Z-bridge

[Colby DuLin]

SolutionYou can drop all off the transistors and resistors if your motors don't drain too much current. You only need a 74F139 or 74S139 NOT the LS version. The chip can provide peak currents up to 100mA for a short time (

The problem with the Z-bridge is that it's standby power (10 - 40 mA for the F and S TTL series) is considerable for solar designs, so it restricts it primarily to battery powered robots. However, it is highly likely that the more efficient 74ACT139 version of this chip can drive and withstand significantly higher inductive loads, and as the 139 is stackable and symettric, all input and output leads can be folded over from one side of the chip to the other to increase the power capacity. This means the Z-bridge can be glued right on the motor itself with minimal modification, reducing lead losses and increasing design flexibility.

A futher advantage is that by using diodes from the four motor drive outputs with paralleled select inputs, permutations on enable states will give different current drive levels. This means trivial and cheap digitally selectable speed control using as many stacked 139s as you want.

Inexpensive small motor drive with enable, short protection and cascadeable control in a commonly available 16-DIP package. Very nice indeed, and I can't believe I've been looking at it for years and never made the connection.

A 47k pull-down resistor is recommended for all enables and A and B inputs as the ACT is a high input impedance device. Current efficiency is not great, but more than adequate for battery powered designs. Standby current (enables high) is less than a milliamp though, so don't count out solar operations if your capacitor is big enough to deliver the juice.

Diode assisance from one side to the other is limited as the 139 is not a tri-state device, however 20% power gains can be made with 4001 diodes from the (dis)abled side outputs to the enabled side inputs if the disabled side takes the negative diode rails. More work will have to be done to see if cascaded 139s can give variable motor power.Anyway, it works, it's cheap, CMOS compatable, and simple as hell.

The original "direct drive" Z-bridge used a 74S139 or 74F139 to drive a motor without an additional h-bridge. We can now use the cheaper and more efficient 74AC139 for direct drive. Stack'm for more current. The z-bridge is also used to make the Tilden h-bridge smokeless. Pin 5 and 6 connect to one h-bridge input resistors and pin 11 and 10 connect to the second h-bridge input resistors.

In a microcore controlled two motor walker using z-bridge protected h-bridges, reversing is accomplished by adding a circuit to swap the Nv2 and Nv4 connections to the Z-bridge. The Nv2 and Nv4 outputs of the microcore control rear motor M2 and swapping the Nv outputs changes the order of the motor rotations and causes the walker to back up.

One mutation of the Z-bridge is the reversable Z-bridge shown below. It avoids the additional reversing chip by using all four decoded outputs and four h-bridge input resistors cleverly summed together to drive the h-bridge. The diode and 1M resistor form a simple OR gate for the Enable line pin 15. If Nv2 or Nv4 is low the '139 is enabled. The A input (pin 14) connects to Nv4 and determines if the even or odd decoded output pins are active low. The REV input on pin 13 (input B) is connected to a tactile switch or collision avoidance IR and when active high, selects pin 9 and and 10 decoded outputs which drive the h-bridge inputs in reverse order and causes the walker to back up.

In this schematic I have also shown the final version of the Power Smart h-bridge which ensures optimum base drive for the motor driver output transistors. The output transistors should be PN2222 / PN2907 pairs or similar for motor currents over 100mA.

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