Ive built alot of h-bridge circuits from the net but I would like to be able to understand the process of designing my own.
Design specifications
Im wanting to use tip 120 darlington NpN transistors for the bridge.
Im driving them with a pic so am limited to 5V and 20 mA
The motor is rated for 1.5A at 5V but runs fine off 6V
Seemed logical to me to start from the bridge first so here goes.
Since the Tip102's have a gain of 1000 (confirmed with a test circuit) each base should only require 1.5 mA but since its being used as a switch I should drive it into saturation correct? 120mA is its saturation point, assuming a 5V source that would be roughly 40 ohms of resistance. The part I dont understand is that typically the low side transistor has a higher resistance value then the high side even though they use the same transistors, why is that?
Since the tip120's drop aprox. 1.4 Volts from collector to emitter I should have a source for the motor thats 2.8V higher then required correct? If so 8-9V should be fine IMO.
Anyhow assuming the above is correct I cant directly drive the transistors with my pics output so I would use a 2N2222 NpN transistor to amplify its current output. My NpN has a gain of 129 so its base current should be roughly 1.9mA to provide enough current to both bases of the darlington transistors to achieve saturation (240mA for both). 5V/1.9mA = 2631 so around 2.5K should be used as a current limiter on the NpN's base correct?
Now for the wiring part
Since the base requires atleast .7Volts should I drive the base of the 2N2222 directly from the pic or build a voltage divider network? Most of my schematics drive it directly but how do you figure the resistance values then? The voltage drop across the resistor when driven directly is extremely low, I would think it should be close to 5V but its been more like 300mV. Also should the NpN's emitter go dirctly to the tip120's bases or should I build a voltage divider network.
Thanks in advance, I appreciate any corrections or additions that will help me understand how to design transistor circuits.
Design specifications
Im wanting to use tip 120 darlington NpN transistors for the bridge.
Im driving them with a pic so am limited to 5V and 20 mA
The motor is rated for 1.5A at 5V but runs fine off 6V
Seemed logical to me to start from the bridge first so here goes.
Since the Tip102's have a gain of 1000 (confirmed with a test circuit) each base should only require 1.5 mA but since its being used as a switch I should drive it into saturation correct? 120mA is its saturation point, assuming a 5V source that would be roughly 40 ohms of resistance. The part I dont understand is that typically the low side transistor has a higher resistance value then the high side even though they use the same transistors, why is that?
Since the tip120's drop aprox. 1.4 Volts from collector to emitter I should have a source for the motor thats 2.8V higher then required correct? If so 8-9V should be fine IMO.
Anyhow assuming the above is correct I cant directly drive the transistors with my pics output so I would use a 2N2222 NpN transistor to amplify its current output. My NpN has a gain of 129 so its base current should be roughly 1.9mA to provide enough current to both bases of the darlington transistors to achieve saturation (240mA for both). 5V/1.9mA = 2631 so around 2.5K should be used as a current limiter on the NpN's base correct?
Now for the wiring part
Since the base requires atleast .7Volts should I drive the base of the 2N2222 directly from the pic or build a voltage divider network? Most of my schematics drive it directly but how do you figure the resistance values then? The voltage drop across the resistor when driven directly is extremely low, I would think it should be close to 5V but its been more like 300mV. Also should the NpN's emitter go dirctly to the tip120's bases or should I build a voltage divider network.
Thanks in advance, I appreciate any corrections or additions that will help me understand how to design transistor circuits.