# How do you use datasheets and that information to determine how many devices supported on a circuit?

I'm a grade 10 student in an electrical club. We are studying fire alarm systems and have come to a question.

In an apartment building fire alarm systems, there are isolators (CIMs) and attached to each CIM is a silence module (CSM) for silencing sounders that are installed in each apartment. When the alarm goes off, the silence module has a button that will let the resident silence the sounder for 10 minutes. The building code says that the sound pattern has to be Temporal 3.

Given the datasheets for the isolator/silence modules (CIM/CSM) and using 24 V DC low frequency sounders without strobes, what is the maximum number of sounders that can be on one CSM? (The datasheet shows two sounders.) Maybe we don't correctly understand the terms in the datasheets, but if the maximum current draw for the sounder (producing Temporal 3) is 138 mA and the output circuit rating of the CIM/CSM is 150 mA maximum at 24 V, then it would seem that only only sounder would be allowed.

Can someone explain? How many sounders maximum can be put safely on the circuit?

• Mr Grade 10 Student, your URLs are missing. – Wesley Lee Nov 23 '16 at 22:50
• exclude http//www prefixes for now – Sunnyskyguy EE75 Nov 23 '16 at 22:56
• Sorry but my linking in the original post didn't work ... I hope this works. CIM/CSM Module datasheet - [1]: www.systemsensor.ca/literature/CIM2B.pdf Sounder datasheet - [2]: systemsensor.com/en-us/Documents/… – Jason Nov 23 '16 at 23:54

It depends on if source is AC or DC for load ratings. Shall see when specs arrive.

I would suggest you choose modules with Temporal loads rated for <=18mA which exist, then the 150mA std output rating can fan out to 8 units.

Update Indeed these do not drive more than one external units. Some have a 1pps Strobe flash with even more load power depending on brightness setting.

I suggest you call/email supplier for interface clarification and alternatives for cost-benefits to suit the requirements.

www.systemsensor.ca/literature/CIM2B.pdf These are 18mA

• Is this a question concerning reliability , functionality or do you want to test or merely theorize for non-conformance? Is it a contact rating or an active switch rating or a battery life related spec? I dont know. – Sunnyskyguy EE75 Nov 25 '16 at 18:11
• The challenge is to prove the existing (installed in my apartment bldg), first confirm that 2 sounders are supported and then find the boundary condition. i.e the maximum number of sounders that can be on one CSM (based on the datasheet provided). We cannot change out the modules, and when we approach suppliers we usually don't get much response because we aren't a company buying products. Tony, can you please expand on your answer with why you use the 18 mA number, the mathematics and logic behind the "8 sounder units" conclusion. Thank you – Jason Nov 25 '16 at 18:22
• Sorry I missed your question. The temporal current spec is 18mA @24V , the 18mA x 8 units is what the 150mA rating guarantees ! Maybe 9. – Sunnyskyguy EE75 Apr 24 at 2:32