Hi! I've been trying to make my own H-Bridge using TIP120 (stupid, I know). Anyway, with the test circuit below, I was expecting to get ~6V at minimum across the resistor. However, I'm getting less than 3V! I have checked and reassembled the circuit numerous times, so I'm a bit clueless on this one. Anyone can help? (bit of a noob with darlingtons here).
The circuit still outputs less than 3V even with a base resistor. I've figured giving 5V at the base yields greater voltage, but I really need to operate it with 3.3V.
That is a "high side switch", which doesn't work well or at all with NPN transistors. For the transistor to conduct, the base voltage must be > (emitter voltage + 1.4V (Darlington connection)).
Move the load resistor from the emitter to the collector circuit. An appropriate current-limiting resistor between the base and the voltage source is required.
The circuit you have there is called an emitter follower. The voltage on the emitter follows the voltage on the base, minus the base emitter voltage drop. Which in your case is a double drop of 0.7 + 0.7 volts or 1.4 volts.
thank you for the answer folks (@jremington@Grumpy_Mike@DrDiettrich@stitech )!! However, as the Darlingtons will be used on a H-Bridge, mind sharing how such configuration could be implemented for the full H-bridge? I am having trouble visualising it
An N-Darlington can be used in the "lower" half of the H-bridge, with the load in the collector circuit, but the efficiency is very poor, as the C-E voltage drop can be as high as 2V. Most people have moved on the modern, MOSFET motor drivers.
I know, people write this advice quite literally on every discussion thread I've seen in the web lol. I already have the darlingtons, so I am kinda stuck with them
Is there an important reason why MOSFET devices are suggested for H-bridges? Is it possible that the TIP120 transistors cannot be used to create a H-bridge that would work for you?
It does, if you use an NPN transistor. You will very commonly see PNP transistors in the upper half.
The design of H-bridges is not at all trivial, and requires a fairly deep understanding of how transistors work. Did you think this was supposed to be easy?
Here is a simplified schematic of an H-bridge made from N-Darlington and P-Darlington pairs. It is horribly inefficient, because the typical internal voltage drop is as high as 4V, wasted as heat.
It also requires very careful timing of the switching, as if that ever goes wrong, the left hand or right hand side, or both can act as a dead short circuit across the power supply (after the transistors melt internally from overheating).
As mentioned this is a 40 year old approach and long abandoned.
