I have a set of sensors and actuators all operating at +5v and draw 10mA to
680mA current. My microcontroller will stwitch these units ON or OFF in various configuration. My microcontroller has enough output port to drive switching circuits.How can I switch these unit ON/OFF without using mechanical relay? What is the electronic equivalent of relay ? (May be FET/MOSFET ? How to drive them?)
MOSFETs are good start. Try something like a MTP2955, IIRC it has quite a low on resistance only about 0.2 ohms and a threshold voltage of about 3 volts. Its the cheapest MOSFET I can think of. This is device is a PChannel device so let me see... tie the source to the 5V, the drain to the Vcc of the things you want to drive and as for the gate, attaching it to the output pin of you MC you give you what you want. Driving the line high should turn it off and driving it low should turn the mosfet on. Depending on the configuration of your MC, you may need pullup a pullup resistor at the output pin of the MC.
I think my advice is correct, but don't take my word on it. Feel free to get a second or third opinion.
-- Wing Wong.
There's nothing wrong with using discrete mosfets for this but if you want some of the work done for you I have found BTS 612 N1 from Siemens handy for similar applications. they need no extra components. A couple of things to watch with this device: they shut down (brownout detection) if the supply goes below 5V. Also if your processor is 3.3V that is right on the edge of the turn on threshold, but I find they work with my
3.3V processors.Johan Harold wrote:
If you use a power MOSFET, put a 100R resistor in series with the gate. This prevents ringing at switchon due to the high gatd capacitance of power MOSFETs
Johan Harold wrote:
Check out Fairchild's FDG312P. Rds(on) = 0.18ohms @ Vgs = -4.5V, Vdss max =
-20V, Id = -1.2A and it's in a SC70-6 package. Sneeze and its gone.. 8-)
Or even Fairchild FDV range logic switching FETs in SOT-23 packages
Rds Id Id (pulsed) FDV304P 1.1R 460mA 1.5A (4.5V Vds) FDV302P 10R 120mA 500mA (4.5V Vds) FDV301N 4R 220mA 500mA (4.5V Vds) FDV303N 0.45R 680mA 2A (4.5V Vds)
Gate switching < 1.5V so ideal for low load switching in ONE device. I tend to use FDV304P and FDV303N for lower Rds and higher Id
Available in small quantities Digikey, Farnell......
-- Paul Carpenter | snipped-for-privacy@pcserviceselectronics.co.uk PC Services GNU H8 & mailing list info For those web sites you hate
The others have responded regarding solid state switches (MOSFETs would be my preference, too, though a ULN2003 array would take up less space if you need more than one or two), but to add to the confusion: is there a reason to shy away from relays? If only 680mA you could probably get away with smaller reed relays. You can't hear them, they last for a lot of switchings, and they don't need heat sinks. One thing, though, is that you can't always drive a bunch of them at the same time from your MCU ports. One at a time is usually okay, but of course, check the specs to be sure.
-- Gordon
Johan Harold wrote:
Yes you can use a logic-level MOSFET (p-channel for switching +5V or n-channel for switching the ground). There are some nice dual low-Rds(on) ones in small 8-pin SMT packages.
Take care with bypass capacitors on the switched side. These parts are so low resistance that if you switch them quickly the bypass caps on the switched side can put a glitch on your unswitched power supply.
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
What are the sensors and actuators? Can you get away with a cheap trasistor?
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