Mosfet motor driver circuit advice

[Rubicon]

Hello,

An old project that I'd been working on is that of an electric fishing
Kontiki. I have a circuit that I think will work O.K. but I hope that
someone could take a look at it and point out any glaring mistakes.
In particular the mosfet motor driver circuit is of concern to me.

http://www.geocities.com/talionis.geo/Temp/temp.html

The servo retract button and timeset reed switch are on the same input
as when the PIC starts up it first checks that input to determine if
it should continue with the startup sequence or simply retract the
release mechanism into the casing. The DPST switch is to ensure rough
waves don't accidentally activate the servo button when in timeset
mode.

The seperate voltage regulator for the servo is to hopefully prevent
any brownout reset of the PIC should the servo need to draw alot of
current if the release mechanism is under load or maybe jammed.

The seconday lines to the servo are in case of PIC failure or bad
programming to allow me to control the servo, retract the release
mechanism and get the whole thing out to be fixed.

The two reserved PIC I/O pins are for a strobe circuit I may add
later. If not then I'll rearrange things on Port A to and figure out
how to allow the PIC to sample the battery voltage six times a minute
and average it for a more accurate reading.

The zener diode I meant to remove before making the page but I do need
to ask if the mosfet needs any over-voltage protection.

Any help is greatly appreciated.

Thanks,

Rubicon.

 

[John Larkin]

Hello,

An old project that I'd been working on is that of an electric fishing
Kontiki. I have a circuit that I think will work O.K. but I hope that
someone could take a look at it and point out any glaring mistakes.
In particular the mosfet motor driver circuit is of concern to me.

http://www.geocities.com/talionis.geo/Temp/temp.html

What's the supply voltage to the PIC? The gate drive to the
motor-driver fet looks scary.

John

 

[Jamie]

Hello,

An old project that I'd been working on is that of an electric fishing
Kontiki. I have a circuit that I think will work O.K. but I hope that
someone could take a look at it and point out any glaring mistakes.
In particular the mosfet motor driver circuit is of concern to me.

http://www.geocities.com/talionis.geo/Temp/temp.html

The servo retract button and timeset reed switch are on the same input
as when the PIC starts up it first checks that input to determine if
it should continue with the startup sequence or simply retract the
release mechanism into the casing. The DPST switch is to ensure rough
waves don't accidentally activate the servo button when in timeset
mode.

The seperate voltage regulator for the servo is to hopefully prevent
any brownout reset of the PIC should the servo need to draw alot of
current if the release mechanism is under load or maybe jammed.

The seconday lines to the servo are in case of PIC failure or bad
programming to allow me to control the servo, retract the release
mechanism and get the whole thing out to be fixed.

The two reserved PIC I/O pins are for a strobe circuit I may add
later. If not then I'll rearrange things on Port A to and figure out
how to allow the PIC to sample the battery voltage six times a minute
and average it for a more accurate reading.

The zener diode I meant to remove before making the page but I do need
to ask if the mosfet needs any over-voltage protection.

Any help is greatly appreciated.

Thanks,

Rubicon.

other than the diode as you have mentioned that would other wise cause
an issue when the PWM was in it's off cycle, It looks good from here
how ever, I was wondering if the signal output of that pic is sufficient
to drive the (SP1) transducer?
P.S.
Using 2 Zeners back to back or a bi-directional TVS diode may
achieve what you were trying for on that Zener.

 

[Rubicon]

What's the supply voltage to the PIC? The gate drive to the
motor-driver fet looks scary.

John

John,

Thanks for your reply.

The PIC gets 5V from the LDO regulator.

Why does the gate drive to the motor-driver fet look scary?

Regards,

Andrew.

 

[Rubicon]

other than the diode as you have mentioned that would other wise cause
an issue when the PWM was in it's off cycle, It looks good from here
how ever, I was wondering if the signal output of that pic is sufficient
to drive the (SP1) transducer?
P.S.
Using 2 Zeners back to back or a bi-directional TVS diode may
achieve what you were trying for on that Zener.

Jamie,

Thankyou for your reply and the advice on the diode.

As for the PIC being able to drive the transducer it can quite well
though I do remember that sometimes they require a seperate drive
circuit. The one I'm using is pulled from a portable phone and may
have inbuilt dirve circuitry. I don't know for sure.

Thanks again for taking a look at my circuit.

Regards,

Andrew.

 

[John Larkin]

John,

Thanks for your reply.

The PIC gets 5V from the LDO regulator.

Why does the gate drive to the motor-driver fet look scary?

Regards,

Andrew.

R15:R16 divides down the pic logic swing to about 1 volt, and then the
transistor Vbe's throw most of that away. So I take it back, it's not
scary; the fet will never turn on, so it's perfectly safe.

Oh, I can't find a datasheet for IRF1405N.


John

 

[Rubicon]

R15:R16 divides down the pic logic swing to about 1 volt, and then the
transistor Vbe's throw most of that away. So I take it back, it's not
scary; the fet will never turn on, so it's perfectly safe.

Oh, I can't find a datasheet for IRF1405N.


John

John,

A little knowledge is a dangerous thing is it not!

So you're suggesting I either remove R16 or dramatically increase its
value for a weak pulldown?

The IRF1405 datasheet is now on the bottom of the Geocities page.
http://www.geocities.com/talionis.geo/Temp/temp.html

Thanks for the infomation.

Regards,

Andrew.

 

[John Larkin]

John,

A little knowledge is a dangerous thing is it not!

So you're suggesting I either remove R16 or dramatically increase its
value for a weak pulldown?

The IRF1405 datasheet is now on the bottom of the Geocities page.
http://www.geocities.com/talionis.geo/Temp/temp.html

OK. Note that Rds-on is specified with +10 volts on the gate. Looking
at figs 1 and 3, it's just starting to turn on with +5 on the gate.

Also, this is one of those notorious IR datasheets where they call
this a 169 amp part, but put it in a rediculous TO-220 case and
provide footnote 6, which says that they really meant 75 amps, which
is still hard to believe.

I'd suggest you use a gate driver chip to give a full 12 volts of gate
drive, and use four or so of these fets in parallel. It's still sort
of scary, as I can image a hundred amps or so of startup motor
current. Some sort of *fast* fet overcurrent sensing/shutdown might be
prudent. Or get a 1200 amp IGBT module on ebay?

I'd go for a lot beefier D5, too; it could average 100 amps for a
while.

John

 

[Rubicon]

OK. Note that Rds-on is specified with +10 volts on the gate. Looking
at figs 1 and 3, it's just starting to turn on with +5 on the gate.

Also, this is one of those notorious IR datasheets where they call
this a 169 amp part, but put it in a rediculous TO-220 case and
provide footnote 6, which says that they really meant 75 amps, which
is still hard to believe.

I'd suggest you use a gate driver chip to give a full 12 volts of gate
drive, and use four or so of these fets in parallel. It's still sort
of scary, as I can image a hundred amps or so of startup motor
current. Some sort of *fast* fet overcurrent sensing/shutdown might be
prudent. Or get a 1200 amp IGBT module on ebay?

I'd go for a lot beefier D5, too; it could average 100 amps for a
while.

John

John,

I've altered the circuit a bit with a driver chip, paralleled motor
diodes and assume 4 mosfets though it currently only shows two.

I was thinking that I could maybe use a subroutine to slowly increase
the PWM output of the PIC as a sort of soft start. Not sure about
this.

I have no specs on the motor but others who build similar things just
use one or two 12V SLA batteries, a 55 Amp relay and a mechanical
timer.

Must go.. 12.10am

Regards,

Andrew.

 

[John Larkin]

John,

I've altered the circuit a bit with a driver chip, paralleled motor
diodes and assume 4 mosfets though it currently only shows two.

It's best to overkill here. Replacing blown fets can be a drag.

I was thinking that I could maybe use a subroutine to slowly increase
the PWM output of the PIC as a sort of soft start. Not sure about
this.

That's a good idea.

I have no specs on the motor but others who build similar things just
use one or two 12V SLA batteries, a 55 Amp relay and a mechanical
timer.

Must go.. 12.10am

Regards,

Andrew.

Night!

John

 

[Rubicon]

It's best to overkill here. Replacing blown fets can be a drag.


That's a good idea.


Night!

John

John

Thankyou for your help.

Regards,

Andrew.