthegridshed

    Thread Starter New Member

    Aug 24, 2015
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    I'm working on a 1930 32v lighting plant that was designed to do the following:
    Provide dc power from a 32v battery bank to a lighting circuit in a house/shed with or without the generator running as needed.
    Auto start the generator engine using the generator windings when current drawn exceeded 7.5 amps (4 lights or more turned on) and remain on until either the load was reduced/removed and/or the battery bank was fully charged.

    These functions were all completed using one or both of two reasonably sized relays (I don't know for sure as there is no documentation available) which I don't have.
    I will tackle each of the functions on the project one at a time and to start with, need help designing something SIMPLE (I'd actually prefer to put relays back, but where would you find 32v relays these days? ) to do the current monitoring task (and therefore switch on the starter circuit).

    I guess I'm going to have to drop the system monitoring voltage down to something modern components can handle and then use a contactor of some sort to switch the higher voltages.

    My battery bank is a 32v bank of 2v 1000ah off grid batteries. The current idle voltage of the bank is 33.6v and one person suggested using a voltage comparator, but I'd rather monitor the current load as it would take some time for the voltage of the bank to drop any significant amount, I. E, I'd prefer frequent top ups of the bank rather than longer charge cycles.

    Hope I have given enough info?

    What do you guys recommend?

    Cheers
    Chris
     
  2. TheButtonThief

    Active Member

    Feb 26, 2011
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    Have you considered using a shunt resistor to monitor the load?
     
  3. offthegridshed

    Thread Starter New Member

    Aug 24, 2015
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    That had been recommended, but what is actually monitoring the output of the shunt? Do I try to find one that matches the input requirements of a standard mechanical relay? Aren't most shunts in the mv range?
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    If those relays are built how I think they are then one will have a very large gauge wire in SERIES with your outgoing supply wire. It is using the current flow as a magnetic trip switch, such that small current draws do not activate it, but a large current flow will create a stronger magnetic attraction to pull in the relay contacts. Careful examination would reveal if this is in fact how it is wired
     
  5. offthegridshed

    Thread Starter New Member

    Aug 24, 2015
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    I don't have them, they were removed years ago, but I do have pictures of what was fitted as standard and that sounds like how they look to have worked (if that makes sense).
    The question is what to replace them with...
     
  6. Kermit2

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    Feb 5, 2010
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    http://www.crmagnetics.com/relays/cr5395
    would be a possible choice. You may have to get creative about powering the relay. The one I found in a 2 minute search requires Household AC or 24 volt DC to operate.
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    A 32V relay is a 24V relay with a resistor in-series with its coil. Get the coil resistance of the 24V relay by measurement or from its data sheet. Calculate a series resistor that keeps the 24V relay coil current the same at 32V.

    Now if you want a voltage detector that trips a relay at 33.6V+-0.1V, I can help with a circuit for that. If you want a current detector that trips at 7.5A+-0.1A, I can help with that, too.
     
    Last edited: Aug 24, 2015
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Kermit's suggestion is a prepackaged version of what many would recommend to replace a current-sensing relay coil: Current sensor (hall effect or resistive shunt), amplifier and conditioner, driver for a conventional relay, power supply and packaging. There is a chance that the resistance of the original relay coil was counted on for proper operation of some other aspect of the control system, but probably not. If you knew the resistance of the original coil, than scaling a circuit to use a shunt of the same resistance would be a good plan.

    ak
     
  9. offthegridshed

    Thread Starter New Member

    Aug 24, 2015
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    [​IMG]Thanks for all your replies.
    Mike I would be very interested in your circuit ideas if you could contact me.

    Cheers
    Chris
     
  10. MikeML

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    Oct 2, 2009
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    As long as we do it here...

    Let's do the current sensor first.

    You want a relay closure when current in a shunt exceeds 7.5A.

    How much voltage can the shunt drop? 10s of mV?

    Does the shunt have to be in the high-side, or can it be in the ground return?

    If high-side, the highest voltage to ground is ~37V, right?

    Presumably, the current sensor needs some hysteresis. If it picks up the relay at 7.5A, at what current should it drop the relay?

    The current sensor needs to be powered from the 37V?

    Do you really need a relay, or can the switching element be a transistor or FET?

    Do you have any problems sourcing parts? (Are you in the US or EU?)
     
  11. offthegridshed

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    Aug 24, 2015
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  12. MikeML

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    What is the maximum current that will ever flow in the current sensing shunt?



    Can you obtain the following parts:

    TI INA168 in a SOT23-5 package?

    A 7.5mΩ (75mV/10A) shunt, like this?

    A IRL530 N-FET?

    A 5V Zener or 78L05 regulator?

    A 24Vdc relay? (Tell me which one)
     
    Last edited: Aug 25, 2015
  13. MikeML

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    Oct 2, 2009
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    Here is a hack at it:

    Plot shows relay coil current vs load current. Using a 24Vdc relay (almost any will do). The relay is energized when the load current exceeds the trip point. Trip point is adjustable (using R2, and must be adjusted to compensate for the actual Vt of M1) as needed. Plot shows what happens as R2 goes from 200K to 250K in steps of 10K. R2 could be replaced with a ~160K fixed resistor in-series with a 100K trim pot.

    R4 drops the battery voltage from ~32V to 24V for the relay. R4 should be 0.3*Rcoil, where Rcoil is the coil resistance of the relay.

    Standby current (Relay not energized) from the battery is 1.2mA.

    Circuit is virtually immune to changes in temperature, and variation of the battery bank from 30V to 37V.

    cs.gif
     
  14. offthegridshed

    Thread Starter New Member

    Aug 24, 2015
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    Mike, I did a little research today about the components.

    The main electronics supplier here doesn't carry an IC for current shunt monitoring so I guess that'd have to come from over your way by mail.
    There are various shunts avaliable on ebay here like that one so I'd say yes to that.
    Your embedded link for the N-FET was the same link as the shunt so I couldn't tell. The parts supplier here has a couple but not an N-FET.
    The 78l05 regulater is fine.
    They have quite few choices of relay.
    http://www.jaycar.com.au/search?text=24v+relay.

    Maximum current should be 22.5 amp, so say 25 for safety.

    Cheers
    Chris
     
  15. MikeML

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    Oct 2, 2009
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    Your not looking very hard:

    Link 1

    Link 2

    Link 3 (We can do something with these).

    To eliminate the high-side current monitor IC, you would have to break the ground side of the 32V power distribution at the battery bank, and install the shunt there. Can you do that? If so, the current measurement can be done with a garden variety opamp, like a LM358. An IRL530 NFET should be common as dirt. (You need a new OZ supplier). Check both RS and Mouser...

    Try this Google search: "Australia industrial electronics suppliers"
     
  16. offthegridshed

    Thread Starter New Member

    Aug 24, 2015
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    Hi Mike,
    I appreciate your effort and input. I apologize if my limited research so far is a little on the lame side for you. A lot of this stuff is new to me and I'm having to learn as I go.
    The IC from RS would be fine, so we can stick with your original idea.
    You may be right about the supplier.... RS has three choices for the mosfet and I imagine everything else as well by the looks of them.
    Let me know if there is anything else specific that you need to know..

    Cheers
    Chris
     
  17. MikeML

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    Oct 2, 2009
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    Chris, here is a ready-made current sensor that would work for you. Look at the description link, and check to see if you could obtain it. Getting it to trip a relay is something I can help you with... I had seen these before at Sparkfun, but I had forgotten where, but this thread reminded me.

    This sensor is one of a many designed for the Arduino environment. The Arduino would be an ideal controller for your generator system. With the module above, it can do current sensing to start the generator. It can do battery voltage sensing both to start and stop the generator. It can do running-detection to see if the generator started. It can do timing to see if the generator failed to start within a reasonable time. It can do control of starting, such as choking as a function of ambient temperature or engine temperature. It can do monitoring of a running generator, such as low oil, over temperature, over/under rpm, etc. It can even send you a text message that the generator failed to start, or needs service...

    Is this something you want to pursue?
     
    Last edited: Aug 28, 2015
  18. offthegridshed

    Thread Starter New Member

    Aug 24, 2015
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    Mike, I did look briefly at the Arduino platform, but it seems a little more involved than I really wanted this to get.
    Can this be used standalone? I'll have a look into the link you provided.

    Cheers
    Chris
     
  19. MikeML

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    Yes, it is tethered to a PC via USB only while you are developing the code that runs on the Arduino. Once the code is debugged, you unplug the USB, and the Arduino becomes a stand-alone controller that looks at inputs, and produces outputs (like relay closures).
     
  20. BR-549

    Well-Known Member

    Sep 22, 2013
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    Or, install a relay that closes at 7.5 amps.

    Much of the magnetic control circuitry of the 1920s and 1930s is superior to the control circuitry used today.

    Magnetic amplifiers can control AC and DC very efficiently and symmetrically.
     
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