Making an ESC out of an ESC...

[cowana]

Hi!

This is for a boat project - I want an ESC that can handle 19.6V and about 30A.

I know about IRF540s, - at 100V and 28A each, 3 of them on a CPU heatsink should power the boat fine. However, I want an easy way of connecting them to the reciever.

I read on a website, you can use a servo amplifier. However, this cannot easily power 2, and definitly not 3 FETs.

You can also make your own PCB. However, this is quite expensive.

My question, is that as the FETs need a PWM input, surley a normal ESC would do?

Lying around, I have a 6V, 10A ESC. Could I use this to run the FETs? Picture attached (with two, I would use 3 or 4).

I don't really want to have to use PICs, when I have a ready working circuit right next to me.

Andrew

http://image.rcuniverse.com/forum/upfiles/105861/Kh17443.jpg

[Phil Cole]

It probably won't work the way you have it drawn. Typically, ESCs switch the -ve output to ground, with the +ve output being connected directly to the +ve battery input.

I'd look for the gate drive signal in the ESC and use that to drive the FETS. However, with only 5 V input, you would possible have only 5 V of gate drive unless the ESC has a voltage booster for the gate drive. The IRF540s will require about 10 V of gate drive to get them turned on properly. There are FETs that only require 5 V gate drive, such as this one:

https://ec.irf.com/v6/en/US/adirect/ir?cmd=catSearchFrame&domSendTo=byID&domProductQueryName=IRLZ44Z

You will also need a Schottky diode across the motor, cathode to the + motor connection. Something rated at 30 V or more and 30 A will do the job.
This one will work well:
http://www.onsemi.com/PowerSolutions/product.do?id=MBR40L45CTG

The diode will require heat sinking as well as the FETs.

[cowana]

Thanks for that. Livivng in the UK, I have been unable to find that particulat FET, but on Rapid Electronics (http://www.rapidonline.com/productinfo.aspx?tier1=Electronic+Components&tier2=Discrete+Semiconductors&tier3=MOSFETs&tier4=TO-220+Logic+level+power+MOSFETs++N-Channel&moduleno=77687) I have found FETs that I (think) have a 5V switchon voltage. Would STP55NF06L work? Also, instead of 5V, I would probably use a separate 7.2V pack (with a common ground). Would this FET still work?

I plan to use a CPU heatsink to cool the FETs and diode. Would the diode have to go across the motor, or wouldn't it matter?

As seen in my (new) circuit diagram, I would also like to have reverse controlled by a relay. Where would the diode go?

[Chippie]

Why not use Eric Behr's design using discrete parts?

http://www.math.niu.edu/~behr/RC/speed-ctl.html

There are several designs on his home page too.

[jeffs555]

STP55NF06L would be a good choice, but your diagram is all wrong. The source pin of the mosfets needs to be connected to ground and the motor needs to be connected between the drain pin and the positive of the 19.5 volt battery(like you had in your first diagram). The schottky also needs to go between the drain pin and the positive of the 19.5 volt battery. You would also want a diode across the coil of the relay.

[cowana]

Which would be better?

1) making this circuit (shematic attached for whole boat)
2) making this (http://norbique.rchomepage.com/esc/schematic.htm) circuit with parts from here. (http://www.dannell.co.uk/mosfet.htm)

Andrew