relay to transisitor?

Discussion in 'General Electronics Chat' started by tsmwebb, Mar 5, 2010.

  1. tsmwebb

    Thread Starter New Member

    May 21, 2009
    3
    0
    I have a solar controller (associated with an energy monitor/external alternator controller) that drives a funky relay with a built in diode. The diode is there to prevent discharge through the solar panels at night. I have no idea where this part was sourced and have never been able to find a replacement. The thing lives on a boat and the switch trips perhaps 100 times a day and it is 10 years old. The boat works in very remote areas for extended periods of time. So, I'm worried about what to do when it fails. I am in the USA visiting in-laws and spent some time in the local Fry's trying to avoid the in-law part of the visit and noticed that they have on the shelf a transistor rated at 30 amps (just about the max amperage the panels can put out) and a rectifier rated at 40 amps. Now, the combined reaction time of the controller/relay is slower than ideal. So, I am curious if I couldn't just hook the transistor and diode up in place of the relay. It would be nice to have a backup and it would be wonderful if the faster switching would give me something closer to PWM. Of course, I didn't think of any of this when I was onboard and I head back out of the country Monday. So, I hope you will forgive me for asking before doing my research: Is this possible? Is it wise? Is there a better way. What mistakes am I likely to make as a total novice?

    Cheers!

    Tom
     
  2. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    I am not sure I understand your question. I understand you have a boat that uses solar panel to charge a battery... I think it also has an engine with an alternator that also charges the battery. I know that the diode prevents the solar panels from discharging the battery at night. I am not sure what the function of the relay is. I think it controls whether the alternator or the solar panels charge the battery. If so it appears that when the engine is running the solar panel is disconnected from the battery.
    If I understand correctly you want to replace the relay with a transistor.
    How am I doing so far?
     
  3. tsmwebb

    Thread Starter New Member

    May 21, 2009
    3
    0

    Sorry I wasn't very clear. Not too surprising since I don't have much of a clue.

    Anyway, Yes, you're right, I do want to replace the relay with a transistor. Since the relay has a diode built into it I would need to replace that as well. The solar array is made up of 4 nominally 85 watt panels. They are not all aimed in exactly the same direction so I think a 30 amp transistor should be safe... In any case, there is a 30 amp fuse in the system now.

    It is probably not important that the controller also controls the alternators. The relay shuts the solar panels off at a set voltage and turns them on again at lower voltage. In this case the high voltage is 13.5 volts and the low is 12.9. The solar keeps up with the basic energy requirements of the boat for up to a week. When the boat is underway or when the batteries have run down the alternators are used to charge the batteries fully (ie. bulk, acceptance and float).

    Is that any clearer?

    Thanks again,

    Tom
     
  4. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    I understand the situation. There must be some control circuitry that senses the voltage to energize the relay. Mechanical relays do have a life span and constant on and off can eventually cause the relay to malfunction. Trying to replace the relay with a high current transistor is not that simple, as the relay requires approximately 12 volts to turn it on, and the transistor uses a lot less voltage to turn it on. With 4ea 85 wattt solar panels it is possible to overcharge the batteries, that is why the relay and sense circuitry is there. I believe the diode is in parallel with the coil of the relay to supress the back emf when the relay coil is deenergized.

    I recommend you get a solid state charge controller that allows a secondary charger to be connected to the batteries as the I calculate the maximum output from the solar cells can be about 18 volts at near 18 amps. One brand of charge controllers that has that capability is Xantrex.
     
  5. tsmwebb

    Thread Starter New Member

    May 21, 2009
    3
    0
    Thanks. I understand. I think an 18 amp controller would be very undersized. I hear you about the rating but pretty often I've seen readings on the amp meter in excess of 18 amps. I suspect the ratings are a bit on the conservative side.

    Tom
     
Loading...