Heatsinking through hole resistors? - Page 1
Forum section: electronics / beginners

author: kalel
author_rank: Frequent Contributor
timestamp: September 02, 2017, 02:37:16 am
content: With SMD resistors, I assume this is different, but how would you heatsink through hole resistors?I'm not sure how commonly this is done, and also don't have a specific project in mind, but it might make sense in some projects. If it doesn't make any sense, that would be good to know too.I got the question from thinking about those battery tester units supplied with resistors for load. But those are different shape and material (ceramic) than e.g. metal film transistors and I'm curious about those too. Surely they don't need to be heatsinked if used properly, but I'm not sure if that might help to keep things slightly more stable. Of course, a simple fan might be helpful as well.

author: tautech
author_rank: Super Contributor
timestamp: September 02, 2017, 04:51:07 am
content: I've seen a few solutions, mostly in old gear where power consumption was not such an issue as it is today.Resistors with stout enough leads are stood off the PCB with no other support provided.Stacking ceramic beads on the leads to provide a rigid standoff.Metallic brackets, mostly used with 5W+ ceramic resistors to stand theminon end.Use a larger W ratings than necessary.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 02, 2017, 05:53:00 am
content: Only metal case resistors need heatsinking. Hmm, there's also SOT-227 resistors available, and such. Anyway, anything that's not meant to be bolted, bonded or pressed to a heatsink, mmmkay?As mentioned above, the usual approach is toinsulate, so the PCB doesn't get stupidly hot. The resistor is made of materials that can operate over 200C, while the PCB turns brown above 150C (and solder melts above 200C).Providing a large copper dissipation area, for the leads, will help reduce hot spots on the PCB, and is often a necessary step to use high power SMT resistors.Tim

author: SeanB
author_rank: Super Contributor
timestamp: September 02, 2017, 04:53:34 pm
content: Regular metal film and carbon film resistors are meant to operate at a relatively high surface temperature, so the best way to run them at full rating is to mount them at least the body diameter off the board on the leads. There were lead formers that did this with a single tool, but a good compromise is some small needle nose pliers to put a kink in the leads at the right point so they are spaced there for soldering. As Tim said, a large copper pour on the solder leads will keep those cool, and help prevent solder joint failure, but also have a large solder mask relief as well, or leave the whole pad free and solder fill the pad entirely.Power resistors ( typically a white ceramic boat with the element inside and filled with a cement, or a green cement coat on the resistor wound on a ceramic or glass fibre former with pressed on end caps) are best mounted vertically, with use of a special tinned steel mounting that provides one lead connection, along with holding the resistor from moving with vibration, that spaces it typically 1cm from the board. Otherwise mount them 3cm from the board, using either small ceramic beads or a ceramic or metal tube, though there are solder in rivets that combine the tube with support as well. Power resistors typically are rated for full dissipation at a surface temperature of something between 175C to 200C, and really should be derated to reduce the high surface temperature so you do not scorch the board, or run them mounted to tag strips so that the heat is off the board, and use wires to connect them.Worst thing you can do is have power resistors mounted right next to electrolytic capacitors, where you cook the capacitors to failure in short order. Even worse id to have them touching, or use silastic or glue to hold them to the board or other components, which will cook the board brown and make tracks peel off when you try to rework there

author: TimFox
author_rank: Super Contributor
timestamp: September 02, 2017, 07:03:25 pm
content: Heathkit used to specify mounting power resistors (typically rectangular "cement" cases) by cutting a bit of cardboard from the shipping box, spacing the resistor from the board during soldering, and then removing the cardboard.

author: cvanc
author_rank: Frequent Contributor
timestamp: September 02, 2017, 07:07:40 pm
content: Quote from: TimFox on September 02, 2017, 07:03:25 pmYou know what this means? It means you're as old as I am because I remember this too

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 02, 2017, 11:38:29 pm
content: Also if you're feeling fancy, remember you can still buy those ceramic spacers to slide over the leads, e.g.http://www.essentracomponents.com.sg/images-line/pdfs/CER-X.pdfGives more support, and sets the component height on assembly without needing kinked leads. (Which you can also get, if you find the right part numbers -- most manufacturers provide lead form options, but distributors mostly stock the straight axial format, so if you want just a few, instead of a box of reels, you have to snoop around carefully for the right part numbers.)Tim

author: 0xPIT
author_rank: Regular Contributor
timestamp: September 03, 2017, 02:11:16 pm
content: You typically don't need to heatsink resistors, if they operate within their design specs.A common way to manage heat at resistors is to let the resistor stand off the PCB as long as possible.Sometimes ceramic standoffs are used.The wires act as a heatsink, this usually seems to be sufficient, at least to prevent self-desoldering.

author: alm
author_rank: Super Contributor
timestamp: September 03, 2017, 02:58:49 pm
content: The wires do not so much act as a heat sink, but rather as a thermal resistance to drop the temperature at the solder joint so the PCB substrate does not get overheated. I doubt they contribute much to the resistor case-ambient thermal resistance. If anything, shorter leads would allow more heat transfer through the PCB, decreasing the case-ambient resistance. Obviously this would heat up the PCB, which is usually the limiting factor. The resistor may easily handle 200°C continuously, unlike most PCBs. By putting thermal resistance (thin wires) between the resistor and the PCB, you protect the most sensitive part (the PCB).

author: retrolefty
author_rank: Super Contributor
timestamp: September 03, 2017, 05:10:11 pm
content: QuoteThis is the 'official' policy one should take.However I know that many a thrifty Ham Radio types have frequently taken an inexpensive route in the construction of 'full-gallon' (1K watts) RF dummy loads out of a bunch of 2-5 watt that is series/parallel wired for 50 ohm load and hanging the resistor array into a gallon paint can and immersing them into transformer oil == 1,000 watt 50 ohm resistor. Works fine for the HF bands but not very good at UHF and above.

author: SeanB
author_rank: Super Contributor
timestamp: September 03, 2017, 06:04:08 pm
content: Oil cooled can dummy loads are also typically only used for a very short time, so the oil and the resistors do not get close to the boiling point of the oil. This allows a short high power input to be handled without making a lot of smoke then fire, as the thermal capacity of the oil absorbs the power. They typically have a duty cycle of under 3 minutes per hour, as the oil takes a long time to cool.

author: colorado.rob
author_rank: Frequent Contributor
timestamp: September 06, 2017, 04:13:57 am
content: https://www.vishay.com/resistors-fixed/heatsink/

author: Beamin
author_rank: Super Contributor
timestamp: September 06, 2017, 04:29:46 am
content: I have wondered this myself. I would take a little roll of copper wrap it around it and leave a tab hanging off one end. Fill the middle with heat sink goop. Simple and effective. Solder one side of the copper to one of the leads.

author: stj
author_rank: Super Contributor
timestamp: September 07, 2017, 01:35:32 pm
content: these days you can get metal-film resistors in a TO220 or TO218 package!

author: Beamin
author_rank: Super Contributor
timestamp: September 10, 2017, 01:54:13 am
content: Quote from: stj on September 07, 2017, 01:35:32 pmThat would be a strange product to tear down. All transistors by the look of it. I might order some of those I have a thing about keeping resistors cool.

author: Simon
author_rank: Unknown
timestamp: September 10, 2017, 07:25:17 pm
content: erm, resistors are in themselves rated to a certain power, if you need more power you specify a resistor for that power, once you get into serious power (over 5W) you have to heat-sink the resistor anyway and it will be designed to accept a heat-sink. You can get resistors rated to hundreds of watts that can't do that power unless put on a heat-sink.

author: Cliff Matthews
author_rank: Unknown
timestamp: September 10, 2017, 11:30:13 pm
content: I thought these 225-watt ceramic resistors didn't need a heat sink.. oh well, MTDOC has 20 of them nowhttps://www.eevblog.com/forum/buysellwanted/fs-ohmite-new-old-stock-225w-2ohm-resistors/msg1082793/#msg1082793

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 11, 2017, 03:14:34 am
content: You can cheat a little bit by making little copper or aluminum tabs a folding them over the leads of the resistor and crimping them in place (or solder if copper is used). I have seen a similar tab shaped around the body of a resistor to buy a little more power dissipation capacity.This might be a chosen approach for a hobby project where you were unwilling to wait for an appropriate part to be delivered. Although I have actually seen it in commercial products, the only way I would personally approve of this would be if the resistor were rated for the dissipated power and the heat sink was intended to lower operating temperature for improved reliability. It is hard to imagine where this would be more cost effective or space effective than just specifying a higher power rated resistor.

author: thermistor-guy
author_rank: Frequent Contributor
timestamp: September 11, 2017, 05:04:33 am
content: Quote from: tautech on September 02, 2017, 04:51:07 amOld (failed) steam irons are a good source of ceramic spacers. Steam irons also have useful items like terminal blocks and high-temperature wiring.Next time your steam iron fails, scavenge it for parts.

author: Simon
author_rank: Unknown
timestamp: September 11, 2017, 06:37:39 am
content: Quote from: Cliff Matthews on September 10, 2017, 11:30:13 pmThere will be some I grant you that do high power all by themselves, but I bet they are bigger than their little aluminium counterparts that are deceptively small because they are meant to be put on a heatsink or can only do less power.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 11, 2017, 08:47:27 am
content: Yes, ceramic resistors are usually somewhat larger than aluminum body resistors.Only somewhat so, because they are able to operate at MUCH higher temperatures. Air is a proportionally more effective heatsink (power per volume) at high temperatures. In fact even more than proportional, because of natural convection.Aluminum body resistors tend to be slightly cheaper, or at least competitive. But one must keep in mind it is a false economy: the heatsinking is NOT OPTIONAL. You must include the cost of extra heatsinking volume, machining steps, hardware and assembly. They're a big waste, overall!Tim

author: tszaboo
author_rank: Super Contributor
timestamp: September 11, 2017, 09:05:57 am
content: For those 5-10W cemented wirewound resistors, you can cool them with just some airflow.There are chassis mount resistors, these go up for some 50-100WThis, you drill holes to the heatsink, and bolt it on it.And then there are the big beasts, which go up to 1KWAgain, no heatsinking.There are TO220 and other transistor case style resistors. IMHO they are not very good price/performance wise. For a single one, you can just use a dozen cemented resistors. They are fine, if you are using them as a precision current sense.

author: Simon
author_rank: Unknown
timestamp: September 11, 2017, 11:40:43 am
content: The aluminium ones are probably used to get heat out of the case by screwing them to the inside of the case wall so that heat can go through the wall and out possibly with an additional heat-sink, a ceramic resistor would heat up the inside of the enclosure unless it is forced air cooled or the resistor is put outside.

author: Beamin
author_rank: Super Contributor
timestamp: September 18, 2017, 11:52:05 pm
content: Heres a variable high wattage resistor with built in heat sink. Caution it gets really hot.

author: kalel
author_rank: Frequent Contributor
timestamp: September 19, 2017, 12:53:17 am
content: Quote from: Beamin on September 18, 2017, 11:52:05 pmNeeds good ventilation.

author: tszaboo
author_rank: Super Contributor
timestamp: September 19, 2017, 08:04:19 am
content: Quote from: Beamin on September 18, 2017, 11:52:05 pmIt is also a PTC, with very interesting characteristics.It gets open circuit when it reaches a certain temperature.

author: Simon
author_rank: Unknown
timestamp: September 19, 2017, 08:14:59 am
content: They are often used as heaters with their intentional self limiting characteristics for safety

author: bd139
author_rank: Super Contributor
timestamp: September 19, 2017, 09:09:01 am
content: Incidentally on this as I cocked up recently when I built a dummy load, watch out for TH resistors - they can actually desolder themselves if you overload them a little bit. Make sure there is a rigid mechanical mount as well as the solder one. I opened my dummy load to find that the entire thing had disintegrated

author: Karlo_Moharic
author_rank: Regular Contributor
timestamp: September 19, 2017, 11:07:43 am
content: There are this things called POWER RESISTOR , they look something like this:https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR0.TRC0.H0.Xpower+resistor.TRS0&_nkw=power+resistor&_sacat=0

author: KL27x
author_rank: Super Contributor
timestamp: September 19, 2017, 10:12:04 pm
content: Some power resistors have a heatsink on them. But in general, you don't often find a scenario where you want to put a heatsink on a resistor.As opposed to a semiconductor, a resistor1. Has an increasing resistance with temperature, so it is somewhat self-limiting. There is no point of thermal runaway.2. Can operate at a much higher temperature. The higher the temp differential to ambient, the greater the heat transfer to surrounding air. This means you don't get as much benefit out of a heatsink. If semiconductors could operate at 150C with no decrease in performance, you would rarely need to use heatsink for IC, either. (A resistor changes resistance, which is a change in spec but not a decrease in performance, per se... I.e., you could use a different resistor to get the resistance you need at 150C)Bottom line, it is usually more efficient (space and $$) to just use more and/or larger resistors than to add heatsinking to a resistor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 19, 2017, 11:30:13 pm
content: You must have shitty resistors if they're going out of tolerance at 150C.(At a glance, NiCr is up by 5% at around 800C?)Speaking of semiconductors, it's too bad that we have SiC and GaN power transistors nowadays, yet we still don't have anything that can do better than 175C in commercial product lines. Semis are limited by packaging more than by material.(200-300C parts are available, but they're ceramic and gold military packages, $100 and up sort of thing.)Tim

author: GigaJoe
author_rank: Frequent Contributor
timestamp: September 20, 2017, 02:56:16 am
content: Had I rush to make some load:It dissipated almost 7W; Realize if wire will be longer, more surface, can pump more

author: KL27x
author_rank: Super Contributor
timestamp: September 20, 2017, 08:06:45 am
content: ^ When I need higher wattage and don't want a burn/fire hazard, I use a piece of copper clad and cut some lines in it and solder w/e value of resistors I have that will give me the right resistance and power rating in parallel... or mix of series/parallel as needed.Nichrome wire and a pair of alligator clips will up the fun factor for quick testing.

author: Zero999
author_rank: Super Contributor
timestamp: September 20, 2017, 10:24:31 pm
content: Quote from: NANDBlog on September 19, 2017, 08:04:19 amYes, it's not really a variable resistor either. Adjusting the dial doesn't vary the resistance. All it does it change the temperature, when the thermostat disconnects the power.