MOSFET for switching 2A 12V?

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 (

Reply to
Mark Hindley
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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

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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.

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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.

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

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Reply to
Jamie

A P channel MOSFET would work fine providing you are driving it from a

12 volt source.

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Mark H> Hi,

Reply to
Wrighty

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

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Reply to
Homer J Simpson

have look at some of the Linear stuff LT1910 LTC1255

martin

Reply to
martin griffith

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!

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Reply to
Winfield Hill

Do you have a link for these ?

$9.00 / 56 ~= $0.16 ea

What are the brightness of these as well.

Thanks

donald

Reply to
Donald

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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

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- 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.

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Thanks for that good advice.

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

donald

Reply to
Donald

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

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

Reply to
martin griffith

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.

Reply to
whit3rd

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

Reply to
Mark Hindley

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.

Reply to
whit3rd

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