Fusible Resistor Design 2

[rameshyrvr]

Hi guys. I'm thinking to use the fusible resistor in my circuit to provide protection by limiting the current. But not able to figure out when the resistor will burn out and provide protection. Also is it possible to put a LED across it to know if the resistor is working or not. The resistor is 650 ohm, 5W and max. voltage across it is 24V,DC. max current though it is 48mA. Please help.

Thanks in advance for replies.

 

[LionelHutz]

I'll take an educated guess. I bet you used a little 1/2W or 1W pot for this experiment which is grossly undersized for that test. Next, your claim of applying 28V directly across this device makes no sense. 28V across a 0.105 ohm device would draw 267A. It should have tripped no problem in that test.

I don't think you understand what the PPTC device does or what the ratings mean. Basically, you want the operating current to be < 3A and the trip or fault current to be > 5.1A. If you read the datasheet, it says that that It(A) is the minimum current the device will trip at, meaning it may require more current to trip. The voltage rating is just the maximum voltage that should be applied across the device. It has nothing to do with the device tripping but you are exceeding this rating by expecting it to work on a 28V circuit.

Overall, you seem to be thrashing around in the dark. You started with a 5W resistor and expected <2W to burn it out and now you messing with >5A trip points on a power supply that doesn't seem capable of sourcing that much current. Your data jsut isn't making any sense. You first claimed 24V and 650 ohms gave 48mA which makes no sense. You're now claiming 28V directly across this device which also makes no sense.

This is Engineering Tips, a site for engineering professionals to get help with a specific issue tehy have run into. It's not a site where you should expect users to spoon feed someone who doesn't even seem to understand ohms law.

 

[OperaHouse]

I use a 4.6A ptc and regularly draw over 20A through it for periods close to a minute. A considerable time delay based on ambient based ambient temp and howmuch heat can pass out the leads. This one I think requires at least a quarter watt to keep it activated, so they do not shut off as you say. I have followed this a while and your fusing and matrix ideas are one of the worst concieved projects I have ever seen here.

 

[IRstuff]

"Since this would result in a current that is only somewhat larger than the trip current"
1. How much will this limiting current will be?
2. Till what value/time will it hold this limiting current in the circuit?

Really? You can't find anywhere on the web what a PPTC R vs. T curve looks like?
What was unclear about "the fault condition must be "removed," "

TTFN
faq731-376

 

[IRstuff]

Is this for school?

TTFN
faq731-376

 

[LionelHutz]

I would hope so because it's sad to think someone is getting paid for this kind of random experimentation in the hopes some kind of solution just happens to fall into their lap.