Hi,
I'm trying to build an under water robot. Most things are completed
except for a way to control and power the robot. I have 6 24vac 1.5a
pumps - 2 rear thrusters, 1 left, 1 right, 1 up and 1 down.

I would like to control it with a joystick (except for the 2 rear
thrusters). The joystick has 2 variable resistors for the x and y
axis. When the joystick is still in the middle both the x & y axis
resistor sits at 60ohms. The resistors range from 0 - 120ohms.

Eg when the x resistor is at 0ohms the left pump is turned on. When
the x resistor is at 120 ohms the right pump is turned on. And when
the x resistor is at 60 ohms neither pump is on.

Could you please provide me with a circuit diagram for the request above?

Thanks

Tristan

Do I take it that you really only want off / on functions for the
thrusters? If this is so, you don?t need a proportional joystick, or
any electronics. Be better with an old switching joystick linked up to
cheap automotive relays or knock one up yourself with four
microswitches which can handle the current.

If you want full proportionality, then you do need a (quite simple)
circuit. In the most sophisticated form, the opposite thrusters would
reverse in direction but that would need motors and impellers designed
for bi-directional function.

However, you state that the motors are AC so presumably you?ve thought
this one out because AC motors do not lend themselves to easy speed
control. In which case, why do you want to use the proportional
joystick rather than switches? No point in gold plating a hammer.

If you *really* want to use the proportional joystick as an on / off
switch I could advise you but dunno how a circuit could be sent
because there appears to be no way to post pictures on Google Answers.
Guess I could post it on an associate?s web site or post my address in
cryptic form.

Best

Simple circiut which does the job should be four MOS transistors or 4 diodes,
where each acts like a switch, say you have 4 transistors, 2 should be
connected to one end of your X resistor and 2 should be connected to
one end of your Y resistor. when you are at 0 ohms in the X resistor
then one of the transistor should be ON but the other should be OFF,
this is achieved if one was an NMOS the other is a PMOS, same is done
in the Y direction too...
so when you are in the middle none of the transistors are ON....this
is a rough diagram for it but I hope you get the idea...
                         
         (down thruster)NMOS ____________________ PMOS (up thruster)
                                      |
                                      |
                                      |---0 ohms point in Y direction
                                      \
                                      /
     NMOS                             \
   (left thruster)                    /
                  |__________/\/\/\/\/\/\/\/\/\/\_____________voltage level   
                  |                   /
   (right thruster)                   \
      PMOS                            /
                                      \
                                      |-- 120 ohms point in Y direction
                                      |
                                      |
                                    voltage level

according to this ckt the Y axis NMOS is on when there is a '1' at its
gate input, where as the Y axis PMOS is OFF at this time thus when
there is no voltage drop across the resistor the NMOS is on in other
words when you have
an input at 5 V then it is ON. the PMOS is ON only when the input is
at 0 v or below the certain threshold voltage. but when the input is
at 2.5 volts(60 ohms) none of the transistors should be ON, you can
use this ckt or you can use forward and reverse biased diodes to act
as switches, since they might perform better for sharp cutoffs.
the same logic is used for the X axis...hope this answers your
question...the circuit I have given is just a raw format....better
performance can be achieved using buffers or inverters after the PMOS
& NMOS switches...& reversing the logic.

You state your motors are AC. MOSFETs are DC devices. It is possible
to design MOSFET circuits to drive AC circuits but it?s complex.
Floating supplies and drives etc.

Also, slowly switching MOSFETs risks problems with dissipation and
worse, SOAR failure (safe operating area).

And also, switching reactive (especially inductive) loads with MOSFETs
requires expertise to avoid failure.

Triacs can be used, but again one has to be aware of the problems
associated with switching reactive loads or they blow up with
alacrity.

Best