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Bipolar motor controller problem

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In2Dusk

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I have connected 2 TIP122 NPN transistor as shown in schematic and switched bases on to 3V and collectors to 6V and everything what happened was that the second transistor was warming up.
half H-Bridge.jpg
But when i connected just one transistor and the other pole of motor connected straight to the ground everything worked just fine.
Motor controller.jpg
Anyone know what i am doing wrong?
 

in the two transistor version , try to change the top one connected to +6v , to a pnp type.
and apply its base input as 0 for turn on and 1 (+6v) for turn off.
 
yes i was thinking about it but it will rise power consumption as i would need to apply current to pnp transistor's base even if the motor is off, so i need to do it without pnp transistors
any ideas?
 

"...as i would need to apply current to pnp transistor's base even if the motor is off..."

no. can you elaborate.
 

as i would need to apply current to pnp transistor's base even if the motor is off
Click to expand...
There would be no power consumption in off state. You only need an additional low power NPN transistor to switch the base current to the PNP transistor.
 
which pnp transistor should i use...or what transistor is pnp opposite of TIP122?
 

TIP127 is the exact complementary type (100V/5A TO-220 PNP darlington)
 
how would the use of 2pnp and 2npn instead of 4npn help? what is theory behind it?
 

If motor control is wired symmetrical ( R3=R4 and BJT`s are matched complement pair )
both power BJT`s have equal Ib and Ic.
and both can be driven to full saturation.

Compl_pair_driv_.jpg
 
The problem with NPN high-side switches is, that you need a base voltage above the 6V supply (about 6.5 V) to fully saturate the transistor. The voltage drop for the base resistor would add to it. With PNP transistors you can easily turn the switch fully on by just pulling the 1 k resistor terminal to GND.
 
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