A better H bridge...

[ikalogic]

okay guys... too much of theory here! how can i apply this?

a simple soft start approach would be much simpler, and a shaft encoder to detect if the motor is not turning can do the trick i guess, but however i want to learn more about current sensing..

i guess all i need is a current to volt converter, then an op-amp can compare this to any reference i chose, but again, how to implement this in my H-bridge?? (practically)

any examples?

thx

 

[Nigel Goodwin]

A 'current to voltage' converter is just a resistor - you pass the current through it and a voltage is generated across it. Assuming it's a low value resistor then the voltage will be very low - you can amplify it with an opamp.

 

[ikalogic]

A 'current to voltage' converter is just a resistor - you pass the current through it and a voltage is generated across it. Assuming it's a low value resistor then the voltage will be very low - you can amplify it with an opamp.

okay.. things are getting clearer. but, where should i put this resistor?.. in series with the motor??!! this is the only place i can think of.. and also the most strange place, as - i guess - it would eat all the current... i dont know....

any practical examples of this?

 

[Nigel Goodwin]

It wouldn't 'eat' any current at all, as it's in series with the motor exactly the same current passes through it - but you need a very small resistance, because you don't want to lose much voltage across it. But it's got to be enough to amplify and measure.

 

[Styx]

it will "eat" volts so use as low a value as you can (trade-off between signal resolution and what volt's you can afford to lose)

The correct place to put the "sense resistor" is actually in the load (ie in series with the machine" since then you will also be able to sense the free-wheel current (EXTREAMLY IMPORTANT if you have a PI-control and periods where no switching occurs => freewheeling, since the current-loop 'I' part will wind itself off and cause nice big current-spikes at the next switching event)

You don't seem to have that problem in this setup so put the low-value resistor on the negative side of the power-rails. That way can easily reference w.r.t. 0V

 

[ikalogic]

it will "eat" volts so use as low a value as you can (trade-off between signal resolution and what volt's you can afford to lose)

The correct place to put the "sense resistor" is actually in the load (ie in series with the machine" since then you will also be able to sense the free-wheel current (EXTREAMLY IMPORTANT if you have a PI-control and periods where no switching occurs => freewheeling, since the current-loop 'I' part will wind itself off and cause nice big current-spikes at the next switching event)

You don't seem to have that problem in this setup so put the low-value resistor on the negative side of the power-rails. That way can easily reference w.r.t. 0V

ok brilliant! i understand.... then i connect the input of the op-amp/ADC between the motor and the resistor.. then any uC can do the trick.. so actually this can be done outside of the H-bridge! maybe i'll add it as an option in my H-bridge...

thx a lot

 

[socio]

Hi Ikalogic,

why dont' you use integrated h-bridge with embedded current sense status?


**broken link removed**


only one component, for sure more expensive, but should be everything inside.

 

[ikalogic]

woowww! that's one powerfull device!! :O!

but unfortunately i 99% sure i can't find this in egypt.. and don't tell me to order from outside egypt!!

thx anyway.. very interesting!