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why does this Hbridge use 2 different types of transistors?

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blackbeard:
so i'm making an Hbridge for one of my robots and i discovered this

http://www.solarbotics.net/library/circuits/driver_4varHbridge.html

since i have all most of the parts (lots of 2n2219 transistors, 7414 ic, resistors) i figured it might be worth while. then i noticed that it uses 2 different kinds of transistors. is this important or can i use the 2n2219 transistors in place of the 2n2905 transistors as well?

galannthegreat:
They are either NPN or PNP transistors. The one with the arrow pointing away from the device is NPN, and the one where the arrow points towards the device is PNP. Just follow what is layed out on that diagram, or check out an H-Bridge IC, such as the L293D or the SN754410 (they are actually quad half H-bridges... aka dual H-bridges) that are good for controlling 2 motors.

You can use two complementary transistors aswell.

blackbeard:
doh >.<

ya that makes sense. thanks for the info

Soeren:
Hi,


--- Quote from: blackbeard on October 15, 2009, 03:17:46 PM ---since i have all most of the parts (lots of 2n2219 transistors, 7414 ic, resistors) i figured it might be worth while. then i noticed that it uses 2 different kinds of transistors. is this important or can i use the 2n2219 transistors in place of the 2n2905 transistors as well?

--- End quote ---
If it weren't important, why would they make it with different transistors?
Designers tend to use as little variety in their component choice within a given project as possible.

If you build it with only 2N2219, Q1(Q2) and/or Q4(Q3) will die screaming the instant input A(B) is going low, as they will both be open and try to shunt the battery.
The only thing excusing the (very poor) original design is the low operating voltage, since both transistors will be open when the output of the inverter gate transitions through the interval from ~0.7V to ~Vcc-0.7V, but it's meant for a BEAM, where voltages are low and, all too often, minimalism takes priority over quality in design.

With an extra inverter, mounted between the base+resistor (i.e. on the far side of the resistors seen from the bases) of Q1 and Q4 (similar for the other side), they should work, although not at max. efficiency, since there will be a short overlap in the switching where both transistors conduct.

blackbeard:
ya i'm not even going to bother with this. the 2n2219 transistors burn out with the power from my USB port! junk they are!

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