What is a fuse resistor? - Page 1
Forum section: electronics / beginners

author: thilo
author_rank: Regular Contributor
timestamp: July 25, 2011, 08:52:10 am
content: Hi,whenever Dave inspects a multimeters input protection he talks about fuse resistors. But what is a fuse resistor and what it's used for in a multimeter? And what are all the other input protection elements used for mentioned in the multimeter teardowns? PTCs, MOVs, gas discharge thingies, etc.Thanks,Thilo

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: July 25, 2011, 09:09:53 am
content: Interesting question ..The Fuse resistor, are one common fuse shaped as resistor.The gain is that it does not need's a fuse holder, and so it has one very small size.It is never used as main Fuse on the DMM, and so it protects parts of the circuitry that use very low current,usually are rated at 100mA or less.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 25, 2011, 09:38:12 am
content: Fusible resistors are resistors that are designed to open without flames when overloaded. Sometimes also called flameproof resistors.

author: A Hellene
author_rank: Frequent Contributor
timestamp: July 25, 2011, 09:47:22 am
content: A fuse resistor is an ordinary resistor (or a specialfusible resistor, as Mike mentioned above) that is used as a fuse when its electrical characteristics are violated, resulting to its destruction --thus breaking the circuit.In order to protect a portion of a circuit against over-current of, say, 100mA, I will simply insert asuitableresistor in he current path, according to the formula below:P = I^2*R = U^2/R <=>R = P/I^2 = P*U^2where P is Power in Watts, I is current in Amperes and U is voltage in Volts.For example, to calculate a 100mA fuse-resistor I simply apply the formula above:R = P/I^2 = 0.25W/(100mA)^2 =25 Ohm / 1/4W, orR = P/I^2 = 0.125W/(100mA)^2 =12.5 Ohm / 1/8W, etc.Of course, it is not that simple; but this is the principle of using fuse resistors.-George

author: thilo
author_rank: Regular Contributor
timestamp: July 25, 2011, 10:38:22 am
content: Thank you!Thilo

author: Zero999
author_rank: Super Contributor
timestamp: July 25, 2011, 06:54:56 pm
content: One reason for using a fusible resistor rather than a fuse, is the resistor will itself limit the current surge, whereas a fuse won't, for example a 100mW 1R fusible resistor will fuse at 316mA but will limit the short circuit current to 12A on a 12V electrical system. This is beater than a glass fuse which will have a maximum capacity of around 35A and doesn't have enough resistance to limit the current to a safe level so may fail dangerously if the circuit's impedance is low enough to allow over 35A to flow.

author: thilo
author_rank: Regular Contributor
timestamp: July 25, 2011, 09:44:34 pm
content: Hero999, thanks, I only realize now that until now it was not clear to me why you would prefer a fuse resistor instead of a fuse.And what are the other input protection elements there for (aside from the obvious: input protection).thilo.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: July 25, 2011, 10:01:58 pm
content: Quote from: A Hellene on July 25, 2011, 09:47:22 amthilo this description of George. was just implying the same thing.Resistor limits amperes .. are you a student or a hobbyist ?

author: thilo
author_rank: Regular Contributor
timestamp: July 25, 2011, 10:54:09 pm
content: Kiriakos I'm aware of that (now), but I guess I just needed to hear it one more time and maybe the formulae were distracting me from the first of his sentences.

author: Fox
author_rank: Regular Contributor
timestamp: July 25, 2011, 11:32:54 pm
content: Quote from: thilo on July 25, 2011, 09:44:34 pmPTCs in this case are used as resetable fuses, if the current through the PTC increases the resistance of the PTC increases also (PostiveTemperatureCoefficent)MOVs are voltage depended resistors they are used in parallel to ones circuit and limit the voltage to their nominal voltage e.g 10V,100V etc. but don't limit the current through them. If the voltage across the MOV increases above its nominal voltage the MOV basically goes to lower resistance values.Without the PTCs in series with them they would short out the circuit one are measuring.Sparcgaps do almost the same thing as MOVs except when the voltage goes above their trigger voltage (e.g. 500V) a sparc is triggered between the electrodes of the sparcgap and shorts out the circuit. (burning voltage of the sparc around 10 to 20V).

author: uuufff10
author_rank: Newbie
timestamp: September 18, 2014, 03:00:47 pm
content: what is a fuse resister?i used 2 ohm Fuse resister insted of .22 ohm is it ok? will it more safe or less safe coz its 10 times more than .22 ohms

author: uuufff10
author_rank: Newbie
timestamp: September 18, 2014, 03:02:37 pm
content: what is a fuse resister?i used 2 ohm Fuse resister insted of .22 ohm is it ok? will it more safe or less safe coz its 10 times more than .22 ohms ,should i need .22 ohms anyhow?and which will pass more current n if it has to blow up for safety which will go 1st (i used it on 30/40watt electronic flurecent ballast)

author: zapta
author_rank: Super Contributor
timestamp: September 18, 2014, 04:41:55 pm
content: Quote from: mikeselectricstuff on July 25, 2011, 09:38:12 amIs the resistance typically part of the circuit functionality or are they typically just for protection (kind of a safer fuse)?One of the topics that I would like dave to have a video on are those resettable fuses because they have a few resistance specification and I am not clear how to choose one. I think those resistances are typically just part of the protection, not an integral part of the circuit.

author: Seekonk
author_rank: Super Contributor
timestamp: September 18, 2014, 04:44:42 pm
content: I investigated the fusing properties of 1/4W CF resistors. You can get them to glow red for hours with less than 1% change in resistance. Yet they are often used in line operated products in series with the voltage dropping capacitor. These commonly fail open from line pulses. Carbon comp resistors are quite hardy in pulse duty.

author: Unknown
author_rank: Unknown
timestamp: September 18, 2014, 06:31:46 pm
content: This thread must have gotten indexed with a very high rating on search engines for this topic, it's got over 14000 views. Also thanks for the explanation of MOVs further up, I too was also curious what they were, now I know. Could they be used for transient suppression in a high power switching circuit? Or would a TVS diode be better for that?

author: macboy
author_rank: Super Contributor
timestamp: September 18, 2014, 06:37:32 pm
content: Quote from: zapta on September 18, 2014, 04:41:55 pmThe resistance is part of the circuit operation, whether by design or as a (benficial) side effect. I have seen fusible resistors used to drive the base of the output power transistors in audio amps. Under different fault scenarios, they will either can the base current to a safe value (protecting the driver circuit), and/or fuse open, stopping the drive current for the output transistor. Of course there are many failure modes, but for a meager cost of this resistor, it affords reasonable protection against some catastrophic failures.

author: SeanB
author_rank: Super Contributor
timestamp: September 18, 2014, 06:59:23 pm
content: This is what a MOV looks like when it has tried to clamp 360VAC with a 20A breaker to interrupt the current.It blew itself apart, and blew the daylight switch it was in apart as well, and left an arc mark on the cabling about 30cm away from it.MOV is normally available in higher power ratings than a TVS diode, but the clamping voltage is less defined, and the slope resistance ( how much the voltage across it rises as the current increases) is generally higher. You typically use the MOV at the input side to limit the input voltage and after some resistors or inductors the TVS diode is used to clamp the remainder to a safe level.TVS diodes typically go up to 5W, and MOV devices can do 100W in a large device. In power distribution you get ones that can handle 1kW or more of power, used in transmission lines to provide lightning strike protection. Those can handle 1MJ of power from a strike with little degradation.