simple hbridge question

[max_imum2000]

hello
attached is a simple h-bridge circuit from "pic microcontroller project book"
can anyone describe whats the function of the 10k resistor to the 2 pair of transistors ?

also in the original circuit hes putting a protection diode accross each transistor. isnt the tip120 already gut those ?

thank you all

 

[max_imum2000]

strange
now i connected the circuit above
supply voltage is 30V (adaptor)
and signal voltage is 6V (another adaptor)
connected common grounds
the circuit works but it seems that it is using the signal voltage to turn the motors , not the supply voltage
why is that ??

 

[audioguru]

why is that ??

The circuit is designed for having the same input voltage as the supply voltage.

Your top two transistors are emitter-followers. If your input voltage is +6V, then their emitter voltages are only about +4.7V instead of +28.7V. Then these two transistors get HOT! The motor is very slow?

Since you don't know why the transistors with their emitters grounded have base current-limiting resistors then you probably also don't know what is an emitter-follower.
You need to learn about transistors.

 

[Hero999]

Lol I've built a similar circuit to this when I was a nube. I wondered why it didn't work so I did some research and learned about transistors, I discovered N and P channel MOSFETs. Later I learned other tricks like using a higher gate voltage for the high side drivers and voltage doublers, etc.

 

[theinfamousbob]

Lol I've built a similar circuit to this when I was a nube. I wondered why it didn't work so I did some research and learned about transistors, I discovered N and P channel MOSFETs. Later I learned other tricks like using a higher gate voltage for the high side drivers and voltage doublers, etc.

Apparantely, great minds think alike. (I'm not at home so I can't use Firefox's wonderful auto spell check). The scary thing is, the bridge worked fine. It didn't get that hot but when I tried to replicate it, it didn't work.

At least I know a ton more now (I was 13 then)...

 

[ikalogic]

the voltage between the emmiter and the base is around 4v in tip122 (+/- .5v depending on the driving conditions)

based on this piece of information, you can see your shcematic, calculate and discover that the base voltage on the upper transistors is the not the same on the lower transisors(atually lower transistors have vrey low base voltage, about 1v)

So the 10K resisotrs are just to mantain the same current on the base's of the 4 transistors.

Also, No you don't need protecion diodes, the TIP122 have built in ones.