transistor switch help

[large_ghostman]

i am not good with transistors but i intend to nail it once and for all! cant give exact reason for a couple of days because this project is a competition between me and dad and i dont want to give him help to cheat! so cant tell you want exactly i am doing yet.

i need to switch 12V inductive load with a pic BUT i have to use the transistors in the following diagram (reason dosnt matter at the mo), now i think the following diagram is how i need to do it

sorry i keep getting there was a error uploading file!!! i will be back with question and drawing soon as i can get it uploaded

ok changed browser from firefox to chrome this should work






i have both data sheets in front of me, so can someone take me through step by step referencing the Data sheets how i go about working out the resistors!

i understand Q2 needs to pass 200 mA tops so rule of thumb means base resistor needs 10 x lower current so 20 mA , first part that confuses me tho is the Hfe for Q2 is 100 min so surely the base of Q2 could be run at 2mA and still be fully switched on??

i also understand rule of thumb 2 says R1 is 10 X R2??
i have no idea in this configuration how R3 is worked out.
i chose this way of switching because i needed to switch a little more than a BC547 can handle on its own, the pnp i have to hand will handle it ok but i dont want to have to invert my code so i chose to switch the pnp with the npn

so can anyone walk me through the calculations and point out how the numbers relate in the datasheets, especially things like max emiter base voltage, is that number the difference in voltage between base and emitter?

many thanks LG

 

[ericgibbs]

hi L,
While you are sorting out your uploads.

Whats the specification of the inductive load, how much current.?
Also is your PIC 3.3Vs or 5Vs supply.?
E

 

[large_ghostman]

thanks eric i have sorted the upload hopefully the post now makes sense

 

[ericgibbs]

thanks eric i have sorted the upload hopefully the post now makes sense

hi.
Calculate R2 to give at least 20mA into Q2 Base, assume that Q2 has a large signal gain of only 10.
E

 

[large_ghostman]

ok thank you, chores to do then i will be back

 

[bbiandov]

Why not simplify things: get an Opto22 module for $5 bucks (I have seen them much cheaper; who knows why 5 today) and any PIC ttl will drive it; at the other end you get 3A up to 60V...The thing has all the inductive stuff built-in; reverse coil parasitic diode, etc, etc

**broken link removed**

I have a bag of those red ones and the black ones too (those are AC 120v load ttl controlled)

 

[killivolt]

**broken link removed**[/quote]

I'm pretty sure this is more of a learning exercise rather than a fix. I'm sure he will appreciate knowing there are more options.

 

[NorthGuy]

$5 is many times more than the combined cost of the components necessary to build the original circuit.

 

[large_ghostman]

hi guys, this is a competition between me and dad (details in a couple of days) the reason i chose those parts is because i still havnt unpacked everything so most the transistors are still somewhere in box's also in the case of the mps i have 2 full reels sitting here with 4000 transistors on each! i also have 1 reel of the BC's, i need to build this circuit 20 times so went with what is here now. Also like was mentioned the stuff i do is for learning and fun, nothing commercial, thanks for the info but i do like building stuff from thr ground up rather than plugging bits together . anyway back to the question's and answers
now working out 20 mA for Q2 seemed straight forward my first thought was take the 12V - 0.7 drop for transistor giving 11.3, then 11.3/0.020 = 565 Ohms so nearest 560 will do for R2 and x10 that for R1 is 5k6
BUT is that the correct way??? because what i got thinking was the two resistors R1 and R2 form a divider? and at the chosen values will give a voltage of 1.091V without taking any off for the transistor??
anyway assuming my R1 and R2 are correct then R3 is 5V -0.7 (transistor) 4.3V and needs to supply base current of 2mA?? (not sure about this as i was thinking gain for Q1 would be higher)
so
4.3/0.002 = 2150 Ohms or around 2K or 2K2 would do it?

can someone tell me if i was correct please then i can ask some more questions about this circuit

 

[large_ghostman]

hmmm i am wrong i know i am! i have made too many assumptions, for a start the mps isnt a switching transistor as such, so trying to make sense of the datasheet the base to emitter saturation voltage is actualy 1.2V qm i now on the right track?

 

[ericgibbs]

BUT is that the correct way??? because what i got thinking was the two resistors R1 and R2 form a divider? and at the chosen values will give a voltage of 1.091V without taking any off for the transistor??

Hi Logan,
This conclusion regarding the 1.091V for Vbe on Q2 is not correct,

Q2 Base to Emitter [internal diode equivalent] will start to conduct at ~0.6Vbe and will not go much above 0.75V, so nearly all the currect thru R2 will flow into the Base of Q2.
I would use a 620R and 6k8, also 2k2 for base of Q1, your values are OK.

E

 

[large_ghostman]

although the datasheet says that i found the following graph in it


wich makes me think the voltage drop is 0.8V

 

[ericgibbs]

hmmm i am wrong i know i am! i have made too many assumptions, for a start the mps isnt a switching transistor as such, so trying to make sense of the datasheet the base to emitter saturation voltage is actualy 1.2V qm i now on the right track?

hi,
Almost any transistor can be operated in a 'switching mode', all that means its either fully ON or OFF, [ conducting fully or not at all]
Some transistors are more suitable for switching because of their lower Vce ON when fully ON.
E

 

[large_ghostman]

sorry posted as you did

 

[ericgibbs]

although the datasheet says that i found the following graph in it
View attachment 81929

wich makes me think the voltage drop is 0.8V

When at 0.8Vbe Q2 Base current will be high, look up a Diode forward voltage versus current plot.

 

[ericgibbs]

hi L,
Look at this LTSpice simulation of your circuit, I have used a BD140 [ no MPS]
Top plot is Vin , 2nd Vbe of Q1, bottom is the inductor current.
E

 

[large_ghostman]

i need to find a good book that explains what all these graphs are for and what info they are actually giving me.
transistor theory seems harder than i thought! but determined to master it
thanks for the help

 

[large_ghostman]

great sim! i must learn LT spice its ace makes more sense seeing it on a sim

 

[large_ghostman]

i noticed on the fairchild datasheet of the mps it has this in it

"
Spice Model
PNP
(Is=12.27p Xti=3 Eg=1.11 Vaf=100 Bf=91.63 Ne=1.531 Ise=12.27p Ikf=1.009 Xtb=1.5 Br=1.287 Nc=2 Isc=0 Ikr=0 Rc=.6
Cjc=48.28p Mjc=.5615 Vjc=.75 Fc=.5 Cje=106.7p Mje=.5168 Vje=.75 Tr=496.3n Tf=865.8p Itf=.2 Vtf=2 Xtf=.8 Rb=10)"

so can i assume that in LT spice i could take a already made transistor model rename it and stick the above in as replacement values or something?
if so then i need to make LT spice learning top of list

 

[ericgibbs]

hi.
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.