My head is still going to explode

c131frdave

Hello again. Newbie here.

In my previous post I explained that I am teaching electronics to myself, and I need some help again.

I want to turn on a DC motor.· Moreover, I want to be able to turn it on with the BS to run in either direction.

So, I purchased two Crydom DM0063 SSR's, and they both test out perfectly.· When I send a high singnal to the coil, I get zero resistance in load circuit, and when it is low, I get close to infiniate resistance (I know, there's no such thing as infinate, but you know what I mean).

So they work for DC current- sweet.

Now comes the hard part for me- it's probly super simple for you all.· I wired it up and the motor immediatly spun in one direction only.· I fired up the Fluke, and it is reading either 11 vDC or 12VDC depending on which relay is on- but no negative.· The way I had it wired is below:




After review, obviously this won't work- and it explains why I was getting positive voltage regardless on the fluke (I put the fluke betweeen the motor and the relays.· Obviously you can not·connect the motor directly to the battery and expect the relays to change polarity.

So I'm trying to draw the circuit out on a piece of paper, and I'm completely lost!· I just can't wrap my brain around this.· No matter how I draw it, I end up with a short circuit.· So obviously some diodes will have to be used, but even then I'm not sure how to do it.· Can anyone point to a schematic that can help me out here?· I'm going to keep working on it and hopefully will figure this out, but it would be nice to hear from some of you.

Thanks.

metron9

You need 4 relays

bat+............relay1...............Motor...........relay3............bat-
bat-.............relay2...............Motor...........relay4............bat+

relays 1 and 2 direction A
relays 2 and 3 direction B

You will need to turn off the relays that are on before you turn on the relays that are off. You will need to add some dead time between the transitions as well.
You can use an inverter if you dont need dead time between transitions but a fuseon both terminals you should use to prevent smoke if the relays fail.

A PTC fuse that resets would work as well instead of buss fuse.

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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

c131frdave

Thank you. I was told to use a DPDT relay, which does the same thing, right? Just use a relay to turn on and off the power and a dpdt relay to change the current flow.

I'm on the hunt for a DPDT relay.

Thanks for the reply.

metron9

Yes, you can use a DPDT or two singles, just wire the trigger in parallel for the pairs.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

c131frdave

The dpdt relay at the local radio shack has a coil rating of 12vdc. How do I trigger it with the basic stamp? Somebody on another forum says, "use a transistor", but I'm new and don't understand transistors yet. [noparse]:([/noparse]

LilDi

Here is a schematic

c131frdave

Thank you sir. Now how do I trigger the 12vdc coil?

Mike Green

Look at the Industrial Control tutorial (from the Stamps in Class downloads area), starting on page 85.

c131frdave

thanks everyone. I have a 12vdc dpdt relay. I'm building the circuit on a solderless breadboard (the little one on the stamps in class board isn't big enough), and will let you know if it works. Thanks for the page location Mike. I appreciate it.

The plan is to use my SSR to turn on/off 12vdc to my DPDT relay, and my second SSR to turn on/off the voltage for the motor.

LilDi

You really should take a look at the H-Bridge chips like the L293 or the 754410 IC. This IC will control a small motor with forward, reverse, stop and brake functions all in the same IC, controled by a Stamp. No external components are needed.

http://www.makingthings.com/resources/datasheets/SN754410NE_slrs007b.pdf

metron9

like this? ·

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

c131frdave

metron9 said...
like this? ·http://www.youtube.com/watch?v=CxJi9qGX-ls

LOL!· That's great- and disturbing.· I think my head will be ok now.

c131frdave

LilDi said...
You really should take a look at the H-Bridge chips like the L293 or the 754410 IC. This IC will control a small motor with forward, reverse, stop and brake functions all in the same IC, controled by a Stamp. No external components are needed.

http://www.makingthings.com/resources/datasheets/SN754410NE_slrs007b.pdf

That's fantastic.· Perfect.

Now to find one that can handle 3A (my motor can draw up to 3A under load- and it will be under a high load some of the time).

Also, do you use a transformer to up the voltage from 12 to 24 in this case, or is there a better way to do it?

Thanks again guys.· I can't tell you how much I appreciate it.

Dgswaner

here is one, 4A 12V H-Bridge Motor Driver on ebay for $17.00

cgi.ebay.com/4A12-V-H-Bridge-Motor-Driver-Basic-Stamp-PIC-AGV-Robot_W0QQitemZ170157700810QQihZ007QQcategoryZ71395QQrdZ1QQssPageNameZWD1VQQcmdZViewItem

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A complex design is the sign of an inferior designer. - Jamie Hyneman, Myth Buster

LilDi

You can stack the 754410 IC on top of one another as shown in the attachment. Stack three of them for a total of 3 amps.

c131frdave

Wow. Never seen that before.

Would it be better to do that, or get something like the HB-25?

Mike Green

The HB-25 has the advantage that it "does it all" for you. It's controlled like a servo with a short pulse that sets the direction and speed of the motor, but, unlike a servo, you only have to supply that pulse when it changes. It is kind of "overkill" for your project in terms of current capacity.

The Motor Mind B can handle continuous currents of 2A and peak currents of 3.5A. Depending on your motor load profile over time, this may not be enough.

c131frdave

I just ordered a Motor Mind B. The two crydom relays and the one DPDT relay works fine, but they take up a ton of room, lots of wire, and the cost is higher than the motor mind B. I wish I would have seen the motor mind B when I started this thing. Oh well. I'll use the relays and see if the MMB will work better when it comes.

skylight

For reference, Larger DC series motors which have seperate terminals for the armature and series field winding can be made to reverse using just two wires·by reversing the polarity of these wires.

As everyone knows you cant usually reverse a series motor by just reversing the polarity as it reverses the current through both the armature and field winding and the motor just rotates in the same direction.

but after working on a project for work many years ago I discovered by experiment that·I could get reversal by making the current travel in the same directon in the field winding regardless of polarity by use·of 4 diodes(heavy duty) across the field winding in a bridge arrangement when the polarity· changed the current reversed through the armature but stayed in the same direction through the field winding and thus achieve reversal.

so in theory with boosting the current/voltage through some device you could drive and reverse·the motor·by swopping highs and lows on two stamp pins.

just be careful to allow the motor to stop fully before changing direction though.

Post Edited (skylight) : 11/14/2007 8:35:04 PM GMT

c131frdave

Well, I can't. I need it to stop on a dime, but there is still a bit of inertia after the power is let off. The motor mind B has a "brake" pin. I suppose this is just for turning the motor off, but I was hoping to stop the motor by briefly sending a polarity change to it. Any sugggestions? Layman language please.... [noparse]:D[/noparse])


BTW, this motor is in an actuator.· The actuator has limit switches and when the switch is closed, the motor stops on a dime.··I suppose there is a physical limitation to the screw inside the actuator.· The actuator moves 2 inches per second, but I need to be able to move it maybe 3/4 inch one way, then a couple seconds later move it another 1/2 inch, then a second later back to zero inches.· As it is now, if I bump the power, it will extend about an inch just because inertia.

I have a cordless drill that has a brake in it.· I read somewhere that when you let off the trigger, some how the polarity in the motor switches for a split second to brake the motor.· I guess I should do a good ol google search and read up on it.

Post Edited (c131frdave) : 11/15/2007 1:30:22 AM GMT

c131frdave

Stumbled across this:

http://www.discovercircuits.com/PDF-FILES/motorwbrake.pdf


It says it's good for 1A DC motors, but you can "upgrade the values" for higher current draw motors. Mine peaks at 2.8 amps under full load (20 pounds). Any thoughts?

LilDi

Braking is nothing more than shorting the motor leads together. Obviously the power/voltage to the motor must be off when the braking transistor is turned on.

J.T.

another newbie here...

I have a 4-rotor RC helicopter that is using a "3-cell 11.1 volt 1320mah Lithium-Polymer high-discharge battery."

Not sure whether this is being regulated before going to the motors, but does anyone see a problem with trying to control·my motors (speeds) with any of these H-bridge options?· How do I test the amount of current the motor draws to know which option will work best?· (Just a voltage/amp meter on the two motor terminals when it's running?)





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