Fusible Resistor Design
Fusible Resistor Design
(OP)
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.
Thanks in advance for replies.





RE: Fusible Resistor Design
And how do you expect a resistor not of your own design and construction to behave like a fuse?
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Fusible Resistor Design
SceneryDriver
RE: Fusible Resistor Design
If I use a fuse, then I would have to use around 60 fuses since I have so many branches. using a fusible resistor sounded a rifht option for me since it will not only break during overload but will also limit the current in case of normal operation.
RE: Fusible Resistor Design
Keith Cress
kcress - http://www.flaminsystems.com
RE: Fusible Resistor Design
RE: Fusible Resistor Design
http://www.littelfuse.com/about-us/~/media/Files/L...
http://www.cooperindustries.com/content/dam/public...
Keith Cress
kcress - http://www.flaminsystems.com
RE: Fusible Resistor Design
Something smells fishy here. If this is a power distribution circuit (similar to a fuse box or circuit breaker panel) then fine. But it would be unusual to raise fusible resistors in that context.
RE: Fusible Resistor Design
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Fusible Resistor Design
RE: Fusible Resistor Design
[quote IRstuff][What are in the 60 legs?/quote]
These are the sensors whose maximum rating is 48mA.
RE: Fusible Resistor Design
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Fusible Resistor Design
Z
RE: Fusible Resistor Design
My circuit is like a 24 V supply resistor and sensor all in series....So If there is a current surge the current will flow across the resistor(fusible resistor) which will break at the overload and will hence provide the protection.
RE: Fusible Resistor Design
RE: Fusible Resistor Design
If you quite reasonably assume that such surges will originate from the power system, then you only need to provide suppression circuit in one location.
If the wiring to the sensors is very long and runs outside (risk of lightning or induced surges on the sensor wiring), then that's a different matter. But you'd require a common mode solution in any case, not series fusible devices.
RE: Fusible Resistor Design
Have you looked at any datasheets? A quick Google search found this in the first few links.
http://www.tokenonline.net/pdf/resistor/fusible-re...
Bottom of page 1 gives wattages and times for burn-out. Pick the lowest value resistor that won't overload (damage) the power supply during a sensor short. Check that this resistor is suitable to be in series with the sensor. If yes, then calculate the power in the resistor which will correspond to an opening time. Then, you can decide if the resistor is suitable for what you want.
Another option is to install the highest value resistor suitable to power the sensor. Check that it won't overload the power supply with the sensor shorted. Make it a high enough wattage so it can handle the power during a short.
RE: Fusible Resistor Design
As per the datasheet, this says the V[max]=16V, I[h]=3A and I[T]=5.1A. I want to verify these parameters and understand the PTC fuse working.
Initially, I rigged up the following circuit:
28V supply, 1k POT(12ohms to 4.5ohms)and a PTC fuse all in series.
As per this circuit, if i vary the resistance from 12ohm to 5ohm,the current in the curcuit will vary from around 2 A to 5.2 A. If my understanding is right, then my PPTC should change the resistance at around 5.1 A. This I'm intending to verify by using a multimeter by measuring the resistance across PPTC.
However I overlooked the power dissipation across the resistor that i was using and i ended up giving 28V to PPTC fuse. Interestingly the PPTC continued to make the circuit. Ain't it supposed to break and open the circuit once i exceed V[max] and I[T].
Somebody please help me crack this.
Also please suggest a circuit to test this PPTC resetable fuse.
Thanks for your help in advance.
RE: Fusible Resistor Design
Huh? You need to explain what this means.
"Ain't it supposed to break and open the circuit once i exceed V[max] and I[T]."
No. A PPTC has a continuous I-T curve, so hypothetically, it will initially go to a very high resistance because of the massive temperature spike from the overcurrent, but it seems to me that it would eventually stabilize at some temperature that induces a resistance that dissipates sufficient power to support that temperature. There is insufficient information in the datasheet to get a good estimate, but a swag would be something on the order of a couple hundred ohms would result in a steady state solution. Since this would result in a current that is only somewhat larger than the trip current, an overcurrent is avoided. Thus, a PPTC is not what EEs might consider to be a true "fuse;" it's more like a current limiter, i.e., it only prevents the current from grossly exceeding the trip current. Per: http://www.thinking.com.tw/documents/en-pptc-intro... the fault condition must be "removed," independently from the PPTC.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Fusible Resistor Design
I meant that power was exceeding the power rating of the resistor(1W) that I was using and this resistor was becoming short.
"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?
RE: Fusible Resistor Design
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.
RE: Fusible Resistor Design
RE: Fusible Resistor Design
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: Eng-Tips.com Forum Policies
RE: Fusible Resistor Design
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Fusible Resistor Design