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Fusible Resistor Design 2

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rameshyrvr

rameshyrvr

Electrical
Dec 15, 2012
13
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.

 
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Well, don't you think that a 5W resistor might have some trouble doing anything with an input power of only 1.5W?

And how do you expect a resistor not of your own design and construction to behave like a fuse?

TTFN
faq731-376
7ofakss
 
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After a lot of googling found that the fusible resistor was the best fit for the protection 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.
 
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Consider PTC resettable fuses. They work great in your range. They need resistors in series with them so you have that covered (probably). One trips nothing burns. Power down and up and you're back in business.

Keith Cress
kcress -
 
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"If I use a fuse, then [highlight #FCE94F]I would have to use around 60 fuses[/highlight] since I have so many branches."

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.

 
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What load, exactly, are you trying to limit the current on? 48mA is an extremely low current load to try and control in this fashion. What are in the 60 legs?

TTFN
faq731-376
7ofakss
 
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I experimented a long while ago using 1/4W CF resistors as fuses. You could crank powr through these for hours burning off the paint and turning them red. After an extended time they wouldn't even change resistance by 1%. They can be used in cases where there will be a dead short and a good 10A will blow them apart. Problem is carbon film are very vulnerable to pulses. I see lots of these used in line applications where they just open up due to spikes on the power line and there is no visable damage to the resistor.
 
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VE1BLL said:
[If this is a power distribution circuit/quote]
Yeah this is a power distribution circuit.
I don't want to char my resistor. So that's the reason I'm going with the fusible resistor which is fire resilient. My working environment must be immune to fire.
IRstuff said:
[What are in the 60 legs?/quote]
These are the sensors whose maximum rating is 48mA.
Click to expand...
Click to expand...
 
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But where is this over-current coming from? If it's from the supply, then a single fuse makes sense. If it's the sensor, then it's essentially already fried.

TTFN
faq731-376
7ofakss
 
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Watch out on the PTCs. The few I've worked with have a higher resistance after recovering (i.e. power cycle) than they had before the event that tripped them. Does your circuit perform well with both values? You will really need to test a few to make sure they behave the way you expect them to. The datasheets can be worded very vaguely.

Z
 
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Thanks for the suggestion. I'l surely look into this.
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.
 
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If you really want to protect at those currents, I would use a three terminal regulator in constant current mode instead of the resistor. Loss can be a s low as 1.3V or with enough headroom a 7805 could be used. ust a resistor will never give enough protection without impacting circuit performance.
 
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"...a current surge..."

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.

 
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24VDC and 650ohms gives 37mA so how do you expect to source 48mA through this resistor? Also, that is 0.9W so a 5W resistor would have no trouble handling that power level without burning out.

Have you looked at any datasheets? A quick Google search found this in the first few links.


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.
 
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I experimented with the resetable PTC fuse GP 16 300 to understand its working.It's details and datasheet is in this link
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.
 
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"However I overlooked the power dissipation across the resistor that i was using and i ended up giving 28V to PPTC fuse."

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: the fault condition must be "removed," independently from the PPTC.

TTFN
faq731-376
7ofakss
 
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"However I overlooked the power dissipation across the resistor that i was using and i ended up giving 28V to PPTC fuse."
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?

 
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