Hi Jonathan, I have links to several h-bridge devices and schematics
The main thing you want to watch out for when setting up any h-bridge
is that your logic doesn't accidentally turn on both transistors on
the same side of the bridge - since this will short the power supply
- dan michaels
Thanks for the links. Is there any way to make a more fool proof system that
having both inputs low or high at the same time would not cause over current
draw? The MCU will be programmed by students, and so it would be nice to
make it as safe as possible.
Ha, well there's the $64 question. Foolproof for students? Sorry,
that's an impossibility! [just let them fry the stuff, and learn -
just kidding. ;-)]
When using an mcu tp drive the inputs, it's too easy to get things
wrong - over and over. A fuse won't really help much with low-current
transistors. OTOH, you can jigger the circuits by adding additional
components for safety purposes, to prevent direct shorts. Maybe some
of those links show this. In this case, sometimes people use a logic
chip like a MUX/etc to drive the inputs. If you're gonna go to this
much trouble, however, you might as well use an integrated h-brige
chip, like L293D, SN754410, or driver like a TC4424. All relatively
- dan michaels
Great replies, all! Thanks. I think I will look into the opto isolator idea,
I have some here. This is to run a latching valve that needs a brief small
current draw. The integrated H bridge chip might be the way to go though,
easy for students to use.
If you have access to a scope, and depending on the type of driver circuit
you are designing, you may be able to use an old trick. You can use two
resistor-diode combinations to control the charge/discharge current going
into a small capacitor. The waveform out of this cap would be used to drive
the motor amplifier stage. Don't think digital, think analog. The charge
and discharge sides are ramps. The on/off threshold of the receiving
circuit is different for the "on" and "off" sides. The slope of the ramp
causes one side to turn off an instant before the other side has a chance to
turn on. You have to be careful to characterize this sort of a design for
temperature/voltage/current/tolerance variations, but it works.
DRIVE PIN ----- CHARGE-RESISTOR ---(A) DIODE (K)--\
\-- DISCHARGE-RESISTOR --(K) DIODE (A)--|--- TO MDA
CAP TO GND
The same basic concept can be applied/modified to control timing of pulses
in various types of circuits.
Another more "digital" approach is to use inverter gates and take advantage
of the inherent gate delay (you may have to cascade a number of them to get
the right delay).
All of these solutions need to be tested and characterized for the desired
operating range and component tolerances (voltage, current, temperature,
etc.). Spice is great for this.
Also, you may want to put one of those PTC resettable fuses in the line just
in case. The transistors will probably do a good job of protecting the
fuse, but it won't hurt to try.
for a discrete circuit with current limiting. It shows the outputs
using power darlingtons which have a parasitic diode enhanced for
back EMF protection (at least, that's what was explained to me
here :-). Use external Shottky diodes if you don't have such
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