Base resistors needed for saturated transistors?
Base resistors needed for saturated transistors?
(OP)
Are resistors needed between the transistors and the PIC microcontroller in the diagram below?

Each transistor will be driving a variable load (actually, multiplexed LEDs) from 0 A to 1.25 A in saturation. The LEDs are connected in series to current-limiting resistors, so the current between the collector and the emitter terminals will be not be exceeded. I am worried about whether the base current would destroy the transistor, since there are no resistors... Does the base draw current when the transistor is "off" (Vc = Ve = +5V, while Vb = 0V)?
Thanks in advance!

Each transistor will be driving a variable load (actually, multiplexed LEDs) from 0 A to 1.25 A in saturation. The LEDs are connected in series to current-limiting resistors, so the current between the collector and the emitter terminals will be not be exceeded. I am worried about whether the base current would destroy the transistor, since there are no resistors... Does the base draw current when the transistor is "off" (Vc = Ve = +5V, while Vb = 0V)?
Thanks in advance!
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Operation Radiation: http://www.mrkenneth.com





RE: Base resistors needed for saturated transistors?
RE: Base resistors needed for saturated transistors?
TTFN
RE: Base resistors needed for saturated transistors?
The datasheet says the maximum Vbe is 5V... I guess the low output of the PIC would be around 0.6V because of the high current? How much current would the base source?
Thanks in advance!
---------------------------------------------------------
Operation Radiation: http://www.mrkenneth.com
RE: Base resistors needed for saturated transistors?
I suggest that you review some basic transistor theory before wiping out a cadre of innocent transistors.
TTFN
RE: Base resistors needed for saturated transistors?
RE: Base resistors needed for saturated transistors?
RE: Base resistors needed for saturated transistors?
The reason I thought the transistors could be connected directly to the microcontroller pins was because the emittor of the first transistor in a Darlington pair is directly connected to the base of the second without any resistors.
I shall add the resistors between the PIC and the transistors. From the datasheet, I can get close to 1.25 A with around 25 mA of base current at a Vce-sat of around 0.3 V.
Will there be a current between the collector and the emittor terminals if the base is left floating (when the PIC pins are set as inputs)? How much current will be sourced or sunk by the PIC in that state?
Thanks in advance!
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Operation Radiation: http://www.mrkenneth.com
RE: Base resistors needed for saturated transistors?
You will never get those transistors to switch off until you reduce Vbe to 0, ie raise the base to 5V the same as the emitter.
It is also bad design to ever have an open circuit base as leakage current (Icbo) and noise can cause the transistor to conduct.
A practical way to make this work is to use NPN transistors, connect the emitters to ground, use a base limiting resistor and one to ground and put the collector load resistors to +5V.
RE: Base resistors needed for saturated transistors?
Also as you drag more current out of a pin the voltage on the pin drops rapidly. If the PIC is sucking the current into a pin then the voltage on the pin rises ...a lot...
Just get a handful of these. Put them in Source to ground.
Hook the Gate directly to the PICs use the Drains on your LEDs. Add a current limit in series with the LED. Have a blast while not thrashing the PIC.
A lot of other FETs would do to..
RE: Base resistors needed for saturated transistors?
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RE: Base resistors needed for saturated transistors?
RE: Base resistors needed for saturated transistors?
RE: Base resistors needed for saturated transistors?
RE: Base resistors needed for saturated transistors?
I should have been more detailed and clear in describing my application in the original post. I have created the following schematic that I hope will clarify the circuit:
The PIC will be controlling an 8 x 8 LED matrix, with eight pins driving an 8-channel NPN Darlington IC and another eight pins driving a set of PNP transistors. (I have only shown one PNP transistor simplify the schematic.) Only one PNP transistor will be driven at any time.
Can I add pull-up resistors at the base of the PNP transistors so that when the PIC pin is set as an input, the pull-up resistor would raise Vb to +5 V? I can just use a bussed SIP resistor for all eight pins.
I am planning to use the MPS751 PNP transistor, which should be able to output 1 A from 25 mA with a Vce of under 1 V.
If the bipolar transistors do not work out, I guess I will try itsmoked suggestion to use FETs. I would prefer to use the smaller TO-92 packages and reduce the cost at the same time.
Thanks in advance!
---------------------------------------------------------
Operation Radiation: http://www.mrkenneth.com
RE: Base resistors needed for saturated transistors?
RE: Base resistors needed for saturated transistors?
Alternately, you need to run from an open collector driver with a pullup to 5V.
TTFN
RE: Base resistors needed for saturated transistors?
RE: Base resistors needed for saturated transistors?
The datasheet for the PIC says that the leakage current is +/-1 uA, but only when the voltage at the pin is between Vss and Vdd...
Do you think I can place a ULN2803A "sink driver" between the PIC and the PNP transistors as an extra gain stage? The leakage current of the device at 85 degrees Celcuis is 100 uA maximum.
IRstuff, doesn't a PIC pin act like an open collector when it is set as an input?
itsmoked, sorry, I have never used FETs before... Not that I have used BJTs much either.
I have added a pull-up resistor to the schematic below:
Thanks again to everybody!
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Operation Radiation: http://www.mrkenneth.com
RE: Base resistors needed for saturated transistors?
RE: Base resistors needed for saturated transistors?
If the output has an active pullup, then just because you tell it to turn off does not remove it from the circuit. The drain connection to the p-channel device is still there.
TTFN
RE: Base resistors needed for saturated transistors?
To turn them off, I'd suggest two things:
1 - Use the TRIS register to set the port to an input and leave the data register filled with 0's;
2 - You'll still have a leakage current of about 1µA, which gives around 0.12 mA on LED's. Not a lot, but they may still be visible in the dark (you can test it); to avoid this, add some 10K? resistors between base and emitter (R10 in the previous figure).
Resistor R1 (output->base) can be 470-1000 ?, depending of the gain of the darlingtons. For ß=1000 a 680? is a good choice.
RE: Base resistors needed for saturated transistors?
I have looked at some MOSFETs, but found that using a gate voltage of only 3.3 V would result in a very low drain-source current.
In the end, I am using sisif's recommendations of using Darlington transistors. Actually, I plan to use Darlington NPN transistors for the 1.2 A side and some cheap PN2907A PNP transistors for the 150 mA side, because using two Darlington transistors would result in too great of a dropout voltage. The light leakage should not be a nuissance.
Thank you all again!
---------------------------------------------------------
Operation Radiation: http://www.mrkenneth.com
RE: Base resistors needed for saturated transistors?
RE: Base resistors needed for saturated transistors?
You are right about the clamp diode to Vdd and to Vss! I guess I will have to power the PIC from the same 5-V supply as the LEDs.
Thank you cbarn24050 for the reminding me of the diode. (Too bad I can only give each member one star...) I was already starting to design the circuit.
Thanks again to everybody!
---------------------------------------------------------
Operation Radiation: http://www.mrkenneth.com
RE: Base resistors needed for saturated transistors?
TTFN
RE: Base resistors needed for saturated transistors?
Keith Cress
Flamin Systems, Inc.- <http://www.flaminsystems.com> - kcress@<solve this puzzle>
RE: Base resistors needed for saturated transistors?
What happened after 25 mA?
Thanks again!
---------------------------------------------------------
Operation Radiation: http://www.mrkenneth.com
RE: Base resistors needed for saturated transistors?
h
RE: Base resistors needed for saturated transistors?
Not a lot, the bond wire melted.
RE: Base resistors needed for saturated transistors?
Keith Cress
Flamin Systems, Inc.- <http://www.flaminsystems.com> - kcress@<solve this puzzle>
RE: Base resistors needed for saturated transistors?
Dan
Owner
http://www.Hi-TecDesigns.com
RE: Base resistors needed for saturated transistors?
RE: Base resistors needed for saturated transistors?
It looks like they only offer open-drain devices though, so I guess I would still have to use some PNP transistors or P-channel FETs.
This is the first time that I have seen such graphs from Microchip! All of the other PICs that I have used (PIC16F687, PIC18F4520, PIC18F4550, and others) just say:
"Graphs and tables are not available at this time." under the section DC AND AC CHARACTERISTICS GRAPHS AND TABLES. Shouldn't all of the PICs have similar characteristics?
Thanks all!
---------------------------------------------------------
Operation Radiation: http://www.mrkenneth.com
RE: Base resistors needed for saturated transistors?
RE: Base resistors needed for saturated transistors?
I think you're very close with the last schematic you posted. Just stick a driver IC (the TI part would work) between your PIC and the PNP transistors like you first questioned. All else can stay the same and this will compensate for the voltage difference.
The last display circuit I looked at was connected to a PIC16C77. It used a common anode display (the +ve terminal is common to all segments). There was a MMBT3906 transistor with a 1k base resistor connected to the PIC that switched the positive side. There were 8 - 220ohm resistors from each segment to the port pins for the negative side. This was a 3 segment display that was multiplexed and it worked fine. The PIC and display were both connected to the same 5V power supply.
RE: Base resistors needed for saturated transistors?
I have a suggestion to simplfiy your circuit:
Get rid of all the transistors.
Replace the 2903 (I actually worked on this part a long
time ago when I was at Allegro). It was designed for a printer source driver for the inkjet heads.
If you search www.allegromicro.com you'd find they have LED drivers that can sink a lot current. They are designed for driving LEDs and relays. They come with serial or parallel inputs.