[OpenTRV-dev] H-bridge transistor selection

Thu May 16 17:40:09 BST 2013

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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