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A. A. van Zoelen's "Z-bridge"
Design, limitations, usage -- straight from the designer himself

A Cheaper H-Bridge Design - the Zoelen Bridge (Z-Bridge)

Copyright A.A. van Zoelen
All rights reserved
December 1997


I need to drive two motors, 30 mA each. If I build Tilden's H-bridge, I need the following:

  • 1 x 74x139
  • 4 resistors
  • 4 diodes [optional.]
  • 12 transistors

Altogether it will cost about US$ 10 and much room on my circuit board. The design is really great but i don't have to room for it and i like to spend less.


You 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 (<1 second) and to one output at a time. This is for normal connections. I don't know how far you can go with this misbehavior. It's nothing special, but you get the idea how it is done.


Circuit Layout

                                       +---------- Reverse
                             Motor     |  +------- Forward
                      GND     |  |     |  |  +---- Engage (connected to GND = YES)
                        |  |  |  |  |  |  |  |
                      | 8  7  6  5  4  3  2  1 |
                      |                       [|
                      | 9 10 11 12 13 14 15 16 |
                        |  |  |  |  |  |  |  |
                           |  |     |  |  |  Vcc
                                   Motor     |  |  +------- Engage
                                    |  +---------- Forward
                                    +------------- Reverse



(reply from Tilden) It is something special. It involves minimality, expandability, elegance, and a horrible, dangerous violation of the original purposes of the chip.

It's a biomech beaut. I think we should call it the Zoelen Bridge and, with permission, would like to enter it into the lexicon.

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.

As well, the leftover outputs can be used to drive "motor-active" LEDs right on the chip without current drains on the MicroCore outputs, giving necessary process status during MicroCore convergence.

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.


Tilden later comments:

I am pleased to announce that the 74ACT139 Zoelen bridge works excellently as an all-in-one, foolproof H-bridge for driving up to 300mA motors between 2.5 and 6volts.

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.

Wilf Rigter later added these comments, along with a slight modification to make the Z-bridge reversible:

The Z-bridge is ideal for a microcore since it is smokeless even if the microcore saturates. Typically pin 2 and 3 go to Nv1 and Nv3 (Motor 1 reverse/forward) and pin 14 and 13 go to Nv2 and Nv4 (motor 2 reverse/forward). The enable pins 1 and 15 go to the PNC Nu output pin (engage).

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.

Since the decoded '139 output pins can never be simultaneously active low the h-bridge is protected from illegal input conditions.

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.

The circuit has been extensively tested on a biobug platform with the original B.I.O. Bug motors and it greatly reduces the motor current while maintaining B.I.O. Bug speed and reverses flawlessly.

For more information...

Wilf's original post of the updated Z-bridge reverser is in Yahoo BEAM group post #22673

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Page authors: A.A. van Zoelen, Wilf Rigter, Eric Seale
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