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Old 28-09-2018, 15:42   #1
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AC12 autopilot running higher output linear drive

I have an AC12 and run a Type 1 raymarine linear drive of it.
Anybody see why I couldnt add a relay to output of AC12 and have a
new circuit from battery to supply increased load of a type 2?
Ummm, what about reverse polarity for retraction/extention?
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Old 28-09-2018, 17:42   #2
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Re: AC12 autopilot running higher output linear drive

Why do you think the ac12 can't drive the type 2 drive directly. The drive uses 72w, or about 6 a. The ac12 is 8a continuous or 16a max. Assuming I'm reading the right specs.
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Old 29-09-2018, 16:15   #3
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Re: AC12 autopilot running higher output linear drive

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Why do you think the ac12 can't drive the type 2 drive directly. The drive uses 72w, or about 6 a. The ac12 is 8a continuous or 16a max. Assuming I'm reading the right specs.
It seems that Navico specify the AC42 over 3A sustained, as a matched component and
these AP computers have a history of flakeyness if you search. Maybe no more than any other brand but I sure as hell don't want to overdrive it.
That 16A is for one second & the 8A is max sustained.
Guess i'm just being over cautious.
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Old 29-09-2018, 17:14   #4
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Re: AC12 autopilot running higher output linear drive

I don't understand. How can it be specd for not over 3a and then also be specd for sustained 8a. This makes no sense.
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Old 29-09-2018, 17:22   #5
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Re: AC12 autopilot running higher output linear drive

A H-bridge motor driver can be added to supply the higher current needed. They are available for well under $100 and are reliably used in robotics. Buy two, they are cheap

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Old 29-09-2018, 17:47   #6
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Re: AC12 autopilot running higher output linear drive

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A H-bridge motor driver can be added to supply the higher current needed. They are available for well under $100 and are reliably used in robotics. Buy two, they are cheap

Matt
Like Matt said. I have done it--with an AC12. It works.
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Old 30-09-2018, 10:58   #7
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Re: AC12 autopilot running higher output linear drive

If you use relays in a motor drive circuit you need to add de-arcing circuitry or carry lots of spare relays. When the motor power is shut off and the relay is in the process of opening, the permanent magnet motor is acting as a generator with no load on it. The voltage spikes and an arc bridges the contacts. After thousands of on-off cycles the relay contacts will be burned.

On an autopilot the motor is cycled hundreds of time per hour.
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Old 30-09-2018, 14:26   #8
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Re: AC12 autopilot running higher output linear drive

A third vote for an H-bridge. If the current AP doesn't snap between 0V and 12V (apparently my RM SX-5 doesn't) it can be compared to a 7V reference to get the clean on/off behavior. I would just buy an H-bridge and give it a try, unless you have an oscilloscope handy.


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Old 30-09-2018, 15:18   #9
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Re: AC12 autopilot running higher output linear drive

Could you provide some specific info on a H bridge pls. They are readily available on Ebay but am wondering what I am looking for with regards to inputs and outputs.
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Old 30-09-2018, 22:55   #10
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Re: AC12 autopilot running higher output linear drive

yep, me too. What are we looking for here?
12-30V switchable to 20A?
https://www.ebay.com/itm/30A-DC5V-12...oAAOSwLQpZuDqF
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Old 30-09-2018, 23:25   #11
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Re: AC12 autopilot running higher output linear drive

I burnt out the output mosfet on a Raymarine 4000 when one of the brushes in the motor failed under high load and had to replace it which turned out to be quiet a job because the torx screws holding the mosfet to the heat sink had to be drilled out. A H bridge is a good idea if you want to shift the probable point of failure to a component more readily accessible.

You might need to place a load resister across the existing motor drive outputs.
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Old 01-10-2018, 02:24   #12
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Re: AC12 autopilot running higher output linear drive

I presume clutch would still be switched from AC12 and motor feed from AC12 would go to In1 & ground, output to motor1 and battery feed as marked (PWR).
Would it pay to fuse the AC 12 to H-bridge quite low to protect the mosfets in the AC12 output?
Should the circuit to the Linear drive be fused, or is there one in the RM drive?
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Old 01-10-2018, 04:29   #13
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Re: AC12 autopilot running higher output linear drive

Does the linear drive want an on/off voltage, or can it be proportionally controlled like a steering reversing pump? It would be a shame to give up the smoother steering control afforded by proportional control, and I think with relays or an H bridge you would do just that.
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Old 01-10-2018, 15:46   #14
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Re: AC12 autopilot running higher output linear drive

Before I went to bed last night I trolled through the eBay listings for an appropriate H-bridge; there were lots that were driven by logic-level inputs (3.3V, 5V) but only one design that I found that could use the 12V output of the AC12; that was the same one that the @lateral found above. I did find a lower price listing: https://www.ebay.com/itm/30A-5V-15V-...MAAOSwuLZY61XS . So typical of eBay - lots of listings but little choice.

The control logic inputs to the H-bridge should be very high impedance; barring an outright short I doubt they would threaten the AC12's MOSFETs. In any event I suspect the AC12 has over-current protection. Personally I think a fuse would be more likely to cause a failure then protect anything in this case (AC12>H-bridge).

I have never heard of a fuse between a driver and motor in an AP; it would be problematic as it would have to be large enough to handle the startup current (with a slow-blow fuse) but then might be too large for a continuous over-current. Again, it seems like more a problem than a solution.

The H-bridge referred to apparently does not have over-current protection, but instead offers a current monitor output, in Volts. You could set up a comparator for a max current which could open the Enable input, which is probably a good idea if a bit more complexity.

@tanglewood: I doubt that the AC12 uses a variable voltage output, although that is just a guess. "Linear" is often part of the description for autopilot drives, usually referring to an in/out ram, not the voltage control. Sufficiently accurate control can be achieved with PWM, there is no value in a slower response, and such control would be less efficient. Of course if that were the case it would call for a different solution.

@RaymondR: I have often wondered if any autopilots monitored the current draw of the drive, for protecting the output transistors, detecting a drive not connected condition, and to detect the end of travel (banging up against the stops) at an elevated current which could still be below an over-current condition. A simple over-current protection wouldn't be a problem for driving the H-bridge, but a resistor might be needed if the current is used for detection of drive presence or end of travel. Good thought - thanks.

It has been a couple of years since I last looked at using an H-bridge, so my memory is fuzzy and in any event this technology is not really in my wheelhouse so please feel free to amend/extend anything I have written.

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Old 01-10-2018, 16:40   #15
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Re: AC12 autopilot running higher output linear drive

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Like Matt said. I have done it--with an AC12. It works.
Did it operate as smoothly as the output of the AC12?
How long did you have it operating?
If it it safeguarded the H-bridge in the Ac12, is it not easier to replace?
Thanks....to all for your input.
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