Current considerations in Electronic design

[Azeemshatp]

Dear all,
Is it possible to supply 5V from a single 7805, to multiple number of ICs, some of which are safety critical?
I have one PCB having ICs meant for the safety critical applications.
And also have another PCB for the Human interface.
Can I directly get the 5V from the first PCB, for supplying power to the second one?

It is an ignition control and safety system for heating appliances. So here the controller have to handle few relays, fan, valves etc. But the only thing we have to bother about is, the worst case current which will be drawn by the Controller right? So if I can estimate it, and if its under the possible limit which 7805 can provide, I can go ahead right?


Can anyone tell the design considerations for such cases.
Please suggest me few links also, so that I can search for it.

 

[berkeman]

The key term you are looking for is Intrinsic Safety. I did a google search, and here is the hit list:

http://www.google.com/search?sourceid=navclient&ie=UTF-8&rlz=1T4SUNA_en___US232&q=intrinsic+safety

We are not going to be giving you much advice here about Intrinsically Safe circuit design -- that is way too specialized, and mistakes cause explosions. Do the research, and hire a consultant if you don't have the expertise within your own company or group.

Yes, the energy available from the power supply is one of the primary considerations for IS designs, but the energy stored in inductors and capacitors also comes into play.

 

[zeitghost]

You might consider asking this question on Eng-Tips.

 

[Azeemshatp]

The intrinsic safety issues are already taken into account I believe.

But I will narrow down the ploblem..
Suppose a microcontroller is used to drive a safety valve may be through some optocoupler logic. This controller is powered through a 7805.
Can I use the same 7805 to power another microcontroller which is meant for the human interface. The same has to give power for the LCD also.

 

[berkeman]

The intrinsic safety issues are already taken into account I believe.

But I will narrow down the ploblem..
Suppose a microcontroller is used to drive a safety valve may be through some optocoupler logic. This controller is powered through a 7805.
Can I use the same 7805 to power another microcontroller which is meant for the human interface. The same has to give power for the LCD also.

What safety regulations govern this application? What agency approvals will be applied for with this system? What does the "safety valve" control?

 

[Azeemshatp]

"safety valve" control cuts down the gas supply in case of a mis-communication or error.
I Think some EN298 certification...I am not very sure abt whether this is the regulation or not.The above mentioned "I believe" also applicable for this.

 

[berkeman]

"safety valve" control cuts down the gas supply in case of a mis-communication or error.
I Think some EN298 certification...I am not very sure abt whether this is the regulation or not.The above mentioned "I believe" also applicable for this.

I'm still not sure we can be of much help, unfortunately. There are so many issues that can go into architecting a reliable system for sitations involving intrinsic safety and explosion hazards. For example, on reliable systems that I've worked on at Bell Labs in the past, there would often be triple-redundant components to the architecture, with a voting scheme and hot-swapping of control. The reliability requirements just could not be practically met with a single uC type of architecture.

It may be that the requirements of EN298 cannot be met with simple designs, and that you will need some redundancy or separation or other design considerations. If there is not sufficient expertise in your company at present, then I think you should consider looking for a local consultant or consulting firm with experience designing systems to EN298 (or whatever safety and reliability standards apply). This is just too important to get (even a little bit) wrong.

Best of luck.