[OpenTRV-dev] H-bridge transistor selection

Stuart Poulton EMAIL ADDRESS HIDDEN
Fri May 17 11:28:26 BST 2013 

It's available from Farnell at least.

Stuart

On 17 May 2013, at 11:14, Damon Hart-Davis wrote:

> Yes, very good fit (other than being impossible for human beings to solder)!
> 
> I've not yet found it with one of my current suppliers...
> 
> Rgds
> 
> Damon
> 
> 
> On 17 May 2013, at 11:03, Stuart Poulton wrote:
> 
>> 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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