Author Topic: High DC Voltage measurment  (Read 1431 times)

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Offline sanwal209

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High DC Voltage measurment
« on: February 12, 2018, 08:56:36 am »
Hi,

I am currently doing a project with 8 Channel HV DC measurement. I used typical topology like first main high voltage divider then instrumentation amplifier. I am getting accurate reading but it takes 2 sec for the output to settle down. It seems like there is some high capacitance but i couldn't figure it out where.


PS: High voltage negative terminal is not connected to ground of op-amp.

Please help me where is the problem.
 

Offline HighVoltage

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Re: High DC Voltage measurment
« Reply #1 on: February 12, 2018, 09:51:58 am »
What HV source are you measuring?
How high is the voltage?
How stable is the source?
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Offline capt bullshot

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Re: High DC Voltage measurment
« Reply #2 on: February 12, 2018, 10:02:45 am »
Most probably the capacitance is hidden in your supply (I assume it is floating).
When connecting the HV, the local ground potential has to be moved to the middle point of your HV. This involves charging the capacitance from your local GND to earth - this kind of capacitance exists everywhere and isn't located in a single component.

You don't have resistors from the "+" inputs of the input amplifiers to your local circuit ground. So the charging current has to go "through" the "+" inputs of the OP-Amps - which is bad design as it won't do so as desired.

You should remove R14 and replace it by two identical resistors from the respective node to HV_M_GND. These resistors will then provide the path for the required charging current. Without these resistors, the "+" inputs will settle anywhere (depending on the leakage of the amplifier inputs), most probable outside the allowed input range of the amps. The resistors will also provide the required path of the input leakage to your local GND (HV_M_GND).
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Offline sanwal209

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Re: High DC Voltage measurment
« Reply #3 on: February 12, 2018, 12:46:14 pm »
Source is 1000V dc battery in electric vehicle. I have attached the basic architecture of the system. So we have one main battery but there are many contactors for power distribution and we want to measure voltage at different nodes in the system which includes measurements before and after contactors.

Resistance of 3Mohm in architecture is actually 3x 1M resistor. @Capt Bullshot yes you are right about it. I used the exact same thing before with separate 30k resistors and ground but i changed it because when S1 is off we will still have 6Mohm resistance between node1 and node2.

Another very solution would be to use 8 separate isolated dc to dc converters and have separate ground for each channel.

What do you think? can we have some solution without having this separate current path?
 

Offline capt bullshot

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Re: High DC Voltage measurment
« Reply #4 on: February 12, 2018, 01:25:00 pm »
If absolutely no current leakage is allowed, you'd need to isolate each sense amplifier by using it's own DC/DC converter. Otherwise, calculate your circuit impedance by maximum allowed leakage current.
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