# High Voltage/Amp Transistors Pinball

#### ScottWang

Joined Aug 23, 2012
7,070
Yeah, your right. We have both numbers.
But the 3.3 would explain why the Darlington didn't work either.
When we want to use the Darlington works as a switch then it is get into a bad situation, the situation maybe like these -- because only the second bjt could works as a transistor switch, but the first bjt is works as two diodes, so the second Vce of njt as Vce2 = Vin - Vbc1= 1.4V-0.7V = 0.7V, W2=Vce2*Ic2=0.7V*3.5A=2.45W, so you can see it why it is getting hot, when the Ic is higher then the Vce also will be higher.

I was tested the situation of darlington pair as above, but I didn't test the circuit as TIP120, probably it has a better situation.

#### ronv

Joined Nov 12, 2008
3,770
When we want to use the Darlington works as a switch then it is get into a bad situation, the situation maybe like these -- because only the second bjt could works as a transistor switch, but the first bjt is works as two diodes, so the second Vce of njt as Vce2 = Vin - Vbc1= 1.4V-0.7V = 0.7V, W2=Vce2*Ic2=0.7V*3.5A=2.45W, so you can see it why it is getting hot, when the Ic is higher then the Vce also will be higher.

I was tested the situation of darlington pair as above, but I didn't test the circuit as TIP120, probably it has a better situation.
If he only uses 3.3 volts and the Vbe of the TIP120 is 2.5 volts, that only leaves .8 volts across his 220 ohm base resistors - less than 4ma. With an Hfe of 500 to 1000 that's just not enough for 10 amps.

#### ronv

Joined Nov 12, 2008
3,770
So @adam450. What voltages do you have available.