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need a small H bridge for motor.

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Patrick

Thu Nov 08 2012, 09:46PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i need a small H bridge to control a 200mA motor, ive looked on the mighty google, yet i find the typical 4 transistor bridges that can be smoked via boith pins being held high, and the so called "smokeless" ones use the 74**14 Schitt trigger IC, which id rater avoid, since i only have two left.

i thought there was a way to use 2 diodes to prevent the deadly continous short from both pins being held high. I may have mis-remembered though .

a schmit protected H brigde:

id like the 2 NPN, 2PNP type bridge with the "shorting case" being forbidden.

EDIT: this does what i want, but uses 8 Bipolars to do it ->

I may have to get out my Forest Mims books again, dam it.

Marko

Thu Nov 08 2012, 10:00PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi

What voltage do you need for the motor?

If 5V is enough then a driver like 74HC245 or 74HC541 will do great as a driver. Just use 4+4 gates per leg, no exra components needed :)

Marko

Conundrum

Thu Nov 08 2012, 10:03PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

What about something like an L293 or similar?
I also ran into someone hacking a chip salvaged from an old CDROM drive (equiv TDA2822) to use PWM drive.

Patrick

Thu Nov 08 2012, 10:08PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

dont have either of the two your suggesting, have about 500 of the BC547 and 556's

Conundrum wrote ...

What about something like an L293 or similar?

way to valuable and heavy for this app. Have several L298 and L297's.

maybe it was a "diode logic" front end i was thinking of, that disallowed the two legs to be high.

testtest

Thu Nov 08 2012, 10:47PM

Registered Member #3271 Joined: Mon Oct 04 2010, 02:29AM
Location: Canada
Posts: 159

Used several L298 from STE over the years in many applications.

Easy to drive from TTL logic, full control over motor topology, fast and well protected.
Even used one to drive my high vacuum turbo pump motor. Also it is available and cheap off the shelve from the major suppliers.

If you are worried about the weight you could diamond saw the heatsink rear pad, even thin down the case. Should then be comparable to using several discrete components+wiring in size and weight I would think....

Ben Solon

Thu Nov 08 2012, 10:57PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

if you have a bridge made with two npn's and two pnp's as shown in the Schmidt example, it's impossible to short the bridge no matter what you do. in fact, both legs low or both legs high is a common way to short the motor out on itself(provided the bipolars have freewheeling diodes) in order to provide a braking function to the bridge. i've even built a quad half bridge(or two full bridges) using 2n3904/6 capable of driving a heavy stepper at low speeds. provided adequate cooling, they would've taken it up to full speed. they barely meet your specs, but temperature is the only concern as they can easily take 200% their current rating if you cool them properly. i'd give it a shot.

Patrick

Thu Nov 08 2012, 11:11PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ben123324 wrote ...

if you have a bridge made with two npn's and two pnp's as shown in the Schmidt example, it's impossible to short the bridge no matter what you do.

with out the schimdt it looks like below, thats a definate smoke plume. I onlyhave 2 schimdt ICs left, and dont want to use them for such a simple purpose.

Ben Solon

Fri Nov 09 2012, 12:40AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

is different from

in the second one you can burn them because you have one high piece of si controlled by one signal and a low piece by the other signal all on the same leg of the circuit. both signals at once short the leg to ground. however. the first circuit works in a completely different way. have a look again.

Patrick

Fri Nov 09 2012, 03:09AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ben123324 wrote ...

have a look again.

i see what you mean, but can i remove the schmidt trigger and still have it all work?

Ben Solon

Fri Nov 09 2012, 04:22AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

Basically all a shmidt trigger does is take the digital(or analog) signal and give an output based on two thresholds rather than a single middle one with minimal jitter as the input jitter has to have a large amplitude to affect anything. Ie it reduces the cross conduction in the linear regeon of the device. It gives a signal that's always lowest or highest. The way that the devices cross conduct is that when the signal is between gnd and B+ both devices are linear.

Without a Schmidt trigger you just have to be extra careful that you never feed the lines an analog voltage. A comparator, logic gate, inverter or anything else that has only two output states will suffice.

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