Question about transistors

Thread Starter

what ever

Joined Aug 3, 2019
11
I need help. I want to know how much current and or voltage I need to apply to the base of a tip 102 and 107 transistor to get it to turn on. Thank you.
 

djsfantasi

Joined Apr 11, 2010
9,156
I need help. I want to know how much current and or voltage I need to apply to the base of a tip 102 and 107 transistor to get it to turn on. Thank you.
Eric Gibbs has it right enough.

The answer depends on what you’re trying to turn on. And what voltage/current that device requires/is specced for.
 

crutschow

Joined Mar 14, 2008
34,282
As bertus noted, the high saturation voltage of those Darlington transistors in a bridge will reduce the voltage to the motor by about 4V maximum (below).
Might be better to consider using complementary MOSFETs in the bridge, as those can have a very low ON voltage.

Is the bridge just going to be switched at a slow rate to turn the motor on and off, and reverse it?

upload_2019-8-3_10-50-40.png
 

Thread Starter

what ever

Joined Aug 3, 2019
11
I got the motor from Amazon. It said the rated current is 1.1AMP. I was planning on using an Arduino to power the base of the transistors and have a separate 12Vpower supply to power the motor. I wanted to use the TIP102 & 107 transistors because I heard they had a built in flyback diode and I thought it would simplify the circuit not to have to include the diodes.
 

crutschow

Joined Mar 14, 2008
34,282
I heard they had a built in flyback diode and I thought it would simplify the circuit not to have to include the diodes.
MOSFETs also have built in intrinsic collector-emitter substrate diodes, which can usually perform the flyback diode function when used in a bridge.
 

Thread Starter

what ever

Joined Aug 3, 2019
11
Thanks, do you know any specific mosfets that have built in flyback diodes that would work for my project? I am also still wanting to know how much voltage and or current I would need to apply to the base of a transistor or a mosfet to turn it on.
 

crutschow

Joined Mar 14, 2008
34,282
do you know any specific mosfets that have built in flyback diodes that would work for my project?
All MOSFETs have a built-in diode that can serve as a flyback diode in a bridge circuit.
voltage and or current I would need to apply to the base of a transistor or a mosfet to turn it on.
The MOSFET full-on Vgs voltage varies with the type.
Logic-level types require a Vgs of 5V (or possible less) to fully turn on.
Standard MOSFETs require a Vgs of ≥10V for full turn-on.
 

Thread Starter

what ever

Joined Aug 3, 2019
11
Thanks. I'm still unclear on why I should use mosfets instead of the TIP 102 and 107 transistors. Also if anyone has any suggestions for the specific mosfets or transistors I should use, I would greatly appreciate it. Also if someone could tell me how to find in a data sheet how much voltage and or current needs to be applied to the base to turn the transistors or mosfets fully on, that would be great.
 

Alec_t

Joined Sep 17, 2013
14,280
A normal silicon transistor needs about 0.7V Vbe to turn on and a Darlngton type about 1.4V, BUT you hardly ever drive a transistor base by applying a fixed voltage directly to it because it is then difficult to control how much base current is drawn and the transitor could easily fry. Instead, it is better to apply a voltage via a current-limiting resistor to the base and consider the transistor as current-controlled.
If the transistor is being used as a switch, then a rule of thumb is to arrange for the base current to be about 1/10 of the collector current required by the load being switched.
Note: The TIP102 is a Darlington type. It has the disadvantage that it drops about 2V Vce when saturated, so will waste (as heat) quite a lot of power when conducting high currents.
 

Thread Starter

what ever

Joined Aug 3, 2019
11
Thanks. So if I still wanted to use the TIP 102 and 107 transistors for a 12V DC motor should I use a 14V power supply instead of a 12V because the transistors burn off of 2V? And would it be possible for me to use an Arduino to power the base of the transistor?
 

djsfantasi

Joined Apr 11, 2010
9,156
Use an Arduino? Sure, if each pin is only asked to supply less than 20mA. That means that the load on a Darlington is less than 2A and the load on a normal BJT is less than 200mA. Plus, you have to budget the load on all pins such that they in total draw less than 200mA.
 

TeeKay6

Joined Apr 20, 2019
573
Thanks. So if I still wanted to use the TIP 102 and 107 transistors for a 12V DC motor should I use a 14V power supply instead of a 12V because the transistors burn off of 2V? And would it be possible for me to use an Arduino to power the base of the transistor?
A minor correction. When using an H-bridge, current to the load always passes through two transistors. Thus the overhead allowance would be 4V (or more) when using Darlingtons.
 

Alec_t

Joined Sep 17, 2013
14,280
It said the rated current is 1.1AMP.
Is that the normal running current (most likely) or the stall/start-up current? Brushed DC motors typically have a stall current several times the running current, so the TIP102 would have to handle the stall current (~5A?) and the Arduino might well struggle to provide enough base current.
 
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