Transistor Selection Help (Experts only please)
beazleybub
Posts: 102
Let me start out by saying in no way am I am electrical engenieer and I need some help here.
Can someone explain to me how to choose the right NPN transistor?
The relay is an OUAZ-SH-105D,405 and runs on 5V http://relays.tycoelectronics.com/datasheets/OUAZ.pdf
The relays coil current @ 5V is 90.9 mA
I know that I need a transistor that the maximum collector current Ic(max) is 100mA
The problem I am having is calculating the required hFE for the transistor.
(Nor do I know what hFE stands for)
What was reccomended to me was that the transistor's minimum current gain hFE (min) must be at least five times the load current Ic divided by the maximum output current from the IC.
The maximum output current for the BS2sx pin is 30mA and the·Load·Current is 90.9mA (Relay Coil)
So that would be 90.9mA / 30mA = ·3.03 x 5 = 15.15mA.
I went to digikey in the bipolar transistor section to look for a NPN and
under the part filter I see the section that says
"DC Current Gain (hFE)" (Min) @ Ic, Vce but thats where I get stuck.
I do not understand how to read that section to decide which to choose.
I know I need to choose hFE that is at least 15.15mA but the filter window says things like 9 @ 500mA, 2V.
I realize that the first part is the hFE but what's the @ 500mA ,2v? This is the part that's stumping me.
Thanks for any help in this.
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How can there be·nothing? Nothing is something!
Post Edited (beazleybub) : 10/23/2008 6:26:46 AM GMT
Can someone explain to me how to choose the right NPN transistor?
The relay is an OUAZ-SH-105D,405 and runs on 5V http://relays.tycoelectronics.com/datasheets/OUAZ.pdf
The relays coil current @ 5V is 90.9 mA
I know that I need a transistor that the maximum collector current Ic(max) is 100mA
The problem I am having is calculating the required hFE for the transistor.
(Nor do I know what hFE stands for)
What was reccomended to me was that the transistor's minimum current gain hFE (min) must be at least five times the load current Ic divided by the maximum output current from the IC.
The maximum output current for the BS2sx pin is 30mA and the·Load·Current is 90.9mA (Relay Coil)
So that would be 90.9mA / 30mA = ·3.03 x 5 = 15.15mA.
I went to digikey in the bipolar transistor section to look for a NPN and
under the part filter I see the section that says
"DC Current Gain (hFE)" (Min) @ Ic, Vce but thats where I get stuck.
I do not understand how to read that section to decide which to choose.
I know I need to choose hFE that is at least 15.15mA but the filter window says things like 9 @ 500mA, 2V.
I realize that the first part is the hFE but what's the @ 500mA ,2v? This is the part that's stumping me.
Thanks for any help in this.
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How can there be·nothing? Nothing is something!
Post Edited (beazleybub) : 10/23/2008 6:26:46 AM GMT
Comments
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Post Edited ($WMc%) : 10/23/2008 11:17:24 PM GMT
A much better choice for control by logic circuitry is the IRLZ34N. With VGS = 5V, its "on" resistance is a mere 0.046 ohms. Compare that with an "on" resistance of 0.400 ohms at VGS = 10V for the IRF510.
-Phil
http://www.nxp.com/acrobat_download/datasheets/PH2625L_2.pdf
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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!
I got my formula here http://www.kpsec.freeuk.com/trancirc.htm
The information I was going by is about 3/4 down the page at the "Choosing a suitable NPN transistor".
The guide points out "The transistor's minimum current gain hFE(min) must be at least five times the load current Ic divided by the maximum output current from the IC"
I see where you are coming from about not counting on 30mA
(Just because your Vette can go 145MPH does not mean you should push it to the limit)
Thank you
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How can there be·nothing? Nothing is something!
Post Edited (beazleybub) : 10/21/2008 7:56:15 AM GMT
1) I have successfully used a 2n222 triode to trigger a relay many times. If it won't do the trick chose a triode that will only draw 2ma or so. Not need to tax the BS output at all.
2) It seems to me a 12v relay would work better and draw less current.
3) I would avoid the use of MOS devices in this application because MOS is susceptible to damage and false triggering from voltage spikes. The coil will create all sorts of voltage spikes as will the ambient environment.
4) I would use a static switch here. I has a high input impedance which will protect the BS from overload, and the BS can be further protected by using a resistor between the static switch and the output pin. 220 ohms would do nicely. In addition the static switch will never wear out and shuts off at the zero crossing of an AC load, thus generation no arcing or high voltage transient on the load side.. I have one in an application where it is turning a resistive load on and off at every cycle of 120 v AC source, or 120 times a seconds, It has worked for 17 years approximately 65 hours a week.· They cost a few dollars more, but last many times longer and do an excellent·job of protecting both your computer and your load.· I even use solid state motor starters now unless it absolutely positively must be insanely cheap.·
Post Edited (Jesse Hasty) : 10/21/2008 12:47:33 PM GMT
I see where you got it. You'll certainly be safe at 5x the minimum needed gain. I don't think you should take it as a requirement so much as a reasonable starting point.
Jesse,
2) 5V is what is readily available, so a 5V coil voltage is the easiest to use here. If you've got 12V already, you can certainly use it, but it wouldn't work "better". Less current is not necessarily better. You're not talking about a lot of current (100mA).
3) MOSFETs are excellent in this application. All transistors are susceptible to damage. You have to be careful about static, but that's not a big deal. Before gate diodes were used to protect MOS gates, this was a real problem. Now most MOS transistors are pretty resistant. Preventing false triggering from noise is not a big problem either. If necessary, use a pulldown resistor. You need a diode across the relay coil to protect against the voltage produced by the magnetic field collapse, but you'd need that for any transistor.
4) What do you mean by a "static switch"? A simple transistor switch is simple, reliable, and cheap.
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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!
Old habits die hard. I was around when the first FETs came into general availability. I once got really burned on a job because they were so susceptible to noise and destruction. That bad experience has colored my thinking to this day. Your are right, there are robust units available. In fact, the input side of the static switches I like so much are MOS devices.
Post Edited (Jesse Hasty) : 10/21/2008 1:51:34 PM GMT
Something that I have not seen mentioned in this post...· Make sure that you have a current limiting resistor on the base of your transistor if you decide to use a bi-polar.
Here is a FAQ that might help... Don't get HOT choosing the wrong transistor
Even though it references a motor in the schematic, a Relay, Light bulb, Solenoid, etc. could just as easily be substituted.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
-Phil
Post Edit -- IC ≈ IE
Post Edited (PJ Allen) : 10/21/2008 6:32:45 PM GMT
As to what the letters "hFE" stand for, I have no idea. A web search turned up nothing.
-Phil
Usually, in digital applications you don't want to keep the transistor in its linear region, you want to either turn it 'off' or 'saturate' it 'on'. When saturated, further current (beyond that needed to 'saturate' the transistor) put in the base junction doesn't result in increased current in the Collector-Emitter path, because the Collector-Emitter path is already maxed out.
Correction: D'oh, 5 * 5 == 25 mA, not 50.· Sorry about that.
Post Edited (allanlane5) : 10/21/2008 9:04:45 PM GMT
Google turned up this page on Yahoo:
http://answers.yahoo.com/question/index?qid=20080327120501AAY6jqp
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-Rusty-
--
Rusty Haddock <=> AE5AE
**Out yonder in the Van Alstyne (TX) Metropolitan Area**
Microsoft is to software what McDonalds is to gourmet cooking
http://answers.yahoo.com/question/index?qid=20080327120501AAY6jqp
P.S. - Phil, while not as efficient, have you ever used the IRL520 MOSFET in logic control?
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Chris Savage
Parallax Engineering
Post Edited (Chris Savage (Parallax)) : 10/21/2008 8:49:53 PM GMT
I've never used the IRL520. From the specs, it looks like a logic-switchable equivalent to the IRF520 (the IRF510's bigger brother), so it should work fine in logic control apps. It has one distinct advantage over the IRLZ34N: much lower gate capacitance, which means it can be switched faster (e.g. in PWM circuits) without special driver circuitry. It's "on" state resistance is about 6x that of the '34', however. Nothing good comes without tradeoffs!
-Phil
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'Just a few PropSTICK Kit bare PCBs left!
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Chris Savage
Parallax Engineering
A 1K-4.7KΩ between the Stamp output and the transistor Base and be done with it.
Post Edit -- IB = ( Vstamp_out - VBE ) / RB
Or, in other words:
· IB = ( Vstamp_out - 0.7 ) / RB
Post Edited (allanlane5) : 10/22/2008 6:52:35 PM GMT
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Chris Savage
Parallax Engineering
Whoops I drew my key wrong! I have it backwards. The actual design is CBE top down.
(I have since deleted those images from this post)
Thanks for pointing that out.
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How can there be·nothing? Nothing is something!
Post Edited (beazleybub) : 10/28/2008 7:36:09 AM GMT
For catching My (typo) "IRF". I have edited My post.....Details,Details,Details................
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·
When I look at a transistors data sheet and see that a particular transistor has a minimum hFE of 35 @ 200mA, 2V and a maximum hFE of 300 I am led to believe that in order for the transistor to become "saturated" you have to at least meet a hFE of 35 but can't exceed 300. This is where all of my confusion is about transistors.
I basically understand how to choose a base resistor and how to determine minimum hFE but can't grasp how to choose which·transistor would suit a particular situation best.
·
However, I may have found a more reliable solution by just simply using a relay driver·IC, eliminating the need for fudge factors and guess work on my part.
·
Here is a link to a relay driver that might work well for a 5 V relay.
·
[url=mhtml:{7335AF52-1042-49AF-80BF-6F4A483FEEE4}mid://00000004/!x-usc:http://www.mouser.com/Search/ProductDetail.aspx?qs=ZXBb0xZ9WeAm1igmTYH1CQ%3d%3d]http://www.mouser.com/Search/ProductDetail.aspx?qs=ZXBb0xZ9WeAm1igmTYH1CQ==[/url]
·
I still am open to anyone that would be so kind to help me fully understand·hFE "Beta" and how to·calculate which transistor to use if I chose to use a transistor.
·
Once again here is the information for my circuit. [noparse]:)[/noparse]
Relay operating voltage 5 V
Relay resistance 55 ohm
Relay current 90.9 mA @ 5 V
Transistor Style NPN
Base Signal 5 V·
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How can there be·nothing? Nothing is something!
Post Edited (beazleybub) : 10/28/2008 7:41:35 AM GMT
1) The maximum voltage between the collector and emitter when the transistor is off has to be less than Vce Max. In other words, the maximum supply voltage to the load has to be less than the rating of the transistor.
2) The maximum collector current Ic has to be greater than the amount of current expected to flow through the load.
3) The current gain of the transistor (minimum hFE) has to be greater than needed to produce the amount of current needed by the load given the amount of base current available. In other words, hFE > maximum load current / minimum base current. Usually the base current is set by a series resistor between the base and the I/O pin and the capability of the I/O pin to supply current. For most microcontrollers, this will be no more than 20mA.
4) The power dissipation capability of the transistor case and any heatsink has to match the amount of heat expected to be produced. If the transistor has enough base current to saturate (which depends partly on the hFE), this is usually the load current x Vce-sat. Typically, the Vce-sat is 0.3V, but depends on the transistor construction and would be in the datasheet.
As you've noticed, transistors can have widely varying gains. For switching applications, you're interested in the minimum gain because you either want the transistor to be off or to conduct "fully" (saturated) when your I/O goes high.
Post Edited (Mike Green) : 10/25/2008 5:17:07 PM GMT
A little tutorial -- may be redundant, for I didn't real all the replies from people who aren't sure. Since we are talking about DC parameters, hFE is of interest and hfe is not. hFE is the DC current gain. You have 30 ma available from the Stamp to drive the base, and you want to drive, say, a 100 ma load (relay coil). The required hFE is 100/30, or about 3.33 -- not a very demanding specification.
Let us assume you select a transistor whose hFE is 10. Then you will need 100ma/hFE = 100ma/10 = 10 ma of base current. Double this and drive the base with 20 ma. You will need a resistor between the Stamp output pin and the transistor base, to limit the current. How many ohms? Well, you're driving from about 4.8 volts or, so, and the drop across the base-emitter junction in many NPN silicon transistors will be in the range 0.4 to 0.7 volts. Assume the worst, 0.7 volts, and the voltage across the resistor is 4.8-0.7=4.1 volts. Calculate the resistance, R = E /I = 4.1/0.020 = 205 ohms. Use a 220 or 330 ohm resistor, because the hFE of any actual transistor is going to be much more than our 3.33.
What transistor? That depends, a little, on the relay. What's the relay coil voltage? Maybe 12 volts. So the transistor needs to withstand 12 volts Vce when in cutoff. Not very demanding.
What power will the transistor dissipate? Assume its saturation voltage (when conducting) is 1 volt. It'll be less, but we need a margin of safety. One volt drop across the transistor when conducting, and the current is 100 ma -- so the transistor dissipates 100 milliwatts. Most silicon transistors will handle this without a heat sink.
So here's our transistor spec: 100 mw dissipation, minimum hFE 3.33, and we'd like a saturation voltage under 1 volt. We need a maximum collector voltage at least twice the relay-coil supply voltage, for safety.
Our specs are not demanding at all -- it's easy to find a transistor that will do. I use 2n3053 for these things, because I have lots of them on hand. But many other types will serve as well, and most are smaller physically.
When the transistor, whatever type it is, shuts off current through the relay coil, the coil's inductance will resist the change in current. That's what inductors do. Place a rectifier diode across the relay coil to carry this current while it decays, so it won't force current through your transistor when the transistor is trying to shut it off. Place the diode in the direction that will NOT conduct when the coil is "on". This is how you rotect your transistor from a quick and mysterious death.
-- Carl Hayes
Nice explanation thanks.
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How can there be·nothing? Nothing is something!
Minimum base current = 100 MA / 100
Minimum base current = 1 MA So any 2n2222a 2n3904 and 2n4401 that you can get for $3.59 at radio shack and you get 15 of them lol would work. And if you used a 1kohm resistor to base it would only sink 5ma that would give you a lot left for led flashing lol I made a h bridge with 2 2n3906 pnp and to 2n3904 and when the motor stalls out it will pull 200ma thay don't even get hot . Running it pulls 100ma
Take a look at this site http://www.rason.org/Projects/transwit/transwit.htm
If it works use it If not try again. Works for me lol
Post Edited (be80be) : 10/26/2008 4:04:13 AM GMT