transistor switch help

large_ghostman · Nov 10, 2013

i should be getting on with building but going to download LT and do the sim you did

ericgibbs · Nov 10, 2013

hi L,
If you are interested in LTS, join the free Yahoo LTS user group .
https://groups.yahoo.com/neo/groups/LTspice/files/ Lib?prop=eupdatehttps://groups.yahoo.com/neo/groups/LTspice/files/ Lib?prop=eupdate
Lots of files and libs on that site.

Learning LTS is not easy, but well worth the effort.
Remember its a simulator, it will not design circuits for you.

E

large_ghostman · Nov 10, 2013

one question i forgot uncle eric is.

you said assume a large current gain of 10, and i originally thought it would be 100 because of the min Hfe. so where on the datasheet do i get the information that tells me what gain to expect from a given load, if you see what i mean
i would originally gone for 2mA at the base not 20Ma as you suggested, so somewhere i have missed something important in the datasheet. dont know why but transistors really interest me hence why i want to learn as much as i can about them. once i get a few basics i will look at trying to figure out some the equations i have seen!!

large_ghostman · Nov 10, 2013

have asked dad and he say's its ok if i join group so will sort that

audioguru · Nov 10, 2013

The graph for Base-Emitter Saturation Voltage and for Collector-Emitter Saturation Voltage shows the beta (hFE) is 10, not 100.
The spec's for hFE show that the transistor is a linear amplifier (not a saturated switch) with a collector-emitter voltage of 1V, not 0.05V.

Nearly every transistor needs a very high base current (10 times the hFE) to saturate but a much lower base current (the hFE) when not saturated.

ericgibbs · Nov 10, 2013

hi agu,
This plot for Vce Sat if from the MPSA56 datasheet, it shows 0.25Vce for Ic=~200mA and a Beta of 10.

Where do you get the 1V from.?
E

KeepItSimpleStupid · Nov 10, 2013

i have no idea in this configuration how R3 is worked out.

LG

You want the transistor in saturation, so the base current has to be Ib > Isat/Hfe(min). There may be another name for saturation current gain and it's sometimes different than the small signal gain. There is a Vbe drop as well.

So R2 < (5-Vbe)/Hfe(min)

This may not tell the full story, because I don't know what that 5V source is. If it's a TTL gate, then you have to include those restrictions too.
e.g. Max current, Vh(min), Vh(max) etc.

killivolt · Nov 10, 2013

ericgibbs said:
hi agu,
This plot for Vce Sat if from the MPSA56 datasheet, it shows 0.25Vce for Ic=~200mA and a Beta of 10.

Where do you get the 1V from.?
E

I think the confusion is coming from LG's post #12. I was thinking the same thing, I also think that's why LG is confused also, I know I am?

I thought in post #12 the sheet shows just over 1v Vce and Ic= ~1amp But, you have a data sheet clearly marked with the Part #.

FYI, I spent the better part of the day yesterday, trying to understand Hfe.

LG, that's why I said stay in your Maths and Make it your second language.

large_ghostman · Nov 10, 2013

really great information everybody, thank you all very much, KISS the 5V is from a pic pin but i want/need to keep the current pulled from it fairly low certainly well below the max 20-25mA, again cant say why yet as crucial information for the other side (dad!).
what i can tell you though is the comp is to see who can build the best incubator, we will run them both for 6 months and keep records of hatch rates etc, Dad thinks a PhD in embryology and nearly 40 years experience will give him the edge over my superior soldering iron skills we have one week each to design then 2 weeks each to build (i have help for that).
Just a fun comp but i need to win at all costs because if i do beat dad he said he would buy me a pic32 dev board of my choice

audioguru · Nov 10, 2013

ericgibbs said:
hi agu,
This plot for Vce Sat if from the MPSA56 datasheet, it shows 0.25Vce for Ic=~200mA and a Beta of 10.

Where do you get the 1V from.?
E

The written spec's say that the minimum hFE is 100 when the VCE is 1V. Many other transistor spec's show the hFE when the VCE is 10V.

large_ghostman · Nov 10, 2013

the graph was from the fairchild version of the mpsA56, maths is not easy for me but i want to learn it and i want to be able to get correct information out of a datasheet for myself, but it takes time! at least i have great help here guiding me as there isnt any other way at my age to learn this stuff

audioguru · Nov 10, 2013

Motorola invented the MPS line of transistors. Motorola Semi today is called ON Semi so I use their datasheet. Fairchild copied them but frequently a copy has errors in it.

alec_t · Nov 10, 2013

It is possible to create 'models' from the manufacturers data.
It often easier to choose a close equivalent from the very wide of transistors that come with LTS.

.... and for many circuits, where transistor gain or frequency response isn't critical, almost any transistor model (NPN, PNP, NFET .... as appropriate) will give a passable circuit simulation.

large_ghostman · Nov 10, 2013

it is a bit confusing so tonight i will sit down and form my questions and hopefully go through the datasheet with you tomorrow , i will download the Motorola one. There is always loads of transistor questions so hopefully if we can go through a Datasheet and sort what all the graphs mean etc then it should helps alot of others in the future. i will ask my LT spice questions in another thread to save confusion

killivolt · Nov 10, 2013

audioguru said:
The written spec's say that the minimum hFE is 100 when the VCE is 1V. Many other transistor spec's show the hFE when the VCE is 10V.

What I don't understand is why take the Hfe of 100 at ~1v Vbe when that's taken from the lowest Temp of -55c. Shouldn't it be taken from 25c which is 150 Hfe ~1v Vbe ?

Edit: Here is the Datasheet I found / On semi, https://www.onsemi.com/pub_link/Collateral/MPSA05-D.PDF

ericgibbs · Nov 10, 2013

audioguru said:
The written spec's say that the minimum hFE is 100 when the VCE is 1V. Many other transistor spec's show the hFE when the VCE is 10V.

This clip from the MPSA56 datasheet shows 0.2Vce sat at 100mA, with an Ib= 10mA, so extrapolating for IC=150mA I would say Vce is 0.25V.

killivolt · Nov 10, 2013

ericgibbs said:
This clip from the MPSA56 datasheet shows 0.2Vce sat at 100mA, with an Ib= 10mA, so extrapolating for IC=150mA I would say Vce is 0.25V.

Never mind, I get it 1/4 of a Volt is equal to around 10-25 Hfe

Edit: So if I pick the lowest side of it to be safe at 10 Hfe when Calculating the Resistance you will still turn it on and keep the current down.

audioguru · Nov 10, 2013

ericgibbs said:
This clip from the MPSA56 datasheet shows 0.2Vce sat at 100mA, with an Ib= 10mA, so extrapolating for IC=150mA I would say Vce is 0.25V.

Motorola invented the MPS line of transistors. Motorola is called ON Semi today and their datasheet spec's are different from a copy-cat one.

ericgibbs · Nov 10, 2013

I would trust Fairchild to know what their product datasheets claim.

Perhaps the later produced MPSA65 are to a better specification compared the old crappy versions.

audioguru · Nov 10, 2013

killivolt said:
What I don't understand is why take the Hfe of 100 at ~1v Vbe when that's taken from the lowest Temp of -55c. Shouldn't it be taken from 25c which is 150 Hfe ~1v Vbe ?

Some are excellent and others (made on a Friday?) are not as good.
The written spec's say that the minimum hFE is 100 at 25 degrees C.
The graphs show "typical" (which is not minimum and not maximum) and the hFE is about 160 at 25 degrees C. The "typical" hFE at -55 degrees c is 100.

transistor switch help

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