[OpenTRV-dev] H-bridge transistor selection

[Stuart Poulton]

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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