At 02:00 AM 10/27/2011, IVP wrote:
>Hi all,
>
>I came across this H-bridge circuit, a design I've not seen before
>
>The only comment is "Note the diode; this allows adequate base current
>from a regulated 5V supply. Have an LED if you are putting in the full
>7.4V as an input, such as with booster transistors"
>
>Apparently it "works well with RC bots", such as this tracked tank

Aside from the diodes being drawn backwards, I think that its pretty=20
cute.  I don't think that this is anything that I'd ever use (most of=20
my stuff is for Industrial customers) but I can sure see a=20
high-volume toy manufacturer going with something like this.

I'm assuming that they are using older (low-intensity) green or=20
yellow LEDs (Vf ~2.1V)     as opposed to red (~1.7V) or=20
high-intensity green, blue or white LEDs (~3.2V).

Let me see . . . You said Darlington transistors.  7.4V - 1.2V (Vbe)=20
- 2.5V (nominal idle point) =3D 3.7V.  Hmm - use 2- red LEDs in series.

I'm assuming that in China, cheap LEDs can be had for about the same=20
price as I would pay for a resistor here in North America.  That=20
would make this a very cost effective design.  Especially if the LEDs=20
were part of the exterior 'bells and whistles' jazzy lighting effects.

Not a great design, in my opinion, but entirely adequate for the=20
consumer toy market.

In fact, the more I study the diagram and work the numbers, the more=20
I think that its a pretty good design.

I'd probably add a single series resistor between the W2 output and=20
the center of the LED string that feeds the upper half-bridge.  You=20
could also probably change the upper half-bridge transistors to be=20
not Darlington if you wished to reduce losses in the output transistors.

dwayne

--=20
Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
www.trinity-electronics.com
Custom Electronics Design and Manufacturing

--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.