Using a small resistor in some water instead of a power resistor? - Page 1
Forum section: electronics / beginners

author: help_me_pick_username
author_rank: Contributor
timestamp: November 02, 2019, 07:52:08 am
content: I don't have any large power resistors, I was wondering whether I could, instead of a power resistor, use a 1/4w resistor dunked in some water to dissipate some power? If this was even remotely possible, how much power would I be able to dissipate?

author: Siwastaja
author_rank: Super Contributor
timestamp: November 02, 2019, 08:51:53 am
content: You need to try it out, and it will heavily depend on the construction of the particular resistor you have, but I'd expect anything between just 1W and maybe even 5-10W.If the voltage is very low (say < 1V) and the operation is short, electrolysis and corrosion isn't a problem. For higher voltages or permanent solutions, you need to properly coat it, and any coating will cause significant penalty in heat transfer; all epoxies and plastic-based materials have poor thermal conductivity. If the internal temperature rises too much, the coating material may be damaged slowly, finally letting water into contact with the leads.For a properly epoxy coated solution, the end result may not be too stunning. It may be good for only, say, 0.5W, in the end, due to the excess thermal resistance added by the coating.

author: AVGresponding
author_rank: Super Contributor
timestamp: November 02, 2019, 10:04:39 am
content: You'd be better off using either auto coolant or mineral oil, but unless you have a flow it'll not help much, and ultimately the surface area of the resistor will still limit the maximum dissipation quite badly.Even with good flow, to a radiator, you probably aren't going to get better than 5x rated power.Unless you're happy to see the coolant boil ofc...Edit: This sounds like a fun experiment!

author: RoGeorge
author_rank: Super Contributor
timestamp: November 02, 2019, 10:45:01 am
content: OK, but careful not to kick the bucket!

author: GerryR
author_rank: Frequent Contributor
timestamp: November 02, 2019, 11:06:31 am
content: This is a 50 ohm dummy load used for RF transmitters:

author: pk7639
author_rank: Newbie
timestamp: November 02, 2019, 11:13:59 am
content: I can vouch for a 5W ceramic resistor being good for 100W if you drop it in your coffee the moment you realise you got the power calcs wrong!

author: SL4P
author_rank: Super Contributor
timestamp: November 02, 2019, 06:21:24 pm
content: I’d be thinking that as the resistor heated up, you’d start getting air bubbles (insulation) forming on the surface.The best/only reliable solution would have the ‘coolant’ flowing across the surface of the element.

author: MLXXXp
author_rank: Frequent Contributor
timestamp: November 02, 2019, 09:31:15 pm
content: Do you have more than one 1/4 watt resistor of the value you need? If you have 4 of them, you can series/parallel them to make a 1 watt equivalent. You can do the same with more, and even use multiples or fractions of the value, to get as much power handling as you need.

author: help_me_pick_username
author_rank: Contributor
timestamp: November 02, 2019, 10:08:46 pm
content: Quote from: MLXXXp on November 02, 2019, 09:31:15 pmI should have more than one... however, isn't there this thing when you put multiple resistors in parallel, the current will just go through the one that has the least resistance...? I only have 5% precision, so this could be an issue, right?I can always do series though... Thanks for the suggestion!

author: Kleinstein
author_rank: Super Contributor
timestamp: November 02, 2019, 10:13:53 pm
content: Precision is not that critical for power sharing. Even of some resistors are 10% off they get some 10% more or less than the average. Not much difference in parallel and series connection.One may have to include the thermal coupling: so 4 resistors with 1/4 W placed close together can not dissipate 1 W. Especially with SMD resistors it needs quite some board space to get the nominal power rating.

author: MLXXXp
author_rank: Frequent Contributor
timestamp: November 02, 2019, 10:14:20 pm
content: Quote from: help_me_pick_username on November 02, 2019, 10:08:46 pmNo. If one is slightly lower, it will conduct slightly more current and use slightly more power but only proportional to the percentage of the difference. You might be thinking of diodes.

author: MLXXXp
author_rank: Frequent Contributor
timestamp: November 02, 2019, 10:27:50 pm
content: https://phys.libretexts.org/Bookshelves/University_Physics/Book%3A_University_Physics_(OpenStax)/Map%3A_University_Physics_II_-_Thermodynamics%2C_Electricity%2C_and_Magnetism_(OpenStax)/10%3A_Direct-Current_Circuits/10.2%3A_Resistors_in_Series_and_Parallel

author: help_me_pick_username
author_rank: Contributor
timestamp: November 02, 2019, 10:50:54 pm
content: Quote from: MLXXXp on November 02, 2019, 10:14:20 pmI think Iwasthinking of diodes. Thanks for the clarification!

author: schmitt trigger
author_rank: Super Contributor
timestamp: November 02, 2019, 10:57:19 pm
content: This can easily become Photonicinduction - worthy experiment.

author: help_me_pick_username
author_rank: Contributor
timestamp: November 03, 2019, 01:28:25 am
content: Quote from: schmitt trigger on November 02, 2019, 10:57:19 pmWhat's photoinduction?

author: Jwillis
author_rank: Super Contributor
timestamp: November 03, 2019, 01:41:57 am
content: Immersing resistors to increase the wattage ? I would be concerned about possible steam developing in any cracks or pours causing a small explosion destroying the resistor. I would be more inclined to use thermal adhesive and glue some heat sinks to the resistor if I didn't have enough to parallel as MLXXXp and Kleinstein suggested.Its Photonicinductionhttps://www.youtube.com/channel/UCl9OJE9OpXui-gRsnWjSrlA

author: ejeffrey
author_rank: Super Contributor
timestamp: November 03, 2019, 02:57:07 am
content: I once used a 1/4 W axial resistor as a 10 watt heater... Immersed in liquid nitrogen. Of course as soon as the nitrogen boiled off it turned into a fuse.

author: AVGresponding
author_rank: Super Contributor
timestamp: November 03, 2019, 09:29:15 am
content: Quote from: ejeffrey on November 03, 2019, 02:57:07 amGood thing nitrogen is relatively inert...Liquid cooled resistors are a thing. Here are a couple of links, for the curious:https://en.wikipedia.org/wiki/Liquid_rheostathttps://www.danotherm.com/power-resistors/cooling-method/liquid-cooled

author: SeanB
author_rank: Super Contributor
timestamp: November 03, 2019, 10:50:23 am
content: I used to use a cup of water and a 2N3055 as a dummy load, using a resistance box to vary the base current to get the desired current flow. Was small, and the water would generally last longer than the 5 minutes I needed to do the testing, plus you could always add a little more if needed when running low. 5V rail, so not too much worry about corrosion, and in any case you could use old transistors that are not good for other uses, or where you replaced one of a matched pair.

author: Zero999
author_rank: Super Contributor
timestamp: November 03, 2019, 10:59:24 am
content: Quote from: help_me_pick_username on November 03, 2019, 01:28:25 amHe was a crazy Youtuber who used to blow stuff up with high voltages.Quote from: help_me_pick_username on November 02, 2019, 10:08:46 pmYes, connecting resistors in parallel causesmost ofthe current to go through the one with the least resistance.Note that he said series in parallel. Suppose you have four, 1 Ohm resistors. Connect a pair of them of them in parallel, to make1/2Ohm, now connect another pair in parallel to get another1/2Ohm, put them in series and you now have a 1Ohm resistor with four times the power rating. You could also put two series pairs of 1R resistors in parallel: the effect would be the same: 1Ohm at four times the power.Don't worry about the tolerances. Some of the resistors will be above an Ohm and others below, so it won't make and difference. In fact, the resulting resistor will have a greater precision, than each resistor, as the errors average out to some degree.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 03, 2019, 12:44:41 pm
content: Quote from: Zero999 on November 03, 2019, 10:59:24 amNo, never say this. I know it gets stuck: it's a very common lie that is taught to us early on (speaking of Youtubers, I think Electroboom had a section about it on some of his videos). It has no merit in reality, not even as a simplification, because it is clearly just plainly wrong, and won't provide useful abstraction for any practical use.I know you know what you are talking about, but phrases like this are extremely confusing, and the beginners must unlearn such widely touted lies.Your words are clear: "thecurrent" "throughtheone". This is just plainly wrong. Correctly would be, more current goes through the smaller resistance.Of course, our complete electrical system is a bunch of things connected in parallel - classically even simple resistances, such as heaters. We all know that turning on a heater does not much affect the others.You are right that tolerances are indeed not that meaningful, but this goes for both parallel and serial connections, so use whichever is easier for the particular set of resistor you have, and based on the ease of making the actual connections physically.Unless the tolerances arereallycrappy, or the system is improperly dimensioned to be really on the edge without any margin whatsoever...

author: Zero999
author_rank: Super Contributor
timestamp: November 03, 2019, 06:47:34 pm
content: Quote from: Siwastaja on November 03, 2019, 12:44:41 pmI appreciate the correction, but resent your implication of lying. It was a simple mistake. I omitted the words "most of". I've edited my post. Don't be so quick to accuse others of dishonesty.

author: m3vuv
author_rank: Super Contributor
timestamp: November 04, 2019, 03:03:43 am
content: why water and not oil??

author: Siwastaja
author_rank: Super Contributor
timestamp: November 04, 2019, 05:06:29 pm
content: Quote from: Zero999 on November 03, 2019, 06:47:34 pmOh, I mean all the textbooks saying this are lying; this is where it gets stuck to our language from. Doesn't require active "lying" on your part, this wasn't intended. It just comes "automatically", I have said that too.The authors of such textbooks may even know their stuff, but think that conceptually, a "convenient" lie is always a good idea, as a simplification. Sometimes it is, but it goes too far in this case. Hence, lying.

author: MLXXXp
author_rank: Frequent Contributor
timestamp: November 04, 2019, 05:09:21 pm
content: Quote from: Zero999 on November 03, 2019, 06:47:34 pmAnd IMHO it's still wrong. Although it depends on what one defines the meaning of the wordmostto be in this case, personally, for two resistors of almost identical value in parallel, I wouldn't consider the small amount of additional current that the lower value one draws, compared to the higher one, to bemost ofthe current.A clear statement for all cases would be to usemore of, notmost of:Yes, connecting resistors in parallel causesmore ofthe current to go through the one with the least resistance.

author: alpher
author_rank: Frequent Contributor
timestamp: November 04, 2019, 05:14:40 pm
content: And I just did a quick and dirty test of your theories.Put a 101/4W resistor in a glass of tap water, run 1A through it (10V) for good 5 minutes, some bubbles and not much else.Same resistor burned in air in less than 10 second.So my conclusion, if you need some quick and dirty 10W capable resistor for a few minutes, go ahead and dump your run of the mill 1/4 watter in a glass of water.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 04, 2019, 05:17:34 pm
content: Quote from: alpher on November 04, 2019, 05:14:40 pmYes. Do note that the speed of the corrosion starts to accelerate. If the tap water is relatively low mineral, the rate of reaction is fairly low first, but the more it reacts, the more metals are dissolved into the water, making it more conductive, again making the reaction happen more quickly. An accelerating spiral of death.

author: Zero999
author_rank: Super Contributor
timestamp: November 04, 2019, 07:20:58 pm
content: Quote from: MLXXXp on November 04, 2019, 05:09:21 pmThis is why one should learn the basic maths behind it, to avoid getting into semantics.

author: AVGresponding
author_rank: Super Contributor
timestamp: November 05, 2019, 03:35:16 pm
content: Quote from: Siwastaja on November 04, 2019, 05:17:34 pm

author: help_me_pick_username
author_rank: Contributor
timestamp: November 06, 2019, 04:41:44 am
content: Quote from: m3vuv on November 04, 2019, 03:03:43 amI don't have mineral oil. I guess I could always use vegetable oil for a quick and dirty test instead...