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I'm trying to build an H bridge to rapidly toggle between a variable voltage from 0 to ±30v.

When I look online for pre-built motor controllers, the lowest voltage they go is around 1v.

This is strange because when I look at a simple four chip MOSFET H bridge design, I don't see any reason why there should be a lower limit on the voltage that actually hits the motor? I understand there needs to be a minimal voltage to activate the MOSFET.

Sample spec sheet: arduino motor shield

Here is another one that goes from 5v to 12v.

What I expect the circuit to contain: Simulation

Four of these chips (863-NDDL01N60Z-1G) should be able to achieve a similar effect, but lets you get low voltage across the "motor" terminal?

Why can't these shields output a small voltage across the motor? For example, 0.2v?

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  • \$\begingroup\$ Google Vgs(th) - aka gate threshold voltage. Also remember that the Falstad simulator is not an accurate simulation tool, the devices in it do not mimic real world devices. \$\endgroup\$ – Tom Carpenter Apr 13 '17 at 20:25
  • \$\begingroup\$ WHat exactly do you mean the lowest they go is 1V? \$\endgroup\$ – Trevor_G Apr 13 '17 at 20:30
  • \$\begingroup\$ They are usually PWM, are you sure that's not the minimum motor voltage they will work at. If so, 1V motors are pretty rare. \$\endgroup\$ – Trevor_G Apr 13 '17 at 20:37
  • \$\begingroup\$ Please post a link to the schematic for the motor driver. (If this one doesn't come with a schematic, than find one that does.) \$\endgroup\$ – Nick Alexeev Apr 13 '17 at 20:40
  • \$\begingroup\$ @TomCarpenter Actually it has a user adjustable threshold voltage, although I am sure what that threshold has to do with the circuit. \$\endgroup\$ – Mikhail Apr 13 '17 at 20:44
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I don't see any reason why there should be a lower limit on the voltage that actually hits the motor?

Without a schematic it's difficult to say, but that board appears to derive its driver supply voltage from the motor supply via a step-down switching regulator, which sets the minimum input voltage. The FETs themselves would probably be quite happy to operate down to 0V, provided they got sufficient Gate drive.

All active components need some minimum voltage to drive them (2.5V for a typical low voltage power MOSFET, 0.6V for a bipolar silicon transistor etc.). If the supply voltage is lower then it can be boosted, but the booster circuit must still be able to work on the lower voltage!

Alternatively the controller could have a separate driver/logic supply voltage input, then the motor supply voltage need not be a limitation.

So the reason your controller has a lower limit on motor supply voltage is simply that it was a design decision to not use a separate supply. Since the Arduino runs on 5V, they could have used that to power the drivers, but chose not to. This probably also applies to motor controllers that operate down to 1V, since they are designed to work from a single supply.

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The problem is not so much the H bridge itself as driving the H bridge.

The N mosfets are relatively easy. They are tied to ground so you can just feed logic signals into them directly.

The P mosfets are where it gets tricky. Their control signals must be relative to the motor supply voltage. To turn them off the control signal must be close to or greater than the motor supply voltage while to turn them on the control signal must be significantly less than the motor supply voltage.

So the control signal you need for the P mosfets is heavily dependent on the motor supply voltage.

If the motor supply voltage is very low then you need a Negative gate drive signal to properly turn on the P-Fets.

If the motor supply voltage is sufficiently high to properly turn on the fets but less than or approximately equal to your logic voltage you can drive the P-Fets directly from your logic.

If the motor supply voltage is significantly higher than your logic voltage then you will need a signal of significatly higher voltage than your normal logic supply to properly turn off the P-Fets.

Motor controller chips are generally designed to operate in the third region.

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