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dc motor controller

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  • #31
    Reverse polarity protection in DC systems is simple a has been described above.

    Protecting systems from out of phase AC systems is somewhat more difficult, it would be unpleasant if you started a large process on Monday morning and 10,000 feet of conveyors tried to run backwards. Phase detecting relays will not let such a system start.


    • #32
      Originally posted by dian View Post

      what do you think about this?
      That circuit is just (supposedly) for short circuit protection. But in fact it relies on the battery to have a high enough internal resistance to withstand a short circuit and have its output voltage drop low enough for the relay to drop out. A 6V DC relay as shown may have a drop-out voltage as low as 1.5 volts, and a 4.3 volt lithium battery may be able to supply 20-50 amps before its terminal voltage drops low enough to drop out the relay (unless it is a "protected" cell). This is a stupid and dangerous circuit, and the contacts of that little relay will likely weld closed. It will not protect against reverse polarity.

      For reverse polarity protection, put a diode in series with the relay coil, and perhaps a green LED and resistor across the coil. And maybe a red LED and resistor from the NC contact to the common (-) connection. The input (+) goes to the armature, and the NO contact is the output. A reverse polarity input will NOT energize the relay coil, and the red LED will light, showing the error. With correct polarity, the relay operates, and correct polarity voltage is available on the NO output contact. Overcurrent and short circuit protection may be best provided by a fast acting fuse. With the fuse in place, you could just add a high current diode across the load, and reverse polarity will just cause it to conduct and blow the fuse. A PTC resettable device could be used, but they have considerable voltage drop and limited voltage specs.
      Paul , P S Technology, Inc. and MrTibbs
      USA Maryland 21030


      • #33
        I have a power supply I built that has over-current protection. The moment you draw more than the set current, it deactivates the output. You then have to disconnect the load (or reduce it) and push the reset button. It also has a setting where it just limits the current, but keeps supplying it. As an experimenters tool it's perfect- in snap-off mode it will protect against accidentally shorted wires, suddenly shorted transistors, etc- and you can walk away unworried that something will be cooking while you aren't looking. Using a power supply like this, the only reverse voltage protection a circuit would need is a reverse connected diode- one part, no fuse needed. No voltage drop to the circuit from the power supply.

        The next simplest is the diode and a fuse. In this case, the power supply has to be capable of enough current to blow the fuse, and if it isn't, then it basically puts a sustained short across the power supply if you hook up your circuit backwards. You still avoid the voltage drop from power supply to the circuit, but you risk burning out the power supply.

        A series connected diode solves the problem, but at the expense of some voltage drop. My favorite in this case is the dual shottky in the TO-220 case, as you get to parallel two diodes, and in that case style they can easily have more than a 10 amp rating. Besides having a lower forward voltage drop by virtue of being a shottky, two diodes in parallel will have a lower fvd for a given current level. At a reasonable current passing through, you can keep the voltage drop down to .4 volts or so, which for many circuits won't affect it much, if at all. And doing it this way also only requires one part.

        As soon as you start adding relays, resistors, etc you build in some complexity and take up more space, reduce the reliability, and increase the cost. If this protection is going to stay with the circuit, then you will need it for every circuit-

        Someone mentioned the polarized connector. One of my recently completed projects used these on each circuit, and as long as the supply voltage is properly polarized on the mating connector, you automatically have proper polarity when the connectors mate. Somewhere along the line you do have to pay attention to the polarity, so you can get away without any of these protection schemes if you just take this one step- get it right the first time.
        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


        • #34
          A schottky is pretty good, not perfect, but pretty good. Only slightly worse than an old-time germanium rectifier, and a good deal more available and robust.

          The MOSFET circuit has lower voltage loss than any diode, and is simple enough that the loss of reliability is not serious, compared to a complicated circuit it may be protecting. But that added complexity does take board space, and cost money, so one needs to think when choosing the circuit most suitable for the application.

          There s also the question of effectiveness, how much does the protection cost, compared to the circuit. Some circuits are not worth the cost, others are worth far more, and in other cases the cost is in the nuisance of having to replace something if it is damaged by reverse polarity, even if the "something" is cheap. You have to have one available, and then you have to swap it for the bad one.

          Keep eye on ball.
          Hashim Khan


          • #35
            Back to the original question, when designing things compromises are always made. How those compromises are juggled is a function of many factors, one of which is the basic purpose of the device. Reverse polarity protection is often one of these factors.

            The actual answer to the OP's question is YES, there ARE power supplies that have built in, reverse polarity protection. But the very fact that it is not often an advertised feature of the available supplies is indicative of the limited value of this feature.

            One observation that I have is that most, if not all reverse polarity protection circuits will serve their purpose a lot better if they are built into the LOAD instead of into the power supply. Once the power rails leave the power supply, the polarity is fixed and can still be connected backwards. And all the protection circuits are of limited or no use at that point in the circuit.

            The simple diode or the FET type circuit will not detect if a load is connected backwards at it's output side. They just won't.

            At least some of the fuse type circuits will detect and, once the fuse has blown, protect the load circuit. But, and this is a big BUT, there is an old saying in electronic circles, "The transistor (or many other devices in the load) will ALWAYS blow first to protect the fuse." I have seen many circuits with blown transistors or other devices where the protection fuse was perfectly OK. Fuses, even the fastest of fast blow fuses are simply a lot slower to blow than almost any transistor, diode, or FET. So the fuse wins and the transistor in the load loses.

            Therefore adding a reverse polarity circuit to the power supply is not a good strategy, IMHO and in the opinion of a lot of others who design electronic equipment.

            The biggest reason, again IMHO, for having reverse polarity protection in a power supply is to prevent a (reverse or even a forward) Voltage from being feed backwards from the load circuit or from another, redundant and "hot" power supply into the output of the power supply and damaging that POWER SUPPLY.

            In a career of repairing electronic and electric equipment, I have seen few, if any, problems where the load fed a reverse Voltage back into the power supply. Yes, I can imagine it, but I can not recall ever seeing it. And if a particular load uses only one power supply Voltage, that back feed is all but impossible. Motors, which can act as generators, may be an exception to this.

            That leaves the case of dual, redundant power supplies. And in that case, I have seen many examples of the use of steering diodes to protect the good power supply and to ensure no interruption in power when one of them goes bad. I have not seen any designs where fuses or things like FET circuits are used for this. Here diodes ruled supreme and they are usually external to the power supplies. Yes, there were small losses, but these designs worked well for the intended purpose, which was reliable operation with NO interruptions. Those small losses were just one of those compromises.

            The OP should state the valid design reason for reverse polarity protection. If it is to protect the load from accidental reverse polarity connection, then my previous suggestion of using a diode while doing the design/prototype work is being done and just removing it when that phase is completed will help. If the purpose is otherwise, then please let us hear it.

            Originally posted by JRouche View Post

            Mind you I have limited electrical.

            I hate diodes for this use. To bleed energy off to a load that is not the load you are driving IMO is a "Net Loss". JR
            Paul A.
            SE Texas

            Make it fit.
            You can't win and there IS a penalty for trying!


            • #36
              Maybe the actual question got lost?????

              The OP has (or wants) a motor controller that takes a DC source, and controls a DC motor speed.

              He wants a controller that has input protection to avoid a failure/damage in the case of an inadvertent connection of the DC source to it in reverse polarity. He is not asking for a protected supply, but rather a protected controller.

              He is aware that diodes can do it. And maybe that is sufficient. But he is asking if anyone knows of a brand/model that has the protection built-in.

              Keep eye on ball.
              Hashim Khan


              • #37
                The PWM controller I recently repaired is probably very similar to most of those similar to the OP's. They have a high current dual Schottky (freewheeling) diode across the motor connections, and several MOSFETs from the (-) output to GND. If the input terminals are connected backward, the freewheeling diodes and the intrinsic MOSFET diodes are both forward biased, and present a virtual short circuit to any input greater than 1 volt. It is unlikely that any of the cheap Chinese PWM controllers from Amazon, eBay, Banggood, or AliExpress, will have reverse input protection. But any such circuit should be protected by a properly sized fast acting fuse or circuit breaker, and the components of a 30A controller like the OP's should withstand the current long enough to trip the protection. But there is no guarantee of that, particularly for a cheap unit which is often far over-rated. A 30A controller should have a fuse no larger than 15-20 amps.
                Paul , P S Technology, Inc. and MrTibbs
                USA Maryland 21030