HB-25 over-volt behavior?

MurphysLaw

Hello all,

I've got a 12V robot using Dimension Engineerings Sabertooth regenerative motor drivers and would like to also make use of some Parallax HB-25's on the same power rail. I'm expecting voltage spikes in excess of 16V from the regenerative drivers during active braking. I'm concerned that may damage the HB-25's. The documentation says they have over-volt protection but isn't specific. Has anyone here dealt with this sort of issue before and/or where can I find more detailed HB-25 specs?

Thanks,

Ryan

erco

I haven't used HB-25s myself, so I'll defer to someone else's expertise. But I had to say welcome to the forum, and based on the user name you chose, I expect to see a lot of posts from you.

I love the smell of Magic Smoke in the morning!

Mike Green

Call Parallax Tech Support after the holiday. They can find out what the maximum rating is for the power rail. If you're concerned, you'd be better off using a filter between the power rail and the regenerative motor driver. Typically a series choke and a filter capacitor on the noisy side of the choke is used to prevent the spikes from affecting the other circuitry. The electrolytic capacitor should be rated for twice the expected peak voltage if possible (like 35V) and a value on the order of 1000uF. The choke value depends on the current involved, but 1uH is a good starting value rated for the current involved and the noise spikes expected.

Matt Gilliland

Hi Murphy - Welcome to the Forums :thumb:

Definitely call our gurus in tech support - they know a lot more than me :-)

However, I've had one running on my desk since I saw your post this morning - it's been running for about 30 minutes @ 19.0 volts...it's working fine. At 19.4 volts the HB25 shuts off. Drop the voltage back to 19 and it runs fine.

I think Chris in Tech support is the expert on the HB25...I'll let him know about this thread.

-MattG

Chris Savage

In a system where an H-Bridge was driving the motors directly you could expect the system to be configured to have a free-running mode and/or a braking mode. However if you're using the HB-25 to drive the motors then I am not understanding how you'll have the other system integrated with the HB-25. Typically you would use one system or the other. Do you have a connection diagram for your anticipated connections using multiple motor controllers/drivers?

MurphysLaw

Thank you everyone for the welcome.

Chris Savage,
I've attached a basic hand drawn "connection diagram" that should disambiguate things a bit. Note that in the full schematic there are 2 Sabertooth 2x12s and one 2x25, plus two hb-25s (rather than just the one shown). So why am I using two different types of motor controllers in the same system? Basically, having dropped the voltage from 24 to 12 and running out of Sabertooths, I remembered I had some hb-25s hanging around. :-)

Matt Gilliland,
I very much appreciate the testing data you've provided. Since they simply shut off at 19.4V without venting magic smoke, it sounds like they'll work just fine for my application. They're being used to drive a gripper that typically won't be actuated except when all the motors are sitting still.

Mike Green,
The Sabertooth drivers have capacitors built in for this purpose. Ultimately the battery is supposed to sink the charge, so adding more capacitance would just give the Sabertooth a larger place to temporarily dump the charge where it would subsequently be slowly absorbed by the battery and/or other components drawing power. At least, I think that's what would happen. :shrug:
In any case, it's obviously still good to add said capacitor/inductor, but I've got power wheel-chair motors connected to the Sabertooth 2x25, and I imagine the amount of energy they'd dump into the power rail upon reversing, for example, would require a very large capacitor to sink. While this may smooth things out a bit, I still think the HB-25s will likely have to deal with being significantly overvolted for possibly a second or two at a time, but based on Matt Gillilands tests, they should handle it ok.

Let me know if you notice anything still seriously wrong with my setup.

Thanks again everyone for your help,

Ryan

Chris Savage

Okay, I see what you mean. They're just on the same power rails so you're wondering about excessive voltage to the HB-25. I'm not sure what a failure point would be because the other consideration is the fan. It's a 12V fan and I'm not sure of the Absolute Max Rating on it.