# H bridge controlling DC motor with polarity for direction

#### slegha

Joined Nov 23, 2016
4
Hello,

I am designing a circuit that will have a DC motor powered by DC voltage (at line 1) and then a control voltage (line 2) that for example:
1- Line 2 = +V, Motor will rotate clockwise
2- Line 2 = -V, Motor will rotate counter clockwise
3- Line2 = 0V, Motor is not rotating.

Line 1 ~ 170V
Line 2 ~ +/- 25V

Motor: 150V, 300mA max

I came with the following schematic. What do you think and what can you advice.

Thanks.

#### crutschow

Joined Mar 14, 2008
30,116
That circuit requires a gate voltage for the N-MOSFETs Q1 and Q3 equal to 10V above the line 1 voltage to turn them fully ON, so it will not work as shown.

You either need a H-bridge such as this or a bridge driver such as one of these.

If you wanted to roll your own, you would need to change Q1 and Q3 to P-MOSFETs with circuitry to drive their gates between the Line 1 voltage (P-MOSFET OFF) and to the Line 1 voltage minus 10V (P-MOSFET ON) such as shown below:

What is the maximum speed that you need to switch the circuit?

#### slegha

Joined Nov 23, 2016
4
That circuit requires a gate voltage for the N-MOSFETs Q1 and Q3 equal to 10V above the line 1 voltage to turn them fully ON, so it will not work as shown.

You either need a H-bridge such as this or a bridge driver such as one of these.

If you wanted to roll your own, you would need to change Q1 and Q3 to P-MOSFETs with circuitry to drive their gates between the Line 1 voltage (P-MOSFET OFF) and to the Line 1 voltage minus 10V (P-MOSFET ON) such as shown below:

What is the maximum speed that you need to switch the circuit?

View attachment 116579
Hello Crutschow,

Thanks for reply. After posting this thread, I was looking at it and I noticed something wrong with it so I think I have it correct now. Check the schematic attached.
Turning it on doesn't have to be fast. Just command to turn CW and CCW with long pause between each.

#### crutschow

Joined Mar 14, 2008
30,116
You didn't fully read what I said.
As I previously stated you need a gate voltage of 10V above the Line1 voltage to fully turn on Q1 and Q3 as a switch (the gate voltage needs to be 10V greater than the source voltage).

#### slegha

Joined Nov 23, 2016
4
Assuming that I can send a voltage 10V greater than line 1. How to handle the Line 2 -V to rotate in the opposite direction?

#### Alec_t

Joined Sep 17, 2013
12,898
Without component values it's impossible to know whether Q5 and Q6 will turn on/off correctly.