H-Bridge Circuit description ---------------------------- ALthough the schematic shows a motor, I'm actually building this to control a Peltier device (heater/cooler). The peltier module I have is rated to 6 amps. I couldn't find an "all in one" H-Bridge chip such as the 4427, L298, etc, that could handle the required current so I had to roll my own. ********* The Parts ********* -Mosfets -------- I used all N-Channel mosfets. P-Channel on the high side would have been more convenient but, I couldn't find any in a style that would fit in a breadboard for experimenting. I chose the IRL520 logic level mosfets because they can handle >6 amps and are "fully on" at >3 volts on the gate regardless of the switched voltage. Normal mosfets are not fully on at 5 volts. The IRL520 includes body diodes so I didn't have to include reverse-voltage protection diodes. - 7660 ------ Since N-Channel requires a gate voltage higher than the supply being switched, I used a TCA7660 to double the voltage. Normally the 7660 (or 7662) is used to generate a negative voltage but with a modified circuit, it serves nicely as a doubler. - 2N3906/2n3904 --------------- I had thought to switch the high side mosfets with only the 3906s. This didn't work! Not quite sure what was going on but I ended up having to switch the 3906s with 3904s, which now work perfectly. I'll have to keep learning and figure out why it doesn't work with just the 3906s. - 74HC4053 Triple, two channel multiplexer ------------------------------------------ In my application, a microcontroller will be operating the bridge. My big fear is that I will inadvertently activate two mosfets on the same side of the bridge, causing a short circuit (shoot-through). To prevent this, I use a 74HC4053. The '4053 is like a set of three SPST switches. the Z pins are common, the Y pins are the switched poles, and the S pins are the switching controls. it also includes an inhibit pin, when low, it disables all inputs and outputs (high-Z). How it works ------------ I control the motor speed through a PWM signal to the low side mosfets. The high side mosfets are simply on/off. With S1 set low, the PWM signal on 1Z is output through 1Y0 and applied to Q8 (bottom right mosfet). Vcc is applied to 2Z and output through 2Y0 to Q6 (upper left). For the opposite direction, S1 is set high, 1Z is output through 1Y1 to Q5 (bottom left) and 2Z is output through 2Y1 to Q7 (upper right). The third channel is not used With this circuit, I simply cannot accidently activate two mosfets on the same side at the same time. ******************* CAUTION: it is important that the S1, S2, and S3 pins are not left floating! It can result in the all ouputs being active at once! I have pulled them low with a 10k resistor to ground. DOesn't really matter if you pull high or low - change to suit your application or sense of logic. ******************* INHIBIT ------- The 4053 automatically disables all output prior to switching so, strictly speaking, ENABLE is optional. If you are as paranoid as me, you'll want to disable the chip while switching or when the motor is stopped. With this setup, the chip will not operate until you pull the enable line low.