diode? 1

[bigheadted]

the reference to an inveter drive at work is 0-10v, the reference under general opperating conditions doesnot exceed 2.5v to achive the required speed. The reference comes from a PLC analogue output. before my time there was an issue that when the plc was first switched on it set its output to -10v which the drive didnt like, so a diode was put in series with the reference to prevent the reverse voltages. my question is, how would the forward bisa volt drop of the diode affect the drive. surely it will drop the refernce voltage the drive sees by 0.6v, which is quite a large proportion of the running refernce level. am i correct? does the volt drop across a diode depend on the currentr flowing through it?

 

[Skogsgurra]

Yes, absolutely. And, what's more, there will be a -2.6 mV/C tempco as well.

Gunnar Englund

http://www.gke.org

 

[bigheadted]

thanks sometimes its good to know youre on the right track

 

[ScottyUK]

Could the reference be changed to use a 4-20mA loop instead of a voltage signal? That would eliminate the diode volt-drop problem while allowing the diode to remain and provide better noise immunity too. Most drives have the option of voltage or mA input.


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[bigheadted]

good idea, although im not sure the company will meet the expense as it would mean either a plc change or some sort of converter device and there are 48 of these drives around the plant. i really have two options, attempting to offset the reference in th plc to account for the volt drop, (measurment today indicated that it averages 0.4v) or i believe you can get low forward bias voltage diodes. both of which are not ideal
thanks

 

[ScottyUK]

Google 'schottky diode' for your low V_f diode. Be aware that they have limited reverse blocking capability, often only 30 - 40 volts.

----------------------------------

Sometimes I only open my mouth to swap feet...