[OpenTRV-dev] H-bridge transistor selection

Fri May 17 11:03:10 BST 2013

This looks ideal....

http://www.ti.com/lit/ds/symlink/drv8837.pdf

Shame about the footprint, pricing is also high 1 off's £1.23, dropping to £0.84 for 10+

Stuart

On 16 May 2013, at 17:40, Damon Hart-Davis wrote:

> Hi,
> 
> I'm not assuming much, I hope.
> 
> By 'we' I meant 'those of us working on OpenTRV goals' for this purpose.  Several people on this list are interested in producing direct drive mechanics.
> 
> Sorry for any confusion from my clumsy wording.
> 
> Rgds
> 
> Damon
> 
> 
> On 16 May 2013, at 17:22, Stuart Poulton wrote:
> 
>> Damon,
>> 
>> My TRV teardown is a know as well. 
>> 
>> I have circuit diagrams, and the drive hardware is a self contained unit.
>> 
>> Please don't assume we all want to do the same thing.
>> 
>> Stuart
>> 
>> On 16 May 2013, at 14:28, Damon Hart-Davis wrote:
>> 
>>> Thanks.
>>> 
>>> I think that we should try and start to push along our own direct motor drive design generally, since I'm/we're in a position to start toying with it in this V0.2-Arduino design.
>>> 
>>> Indeed, we may be able to do an early board prototyping round so that we all have a couple of boards to play with, with my software framework in place.
>>> 
>>> If I'm to prototype direct-drive hardware and software I need some vague motor and sensor specs to work to, possibly starting by taking apart a Comet which is known, and working from there.
>>> 
>>> Rgds
>>> 
>>> Damon
>>> 
>>> 
>>> On 16 May 2013, at 14:11, Mike Stirling wrote:
>>> 
>>>> I'll sketch something out later. The one you have in your head is probably right, though. You need to pull down the low-side and pull up the high-side, and remember that no current will flow through any of these resistors with the bridge turned off because there is no voltage across them.
>>>> 
>>>> ----------------original message-----------------
>>>> From: "Damon Hart-Davis" EMAIL ADDRESS HIDDEN 
>>>> To: "Closed list for developer discussions" EMAIL ADDRESS HIDDEN 
>>>> Date: Thu, 16 May 2013 14:03:13 +0100
>>>> -------------------------------------------------
>>>> 
>>>> 
>>>>> I will try hard to avoid the last option!
>>>>> 
>>>>> Thanks
>>>>> 
>>>>> Damon
>>>>> 
>>>>> PS. Do you have a diagram to hand for a power sipping design? In my head I'm seeing 
>>>>> 'leaky' pull-ups, though they could at a pinch be pulled up via my intermittent 
>>>>> I/O rail for example...
>>>>> 
>>>>> 
>>>>> On 16 May 2013, at 13:40, Mike Stirling wrote:
>>>>> 
>>>>>> Yes. Drive each high-side transistor from the collector of the opposite 
>>>>>> low-side one. This also solves the problem of high-side drive when V+ is higher 
>>>>>> than logic level - not an issue in our case.
>>>>>> 
>>>>>> This topology leads to the following truth table (and no need for 
>>>>>> tri-state):
>>>>>> 
>>>>>> 00 - Idle (all off)
>>>>>> 01 - Motor run forward
>>>>>> 10 - Motor run reverse
>>>>>> 11 - Magic smoke (all on)
>>>>>> 
>>>>>> Mike
>>>>>> 
>>>>>> ----------------original message-----------------
>>>>>> From: "Damon Hart-Davis" EMAIL ADDRESS HIDDEN 
>>>>>> To: "Closed list for developer discussions" 
>>>>>> EMAIL ADDRESS HIDDEN 
>>>>>> Date: Thu, 16 May 2013 12:30:46 +0100
>>>>>> -------------------------------------------------
>>>>>> 
>>>>>> 
>>>>>>> Hi,
>>>>>>> 
>>>>>>> I *should* be confident with this, but I am not.
>>>>>>> 
>>>>>>> Is it possible to run a bipolar H-bridge from 2 tri-state outputs properly 
>>>>>>> 'class 
>>>>>>> C' ie avoiding ever having shoot-through by being able to let all four 
>>>>>>> transistors be off at once?
>>>>>>> 
>>>>>>> It's trivially possible with 4 tri-state outputs, or even 4 
>>>>>>> non-tri-state 
>>>>>>> outputs.
>>>>>>> 
>>>>>>> Rgds
>>>>>>> 
>>>>>>> Damon
>>>>>>> 
>>>>>>> 
>>>>>>> On 16 May 2013, at 11:38, Stuart Poulton wrote:
>>>>>>> 
>>>>>>>> On 16/05/13 11:32, Mike Stirling wrote:
>>>>>>>>> Interesting. The poor performing transistors do so because he is 
>>>>>>>>> running 
>>>>>>>>> them far too hard, although he does acknowledge this. He is also 
>>>>>>>>> running out 
>>>>>>>>> of gain at the lower voltages, and would get an improvement there by 
>>>>>>>>> reducing 
>>>>>>>>> the value of the 1k base resistor. This is evidenced by the 
>>>>>>>>> reduction in 
>>>>>>>>> voltage drop with increasing supply voltage. The important 
>>>>>>>>> figures in the 
>>>>>>>>> datasheet are Ic from the absolute max ratings, which needs to be 
>>>>>>>>> well above 
>>>>>>>>> the expected stall current of the motor, Vce-sat, which should be 
>>>>>>>>> small, and 
>>>>>>>>> the worst case hFE (large signal current gain), which should be 
>>>>>>>>> high enough 
>>>>>>>>> to allow for an acceptably low base current during motor stall 
>>>>>>>>> conditions. 
>>>>>>>>> Mike 
>>>>>>>> 
>>>>>>>> Mike,
>>>>>>>> 
>>>>>>>> Coming up with a nice H-Bridge for battery operation would be great, 
>>>>>>>> esp if it 
>>>>>>>> could be driven with a low pin count from a uP.
>>>>>>>> 
>>>>>>>> I did measure the stall current of my dissected TRV head,
>>>>>>>> 
>>>>>>>> 2 - Motor run current (no load) 13-15mA
>>>>>>>> 3 - Motor Stall current - 95-100mA
>>>>>>>> 
>>>>>>>> Cheers
>>>>>>>> 
>>>>>>>> Stuart
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>>>>>> 
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