Good day everyone.
I have just joined ABC and it looks like I will enjoy exploring the wealth of information available.
My present problem involves making the output from a small autopilot, on a boat, drive a much larger load. Now it drives a motor of approximately 2A load and I wish to make it drive a hydraulic motor load of approx. 20A.
The output from the autopilot is DC 12v and it changes polarity depending on the direction selected.
I have come up with two DPST relays whose coils are grounded on one side and the other side of the coils alternately go to the output of the autopilot. Hence when either coil is activated the contacts, which are connected to a 12V source, give a switched polarity and drive the motor.
The problem I am seeing with this idea is that if either of the relays were slow in operating a short would occur.
I hope my convoluted description of what I am trying to make makes some sense and someone can help me.

Regards
Brian

 

@Btheozz
Your current drive is likely an H-Bridge of 4 transistors. 2-N-channel mosfets and 2 P-channel mosfets (or) 2 NPN and 2 PNP transistors.

Your plan might be better execuited if you pull out the existing H-Bridge and add MOSFETs of higher load-handling (current handling) capability and a power supply that can provide more current. From there, the existing drive mechanism is likely fine and can apply voltage to the base of the higher-current handling MOSFETs as well as it does to the existing mosfets.

 

The Auto pilot ive worked on was hydraulic & the pump was run continuously. The steering was controlled by 2 hydraulic solenoids, very little current required. The 2 solenoids are in the bottom RH corner .

 

I designed and implemented a similar function some years ago on a transom platform.
If you use quality relays, and wire them so they are open in the off position, you should be fine.
I am attaching the schematic I used. As you suggest, one end of each coil would be grounded and the + terminals would be connected to the two motor drive wires from the autopilot.
I suspect the AP is using MOSFETS to switch the outputs so I would suggest adding a diode between the AP output and the relay coils... Band towards the relay coil. This will avoid problems if there is some leakage through the MOSFETS. Also, don't forget the circuit breakers/fuses in the relay outputs. These will protect from a stuck relay failure.

This system was 24V and used the following relays:
DPST relays for Transom Lift Motor
2 x G7L-2A-BUBJ-CB-DC24 Omron 24VDC 25 amp relay Digikey Part # Z2258-ND Qty 2 @ $16.74
You would need to substitute relays with 12VDC coils.

Graham

Good day everyone.
I have just joined ABC and it looks like I will enjoy exploring the wealth of information available.
My present problem involves making the output from a small autopilot, on a boat, drive a much larger load. Now it drives a motor of approximately 2A load and I wish to make it drive a hydraulic motor load of approx. 20A.
The output from the autopilot is DC 12v and it changes polarity depending on the direction selected.
I have come up with two DPST relays whose coils are grounded on one side and the other side of the coils alternately go to the output of the autopilot. Hence when either coil is activated the contacts, which are connected to a 12V source, give a switched polarity and drive the motor.
The problem I am seeing with this idea is that if either of the relays were slow in operating a short would occur.
I hope my convoluted description of what I am trying to make makes some sense and someone can help me.

Regards
Brian

 

In an Auto pilot I can see relays having a long term reliability problem, due to the constant switching. An Auto pilot is constantly making corrections.

 

Daryl,
That's a very good point.
Other than just replacing the control unit with one designed to support the OP's desired drive, I guess the best approach would be GopherT's suggestion; or if the control unit is sealed, then building an external H-bridge with suitable MOSFETs. Neither would be good for the faint of heart or inexperienced as there are several pitfalls that must be designed out.
Graham

 

Thanks everyone for your suggestions.
Reliability of the relays is definitely a concern I had but I am not confidant I am capable of implementing the MOSFET H-bridge and also as I am presently in Panama, ever finding a source of the MOSFETs.
I guess if I sourced them, I could have them shipped to Panama as I would also have to have the ram drive shipped in.
Has anyone got a schematic for the H-bridge and what sort of pitfalls can I expect?

Cheers
Brian

 

I would take the easy (but more expensive) route of using 4 DC solid-state relays (SSR) to form the bridge, such as one of these.
Here's a paper on doing that (last page).
Note the addition of the protective diodes across each relay (a 3A diode, such as the 1N5400 with a surge rating of 200A, should be sufficient for that purpose).

Question: Does the autopilot output have a deadband (no output) when in a steady heading and not going from left to right?
If so, that should prevent any possible problems with both sides of the bridge being on at the same time.

 

Auto pilots using a H bridge have a Dead band. This is a Auto Helm 1000 from 1981 that used a H bridge to drive a small motor & screw push /pull rod as a Tiller pilot.

 

Would help if OP posted some pictures of his Auto pilot & what model it is & a bit more detail.

 

Thanks again to every body.
I am feeling a little slow this morning after my birthday party last night so please bear with me.
The autopilot is an old Autohelm 4000. Approximately 10 years old. At present it drives both the small motor on a wheel drive or the motor on a tiller drive which in turn drives the rudder on a Hydrovane.
But....I had a flash this morning.......I do not need to up the output of the H-Bridge on the auto pilot as all I am wanting it to do is just drive a relay coil which would be less than what it drives now.
Or is this thinking just my thick head this AM?
Again thanks to all, I appreciate your efforts and input.
Brian

 

Yes, I believe driving relays was already suggested, but reliability of a mechanical relay would be a concern, since it would be constantly switching.
That's why I suggested SSRs be used in my post #8.

 

If its this model Autohelm ST4000 circuit of the H bridge. Then just mount 4 suitable sized MOSFETs on a heatsink externaly.

 

Thank you all for your help and suggestions.
It appears to get stuff into Panama here is VERY costly. Example, I would have to pay almost double to get the hydraulic ram here so I have decided to put this project on hold for now but again I am saving all this information for a later day.

Thanks
Brian