Motor Controllers

Looks like the Lynxmotion Dual H-bridge can handle the motor just fine as long as the current doesn't exceed 2 amps. The Solarbotics kit looks good, too, but make sure your skills can handle the kit.

If I were buying these motors, I'd want to know what the current is with the motor stalled, too, and make sure the controller could handle that also. Wait on other responses to your post to make sure I am telling you right!

Parallax has some nice motor drivers, too.

Hope that helps! JCD

I took a look at the specs, and what it doesn't show is stall current. That is sort of the most important aspect, if the motor stalls and will sink more current than your drivers, you may have a pretty nice smoke show.

The motor voltage range is 3 to 12 volts, stall current is largely limited by the internal resistance of the motor windings. (inductive current hysteresis exceeded). As such, stall current could vary quite a bit, but if the H-Bridge circuit you choose offers "current limiting" you should be OK.

The L298 bridge has current sense inputs, and if you use them (they're optional) you can protect against the motor pulling too much current. The chip also has overtemp protection that forces it to disable itself should it exceed a pre-set temperature.

The motor you reference is the fairly common Chinese-made motors often used in robotics, as is the L298. Haven't heard too many people say this combination has resulted in smoke, but if it does in your case, be sure to post! In any case, you'll want to attach a heat sink to the chip should you notice it getting too hot.

-- Gordon

kitmor wrote:

kitmor a écrit :

what about TC4427A from microchip ?

Brian Dean has just released his RX50 motor controllers

0 to 50 Amps # 5 to 24 Volts (30V absolute max) # high frequency PWM, frequencies over 100 kHz # ATO style fuse on-board # status and power indicator LEDs # convenient FAST-ON blade type connectors # convenient screw terminals for control logic # mounting holes matched for 60 mm fan # power header for fan (uses motor supply voltage) # locked anti-phase native control method # sign magnitude / synchronous rectification also possible # small size - 2.5 x 2.6 inches # high quality 4-layer board, large ground and power planes

Just ordered a couple for myself.

Alex

I also have a pair on order, and maybe more depending on the motors I need to control.

I used the prototypes of these controllers for a previous incaration of Groucho and they worked extrmely well.

-- D. Jay Newman

Oh no, I wasn't quite ready yet!

Oh well, close enough I guess. However, I'm being a little more conservative on the specs:

Based on bench testing with the supplied heat sink, the one in the photo above, I'm rating this h-bridge at 30 Amps continuous, with 50 Amp peak. If you use the optional fan kit, you can increase that to

40 Amps continuous, with 60 Amps peak.

Things get pretty warm at 30 Amps - not just the h-bridges, but the wiring and connectors too. If you are running that high continuously, I would probably recommend the fan.

Note that for most robotics work, 30 Amps is really quite a lot of current.

I'm still polishing the manual and the web page, but since the cat is out of the bag, feel free to browse and any feedback is welcome.

Cheers,

-Brian

I just have a couple of questions - trying to bring my knowledge of electronics back up to speed from a few years ago:

Are those mica insulators between the power regulators and the heatsink ? If so, why? Wouldn't mounting them directly to the heat sink result in better heat transfer ? Just curious about this - not a critique at all.

Thanks !

Yes.

The reason is that the MOSFET tab is energized with the voltage passing through it. If all the tabs were mounted directly to the electrically conductive heat sink, you'd have a short circuit within the bridge and likely some burned out components.

The mica is thermally very conductive, however, and does a good job of passing the heat from the MOSFET through to the heat sink. This is facilitated by a light coating of heat sink grease between the MOSFET and the mica, and between the mica and the heat sink.

Thanks for looking!

-Brian

Another reason, is that you often get better thermal conductivity when you use a "soft" insulator. When you look at the surface of the heat sink and the surface of the MOSFET at high magnifications, you'll find that they're both very rough and irregular. This causes lots of tiny little air pockets to be formed, which act as thermal insulators. By using a product like sil-pads or mica it will help to eliminate the air pockets, thus increasing the thermal conductivity. If you didn't care about electrical insulation, you could use a thermal grease.

Hmmmm. It * looks * to me like the tabs are mounted with metal screws. I guess that is not correct ? Otherwise you'd have the short circuit. That's kind what led me to ask this question to start with, but I didn't state it originally.

Thanks again ! JCD

Yes, they are metal screws. But the white ring just under the head of the screw is a plastic insulator that insulates the screw from the MOSFET tab. The insulator also has a neck so it insulates not only the screw head, but also the screw shaft and keeps it from touching the inner wall of the MOSFET tab hole. Here's a photo of the white insulator so you can see what I am trying to describe:

-Brian

guess that is not correct ? Otherwise you'd have the short

didn't state it originally.

Plan on building your own ?

The screw is insulated as well.

Look for the white insulator around the screw.

or if needed See: "insulated shoulder washer".

Typically you'll see a lock washer/thread lock compound used- as thermal expansion tends to loosen screw. Thermally compensated plastic 'snap-in' s are available but I prefer screws for their holding. A problem neophytes may make is over-tightening and breaking or excessively compressing the insulators which may allow arcing(read as possibly letting magic smoke out). Not so much so with the rubberized-thermal-transfer as the mica. As such the rubberized is more popular for application(s) where there could be vibration/shock as the mica insulator can be cracked. A thermal grease compound, (available in isolating and non-electrically isolating- so read the damn label) is also applied to increase thermal conductivity. Also look into aluminum-oxide-ceramic. Don't have much info on those; you or somebody else will have to fill in. FWIW - For rubberized insulators one should check with the manufacturers' recommendations to see if heatsink compound can be or should be used and what types are acceptable. Available in sheets or pre-cut.

I could direct you to :

*How to Select a Heat Sink *Optimum Design and Selection of Heat Sinks

You'll want the*.pdf That might help in selecting heatsinks for your own design(s).

have fun damnit.

• That * explains it! Thought I was losing all of my faculties there for a while ! :-) Thanks !