MOSFET for switching 2A 12V?

[Mark Hindley]

Hi,

I need to switch an array of 5 lamps (sailing boat navigation lights) in
6 different configurations on at nominal 12V (lead acid) system. The
highest load on a single circuit is 25W. I need to switch on the high
side with a common negative ground.

There are plenty of 6 position rotary switches that will carry the load,
but they are not rated to switch it. The few that are are very
expensive (>£30).

I need minimal (<0.1 Ohm) loss in the switch as the cable runs to the
lamps are long (up to 10m to the top of the mast)

Battery capacity is limited (c 20Ah usable) so I don't want the extra
drain of 5 relays which would be significant over 12 hours. I have also
seen the smart protected mosfets, but they still have that same
disadvantage of additional drain.

It has occurred to me that I might be able to do this with a lower rated
switch and power mosfets. I have never used them before, so would be
grateful for someone more experienced to advise.

Plan/questions:

1) Presumably to switch high side I would need P channel with source pos
and drain to load.

2) Ground gate for on. Do I leave gate floating or tie it high for off?

3) Is it right that once the capacitance is filled, the gate current
should be virtually nil?

4) I have found FAIRCHILD FQP17P06
(http://www.fairchildsemi.com/ds/FQ/FQP17P06.pdf) which will handle
the load and also has a Vgss of 25V which I assume means I can drive it
directly.

5) If I have a 5A fuse in the supply and reverse bias a diode between
the supply and ground will that give enough protection against
accidental reverse polarity connection and short?

6) Does the system require any other protection?

Any other thoughts/better solutions/things I have forgotten?

Thanks,

Mark

 

[default]

Hi,

I need to switch an array of 5 lamps (sailing boat navigation lights) in
6 different configurations on at nominal 12V (lead acid) system. The
highest load on a single circuit is 25W. I need to switch on the high
side with a common negative ground.

There are plenty of 6 position rotary switches that will carry the load,
but they are not rated to switch it. The few that are are very
expensive (>£30).

I need minimal (<0.1 Ohm) loss in the switch as the cable runs to the
lamps are long (up to 10m to the top of the mast)

Battery capacity is limited (c 20Ah usable) so I don't want the extra
drain of 5 relays which would be significant over 12 hours. I have also
seen the smart protected mosfets, but they still have that same
disadvantage of additional drain.

It has occurred to me that I might be able to do this with a lower rated
switch and power mosfets. I have never used them before, so would be
grateful for someone more experienced to advise.

Plan/questions:

1) Presumably to switch high side I would need P channel with source pos
and drain to load.

2) Ground gate for on. Do I leave gate floating or tie it high for off?

3) Is it right that once the capacitance is filled, the gate current
should be virtually nil?

4) I have found FAIRCHILD FQP17P06
(http://www.fairchildsemi.com/ds/FQ/FQP17P06.pdf) which will handle
the load and also has a Vgss of 25V which I assume means I can drive it
directly.

5) If I have a 5A fuse in the supply and reverse bias a diode between
the supply and ground will that give enough protection against
accidental reverse polarity connection and short?

6) Does the system require any other protection?

Any other thoughts/better solutions/things I have forgotten?

Thanks,

Mark

There are plenty of toggle switches that will probably do what you
want with a lot fewer problems.

Any short may destroy a mosfet. I have my turn signals on a 30 amp
mosfet with a 10 amp fuse - when the fuse blows the mosfet shorts.
There may be some inductive kick in the 3 feet of wire involved, and
that may be causing the mosfet to fail when there's a short (normally
only a 4 amp load).

You shouldn't let the gate float - the gate impedance is very high and
it will toggle with RFI and the high switching speed and inductances
involved may destroy the mosfet - and the mosfet may stay on for a
long time with no drive once activated until the charge bleeds off -
while operating in its linear range. bias it high in the case of a P
channel.

Yes, the gate current is virtually non existent - nano amps. Leakage
in the wiring may be higher.

Check out
http://www.alliedelec.com/Images/Products/Datasheets/BM/AROMAT/Aromat_Industrial-Control_7880258.pdf

That is a device that can be used to isolate an N or P channel device
in a high side switch - then your choice of mosfets is greater.
Another possibility is to locate the mosfets closer to the point of
use and run very heavy wire out before branching to individual
circuits. Notice the internal gate discharge resistor in the
schematic?

All that said - I'd use toggle switches. Easy to find to switch the
current you need, no semiconductors involved, easy to find a
replacement anywhere. Then there's the idea of putting a mosfet
anywhere near a mast where static electricity or lightening could be
involved.

And you can use steering diodes to make the toggle switches switch
different combinations of lights. A lot more rugged and foolproof in
my opinion - especially out on the water.

 

[default]

I need to switch an array of 5 lamps

How about an array of LEDs? I replaced two 1157 lamps in my brake
lights with Four Cree 1.5 watt LEDs and have about twice the
brightness with less than half the power - Two LEDs would have been
more than enough. Current with the brake on went from 4.2 amps to .7
amps and running lights from 1.2 amps to .05 amps. They come in red,
green, amber and white.

I added a really impressive white running light to my motorbike - 56
white leds - 1/4 amp - cost about $9 US.

Kerosene lantern makes a good anchor light run it up on a halyard
attached to the spreader between the mast and shroud.

 

[neon]

You should consider LED as opposed to 12 v lamps a cluster of these guys will surpass the lighting requirement, expecialy super bright and the power can be reduced to 1/4 of an amp.

 

[Jamie]

Hi,

I need to switch an array of 5 lamps (sailing boat navigation lights) in
6 different configurations on at nominal 12V (lead acid) system. The
highest load on a single circuit is 25W. I need to switch on the high
side with a common negative ground.

There are plenty of 6 position rotary switches that will carry the load,
but they are not rated to switch it. The few that are are very
expensive (>£30).

I need minimal (<0.1 Ohm) loss in the switch as the cable runs to the
lamps are long (up to 10m to the top of the mast)

Battery capacity is limited (c 20Ah usable) so I don't want the extra
drain of 5 relays which would be significant over 12 hours. I have also
seen the smart protected mosfets, but they still have that same
disadvantage of additional drain.

It has occurred to me that I might be able to do this with a lower rated
switch and power mosfets. I have never used them before, so would be
grateful for someone more experienced to advise.

Plan/questions:

1) Presumably to switch high side I would need P channel with source pos
and drain to load.

2) Ground gate for on. Do I leave gate floating or tie it high for off?

3) Is it right that once the capacitance is filled, the gate current
should be virtually nil?

4) I have found FAIRCHILD FQP17P06
(http://www.fairchildsemi.com/ds/FQ/FQP17P06.pdf) which will handle
the load and also has a Vgss of 25V which I assume means I can drive it
directly.

5) If I have a 5A fuse in the supply and reverse bias a diode between
the supply and ground will that give enough protection against
accidental reverse polarity connection and short?

6) Does the system require any other protection?

Any other thoughts/better solutions/things I have forgotten?

Thanks,

Mark

looks good, make sure you use a pull up resistor on each input.

 

[Wrighty]

A P channel MOSFET would work fine providing you are driving it from a
12 volt source.

http://www.ckp-railways.talktalk.net/pcbcad17.htm

 

[Homer J Simpson]

Battery capacity is limited (c 20Ah usable) so I don't want the extra
drain of 5 relays which would be significant over 12 hours.

Have you looked for 2 amp latching relays? DC rated of course.



--
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+ damaged idiots.
+
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+ Port: 119, Secure(SSL): No, Server Error: 441,
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[martin griffith]

Hi,

I need to switch an array of 5 lamps (sailing boat navigation lights) in
6 different configurations on at nominal 12V (lead acid) system. The
highest load on a single circuit is 25W. I need to switch on the high
side with a common negative ground.

There are plenty of 6 position rotary switches that will carry the load,
but they are not rated to switch it. The few that are are very
expensive (>£30).

I need minimal (<0.1 Ohm) loss in the switch as the cable runs to the
lamps are long (up to 10m to the top of the mast)

Battery capacity is limited (c 20Ah usable) so I don't want the extra
drain of 5 relays which would be significant over 12 hours. I have also
seen the smart protected mosfets, but they still have that same
disadvantage of additional drain.

It has occurred to me that I might be able to do this with a lower rated
switch and power mosfets. I have never used them before, so would be
grateful for someone more experienced to advise.

Plan/questions:

1) Presumably to switch high side I would need P channel with source pos
and drain to load.

2) Ground gate for on. Do I leave gate floating or tie it high for off?

3) Is it right that once the capacitance is filled, the gate current
should be virtually nil?

4) I have found FAIRCHILD FQP17P06
(http://www.fairchildsemi.com/ds/FQ/FQP17P06.pdf) which will handle
the load and also has a Vgss of 25V which I assume means I can drive it
directly.

5) If I have a 5A fuse in the supply and reverse bias a diode between
the supply and ground will that give enough protection against
accidental reverse polarity connection and short?

6) Does the system require any other protection?

Any other thoughts/better solutions/things I have forgotten?

Thanks,

Mark

have look at some of the Linear stuff
LT1910
LTC1255


martin

 

[Winfield Hill]

Mark Hindley wrote...

I need to switch an array of 5 lamps (sailing boat navigation lights) in
6 different configurations on at nominal 12V (lead acid) system. The
highest load on a single circuit is 25W. I need to switch on the high
side with a common negative ground.

I need minimal (<0.1 Ohm) loss in the switch as the cable runs to the
lamps are long (up to 10m to the top of the mast)

Battery capacity is limited (c 20Ah usable) so I don't want the extra
drain of 5 relays which would be significant over 12 hours. I have also
seen the smart protected mosfets, but they still have that same
disadvantage of additional drain.

I think intelligent hi-side MOSFET switches are a good choice,
because they are robust against all kinds of possible issues.

The Infineon BTSxxx types I like draw only 15uA when off, and
2mA per switch when on. That should be acceptable. You could
consider the BTS611 or the BTS621. They're inexpensive dual-
switch ICs and are in stock at DigiKey, about $1.60 per channel.

The '611 switches have an Ron of 0.16 ohms and a current-limit
of 4A, compared to 0.08 ohms and 8A for the '621. They feature
a useful STatus fault output signal, which includes open-load
detection (under 200 or 400mA, resp.), i.e., for a burned-out
light-bulb. This signal is open-drain (like open-collector),
so you can wire them together to create a single FAULT signal,
telling you to check all the lights. Nice!

 

[Donald]

I added a really impressive white running light to my motorbike - 56
white leds - 1/4 amp - cost about $9 US.

Do you have a link for these ?

$9.00 / 56 ~= $0.16 ea

What are the brightness of these as well.

Thanks

donald

 

[default]

Do you have a link for these ?

$9.00 / 56 ~= $0.16 ea

What are the brightness of these as well.

Thanks

donald

http://www.ledshoppe.com/ they were 12 cents each - I had some epoxy,
resistors and some aluminum flashing for a case so I estimate about $9
total.

5mm 13,000 mcd white 10 degree beam angle

http://www.ledshoppe.com/led5mm.htm - the page with all the leds

They are in Hong Kong, ship for free, takes about 14 days to receive -
I've placed two orders with them and have been happy with the results.

Word of caution - they or their credit card clearing house, sent an
awful lot of spam to my throw away email address. Credit card is
handled by CCnow in the US. I didn't give them my correct phone
number - SOP with me.

 

[Donald]

http://www.ledshoppe.com/ they were 12 cents each - I had some epoxy,
resistors and some aluminum flashing for a case so I estimate about $9
total.

5mm 13,000 mcd white 10 degree beam angle

http://www.ledshoppe.com/led5mm.htm - the page with all the leds

They are in Hong Kong, ship for free, takes about 14 days to receive -
I've placed two orders with them and have been happy with the results.

Word of caution - they or their credit card clearing house, sent an
awful lot of spam to my throw away email address. Credit card is
handled by CCnow in the US. I didn't give them my correct phone
number - SOP with me.

Thanks for that good advice.

I have seen this web site before, but have been nervous about ordering
through them.

donald

 

[martin griffith]

On 5 Nov 2006 11:41:45 -0800, in sci.electronics.design Winfield Hill

The Infineon BTSxxx types I like draw only 15uA when off, and
2mA per switch when on. That should be acceptable. You could
consider the BTS611 or the BTS621. They're inexpensive dual-
switch ICs and are in stock at DigiKey, about $1.60 per channel.

The '611 switches have an Ron of 0.16 ohms and a current-limit
of 4A, compared to 0.08 ohms and 8A for the '621. They feature
a useful STatus fault output signal, which includes open-load
detection (under 200 or 400mA, resp.), i.e., for a burned-out
light-bulb. This signal is open-drain (like open-collector),
so you can wire them together to create a single FAULT signal,
telling you to check all the lights. Nice!

just looked up the 611's. Might be better value than the Linear stuff

"In order to provide you with a better service, we are currently
upgrading our servers."


I wonder what better service means....like the Philips/NXpee website?

Happy election, hope the crooks dont get in


martin

 

[[email protected]]

I need to switch an array of 5 lamps (sailing boat navigation lights) in
6 different configurations on at nominal 12V (lead acid) system.

There's no need for MOSFETs, a simple PNP darlington transistor will do
fine; TIP32C is about fifty cents. Use a base resistor to limit the
current
and a base/collector capacitor to prevent RF oscillation. A high-side
switch
P-MOSFET gate will require some protection in case of voltage surges
(I'm assuming
the boat has similar load-dump power issues as an automobile), but
the TIP32 will tolerate higher voltages and spikes fine.

Use one darlington/base series resistor for each lamp, emitter to +12,
collector to lamp, base through limiting resistor (4.3 kOhms) to input.

You have to select configurations so a circular switch with six
positions
will be the selector; ground the common point of the switch, and
connect
the switched points through diodes to the inputs corresponding to the
lamps to be lit for that configuration. You could open a switch on the
grounded common terminal to douse all lamps.

Is that clear? For each switch position, a single terminal is grounded
which
pulls current through isolation diodes from some subset of the switch
transistors;
the diodes keep the switch connections from configuration 2 from
cross-connecting
the lamps when configuration 4 is selected...

I'd consider a fuse for each section, or a thermal-reset circuit
breaker for the
ensemble, if it were up to me. It's easy for a bulb to get smashed and
short out.

 

[Mark Hindley]

There's no need for MOSFETs, a simple PNP darlington transistor will do
fine; TIP32C is about fifty cents. Use a base resistor to limit the
current

Thanks.

The problem with that is that I would lose about 2V across the
darlington pair. With long cable runs already reducing useful output, I
would be reluctant add more wastage.

Or have I misunderstood?

Mark

 

[[email protected]]

The problem with that is that I would lose about 2V across the
darlington pair.

It's actually more like 1V (and I goofed the part number; it's TIP127
that's the
PNP darlington...); the 2V number is a worst-case across temperature.
As others
have noted, there are integrated high-side switches that might be the
best solution.

MOSFET drive sounds so easy, but a sump pump's brush noise can
blow that sensitive gate in a millisecond if one doesn't design in
protection.
Another possibility is a two-transistor pseudo-darlington (NPN first,
PNP second)
but adds biasing resistors.