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'[EE] Switching solenoids'
2011\05\06@115331 by PICdude

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I need to switch a number of solenoids from a PIC, and the solenoids  will be normally-off unless told otherwise.  I'm considering on-board  relays vs. MOSFETs.  MOSFETs would be nicer due to the smaller space  requirement, but I understand that in case of failure, MOSFETs usually  fail closed whereas relays usually fail open (unless contacts get  welded, etc).

How true is this?

I'd think perhaps I can come up with some arrangement of two MOSFETs  in series to drive each solenoid (for much higher reliability), but  that drives cost up noticeably, and then I'm not sure there'd be any  space benefits.

Are there any special MOSFETs that are different in this respect?

Thanks,
-Neil.

2011\05\06@121117 by Michael Watterson

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On 06/05/2011 16:53, PICdude wrote:
>    I'm considering on-board
> relays vs. MOSFETs.

But you need MOSFETS or BJTs anyway to drive the relays.

If you choose suitable MOSFETS, snubber and/or diode etc driving the solenoids direct can be more reliable.

But there is a big difference between say 12V 500mA solenoids and 110V 10A ones.


You don't say what voltage/current of solenoid.

You can also monitor the MOSFET output via resistor divider (even OR them all with diodes)  and have an alarm or something if a FET fails.

900V 10A MOSFETs are < $

2011\05\06@132157 by Dwayne Reid

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At 10:11 AM 5/6/2011, Michael Watterson wrote:

>900V 10A MOSFETs are < $2

OTOH - JS1 relays (SPDT 10A max) cost us less than $0.50 in quantity <grin>..

dwayne

-- Dwayne Reid   <spam_OUTdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2011\05\06@142520 by Olin Lathrop

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PICdude wrote:
> I need to switch a number of solenoids from a PIC, and the solenoids
> will be normally-off unless told otherwise.  I'm considering on-board
> relays vs. MOSFETs.  MOSFETs would be nicer due to the smaller space
> requirement, but I understand that in case of failure, MOSFETs usually
> fail closed whereas relays usually fail open (unless contacts get
> welded, etc).

A mechanical system like a relay is going to be less reliable than a
transistor IF the transistor is not being abused.  A properly used
transistor should last indefinitely for most practial purposes.  With a
inductive load like a solenoid, you have to make really sure the stored
energy in the coil is handle properly when shut off.

The best thing would be to use transistors and make sure they are allways
run well within specs.  What are the voltage and current requirements of the
solenoids?  Why do you think you need FETs?  Why wouldn't a bipolar be able
to do the job just fine?  How fast do the solenoids need to respond to off
and on switch commands?


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000

2011\05\06@143403 by PICdude

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Good point, but somehow I'm thinking/expecting a mosfet switching a  larger load such as a solenoid failing easier than one switching a  small relay.  Of course, the choice of mosfet in each situation would  make a big difference.

I need to switch 12V 10A.  The cost of any average MOSFETs that can  handle this, is similar to that of the relays.

Cheers,
-Neil.



Quoting Michael Watterson <.....mikeKILLspamspam@spam@radioway.org>:

{Quote hidden}

>

2011\05\06@143843 by PICdude

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The relays I'd use are automotive-spec relays, engineered to handle  voltage spikes, vibration, etc, handle 20A, and will be around $1 each  in quantity.

Cheers,
-Neil.


Quoting Dwayne Reid <dwaynerspamKILLspamplanet.eon.net>:

{Quote hidden}

>

2011\05\06@184048 by Brent Brown

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Would you consider "intelligent" MOSFET's? Would give you a space saving over relay, but possibly more expensive. One I've recently used (for driving solenoids) is the ST Microelectronics VNQ690SP, 4 channel high side solid state relay, nominal 10A, 36V, fully protected.

www.st.com/internet/automotive/product/75329.jsp
www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERAT
URE/DATASHEET/CD00003299.pdf

On 6 May 2011 at 11:34, PICdude wrote:
{Quote hidden}

-- Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
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2011\05\07@000806 by V G

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On Fri, May 6, 2011 at 2:25 PM, Olin Lathrop <@spam@olin_piclistKILLspamspamembedinc.com> wrote:
> A mechanical system like a relay is going to be less reliable than a
> transistor IF the transistor is not being abused.  A properly used
> transistor should last indefinitely for most practial purposes.  With a
> inductive load like a solenoid, you have to make really sure the stored
> energy in the coil is handle properly when shut off.
>
> The best thing would be to use transistors and make sure they are allways
> run well within specs.  What are the voltage and current requirements of the
> solenoids?  Why do you think you need FETs?  Why wouldn't a bipolar be able
> to do the job just fine?  How fast do the solenoids need to respond to off
> and on switch commands?

In this case, what would be the difference here in using a MOSFET or a BJT?

2011\05\07@010747 by IVP

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> In this case, what would be the difference here in using a
> MOSFET or a BJT?

Depends. For the clocks I build and service which have solenoid
ratchet pulse drives (about 3A continuous), I use medium-power
Darlingtons, like the TIP121, with a catch diode across the coil

They require around 100mA base current for an Ic of 5A. A large
BJT like a TIP3055 needs many more times that for 10A Ic because
of the low gain. Plus there's the voltage drop across the transistor,
which would be at least 1V at high current. This will generate heat
and need to be dissipated

OTOH, a FET needs little gate current and when turned hard on
will not generate much heat if it has low Ron

Jo

2011\05\07@100902 by Olin Lathrop

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V G wrote:
> In this case, what would be the difference here in using a MOSFET or
> a BJT?

Like everything, its a tradeoff.  There are mainly two issues to consider in
this case, power dissipation when on and driving the transistor.

A FET looks largely like a low value resistor when on.  In fact, that's what
the Rdson spec is (resistance drain to source when on).  If Rdson is
100mOhms, then at 10A the FET will drop 1V and dissipate 10W.  Of course
100mOhms is quite high for a good FET rated only for low voltage, like 20V.
If it's 10mOhms then the voltage drop will be 100mV and it will dissipate
1W.

A bipolar looks more like a voltage source when saturated, although there
will be enough of a resistive component to matter at 10A.  At low currents a
power transistor can probably do less than 100mV.  At 10A a good one
probably a few 100mV.  At 500mV it would dissipate 5W.

Both FETs and bipolars will have better on-state specs when rated for lower
off-state voltages.  Bipolars can be made to have higher gain when the C-E
voltage doesn't need to be high, and FETs lower Rdson when the D-S voltage
doesn't need to be high.

The other major issue to consider is how to drive the transistors.  A FET is
driven with voltage on the gate while a bipolar requires current thru the
base.  The problem with the voltage drive of a FET is that the voltage is
often inconvenient for a microcontroller.  To get otherwise the best specs,
a good FET will want 10-15V on the gate to be fully on.  Particularly at low
D-S voltage ratings, FETs can be made to require lower gate voltage.  Those
that work well with only 5V on the gate are sometimes called "logic level"
FETs.  You can get some pretty amazing FETs if you only require 20 or 30
volts capability.  Take a look at the IRLML2502 or IRLML0030 for example.
However impressive, they can't do 10A.  10A is a lot, and to keep the
disspation down you probably buy into the more complicated full voltage gate
drive in this case.

A bipolar can be driven easily from low voltage circuits since the input
signal is essentially the current thru what looks like a silicon diode to
the driving circuit.  The base voltage even at high base currents will be
under 1V, usually around 750mV.  The problem here is that this current must
be sustained continuously while the transistor is on, and can be substantial
in the context of a ordinary logic circuit.  Small signal transistors can
easily have gains of 100 or more (1A collector current requires only 10mA
base current), but transistors designed for high current like 10A are going
to have lower gain.  The fact that it only needs to be rated for 20 or 30
volts will help.  That allows tradeoffs in the design of the transistor in
favor of higher gain.  You can probably find a NPN transistor with a gain of
around 100 in this range.  However, that still means you have to come up
with 100mA base drive to get 10A collector current.  That's doable, but not
straight out of a microcontroller pin.

There is no free lunch.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000

2011\05\07@120643 by RussellMc

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> Would you consider "intelligent" MOSFET's? Would give you a space saving over
> relay, but possibly more expensive. One I've recently used (for driving solenoids) is
> the ST Microelectronics VNQ690SP, 4 channel high side solid state relay, nominal
> 10A, 36V, fully protected.

I tried connecting a similar device directly across a  car battery and
turning it on. It survived without complaint. That sort of protection
is "useful".

A properly rated and protected MOSFET would work well in your application.

I'd personally usually use a MOSFET rather than a bipolar (both being
"transistors") in that sort of application, due to the greater ease of
drive in most cases wrt bipolars.

MOSFETs can and do fail any way they wish BUT do seem to have a soft
spot for S/C Drain-Source.   Sometimes hard S/C Drain-Source-Gate.
Designing the circuit so they are not over rated usually results in
very high reliability.



    Russel

2011\05\07@190848 by PICdude

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I was thinking FET's would be better on power dissipation.  Why would  you veer towards bipolars?

Cheers,
-Neil.


Quoting Olin Lathrop <KILLspamolin_piclistKILLspamspamembedinc.com>:

{Quote hidden}

2011\05\08@093424 by PICdude

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Not sure what an "intelligent MOSFET" is, but you're linking to a  solid-state relay.  These specific ones don't seem to be available  from my usual sources (Dk, Mous, etc), but other solid state relays  are in the US$25 range, and that's a significant jump from MOSFET's,  transistors, or relays at ~$1 each.  With several of these in each  unit, the price diff is significant.

Cheers,
-Neil.



Quoting Brent Brown <RemoveMEbrent.brownTakeThisOuTspamclear.net.nz>:

> Would you consider "intelligent" MOSFET's? Would give you a space saving over
> relay, but possibly more expensive. One I've recently used (for  
> driving solenoids) is
> the ST Microelectronics VNQ690SP, 4 channel high side solid state  
> relay, nominal
> 10A, 36V, fully protected.
>
> www.st.com/internet/automotive/product/75329.jsp
> www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERAT
> URE/DATASHEET/CD00003299.pdf

2011\05\08@154236 by cdb

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:: Not sure what an "intelligent MOSFET" is

It is a MOSFET which has various types of protection circuitry built in. TOPFET's are one kind - here is the datasheet on the BUK202

<http://docs-asia.electrocomponents.com/webdocs/009a/0900766b8009a48a.pdf>

Also check out BTS240A and ZXMS6003GTA. Last one is about 40c each and is SMD.

Colin
--
cdb, spamBeGonecolinspamBeGonespambtech-online.co.uk on 9/05/2011
Web presence: http://www.btech-online.co.uk   Hosted by:  http://www.justhost.com.au
 

2011\05\08@182827 by Brent Brown

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Hi Neil,

OK, solid state relay is something different I think... they are a drop in replacement for a relay and switch AC or DC and provide isolation, and are coniderably more expensive than a relay (but offer the advantages of silent operation, longer life, and lower drive current).

On the other hand an Intelligent MOSFET, smart high side driver, or other variation of these names, is DC only, consists of a N-ch MOSFET switch and supporting circuitry to do high side driving and a bunch of protection features such as over current, over voltage, under voltage, over temperature. Some have diagnostic feedback.

http://www.mouser.com/Search/Refine.aspx?Keyword=vnq690

VNQ690SPTR-E USD3.91 each 500+, so if your quantity is up there then it's in the ballpark. One device replaces 4 relays. They nicely handle the clamping of inductive loads at switch off, so you don't need any suppression components (although you do have to consider the energy being clamped and associated power dissipation).

As Russell noted, they do what they say they do in terms of handling short circuit etc, which is nice... typically no need to put a fuse on each output. More or less bullet proof for most low voltage DC applications, but not appropriate for use where a potential fault could see mains AC power applied to an output.

In the comparison the main advantages of a relay are isolation, AC switching capability, and often price as well.

On 8 May 2011 at 6:34, PICdude wrote:

{Quote hidden}

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