Electronics > Projects, Designs, and Technical Stuff

DC Bus Voltage sensing cirquit

(1/4) > >>

eliafavero:
EDIT 21.08.19: scroll to the bottom to read about the recent consideration on the topic.

Hi All

----disclamer----
first time posting on the blog and i am not sure this is the right section for this topic. Feel free to point me to the more appropriate section or to a post that already discusses the topic.

General / open question (Below more context is given):
Can anyone point me to documentation or suggest me some DC voltage sensing cirquit? I am working on a project where i need to measure a voltage of roughly 270V.
Design restriction:
   - No microcontroller, all analog
   - Low budget
   - Accuracy of ~1% with the voltage between 270 and 320V
   - Low power consumption 

context:   
I need to design a voltage sensing cirquit for a DC bus of 270V. Multiple AC motors and the respective driver are connected to the DC bus. The circuit needs to "regulate"  the dc voltage by powering a "braking resistor" (Pnominal = 500W) in case that the dc voltage raises above 300V. The option that i found to be the best at the moment is the "typical application Circuit" proposed on the datasheet of the ACPL-C87 sensor with some sort of schmit trigger or comparator stage at the end to drive the Mosfet of the braking resistor (see the attachements).

My concern with this design is precision. To measure the voltage i first need  a voltage divider which is already pretty inaccurate. The ACPL-C87 accepts an imput voltage of 0-2V. The variation of the DC voltage is about 10% (((300-270)/270)*100) and since Vin is proportional i have roughly 200mV to work with.
Now,to me, it sounds a bit unrealistic to have a reliable measurement in an environment with high AC Noise.
Should i not be worried? Is there a valid alternative? 

if something is unclear(maybe i should delete the "if") just ask, i'll be glad to give further explanation.
Thanks! :)

schmitt trigger:
A rssistor divider is as accurate as the tolerance of its individual resistors.

The 1% tolerance resistors are cheap as dirt nowadays, and you can also get 0.1% easily (at least in major countries).

Using exclusively 0.1% resistors, and by selecting the proper values, the resulting divided down DC sample will retain that accuracy level.

Zero999:
There's no need for the ACPL-C87, as the circuit being switched, isn't isolated for the voltage being measured.

I would go for the TL431 controlling some power transistors, with most of the power dissipated by a resistor.

Marco:
I'd call it a bleeding resistor since the bus voltage is already providing the breaking.

You can use a TL431, but it will need a few auxiliary components. How about something like this? Switching won't be very fast.

eliafavero:

--- Quote from: Zero999 on April 08, 2019, 02:09:09 pm ---There's no need for the ACPL-C87, as the circuit being switched, isn't isolated for the voltage being measured.

I would go for the TL431 controlling some power transistors, with most of the power dissipated by a resistor.

--- End quote ---
you are definitly right! Don't know why I came up with the isolated OP amp.:palm:

I am not very familiar with the TL431, is the circuit proposed by Marco the same you had in mind? or would you propose something different? i'd really appreciate some schematics :)


--- Quote from: Marco on April 08, 2019, 04:14:20 pm ---I'd call it a bleeding resistor since the bus voltage is already providing the breaking.

You can use a TL431, but it will need a few auxiliary components. How about something like this? Switching won't be very fast.

--- End quote ---

I think i haven't done a good job at explaining the situation.
the AC/DC converter actually generates a DC Voltage of 540V which is then divided in two with capacitors in series. this gives me virtually 2 dc lines: 270-->"0" and "0"--> (-270). I then have several motor units on the "High"dc bus and the same number on the "low" dc bus. in normal operation every motor has the same mechanical, and therefore electrical, load. this keeps the bus and the "0" stable. so far so good.
it can occour that one or several motors need to stop for a while. This causes an unbalanced load and the vitrual "0" drifts. which leads the "high" dc bus to go up to 350V and the "low" one to go as low as 190V. Is there some sort of circuit used for this applications? does anyone have an idea on how to adress the problem or which approach should i take?

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod