# question about resistors



## bonuts (Apr 25, 2010)

Hi. One of the resistors in my Hitachi calls for a 3.9 ohm 1watt resistor. I couldn't find a 3.9 ohm 1 watt, so I bought a 3.9 ohm 5 watt. Will this be ok? 

It looks different, in that is not a cylinder shape like more resistors, rather a big honkin white box, about the size of a piece of gum. Thanks!


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## lcaillo (May 2, 2006)

I will work , but is not a great idea. If something goes wrong, rather than opening up at a lower power, it may keep cooking, creating other problems. Electically it will work the same under normal conditions, but for safety it is better to use the same power rating.


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## bonuts (Apr 25, 2010)

Thanks for the reply! That makes sense to me now. I went ahead and used it, but now I guess I'm worried. Maybe next time I place an order with mouser I'll get a 1 watt and change it out.

BTW, thank you so much for the information you've provided! I am watching TV again, and it is a great feeling to DIY + only spend $30 in parts. Thanks!


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## dyohn (Apr 17, 2008)

Where did you look? 3.9-ohm 1-watt resisters are very common.

http://www.mouser.com/Passive-Compo...istors-with-Leads/_/N-2gra1?P=1z0x74hZ1z0vocv


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## goyop (May 4, 2010)

Don't sweat using a higher wattage resistor. They are not supposed to act as fuses. The wattage is based purely on what is happening in that circuit. It needs to handle up to one watt. If it can handle five then it still serves the purpose.


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## lcaillo (May 2, 2006)

While that is mostly true, there may be other considerations. In many circuits resistors serve two purposes, acting as a resistor and opening to protect other components when something creates a problem. That is mostly not true in these circuits, but it is remotely possible that by not having a resistor that will open soon enough, you could open a yoke instead which would be a lot bigger problem than a shorted IC.

The best practice is to put back in what came out, unless there is a clear understanding of why there should be a change or the manufacturer suggests it.


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## goyop (May 4, 2010)

I have been mulling this one over and do not want to upset anyone by challenging their posts so please keep that in mind.

It is possible that I am mistaken but having studied circuit design, worked in circuit design, trouble shooting thousands of circuits, and designing automated test equipment I have never even heard the idea that resistors are used as fuses. Fuses are used as fuses. Resistors burn, catch fire, and give off toxic smoke. I cannot imagine them ever being designed into a circuit as a fuse.


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## lcaillo (May 2, 2006)

Actually, it happens rather frequently. You see low resistance values at low power ratings in flyback secondaries all the time that are intended to open if a vertical outpur fails, for instance. It is simply not cost effective to put fuses in every circuit that might have a failure that could draw too much current from another. 

The resistors in convergence outputs are not intended to be fuses specifically. They are there to affect linearity and to work with the yoke. Regardless, unless one understands the specific circuit, it is bad practice to make changes that may have unintended consequences. Many years of repair experience tells me that there are lots of designs that are surprising and that may involve factors that are not documented nor understood without greater consideration.


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## goyop (May 4, 2010)

As mentioned, I could be mistaken, but in my experience I have never heard of a resistor being intended to open to protect a circuit. I worked exclusively with TV circuitry for a couple of years. In study and practice I have never heard it mentioned. Protection circuitry that is built in may be a thermistor or some device which changes properties based upon conditions but not destroy itself by overcurrent.

We are probably mixing apples and oranges here since I believe your point is that unless you know what you are doing and what the circuit is designed to do then you should be careful to replace like components for like. I would never disagree with that.

My point was more that oversizing a resistor is no cause to lose sleep because they are not used as fuses. But I am an experienced engineer and technician. Probably you are urging caution for those who don't have a technical background.

Cheers


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## Syd26147 (Jul 4, 2008)

If you bought something similar to this:








That's a wirewound resistor



Syd


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## lcaillo (May 2, 2006)

Google fusible resistors. You will find many low ohm resistors are specifically designed to fuse as protection devices. They are typically metal film devices designed not to catch fire.

Fuses, really, are resistors themselves.

You are largely correct, that most resistors that serve a purpose as such in a circuit are not intended to fail. And, in these circuits, it probably does not make much difference that higher values are used. We agree that without design knowledge it is best to replace parts with what is intended by the manufacturer, unless some specific reason is well understood.


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## goyop (May 4, 2010)

Up to now I did not know that you were speaking specifically about fusible devices. Obviously that makes all the difference since as you know it is designed to protect something. I was thinking purely of the various types of resistors used in circuit design purely for their R value.


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## lcaillo (May 2, 2006)

Just because a resistor does not have in its description "fusible" does not mean that the power rating was not chosen to account for the possibility that having it open may have protective value for other compnents. This is largely why carbon resistors are rarely used these days. They flame out rather than simply opening with less thermal effects. Again, one cannot make sweeping assumptions about design issues. It is best to replace parts with the specified values unless one makes a thorough analysis of the application and has specific information that a change is appropriate. In this case it likely does not make much difference, but it is generally bad practice to change values for reasons of supply convenience without more consideration, and generally bad practice to assume one understands all of the design considerations when one was not involved in the design nor has extensive experience with the type of circuit involved.

You can be correct and still be giving advice that can be dabngerous for other less informed people, or that might be taken out of context. I tend to be a bit more conservative in my advice.


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## randtek (Jun 3, 2010)

Resistors are quite commonly used as fuses in modern equipment. Any time you have a low value, higher wattage resistor, there is a decent probabililty that it is being used at least partly for fusing. A 3.9 ohm 1 watt resistor falls exactly into this category. Another thing to consider that has not really been mentioned is why the resistor was opened in the first place. It is uncommon, but not unheard of, for these resistors just to fail on their own. More likely, it failed because something else has failed, causing excessive current draw. I would not replace a 3.9 ohm 1 watt resistor with a 3.9 ohm 5 watt resistor for this reason.


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## lcaillo (May 2, 2006)

What can happen in these circuits, in fact I have one in the shop right now, is that when there is a channel that is poorly adjusted, you can have a resistor that runs hotter and opens. If you don't correct the underlying problem, and just put in a larger power rated device, you will likely eat the chip eventually. I have a Samsung in the shop right now that was eating one pair of resistors. That channel was offset quite a bit and needed to be recentered. Some sets have the edge or point correction so far out that a channel might overheat as well. It is sometimes necessary to recenter the controls for some or all of the parameters to get a system working most efficiently.

Regardless, as has been said before, it is best to replace parts with what was designed by the manufacturer to be there unless there is a really good reason to do otherwise and the consequences for that particular circuit application is well understood. Simpy assuming that a change will not be a problem is often a good way to get into trouble. 

And while smoke signal troubleshooting that results from simply replacing open fuses and resistors can be effective, it can also create more problems, partiucularly for someone who does not understand troubleshooting in modern electronics.


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## randtek (Jun 3, 2010)

lcaillo said:


> And while smoke signal troubleshooting that results from simply replacing open fuses and resistors can be effective, it can also create more problems, partiucularly for someone who does not understand troubleshooting in modern electronics.


Just to clarify, I was not suggesting or condoning using a larger value resistor to see what smokes. I was doing just the opposite. I was attempting to use understated sarcasm to make that point. I hope my post was not taken otherwise. Using an over rated fuse or fusible resistor to replace a failed one is ALWAYS a very bad idea that will most often result in further circuit damage, and should NEVER be done. If you do not have the knowledge and skills necessary to determine what caused a fuse or resistor to fail, you should take the unit to a qualified technician. This is the point I was trying to make, not suggesting using an overrated fuse as a troubleshooting technique. 

I am sorry if my failed attempt at humor caused any confusion about this. That was not my intent.:doh:

I will be editing my previous post to remove that unclear sarcasm line.


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## lcaillo (May 2, 2006)

Randy, please do not take my post as a critique of your own. There is no need to edit your post. I was simply re-inforcing your point. I understood your humor and sarcasm precisely, but wanted to make sure that other readers, who might not get it as clearly, understood.

I welcome your contribution as a working tech and hope that you will participate and be a great asset to the forum.

Have you entered your company's info in the form for listing it as a vendor? That listing is free if you want it. Just go to the vendor list and look to the right side for the link.


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## randtek (Jun 3, 2010)

I did not take it as a critique per-se, but it did make me realize that to some, my sarcasm could come across as a "suggestion". I sometimes forget that these forums are worldwide, and that people for whom English is a second language may not easily recognize that type of sarcasm for what it is. After reading your response to my post, I decided that my ambiguous attempt at humor could result in someone doing something that could be dangerous, and that is not the way I wanted to start out here. 

This is the first technical related forum that I have found that is up to the standards I was looking for. As soon as I found it, I recognized that, and immediately joined it. The moderators/administrators here are quite obviously very knowledgeable, and I like a more tightly moderated forum such as this. I will not frequent forums that have cursing or other disrespectful postings, and I hate "fan-boy" type postings too. My first visit here, I found information that was useful to me professionally, pertaining to a Samsung DLP I was working on. It helped me to quickly confirm my assessment of the unit I was looking at. I look forward to both using this forum as a resource, and contributing knowledge where I can. 

Sorry for the unintentional mini-hijack. We now return you to the original intent of this thread, which is resistors!


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## goyop (May 4, 2010)

Gentlemen,

Forgive me but I am still a bit stuck on the idea of a resistor being designed into a circuit as a possible fuse. (Except if it is in fact a fusible resistor) Let's all agree as we have already done, that no person should replace components without having an understanding of what they are doing and the implications to the original circuit design. And further, my comments are only for the sake of understanding. I may very well be uninformed. I do not care if I am right or wrong but I am interested in this topic.

During my education and practice in circuit design, and work in television circuitry I never saw any instance of using a resistor as some sort of protection device. Fuses and fusible devices are fuses not components. I will say that I have been away from design for a while and I have no experience in Plasma TV design or troubleshooting so things may have changed. But based upon the basics of good circuit design I find it unimaginable that a good designer would look to a regular resistor as a fail safe of some sort. Especially for an expensive chip or component.

How do you know that these resistors are intentionally being used as fuses? Is it indicated on the schematics or has it been noted somewhere? Again, I am not trying to be smart I genuinely want to know so please take no offense. 

Thanks for the discussion.


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## lcaillo (May 2, 2006)

Some are listed as fusible, some are simply listed as critical value parts, some are identified as such in training. I recall in Sony training the point being made very specifically about the small resistors in the flyback secondary that are supplying the secondary rectifiers. They are not listed as fusible resistors, just metal film, but we were warned against using larger wattage values to prevent damage to the flyback secondary windings. Are they fusible resistors, fuses, or resistors? You could argue any of these as appropriate. From the view of a servicer it really does not make much difference what you call them. What are fuses anyway, but a resistor that is designed to open at a particular current and time relation?

What the designers are thinking is hard to know. Some of the things we see simply do not make sense, but we have to work with what comes our way.


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## randtek (Jun 3, 2010)

When you have a low value resistor in series with a primary or secondary winding in a power supply or switching regulator circuit, it's probably being used as a fuse. Same with in the secondaries of a flyback circuit, as Leonard stated above. In the schematic section below, 3 fusible resistors are circled in red. They are 1 ohm, 1 watt metal film resistors. This schematic is the flyback section of a recent model RCA TV.


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## goyop (May 4, 2010)

Thank you for the explanation. I can appreciate the service aspect since I serviced televisions for two years and lab equipment for several more. Probably due to my design education and experience for several years I have been looking at it from a design perspective. If that is what Sony says then obviously one would be careful to heed their advice. From a pure design perspective it would never be taught to rely on any type of resistor to protect components. That is just bad design. It is saying that we think we know what we are doing but just in case let's throw in a resistor to blow if there is too much current.

What is taught and is practiced in design is to understand what the circuit needs to accomplish first. Then the circuit is designed to accomplish that task. Then everything is sized according to voltage, current, and then adjusted for heat and other extremes. If a component needs protected against a failure somewhere like a dead short or heat issues or whatever, then protection is built in by way of fuses, thermal devices, fusible components, etc. So while in theory fuses are resistors designed to open at some current / time situation I am pretty sure actual resistors, thin film or otherwise, are not relied upon as overcurrent devices. There are numerous better ways to provide this protection which will not require physical replacement of a part. So for all of those reasons I do not believe they are designing circuits and using resistors as fuses.

However, if in practice after they have designed their circuits and put them into production they find that certain problems are surfacing and a certain resistor is opening when there is overcurrent then they would pass that along to everyone. This is probably why Sony made the point about that situation and why it is confusing. Because from a design standpoint you could put in a 500 watt resistor and it does not affect the circuit function at all. The wattage only accommodates the expected current values in that particular circuit. It is not selected as an overcurrent device. That is why in general it makes no difference if a resistor is replaced with a higher wattage device. 

I think your point is one of safety - don't change things unless you are 100% certain that all is understood. This I have always agreed with. My point is that resistors are not designed into systems as fuses. Reality is that bad designs may prove that a certain resistor is unintentionally acting as circuit protection. Those two points are not mutually exclusive. And we do want to be as accurate as possible so that members can benefit from our combined experience.


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## randtek (Jun 3, 2010)

A fusible resistor is just what the name says it is. It is often not a resistor designed specifically as a fusible device, rather it is the designer using the known operating parameters and specifications of a given type of resistor to use that resistor as a fuse. A metal film resistor WILL open up if the wattage rating of it is exceeded, which allows it to be used as a protection device. A piece of wire can be used as a fuse as long as you know the specs of the wire. I am not sure why they choose to use a resistor instead of a fuse, but I'm sure there are reasons, as it is being done in most modern power supply circuitry. This is really not a new idea, I have seen it used since the early days of solid state TV. I don't think it is bad design at all. And it is not done as an after thought. They are designed into the circuit as fuses. They are not being used for their resistive component at all, other than how the resistance will affect the power equation (E*I). When the current in the resistor reaches the point where the voltage drop across the resistor times the current flowing through it exceeds the wattage rating of the resistor, it opens. Since this is a known characteristic of this type of resistor, there is no reason why they cannot safely be used as a fusing device. Resistors as components are much cheaper than fuses. This is probably a big factor in the decision to use a resistor as a fuse. this is used to prevent complete destruction of the power supply if something shorts on one of the supply lines. They have no purpose in the circuit when things are working properly. But components such as solid state devices and capacitors can short. That is when these components become active in the circuit, to prevent further circuit damage when something fails.


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## goyop (May 4, 2010)

Thanks for the schematic. My point from early on was that unless it is a fusible resistor like shown on the schematic, then a resistor is not designed into a circuit as a fuse.


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## goyop (May 4, 2010)

You are right about the cost and also a fuse is another component and requires more real estate on the board.


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## goyop (May 4, 2010)

In looking at that schematic, if in fact those are 1 ohm resistors then they are simply using the fusible resistor as a fuse. As you know the hardware for even the smallest fuses is much more complicated than slipping a resistor into a board. They also wouldn't use a fuse if they thought that the likelyhood of it blowing was minimal. It is more a "just in case" situation. If it blew regularly then no one would buy the TV.


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## lcaillo (May 2, 2006)

goyop said:


> Thanks for the schematic. My point from early on was that unless it is a fusible resistor like shown on the schematic, then a resistor is not designed into a circuit as a fuse.


I don't think that we have any problem with your point, in principle. The distinction of whether a resistor has any value as a fuse, however, simply has little value at the level of repair. You, nor I, have a clear understanding of what the design of a circuit may have involved nor the decisions that went into the product at production. The reality is that all fuses and resistors are really resistors. Fuses are designed to fail and resistors have to be designed to operate under the normal conditions in the set. When those conditions change, it is possible that resistors will open. If that occurs and there is the possibility that a properly designed resistor can safely open and protect other devices, the values are often chosen to be appropriate for that task. Sometimes these are called fusible resistors, other times they are not. In repair practice, we all agree that there is little reason to change values if you don't have a good reason to do so. 

In these repairs specifically, let me give an example of why changing power ratings may be a bad idea. I serviced a Samsung set a few weeks ago. It had a bad output IC and a pair of resistors on the output that were obviously cooked but still measure correctly. I changed the chip, a fuse in the PS, and the resistors. I checked the offset and on that channel it was about 30mv d.c., acceptable. I aligned the convergence and tested the set and it seemed fine. It failed with the same two resistors open in about two weeks. The resistors opened and blew the fuse in the supply. The chip, however, was still good. Had I increased the power rating on the resistors, they would have continued to allow the set to run and the IC would likely have eventually failed. The larger resistors would have masked a problem. They may have allowed the set to continue to operate and it might have never failed also. The reason for the problem appeared to be several of the point convergence registers were adjusted to the point that they were well beyond their range and the convergence was corrected using other adjacent points. I have seen this issue before on Samsungs, often combined with significantly higher d.c. offsets. It is possible that we have an intermittent short in a yoke or an intermittent output IC, so I swapped the two ICs and am testing the set for a longer period of time. I am not completely sure, but I feel better knowing that I did not cover up a potential problem by increasing the rating on the resistors.

The point is that while these resistors are not specifically designed as fuses, they can still play that part in exceptional circumstances that occur with failures of other components. That role may or may not be by design, but this is irrelevant. The complexity of design and production decisions simply makes the distinction between resistors, fuse resistors, and pure fuses less meaningful. We fix this stuff, we don't design it.

We all agree that it is bad practice, in general to change the values without good reason. Beyond that the discussion is pedantic. Pedantry can have value itself, but I think we are beyond that here.


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## goyop (May 4, 2010)

Your technical experience gives good insight into the real world issues. 

My entire point is that the original notion put forward about resistors being used as fuses is absolutely incorrect. They may end up acting as a fuse and may protect something down the line but unless it is specifically a fused device it was not designed in as protection. No designer counts on a resistor as a fail safe. If the designer wants this extra protection they will put in a fusible device. This is fact and how design is done.

One can easily understand most aspects of the design decisions that went into a circuit because components have specific functions singularly and also in combination. Most designers can look at an unfamiliar design and grasp it fairly quickly. In relation to this discussion, if a resistor is 1 watt then that is the power rating for the circuit and the wattage was chosen to handle the voltage and current present while doing its job of offering a specific resistance. It is based on standard design practice. Unless it is a fusible resistor there is no problem in replacing it with a larger resistor because its sole job is to offer resistance period. You mention a properly designed resistor and values being chosen to open in certain situations and this is not true. This is not how circuits are designed. The difference between devices and their design purpose is relevant - especially in repair.

I believe those using this forum are here to learn. So I am attempting to clarify a mistaken view of circuit design. It is not clear to me how that is pedantic.


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