Increasing Rx18 power output

[Alstone]

This mod is for a tank fitted with Heavy duty gear boxes with more powerful motors than standard fitted.

Since blowing up a RX18 board on my Bulldog I decides to make sure it didn’t happen again, the first option is to fit a fan on top of the Rx18 unit this I did and cut a hole in the top to let the air out, no problem so far. The power mosfets can handle up to 35 amp at 4.5v provided they are cooled down enough, but on a setup such as a model tank this performance is no possible. The motors on the heavy duty gearboxes I have fitted take  more power than a standard motor, so I have been doing tests to see how many amps a motor requires to operate over its full range, and it appears that these motors draw 3 to 4 amps when doing a spin turn on grass, which will blow a un-cooled Rx18 board.

The cooling is critical and using a thermal probe on components of the RX18 circuit board, I have found that with just an exhaust hole in the top of the unit some parts do not get enough air flow past them in particular the mosfet power drivers, so I have cut holes in the sides of the unit to get the air flow past these.
Just to on the safe side I have fitted a 5amp fuse in the power supply, ( I have tried a 7 amp fuse and this blows before any damage to the Rx18, the only difference is that the stall time is longer which is not good ) and with this setup I can do anything with the tank with no problems, if I do run up against a wall or plant pot and stall the motors the fuse blows and not the board. I intend to change the fuse unit for a 5 amp re-settable push button trip as soon as I can find one at a reasonable price.

The cooling fan I used is a 5v high output 40mm unit.

Al

[swathdiver]

Thanks for sharing!  Do you happen to have dimensions of the board?  Sure looks smaller then the 13s I deal with!

[Alstone]

Thanks for sharing!  Do you happen to have dimensions of the board?  Sure looks smaller then the 13s I deal with!

Including the case it's 65mm x 55mm x 20mm high.

Al

[swathdiver]

Thanks!

[Jakeand]

Nice work there Alstone, very tidy!

[wibblywobbly]

Which motors have you fitted?

[Alstone]

To be honest I'm not sure all I know is there the ones supplied with the heavy duty gearboxes, they look like black cap 400s?.

With the standard metal gearboxes and motors fitted doing a spin turn on grass blew the Rx18 box. With the new boxes it has more power but still struggles to do a spin turn on grass to the point of stall, but the Rx18 box handles it ok, I do have a 5 amp fuse fitted in the main supply and so far it hasn't blown, when I used a 4 amp fuse it did blow twice. I did try a resettable push button 5 amp circuit breaker but the reaction time was to long so I took it out.

I find that provided I don't do spin turns on grass (which requires both motors on full power) I can do anything no problem on any type of terrain. I do find that the better gear boxes put more power on to the ground than the standard ones, I think this is because they run smoother plus the tank handles more naturally. 

Al

[wibblywobbly]

That's interesting information. I have a 10amp reset fuse in my Elmod Tiger, but it hasn't tripped out yet. Now I am wondering whether I should revert to a standard one?

I am guessing that your new gearboxes are 3:1, hence more torque/less speed, though looking at where the motors are mounted I am not going to have room for them in mine, which is a shame, unless the overall length is the same as a standard HL box? ...and do those metal driveshaft covers come with the boxes?

Rob G

[STEINER]

To be honest I'm not sure all I know is there the ones supplied with the heavy duty gearboxes, they look like black cap 400s?.

With the standard metal gearboxes and motors fitted doing a spin turn on grass blew the Rx18 box. With the new boxes it has more power but still struggles to do a spin turn on grass to the point of stall, but the Rx18 box handles it ok, I do have a 5 amp fuse fitted in the main supply and so far it hasn't blown, when I used a 4 amp fuse it did blow twice. I did try a resettable push button 5 amp circuit breaker but the reaction time was to long so I took it out.

I find that provided I don't do spin turns on grass (which requires both motors on full power) I can do anything no problem on any type of terrain. I do find that the better gear boxes put more power on to the ground than the standard ones, I think this is because they run smoother plus the tank handles more naturally. 

Al

Although I haven't got a fan fitted to my RX-18 board (Yet), I do have a 5amp fuse fitted in the main supply. I also have a 3 amp quick blow fuse installed  in two in-line fuse holders, these are fitted to the positive side on each of the 3-1 motor/gearboxes.   With a fan fitted as in your excellent mod and my 3 fuse set up, I feel you would have the ultimate protection for the RX-18 board. Some people may say its overkill, but I say paying for a fuse once in a while is better than paying for a £20 board. I have went through plenty of RX-13s in my time !!.  In 3 solid months running I have blown one of the 5 ampers   (turning on grass ) and not one of the in line motor fuses. 

[Alstone]

That's interesting information. I have a 10amp reset fuse in my Elmod Tiger, but it hasn't tripped out yet. Now I am wondering whether I should revert to a standard one?

I am guessing that your new gearboxes are 3:1, hence more torque/less speed, though looking at where the motors are mounted I am not going to have room for them in mine, which is a shame, unless the overall length is the same as a standard HL box? ...and do those metal driveshaft covers come with the boxes?

Rob G

Hi Rob, The gearboxes are a  direct replacement for the HL boxes they even have the correct locating holes but they are wider than standard boxes, the output shafts that are supplied are short ones for German tanks, evidently for Allied  tanks they need to be longer so you have to get them made or make them yourself.

The “metal drive shaft coversâ€