transistor selection

Mr. Richard

I am looking to use a transistor to control a 12 volt 1 Amp load. I have am using a Board of Education with a BS2 stamp for the control leg of the transistor. Does someone with more experience than myself have any suggestions of an easily located (Radio Shack or similar) transistor I could use?

Is there a web site to help is this type of selection?



thanks for the help

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Magic Smoke Theory of Electronics –
Inside every electronic part there is magic smoke.
The magic smoke is what makes everything work.
If you release the magic smoke, the part stops working!

Mike Green

The easiest device to use would be an IRF510 MOSFET (Radio Shack www.radioshack.com/product/index.jsp?productId=2062618&cp=2032058.2032230.2032279&parentPage=family). You connect the gate to the BS2 pin, the source to ground and the drain to the load. The other end of the load connects to +12V. The IRF510 already has a protective diode built-in for inductive loads (relays, motors, etc.) although it's always a good idea to put a separate one across the load (backwards, with the cathode to +12V and the anode towards ground). Radio Shack also carries these (1N4001 diodes www.radioshack.com/product/index.jsp?productId=2036268&cp=2032058.2032230.2032269&pg=2&parentPage=family).

I'd suggest connecting a resistor between the gate and ground to keep the IRF510 turned off until your program does a HIGH. Any value from 10K to 100K should work.

In terms of selection: You want a device with a high enough voltage rating (probably 2-3 times that of the circuit involved) and a high enough current rating (consider that many loads have a peak current and an average current ... the transistor should be rated for the peak current). Any switch has some resistance including transistors. You're generally interested in the saturation voltage drop or the "on" voltage drop. That times the peak current gives the peak power that the switch has to dissipate. The IRF510 has an "on" resistance of about 0.5 ohms. At 1 amp, that's a 0.5V drop which comes out to 1/2 watt dissipation. You may want a small heatsink if the load is going to be on for long periods of time.

Post Edited (Mike Green) : 3/14/2007 3:20:07 PM GMT

MikeK

The 0.5 ohms Drain-Source ("on") resistance is spec'ed for Gate-Source voltage of 10 volts. Driving this with logic levels (5V) will give a higher "on" resistance. The IRL510 (L=Logic) is made for use with logic-level inputs. For the IRL510, the "on" resistance is 0.5 ohms when the Gate-Source voltage is 5V (0.75 ohms at 4V).

Mike

Mr. Richard

Thanks Mike and Mike

I really have no experience with the MOSFET, but will gladly buy the IRF510 and have a go at it.

I will not be closing the switch for that long of a time. I have a bot I am building that will drive around a CAD classroom and "shoot" kids with Starburst candies. This will be used to control a solenoid to fire a pneumatic piston.

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Magic Smoke Theory of Electronics –
Inside every electronic part there is magic smoke.
The magic smoke is what makes everything work.
If you release the magic smoke, the part stops working!

Mike Green

MikeK makes a good point. You should get the IRL510 rather than the IRF510. Radio Shack just carries the IRF510. Jameco (www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=209058&pa=209058PS) carries the IRL520N which is similar to the IRL510 that MikeK mentioned and should work better for your application.

Another option would be to use a conventional NPN power transistor like the TIP3055 from RS (www.radioshack.com/product/index.jsp?parentPage=family&summary=summary&techSpecs=techSpecs&currentTab=summary&cp=2032058.2032230.2032279&custRatings=custRatings&features=features&accessories=accessories&productId=2062611&support=support&tab=techSpecs). This can also handle the voltage and current involved and would need about 20ma of base current. You would a 220 ohm series resistor in the base lead (to the Stamp).

With a short duty cycle (firing a pneumatic piston), you should not need a heatsink.

Mr. Richard

thanks for the help Gentlemen

Chris Savage

If you really want to use a Transistor the TIP120 would be able to handle that load and is common (Radio Shack may still carry them).

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Chris Savage
Parallax Tech Support

QuattroRS4

There is another option here - you could use a small photovoltaic (SSR) instead ...
I posted a sample schematic and PDF of a 'pvdz172n' in the following thread

http://forums.parallax.com/showthread.php?p=632696

this has the following advantages -
1. Electrical isolation - as it is optically coupled
2. Capable of driving up to 1.5amps
3. 60v DC capable
4. Integrated reverse biased diode for protection when using inductive loads..
5. Uses a MOSFET switching stage.

Quattro

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'Necessity is the mother of invention'

Post Edited (QuattroRS4) : 3/16/2007 7:32:53 PM GMT

OzStamp

Hi quattro

Re IR HEXFET PVDZ172N unit you suggested..
Have you used these on high speed solenoids ?? MAC valve approx 170mA at 24VDC

I looked at the spec sheet and a little surprised about the 2mS response ..
that seems a little slow ..

Ronald Nollet Australia

QuattroRS4

OZ,
I specifically use these for standard operations i.e. DC motor drives or pneumatic solenoid switching where time is not critical.
As you are aware - mission critical or time sensitive operations do not lend themselves well to opto isolation due to the inherent time delays.

These are ideal for the task outlined here .
I would be interested in your feedback if you were to try this device on a few applications..I have used them for years and have yet to have a failure..

Quattro

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'Necessity is the mother of invention'

Mr. Richard

so much silicon, so little time! Thank you all for your help.

Is there a resource to help in the selection of solid-state switches? I am looking for a web page or a book where I can select options like current, power consumption, ect and have a few suggestions of switched to use.

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Magic Smoke Theory of Electronics –
Inside every electronic part there is magic smoke.
The magic smoke is what makes everything work.
If you release the magic smoke, the part stops working!