[OpenTRV-dev] H-bridge transistor selection

[Damon Hart-Davis]

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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