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Extra large h-bridge

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(@dubbadan)
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I'm currently using IBT-2 h-bridges to drive 24V 250w wheelchair motors, but the over-current protection is kicking in at full load. I'm having trouble finding an off-the-shelf h-bridge that has better power specs, apart from these (I've been unsuccessfully trying to get information about the fets in order to verify the claimed power specs). Does anyone know of others?

Failing this, I will have to build one myself which would be fun but time consuming.


   
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(@dronebot-workshop)
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Have you checked any of the Cytron motor drivers? They have a few models that might be suitable, such as this one.

😎

Bill

"Never trust a computer you can’t throw out a window." — Steve Wozniak


   
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Ron
 Ron
(@zander)
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@dubbadan Wow, those are beasts. How much current do your motors pull? When it comes to reading datasheets @davee is my goto guy, but I haven't seen him on today. He is usually here most of the day though. Are you in Australia?

First computer 1959. Retired from my own computer company 2004.
Hardware - Expert in 1401, and 360, fairly knowledge in PC plus numerous MPU's and MCU's
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Sure you can learn to be a programmer, it will take the same amount of time for me to learn to be a Doctor.


   
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(@dronebot-workshop)
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Posted by: @zander

How much current do your motors pull?

Well, he said 24 volts and 250 watts, so I assume about 11 amps.

😎

Bill

 

"Never trust a computer you can’t throw out a window." — Steve Wozniak


   
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Ron
 Ron
(@zander)
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@dronebot-workshop @dubbadan Right, he was showing us 100A drivers. Overkill for sure. Cytron would be a good source, well established name.

First computer 1959. Retired from my own computer company 2004.
Hardware - Expert in 1401, and 360, fairly knowledge in PC plus numerous MPU's and MCU's
Major Languages - Machine language, 360 Macro Assembler, Intel Assembler, PL/I and PL1, Pascal, Basic, C plus numerous job control and scripting languages.
Sure you can learn to be a programmer, it will take the same amount of time for me to learn to be a Doctor.


   
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(@dubbadan)
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Posted by: @zander

@dubbadan Wow, those are beasts. How much current do your motors pull? When it comes to reading datasheets @davee is my goto guy, but I haven't seen him on today. He is usually here most of the day though. Are you in Australia?

I'm yet to do a current draw test. I'm worried that the motors might be pulling more than they're supposed to.

And yes, I'm in the land of Oz.

 


   
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Ron
 Ron
(@zander)
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@dubbadan It's unlikely they will pull more than they are designed to unless they are broken but easy enough to check with a VOM. 

As far as Oz goes, we won't hold that against you too much LOL!

First computer 1959. Retired from my own computer company 2004.
Hardware - Expert in 1401, and 360, fairly knowledge in PC plus numerous MPU's and MCU's
Major Languages - Machine language, 360 Macro Assembler, Intel Assembler, PL/I and PL1, Pascal, Basic, C plus numerous job control and scripting languages.
Sure you can learn to be a programmer, it will take the same amount of time for me to learn to be a Doctor.


   
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(@davee)
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Hi Ron @zander, (and @dubbadan)

re: easy enough to check with a VOM.

 

Careful Ron, most VOMs I have seen have a max current limit of 10A. IF these are taking 11A (or more), that may not turn out well for the meter, unless you find an extra shunt!

-----

There are cheap digital panel meters on AliExpress (and probably elsewhere) with 50A shunts fitted. I have only played with the 10A versions, which are not brilliant, but the one I tried gave a reasonable indication. Try to do an offline calibration test, possibly up to 10A with a VOM as reference, before trusting to a 'real' measurement.

Unfortunately, measuring with a shunt involves breaking into the power wiring, but cheap enough to consider building into the wheelchair as a permanent current (or possibly voltage and current) monitor, if you can find somewhere to mount them.

I don't know if there are any cheap clamp meters .. my 2 second Google suggested that the cheap ones are limited to AC, the DC ones costing a lot more, but others may be able to advise.

I definitely suggest you try to get some form of reasonable measurement, before investing in new H-bridges, etc. If they are consistently taking more than the specified current, then that suggests the motors are overloaded, and liable to overheat. That would not be good news, but not knowing could have worse long term consequences. 

Of course, the overload might only be transitory (fraction of a second), when starting from stationary, in which case the motors will probably be fine, whilst the H-bridge is panicking. Hence, the need to investigate before jumping to fix a poorly understood problem, albeit I think @dubbadan already knows that.

Good luck with your project, which has great potential benefits, Dave


   
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Ron
 Ron
(@zander)
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@davee @dubbadan My bad, I forgot about the 10A limit. My clamp on does both AC and DC up to 400A and costs a bit over $100CDN.

The top names are Klein and Fluke, for $110CDN you can get a fully featured Klein with a 40A and 400A AC and DC current mode plus more. I don't see that model on the AU amazon site, you might try an electrical supply store. There are more expensive meters up to about $200AU but a lot under $100AU. You might recognize some brands, I don't. DOUBLE CHECK it does AC and DC Volts and Amps. After that pick the options you might find useful.

First computer 1959. Retired from my own computer company 2004.
Hardware - Expert in 1401, and 360, fairly knowledge in PC plus numerous MPU's and MCU's
Major Languages - Machine language, 360 Macro Assembler, Intel Assembler, PL/I and PL1, Pascal, Basic, C plus numerous job control and scripting languages.
Sure you can learn to be a programmer, it will take the same amount of time for me to learn to be a Doctor.


   
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(@dubbadan)
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Posted by: @davee

I definitely suggest you try to get some form of reasonable measurement, before investing in new H-bridges, etc. If they are consistently taking more than the specified current, then that suggests the motors are overloaded, and liable to overheat. That would not be good news, but not knowing could have worse long term consequences. 

Agreed


   
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robotBuilder
(@robotbuilder)
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@dubbadan 

Maybe visit an electric wheel chair outlet that has beach going versions and look at the specs?

 


   
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(@dubbadan)
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So, on testing the current draw of the motors, it seems that the motors themselves are behaving as they should but the IBT-2 drivers are cutting out way before the current exceeds their claimed maximum tolerance. I even put extra heatsinks on the chips and a fan in the box, to ensure that it's not an overheat-protection function that's causing the cut-outs. I'll try getting some Cytron drivers and see how they go.

I guess the moral of the story is don't trust the specs given for electronic devices on ebay.


   
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(@davee)
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Hi @dubbadan,

  I don't know how you measured the current, but there might be high current spikes tripping something which would be 'invisible' to a meter, and need a scope or similar to find them.

On the other hand, not all that is sold on ebay and similar, will meet its advertised clams...

-----------

I had a quick try to look up IBT-2, as I am not familiar with them. I found a 'data' sheet at:

https://www.datsi.fi.upm.es/docencia/Informatica_Industrial/DMC/IBT2.pdf

And a slightly different version at:

https://www.handsontec.com/dataspecs/module/BTS7960%20Motor%20Driver.pdf

The latter includes:

image

which shows the two BTS7960 motor driver chips, and one 'standard logic' 74hc240 chip, which is presumably used as a buffer between the control pins and the motor driver chips. Oddly, this does match the schematic also included in the sheet ... the schematic is copied from the BTS7960's data sheet - https://www.digikey.com/htmldatasheets/production/70497/0/0/1/bts7960.html.

There does not appear to be any power supply filtering or other circuitry which might be expected with circuits driving high current motors. Hence, it is possible, that current transients are interfering with the control signals between the IBT-2 and the computer control. Please note, I am not saying this is the problem ... just a possible problem. Some clues are given in the BTS7960 data sheet schematic and text, which shows the microcontroller and power supply as a combined circuit, with the aim of minimising such problems.

-------------------

Sorry, I don't have any experience with Cytron devices either, but a quick look at a 30 Amp device states that care must be taken if the motor may be mechanically driven.

This is from the MDDS30 User's Manual:

IF YOU’RE DEVELOPING A MOBILE ROBOT, PLEASE TAKE NOTE ON THIS:

Pushing the robot without connecting the batteries to the driver is very risky and might burn the motor driver. When the robot is being pushed, power will be generated by the motor. If the battery is connected, the generated power will flow to the battery and charge it. If the battery is not connected, the generated power has nowhere to go and the voltage will rise. If the generated voltage is higher than what the motor driver can handle, components may burn!

In addition, I would be more comfortable if the logic input circuitry was electrically insulated from the motor driver current part ... unfortunately, I didn't find any such information. Of course, it might be isolated, but I didn't find it. However, I fear it may be vulnerable in a similar manner to the IBT-2.

-----------------------

SoI recommend some research before buying, to make sure it will meet your requirements!!

---------

Keep trying and best wishes for 2024, Dave


   
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(@dubbadan)
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Thanks so much Dave! I measured using a digital current meter, so definitely low resolution.

I already have some Cytron drivers on the way, but I will do some more research as per your advice re power supply filtering and motor isolation.


   
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(@dubbadan)
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Ugh. I blew up one of my shiny new Cytron 13A drivers. I was excited about the new hardware and just connected them up presuming that the problem was with the existing IBT-2s. 🙄 

So then I decided to actually LISTEN to the advice I'd been given and ran a test using an oscilloscope (Scoppy). It looks like there are some HUGE spikes happening - I used a 50A/75mV shunt and some of the spikes (with the motors being partially and intermittently loaded) were over 200mV. I'm only just learning how to use a scope though, so I can't be sure that the measurements are correct.

Hopefully my experience can be a lesson that others will learn before making the same stoopid mistake that I did! I thought the problem was with the IBT-2s but it turns out they were doing a stellar job! The over-current cut-out function was preventing them from being fried. Also, I bought the Cytron drivers that were rated to just over what I thought my motors' maximum draw was - another mistake. 

Posted by: @dubbadan

I guess the moral of the story is don't trust the specs given for electronic devices on ebay.

There's a new moral to this story now: do your research - test, measure, read, etc. I'll need to do some more testing of the motors, make sure my measurements are ok, and maybe read some more about DC motors.


   
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