Transistor H-Bridge pulling great deal of current and affecting BS2...

Steel

This is sort of a run off of my last thread dealing with DC motors, but directly to the point...If you (parallax) could delete my "·Need help with DC motor control··" thread, I would really appreciate it (it veers off topic, and ends.)



Right now, My BS2 is connected to a 74595 shift register.· That series/parallel register (8 pins) is connected to 2 zetec H-Bridges:
http://www.zetex.com/3.0/pdf/zhb6718.pdf

The H-Bridges are connected to 2 DC motors as in the attached drawing (done so wonderfully in ms paint).

With the H-Bridges attached, the circuit is pulling 250mA and there are odd behaviors in the stamp.· I was under the impression that I connected the H-Bridge Correctly, but it does not seem operational.

Has anybody worked with this H-Bridge?· Did I just mess up on the wiring?

Shaun

PJAllen

· What exactly are you using for the +5V?· (Hopefully not the STAMP's on-board 5V [noparse]/noparse]V[sub]CC[/sub.)

· Post Edit -->· I guess I thought the 595s were/are supplying control to the H-bridges?· It's unclear here.· How about getting it all out there?

·


Post Edited (PJ Allen) : 4/18/2006 12:46:25 AM GMT

Chris Savage

As an addendum to PJ, the 74595 is not designed to run a Motor for two reasons.· First of all it doesn't have the current drive capability.· It looks like you're trying to sneak around that by doubling up outputs, but this is unwise.· The other reason is that the 74595 doesn't have any protection diodes for inductive loads such as DC motors and the part will likely fail after some use.· Jon Williams already suggested the L293D, which is a good choice on a budget.· I would heed that advice and use that chip or something similar that is designed for the task.

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Chris Savage
Parallax Tech Support
csavage@parallax.com

Steel

The 74595's are only used to control the 'switching' of the transistors in the H-Bridge.· They are not providing anything more than just current to the transistor bases.

The 5V is coming from an external·5V Voltage regulator, which is also giving the BS2 5V.· The·motors and the BS2 are in Parallal from the 5V supply.

Shaun

PJAllen

Steel saith...
the circuit is pulling 250mA and there are odd behaviors in the stamp

So, this external +5V is running the motors and the STAMP.· More Questions:

• What is the rating of the supply being used?·
• Is it Regulated?·
• What is the current·demand of the motors (at 5V)?
• Does the STAMP work without the Motors, etc.?

Post Edit -- I've looked at the XETEX PDF and I'm concerned, since you haven't shown a complete schematic.· You'll need resistors between the STAMP outputs and the H-bridge inputs (unless you're using the HomeWorkBoard, which has built-in limiting resistors), otherwise you're running the outputs raw through a diode (not good.)· Likewise, more so, the outputs of the '595.· Also, you may be coming on in an "illegal" state (with both sides turned on, something like that, which would surely result a high-current state, taking down the STAMP, etc.)· Maybe your drawing is abbreviated, but we could make assumptions all week and never address your trouble.

The PDF pinout shows the device "upside-down", with pins 1 and 8 on the bottom.· Didn't notice that at first, but it's right in my drawing.

hb_nis.jpg is mistake-proof, no illegal state/s possible.· The other is just a schematic for a low-level test (no STAMP used for either.)

One last matter, there are no free-wheeling diodes in parallel with the transistors (internally) and there are none in my schematics (or yours) -- this, too,·can be problematic.

Post Edited (PJ Allen) : 4/19/2006 1:03:01 PM GMT