Issue with characteristics of silicone heating pads - Page 1
Forum section: electronics / projects

author: DrG
author_rank: Super Contributor
timestamp: September 21, 2020, 08:09:48 pm
content: I can’t figure out something that is happening and wanted to know if someone else has a definitive answer.I had a project that used a silicone covered heating element. Along the way, I tried a couple of different “pads”. I am assuming, perhaps erroneously, that these pads have similar kinds of wire grids under them. I know they have different rated wattages (which are not terribly accurate).This one,https://www.banggood.com/12V-DC-80x100mm-20W-Flexible-Waterproof-Silicon-Heated-Bed-Heating-Pad-For-3D-Printer-p-1265231.html?rmmds=myorder&cur_warehouse=CNworked as I expected.I feed it 12V DC and it achieve a relatively stable current draw after a short time. It showed a range of ~1.29A-1.39A, or about 100 mA, settled at the lower current and, basically stayed there during hours of testing. Note that the high initial current is after 30 seconds or so after power is applied.This onehttps://www.banggood.com/100+100mm-12V-50W-Silicone-Heated-Bed-Heating-Pad-w-or-Thermistor-For-3D-Printer-p-1266879.html?rmmds=myorder&cur_warehouse=CNbehaved differently.It has adhesive on the back (which could not be removed by any solvents I had on hand) and also has an embedded thermistor (ostensibly a NTC 100K 3950).When I feed it 12V DC, it has a stink which I am assuming is from the adhesive, but I don’t know.The measured current declines over the next ~15 min until it reaches a resting point, as did the other, but the drop is much greater with a range of ~700 mA.Looking at the resistance of the thermistor (starting at unpowered ambient of 112.5K), I get: (kohms / amps)46.0, 3.3710.8, 3.006.0, 2.902.8, 2.801.9, 2.741.5, 2.67I know they have different rated wattages, but it is a large difference - why is this happening? Is it simply that the resistance of the wire is falling as it heats up and I am stubbornly resisting that they used a different kind of wire that has different properties, or ?

author: CaptDon
author_rank: Super Contributor
timestamp: September 25, 2020, 03:12:44 pm
content: That would be the fabulous stench of Chinese quality.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 25, 2020, 04:21:47 pm
content: Quote from: DrG on September 21, 2020, 08:09:48 pmThe resistance of the heating element is increasing as it heats up.There are at least two reasons you might see such a difference. The first is that the element temperature might be much higher in the second. For instance, a thinner, shorter element would have a lot less surface area to transfer heat to the silicone so it would get hotter for the same wattage. The second is that the alloy is different. One might be nichrome and the other brass. The latter will have much higher tempco.It should be fairly easy to see the difference if you cut into them and get a look at the actual heating element.The second one also has an NTC so presumably is intended to be used with feedback to stabilize the temperature. Thus, reduced max output power at maximum temperature is not a big deal as you would only operate at full power while trying to reach the setpoint, then reducing power to hold. So in practice this behavior shouldn't have any effect in its intended application.When paying $3 in unit quantity don't expect the one you buy today to work the same as the one you buy next month.

author: DrG
author_rank: Super Contributor
timestamp: September 25, 2020, 07:00:31 pm
content: Quote from: ejeffrey on September 25, 2020, 04:21:47 pmI can state up front that my knowledge of anything that sounds like material sciences is painfully deficient and maybe I should have posted this in the begginers forum.I do, however, appreciate the response. I understand what you said and I think you are right - Basically, the "wire" increases resistance as it is heated and that accounts for what I am seeing. Having done some reading, I get that a little better now. I also get that it is going to depend on the wire material and dimensions.So, how can I make some assumptions from the adverts? The wattage ratings do not make sense to me - are they supposed to be maximums, which would be before heating up? Is the cold resistance a valuable measure...there is a lot of wtf running around in my brainIf I use a big-ass high wattage wire wound resistor as the heating element, will I get the same effect?? If not, why not? (it is a fixed resistor and if I stay within the wattage rating, I would expect the ohms to remain - no?). Does "wire wound" some how exclude the "wire" from changing resistance when heated?edited to add: I know that that third terminal/band is to adjust the resistance - but with that adjusted or removing it altogether, it is a "fixed" resistance as I am using the term.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 25, 2020, 07:29:07 pm
content: Wirewound resistors will still change resistance with temperature. A good wire wound resistor will have a datasheet that specifies resistance tolerance (at room temperature), temperature coefficient of resistance, and thermal impedance to ambient. From these you can calculate the operating temperature and worst case change in resistance at a given load.Cheap wirewound resistors sold without a datasheet will be in the same boat as this: you get what you get. Probably it won't vary as much as the second pad you showed -- generallyAn important question: why do you care?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 25, 2020, 07:29:57 pm
content: Well, what for?If you need constant power, you may prefer a better-constructed one, or perhaps just use a handful of resistors on a PCB, or a power resistor like that (perhaps one with a metal plate to spread heat over the desired area). This still assumes the supply voltage is reasonably constant, which might not be the case, in which case a controller is desirable.If you just need some temp rise, you probably want a temperature controller anyway.The controller can be as simple as a thermistor, comparator and switch (several of which can be jammed together, e.g. using a small and a power transistor together as a Schmitt trigger: doing comparison (threshold with hysteresis), and switching, all at once), or as complicated as a sensor (analog or digital), a proper controller (error amp / PID loop) and a linear or PWM driver. And, let's say, a protected switch too, just so it's extra safe or something.A controller won't much mind the changing resistance of the heater, so it becomes a moot point, and even these parts are usable.Depends what you're doing.Tim

author: DrG
author_rank: Super Contributor
timestamp: September 25, 2020, 08:14:40 pm
content: OK, there is always a back story.I decided to build a particular kind of food dehydrator. One specifically to dry whole peppers. I did design one and it had a control board and a controller outside of the heating compartment.It had a fan, with PWM. An internal temp sensor and an ambient temp sensor on the control board (ds18b20s). The heating elements were also PWM controlled (which I never had to use but it worked fine in testing). Also had an sd card reader/writer to record temps, which were also sent a serial monitor. Arduino UNO. One other thing, I wanted everything working off of a 12V DC power supply (old laptop PS) because that is what I had and that is what I wanted to use.I made two large "errors". First, the size of the heating chamber was very large, meaning that I needed a relatively powerful heating element (on the bottom of the chamber) with the fan pulling air up through the chamber.Second, I made no attempt to make it "convenient" to use...no door, just some chicken wire near the top of the chamber to hang the whole peppers from (and which frequently slipped off of the shelf inserts that were holding it in place). The heating pad sitting on chicken wire on the bottom of the chamber. All kinds of stuff like that. No problem, you learn a lot by doing a lot. I also proved that drilling through stainless steel with whatever bits you have can be problematic).The first pad held the temperature extraordinarily well, with ~ 1 degree variation. I was impressed. But I never had to use PWM, just on and off with 1 degree on the hysteresis. So, when above 95.5, heater off, when below 94.5, heater on. No jitter (I could see by an led when the heater was on). But, 95 degrees (all F) was all I could get. Up at 98, even fluctuations in ambient would causes issues (I would write the temp every minute or so).Then I got the second heating element. Same exact story, except, I could get it to set at up to 105 degrees, but much more than that, the same issue.I am not a professional food dehydrator but there is very little accurate testing on these food sites, just a lot of "I used 120 degrees" or "use 140 degrees", plus, almost nobody uses the whole pepper and drying slices is a lot different. Ideally, you want (as far as I was concerned) a low drying temp with enough air flow to prevent mold. But, I also did not want to "cook them". So, going in, I was shooting for an adjustable 110-140, something like that. BTW: hanging them outside was a no-go because of the humidity - a sure invitation to rot.Testing with some peppers, the 95 worked, but took a LONG time (some 20 days). Not good.So, I went for an even more bad-ass heater:This was rated for 12V at 50W. My bench power supply is a cheap 0-5A constant current. Attaching this to the supply set at 12V, it dropped the voltage to 8.5V and drew 5.16A. This meant that the 3.5A, 12V supply I was going to use would not work.It was at this point that I said FTS and bought a commercial dehydrator for US$150 and it works like a charm. For the peppers that I wanted to use it for, 140 at 4 days does the trick.BUT, when I do a project like this, it is for fun and learning (that is a luxury that I have). I wanted to know what was going on with these heating elements and I wanted to learn more, so I asked. It will pay dividends next time.edited for typos/grammar

author: ejeffrey
author_rank: Super Contributor
timestamp: September 25, 2020, 09:05:39 pm
content: Quote from: DrG on September 25, 2020, 08:14:40 pmThat just sounds like an underpowered heating element for the chamber, possibly complicated by a decrease heater power (due to rising resistance) at higher temperature. The closer you get to the maximum temperature the less regulation you will have. It also sounds like you had an underpowered power supply. You can also likely increase the voltage a bit to compensate for the resistance change if you really want to.Ideally you want to operate your heater at ~50% duty cycle -- whether PWM or on-off. That gives you the best regulation. Doing so is often too expensive since it requires a considerably larger power supply and heater element, so maybe you want to target ~80%, but if you go at 95% you won't have much headroom and your response time will be slow.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 25, 2020, 11:06:46 pm
content: Temp rise is balanced by heat loss. If the enclosure is, say, bare metal, and for that matter if the ventilation is basically open air, or fan cycled to ambient, well, you've got a damn good heatsink there but an oven it is not!Obviously, you have the limiting factors of:1. You can't trap too much air, lest humidity rise into the danger zone; some cycling is necessary.2. Evaporation itself requires heat input.These set the minimum, and any heat lost through the enclosure adds to the total. This is how much power you need to deliver.This may not seem very straightforward (how do you get from some mass of peppers, to a heater wattage rating?), but I think that can be solved. Let's see:- What mass is being dried, and how wet is it?- How fast is it being dried? (Note it can only be dried so fast, due to diffusion through the skin; the peppers could be slitted to improve that.) Multiply these by water's latent heat of vaporization to get that heat flow.- How much vapor is being removed, divided by the vapor content of x% RH air (for some desired relative humidity in the box)?- Which determines how fast you need to cycle the air, in terms of volume rate (likely very little, which is fine -- a leaky door might suffice, or a baffle diverting some of the fan's internal circulation outside -- you'd still want a fan inside to ensure even temperatures). Take the heat capacity of air times the exchange flow rate and desired internal temperature to get that heat flow.- Take internal temperature and figure the heat dissipation rate for the enclosure. A figure of 150 C in^2/W is a typical guess for passive convection; this applies to the outside (in free air), and a somewhat smaller figure applies on the inside (where the fan is moving air around somewhat faster). Add to this thermal resistance, the insulation value of the enclosure, if any. Finally, divide temp rise by the total resistance to get that heat flow.Add them all up, and that's how many watts you need.Probably it's not even worth going to this trouble, of actually designing a thing. Put in a lamp socket and try different size incandescent bulbs, 60W, 100W, 200W... E-Z Bake(TM) it until it measures right and does what you're looking for. Or use a heating element (erm,maybesalvaged from an appliance, but be very careful of shock hazard and mounting..) and a lamp dimmer, and adjust it until it looks right.Also, since this is food, do look for something that can be cleaned very well (e.g., ceramic, metal), or is food-safe. And avoid leaded solder anywhere near food (control circuits in a separate compartment can be leaded, who cares). The funky smells mentioned from the one silicone pad heater is probably not a good idea to smoke your peppers with... (save that for some delicious woodsmoke instead... mmm, chipotles..).Tim

author: DrG
author_rank: Super Contributor
timestamp: September 25, 2020, 11:07:33 pm
content: Quote from: ejeffrey on September 25, 2020, 09:05:39 pmYeah, I think you are right. The biggest mistake was too large of a chamber, but I was making use of what I could get my hands on. Smaller and better insulated might have worked.All-in-all though, it was a good learning experience and I know more about some of the issues. This will come in handy when I want to build, for example, a heated seed starter or some other little project. Plus, it is fun

author: DrG
author_rank: Super Contributor
timestamp: September 25, 2020, 11:32:34 pm
content: Quote from: T3sl4co1l on September 25, 2020, 11:06:46 pmThose are all legitimate issues. I would put it this way, a professional environmental chamber is the ideal, but you quickly come up against some realities and some limitations. Do you know how difficult it is to keep an internal RH of 30% when the ambient is 65% and while keeping RH=30% keep the temp at 140 (rhetorical question)? There is a lot to all that.At the risk of embarrassment (not really), here is a shot of what I kludged together.Some of my motivation comes from just doing what I feel like doing. When I was very young, we would get these specific dried peppers (thin skin and thin meat) from my grandparents and they would be fried (they puff back up) and salted and they were crispy and delicious. I tried various commercially available products and they just did not work. Rubbery and they barely enlarged. I decided that I would grow them and dry them myself this year. Got the seeds from the country of origin and I grew them. Now, I am drying them them and eating them (I will send some to some folks) and some close friends are in for a treat. As odd as it might sound to some people, I love doing that kind of stuff and now, I can - there are a lot of pluses of being "successfully" retired. I have to add that, while we think about memories for places and so on, we also have taste memories and when I bit into the first one, I said out loud, "Oh yeah, that's it".Further, my kludgy mess (that looked like a 5-year old made it) did, in fact, work, But, in the end, it was much easier for me to simply buy this:https://www.amazon.com/gp/product/B07B9KZXLS/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1Works great has more than enough control to get the job done and done safely and takes less than 500 watts. Still, I am glad I did the first one - because I could and the experience will pay off.That's my story and I'm sticking to it

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 26, 2020, 10:16:17 am
content: Neat.Yeah, RH and T aren't exactly independent. But you can get a sensor for each, and a servo on a vent would cover that. An analog controller might not be so practical at that point, but it's nothing a microcontroller can't do.Tim

author: DrG
author_rank: Super Contributor
timestamp: September 27, 2020, 02:45:24 pm
content: Quote from: T3sl4co1l on September 25, 2020, 07:29:57 pm@T3sl4co1lThis is a bit off the thread topic, I know, but I thought I would add this. I have seen many of those thermistor / comparator circuits and here is an alternative that I did a while ago that is a bit more "digital" and much simpler (IMO). It uses a AT30TS750Ahttp://ww1.microchip.com/downloads/en/devicedoc/Atmel-8855-DTS-AT30TS750A-Datasheet.pdf.A really remarkable little chip, although not terribly popular. It does need to be programmed (I2C) but offers a good deal of flexibility and there are some simple implementations like this one (which I used with one of those silicon heating pads).