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
I need to drive two motors, 30 mA each. If I build
Tilden's H-bridge, I need
- 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
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.
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
(reply from Tilden) It is something special. It involves
minimality, expandability, elegance, and a horrible,
dangerous violation of the original purposes of the
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
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
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
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
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
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
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
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
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