A customer let the smoke out of a H-bride motor driver I made for him,
so now I want to add reverse-power protection. My standard anti-parallel
1N4004 won't do much good when connected to a lead accu, so I am
considering a series diode (1N4004, or a schottky). But now the back-emf
from the motor, which is fed to the power rail by the 4 diodes, has
nowhere to go. What is the solution to this? A big zener on the power
rail, for let's say 150% of the maximum power supply? If so, which
wattage would be needed? Or maybe a consensator? If you, what capcity
and what type? Or are there other solutions, maybe with a polyfuse?
Wouter van Ooijen
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Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu
A series fuse say 1/4 amp fast acting followed by a 1N400x reverse biased diode would seem to be the simplest solution. The fuse should be heavy enough for max peak load, but still blow in a reverse power situation.
> A customer let the smoke out of a H-bride motor driver I made for him,
> so now I want to add reverse-power protection. My standard anti-parallel
> 1N4004 won't do much good when connected to a lead accu, so I am
> considering a series diode (1N4004, or a schottky). But now the back-emf
> from the motor, which is fed to the power rail by the 4 diodes, has
> nowhere to go. What is the solution to this? A big zener on the power
> rail, for let's say 150% of the maximum power supply? If so, which
> wattage would be needed? Or maybe a consensator? If you, what capcity
> and what type? Or are there other solutions, maybe with a polyfuse?
>
> Wouter van Ooijen
>
> -- -------------------------------------------
> Van Ooijen Technische Informatica: http://www.voti.nl
> consultancy, development, PICmicro products
> docent Hogeschool van Utrecht: http://www.voti.nl/hvu
> A series fuse say 1/4 amp fast acting followed by a 1N400x
> reverse biased diode would seem to be the simplest solution.
> The fuse should be heavy enough for max peak load, but still
> blow in a reverse power situation.
Yeah, that is one solution, but it is not self-repairing, which I would
prefer. I dunno, the fuse might not even be fast enough to prevent the
diode from exploding.
Wouter van Ooijen
-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu
> A customer let the smoke out of a H-bride motor driver I made for
> him,
> so now I want to add reverse-power protection. My standard
> anti-parallel
> 1N4004 won't do much good when connected to a lead accu, so I am
> considering a series diode (1N4004, or a schottky). But now the
> back-emf
> from the motor, which is fed to the power rail by the 4 diodes, has
> nowhere to go. What is the solution to this? A big zener on the
> power
> rail, for let's say 150% of the maximum power supply? If so, which
> wattage would be needed? Or maybe a consensator? If you, what
> capcity
> and what type? Or are there other solutions, maybe with a polyfuse?
MOSFET in series with line.
If N Channel, place in negative lead with drain to battery, source to
load negative lead. (Unintuitive connection. Has to do with body
diode. Works).
gate goes to positive battery connection. Gate divider may be needed
depending on battery voltage.
If P Channel place in positive lead, keep source towards load, gate to
opposite supply line, proceed as per N channel.
When battery is correctly connected the gate is biased positive
relative to the source and the FET is enhanced. Operation is in a
quadrant not usually used but the FET doesn't mind.
When battery is connected backwards the gate is biased negative
relative to source and FET is off. Looking at the arrangement in this
state you see why you have the FET "backwards" to usual as the body
diode now also blocks.
A low Rdson FET allows very low voltage drop when on.
This is a brilliant idea and decades old. some turkey claimed to have
patented it quite recently.
In you case it is even more brilliant than usual as the FET is a
bidirectional channel and will pass motor back en=mf into the load.
Some design may be required with the gate circuit to provide a small
"hang" time delay. Once you have the general principle the rest is
easy to adapt to your needs.
Bidirectional, able to take a pretty good joke, and a diode in series
with the coil keeps it from pulling in until the input is the right polarity.
You could even do a zener in series, and get undervoltage protection.
At 07:14 AM 6/1/2005, Wouter van Ooijen wrote:
>A customer let the smoke out of a H-bride motor driver I made for him,
>so now I want to add reverse-power protection. My standard anti-parallel
>1N4004 won't do much good when connected to a lead accu, so I am
>considering a series diode (1N4004, or a schottky). But now the back-emf
>from the motor, which is fed to the power rail by the 4 diodes, has
>nowhere to go. What is the solution to this? A big zener on the power
>rail, for let's say 150% of the maximum power supply? If so, which
>wattage would be needed? Or maybe a consensator? If you, what capcity
>and what type? Or are there other solutions, maybe with a polyfuse?
How much current does it have to handle?
The two solutions that come to mind are:
1) reverse-connected MOSFET in series with battery, biased so that correct
battery polarity biases it full on. (Bob Pease from National Semiconductor
is the first person I saw mention this - he says that he should have taken
a patent out on the concept!)
2) relay with NO contacts connected in series with battery. Coil fed with
blocking diode from battery.
Both techniques work very well - its a matter of cost, available space,
battery consumption.
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> Yeah, that is one solution, but it is not self-repairing, which I
> would
> prefer. I dunno, the fuse might not even be fast enough to prevent
> the
> diode from exploding.
A great thing about my FET solution is that it is off by default when
the battery is connected and will only turn on with correct battery
polarity. Honourable mention goes to zero operating power and minimal
on voltage drop.
Or use a series PTC thermistor instead of the fuse for overcurrent
protection.
Harold
> A series fuse say 1/4 amp fast acting followed by a 1N400x reverse biased
> diode would seem to be the simplest solution. The fuse should be heavy
> enough for max peak load, but still blow in a reverse power situation.
>
> KF4HAZ - Lonnie Underwood
At 04:22 PM 6/1/2005 +0200, you wrote:
> > A series fuse say 1/4 amp fast acting followed by a 1N400x
> > reverse biased diode would seem to be the simplest solution.
> > The fuse should be heavy enough for max peak load, but still
> > blow in a reverse power situation.
>
>Yeah, that is one solution, but it is not self-repairing, which I would
>prefer. I dunno, the fuse might not even be fast enough to prevent the
>diode from exploding.
You could use a 1N5403 and it would be more rugged-- would probably work,
until they replace the fuse with a 30A type anyway.
>Wouter van Ooijen
Perhaps the series inverted MOSFET method, but I'd suggest a TVS across the
power supply in case it gets suddenly disconnected, depending on how
much capacitance you have there already. You can calculate the energy
stored in the inductance and the resulting voltage change with a given
capacitance, once you know the inductance and the current (energy = L*i^2/2,
energy = C*v*2/2.
The problem with fuses (and resettable "polyfuses" on an automotive battery
is that the fault current is rather high (the latter are typically limited
to protecting circuits with something like 40A maximum fault current).
>> How much current does it have to handle?
>
> Motor is rated 24V 250mA or something like that, peak maybe 1A.
>
>> 1) reverse-connected MOSFET in series with battery, biased so
>
> I'll try that.
>
> Wouter van Ooijen
>
> -- -------------------------------------------
> Van Ooijen Technische Informatica: http://www.voti.nl
> consultancy, development, PICmicro products
> docent Hogeschool van Utrecht: http://www.voti.nl/hvu
>
>
> Hi Paul.
>
> You will loose the ground through the bridge.
>
>
>> Why not use a bridge rectifier?
>> That way it doesn't matter which way
>> it's connected. And you can use either AC or DC.
> --
> A customer let the smoke out of a H-bride motor driver I made for him,
> so now I want to add reverse-power protection. My standard anti-parallel
> 1N4004 won't do much good when connected to a lead accu, so I am
> considering a series diode (1N4004, or a schottky). But now the back-emf
> from the motor, which is fed to the power rail by the 4 diodes, has
> nowhere to go. What is the solution to this? A big zener on the power
> rail, for let's say 150% of the maximum power supply? If so, which
> wattage would be needed? Or maybe a consensator? If you, what capcity
> and what type? Or are there other solutions, maybe with a polyfuse?
How much power is in that line and can you afford a breaker ? A simple
circuit using a crowbar mosfet or thyristor can activate the breaker. Or
pop it if it has a popping coil. There should be something there because
lead acid batteries + plastic insulated wiring = fire hazard.
>
> What ground?
>
> > Hi Paul.
> >
> > You will loose the ground through the bridge.
> >
> >
> >> Why not use a bridge rectifier?
> >> That way it doesn't matter which way
> >> it's connected. And you can use either AC or DC.
> > --
> How much power is in that line and can you afford a breaker ?
> A simple
> circuit using a crowbar mosfet or thyristor can activate the
> breaker. Or
> pop it if it has a popping coil. There should be something
> there because
> lead acid batteries + plastic insulated wiring = fire hazard.
I must say that I don't know. This is for a relatively small motor
(24V/500mA) but it is powered from the battery of a wheelchair -
definitely some power there! But as said, I will go for the P-FET
option.
Wouter van Ooijen
-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu
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