I would like to assess a circuit for intrinsic safety with respect to capacitance and inductance. It is a fairly complex circuit and would like to make things as easy as possible so wondered if I could purchase a LCR meter and plug it into the power +/- terminals of the board to measure all the total C and L in the PCB + all components. Would this work they I imagine it to, or are these meters only intened for measure single components?

Thanks

 

they will not work if the circuit includes paralell resistance. most of the measuring techniques will not analyze complex impedance. possably a vector analyzer.

 

i seen one that has the option to switch between serial and parrallel, would that suffice?

 

no, the lcr meter would have to compensate for all other componants in the circuit. simple lcr meters do not have the ability to seperate out other componants that might upset the readings. for instance, how much would the lcr meter read if connected to a 1 micro henry inductor with a 1000 microfarad capacitor across it paralelled with a one ohm resistor?

 

Would you be able to offer guidance on how you would asses the following circuit for its equivilant capacitance then please?

many thanks

 

I’ve found it difficult to break the circuit down into an equivalent circuit and rightly or wrongly I have assumed all capacitors are in parallel. Taking a bom from orcad I have summed them all up and concluded that the equivalent capacitance in this circuit is 14uF.
Is this wrong?

Thanks

 

apacitance from what to what? from + to_ on supply? cap to ground on an input? couping caps do not add to supply capacitance.

 

Well this is a loop powered instrument, which is supplied from a 30V zenner barrier in a safe area. For the system, i.e. the cable and the device connected to the barrier to be considered intrinsically safe, they combined must fall within the specifications for the barrier which sets the limits for the system being intrinsically safe. Looking at capacitance in isolation, the barrier may have a Co of 83nF, and the a device (loop powered instrument) might have a fixed Ci of 3nF, which means the I could have a cable length that equalled up to 80nF of capacitance.
With this in mind I presume the answer to your question is from the +/- power terminals. I need to determine the Input capacitance of the device. Why are decoupling capacitors not taken into account?

 

I would like to assess a circuit for intrinsic safety with respect to capacitance and inductance...

In my experience with intrinsic safety, which included many dozens of hours arguing via email and over the phone with UL engineers, the "total" capacitance and inductance is simply the sum of the individual capacitors and inductors, those totals being required to be less than a maximum value computed based upon the max failure mode circuit voltage and currents, and total energy storage limit, regardless of where each component is located or its purpose.

Short answer: add the caps, add the inductors. Then compare with the limits.

 

This is what I have done . I have no inductive loads in the circuit that I'm aware so Hopefully my 14uF sum is the correct assessment. I of course don't want to overcook this assessment either so if anyone else has anything to add it would be greatly received

 

Alternatively, I believe it is permissible treat each reactive component individually, on the condition that each is protected by a UL recognized intrinsically safe energy limiting device.

That would mean redundant zeners in parallel with a cap, or a wire-wound resistor in series with an inductor.

In that case, you can compute maximum energy storage of each device using CV^2 or LI^2, sum all the energies, and check against your limits.

EDIT: BTW, intrinsic safety is *hard*. You'll need plenty of aspirin -- and beer -- to get through your certifying authority...

 

Lol... Oh I know.. Just sat my compex ex12 last week.

Thanks

 

Lol... Oh I know.. Just sat my compex ex12 last week.

Ha! So you've got the advantage of actually taking a class...

I learned in the school of hard knocks. And they were some hard, expensive knocks, to be sure...

 

if that poard is made where you work, just get an unpopulated board, put in all teh caps and measure. will give you total c, but not actual storeage c, the loads on the caps will reduce that a bit.

 

if that poard is made where you work, just get an unpopulated board, put in all teh caps and measure. will give you total c, but not actual storeage c, the loads on the caps will reduce that a bit.

I don't think that will account for all the caps, especially in the case of those that are not connected between power and ground.

Besides, I believe he must take into account the maximum capacitance values, including tolerance, not the nominal values. Pencil and paper, or a spreadsheet, is the only proper way to do this analysis. That is the way the certifying agency is going to do it. And they'll take their data over his.

 

FYI, for those who don't know about intrinsic safety:

There is a science behind all of this, including physics, chemistry, and engineering.

In a nut shell, all flammable gases have what's called an LEL, or lower explosive limit -- the concentration of gas and air at which ignition can occur by the addition of an external source of energy.

Adding energy though, is not enough to cause ignition. The energy must be equal to or higher than a certain magnitude in order to ignite a particular gas.

By ensuring that a circuit, under any fault condition, cannot discharge energy at or above that level, the circuit is essentially guaranteed not to cause an explosion when it is operated in an atmosphere likely to contain LEL or greater levels of gas. Thus, the word intrinsic.

There are many approaches to intrinsic safety. One would be, say, a hermetic seal, where all sources of excess energy are contained in there own atmosphere, and cannot escape to the outside (or explosive gasses let in).

Another might be a flame arrester. This could be as simple as a metal screen that contains ignition within a defined volume.

The most fundamental method, though, of achieving intrinsic safety, is to simply insure that there is never enough energy to cause combustion. This is easy to say, but hard to do, especially with modern semiconductor devices where peripheral caps and inductors proliferate.