[OpenTRV-dev] H-bridge transistor selection

[Damon Hart-Davis]

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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