Ideas for very compact heater power controller? - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: July 04, 2022, 10:04:00 pm
content: Hi,I need to control the temperature a physically very small 5W heater directly connected to mains (80-250V).The enclosure is tiny and the volume available fo rthe PCB is very little.The control is done by a 3V3 micro. The enclosure has a tiny fan so it can provide a little cooling of any switching components.The heater's response time is not important, even few seconds to adjust to the correct value is ok.I was thinking about either a phase control (variable delay in switching from zero-crossing point) or burst firing control (turning on a cyecle evey x-cycles). Which one would be preferred and why?Considering that space is the absolute first priority, are there any other/better solutions?For the same space-constrained reason, are there some ready available solutions/ICs to do such control?Thank you

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 05, 2022, 12:54:41 am
content: Phase or burst is fairly irrelevant as you say; control it however you like. Both are amenable to a single tiny TRIAC, make sure it's rated for low enough holding current -- 5W is quite small indeed. Drive it like a BJT, MT1 as "emitter", G as "base", MT2 as "collector" (to load to HOT). This gives quadrant I/IV drive, which is fine for most any TRIAC.MCU should probably sense line phase regardless, obviously for phase control it's needed to synchronize, but for burst mode, zero-crossing is slightly preferable.Can also use a pair of back-to-back MOSFETs, which will need isolated supply to the MCU/driver instead (the TRIAC and control can be common to NEU, easy use-case for a capacitive dropper kind of circuit).Phase vs. burst, probably depends on how you want to control it. Phase is smoother, at least in principle; but it sounds unlikely that the ripple (or accuracy?) needs to be very much. Basically, phase you get a continuous* number for delay, and that's your control (PID?) output variable. Burst mode, you probably use a sigma-delta encoder; which itself can use an accumulator of arbitrary resolution, and you just take the top (sign) bit off it as your output on/off signal. S-D can therefore be hysteretic (especially if you add a hysteresis band to that signedness check) or fed by PID, whatever you like.*If it were analog; of course being digital this will be some finite timer register value or whatever. The resolution of which therefore defines how ""continuous"" it is in practice.I don't think you're going to get any more compact than a switch (required for handling the voltage), MCU (required for unstated functionality?), and some kind of power supply (C-dropper, LinkSwitch, etc.). So, about a dozen components minimum, all told? Plus whatever is required for "functionality".Tim

author: ricko_uk
author_rank: Super Contributor
timestamp: July 05, 2022, 06:38:50 pm
content: Thank you Tim,the LinkSwitch are very nice and compact parts.Thank you

author: Siwastaja
author_rank: Super Contributor
timestamp: July 05, 2022, 07:12:26 pm
content: Look at photo MOSFETs. They are basically just optoisolated back-to-back MOSFETs, so can control AC or DC. Compared to TRIAC SSRs, current * voltage ratings are not as good, but you don't need much current, just enough voltage rating. And you don't need to think about leakage current or minimum load which prevent usage of TRIAC SSRs in this application.I would add a fusible series resistor + small MOV to protect against voltage transients, if you have enough room.With such tiny current, random firing is OK.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 05, 2022, 09:55:15 pm
content: The TRIAC is probably cheaper and more efficient -- but the photo MOS is perhaps easier to drive (it's an isolated LED, no nonsense -- though it will take more current than a sensitive-gate TRIAC), and they are readily available in such ratings, in small packages (DIP6 at least, not sure about more compact SIP or SOP, but probably?). One important difference: they're slow. Really slow. Like rising edges of fractional to multiple ms slow. Whereas TRIACs can fire in whole to fractional µs. Their performance is greatly hindered at the high-power end (as well as an LED at <= 20mA just not being enough to photo-drive really big MOSFETs at all, let alone at a useful time scale), but here, it's an advantage, low EMI emissions.Tim

author: ricko_uk
author_rank: Super Contributor
timestamp: July 05, 2022, 10:13:07 pm
content: Thank you both!

author: langwadt
author_rank: Super Contributor
timestamp: July 05, 2022, 10:28:56 pm
content: https://www.mouser.dk/datasheet/2/365/SHARP_PR36MF21NSZH-1360470.pdf?

author: shakalnokturn
author_rank: Super Contributor
timestamp: July 05, 2022, 10:29:52 pm
content: If triac hold current is a problem you should be able to get away with driving the gate all the time it needs to be active rather than just a trigger pulse.Phase angle will cause more EMI than burst fire although it shouldn't be of much concern in this case.

author: ricko_uk
author_rank: Super Contributor
timestamp: July 06, 2022, 02:52:36 pm
content: Thank you boththose optos (mosfet or triacs) with built in zero crossing would save a little more space too. I never used them, I assume I just apply a random-in-time pwm? So that without the micro having to sense the zero crossing, and if my control signal is high the next zero crossing then it turns on, and the same for turning off? So both on and off occur only during zero crossing regardless of when I turn on the LED, correct?Thank you

author: Siwastaja
author_rank: Super Contributor
timestamp: July 06, 2022, 04:47:41 pm
content: I would not bother with zero crossing in such tiny, resistive load. Heater also doesn't need actual phase angle control because of thermal inertia. You can use some slow PWM frequency like 1Hz, doesn't need to be synchronized to mains.TRIAC can be turned on anytime, but it can be turned off only when current zero-crosses, which with resistive load happens when voltage zero-crosses. If you use non-synced 1Hz PWM for example, this TRIAC turn-off delay (time from gate driven off -> mains voltage zero crossing) causes some random jitter on the PWM duty cycle, making a tad less accurate, but this probably isn't a big deal.MOSFET can be turned on and off anytime.

author: langwadt
author_rank: Super Contributor
timestamp: July 06, 2022, 05:10:10 pm
content: Quote from: Siwastaja on July 06, 2022, 04:47:41 pmat such a small load the triac might not stay on

author: Siwastaja
author_rank: Super Contributor
timestamp: July 06, 2022, 06:45:15 pm
content: Quote from: langwadt on July 06, 2022, 05:10:10 pmYes, hence careful with reading the datasheet. A small TRIAC would work. I suggested photo MOSFET as an "easy" solution, partially for this reason. With photo-MOSFET, basically the only parameter you would need to look at is maximum voltage over the switch.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 06, 2022, 09:18:15 pm
content: Quote from: langwadt on July 06, 2022, 05:10:10 pmLike I said, drive it like a BJT, and/or use a sensitive gate type -- thyristors below Ih are just BJTs after all, it doesn't stop working as long as you're trickling in some gate current.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 06, 2022, 09:23:47 pm
content: Quote from: Siwastaja on July 06, 2022, 04:47:41 pmWhat thermal inertia are we talking about here, anyway? At such low power levels, it could be quite small, actually, and even line modulation could be annoying (in which case a filtered DC supply might be desirable). The extreme case would be a pretty small (and hot!) heater; but entirely possible. (Remember, you can "listen" to incandescent light bulbs with a photosensor!)Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: July 07, 2022, 06:32:46 am
content: Yes, I'm assuming this "5W heater" is roughly equivalent to some 5-10W rated power resistor with ceramic case, in which case a 1-second PWM period would be fine. But this assumption could be wrong; if it is a free-hanging resistive wire radiating heat, even 10Hz might be too slow.You can also "see" the incandescent light bulb flicker: just take series of photos at high shutter speed like 1/1000s, and the photos do not have equivalent illumination. You see this if you try to do some stop-motion animation, you either need to drive lightbulbs with DC, or use slow enough shutter speed to average out full cycles.

author: ricko_uk
author_rank: Super Contributor
timestamp: July 07, 2022, 09:32:00 am
content: Thank you boththe thermal inertia is not that much. The heater is wrapped around a small glass vial to heat small liquid amounts for evaporation. The maximum temperature is around 70degC and the physical size is just 8mm diameter and 20mm long. And the external temperature is just ambient 10degC to 30degC.Tim might have a point about line modulation, I am thinking to perhaps add a metal sleeve the "mechanically" smooth the thermal variation. That would be more compact than adding more electronics mainly because there is more space around the vial/heater volume. Any thoughts on that?Thank you

author: langwadt
author_rank: Super Contributor
timestamp: July 07, 2022, 09:51:59 am
content: for such low power why use mains?if you already need a supple for a micro adding another 1A at 5V shouldn't be a big issue, then it also all isolated and you don't have to worry about varying voltage

author: Siwastaja
author_rank: Super Contributor
timestamp: July 07, 2022, 10:14:46 am
content: Indeed, at 5V you could just roll some thin magnet wire around the tube and call it a day. Low-side MOSFET is then all you need, maybe a freewheeling diode for stray inductance. Could be powered from 5V USB power supply.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 07, 2022, 11:29:42 am
content: It starts with the vague specification of "very small", without any further details. what is "very small"?There are several options of creating a very small non-isolated power supply from mains voltage, but it generally is a headache because you have to isolate your whole circuit (both uC and heating element) from being touched. A built-in isolated power supply is probably the bulkiest part of the whole project. It does not matter much if you only need a few mA for the uC, or 5W for the heater too.Using some isolated low voltage power supply makes the design of the circuit much simpler, and indeed, running the heater itself also from that low voltage power supply makes much more sense for such a low power application.Maybe you can even delegate the power supply to an external wallwart. In that case you just need a voltage regulator for your uC and an SMT MOSfet to switch your heater element. Everything could fit fairly easily on a 2 square cm PCB. You probably also need some chicken fodder (resistors, decoupling capacitors, etc) and something to provide temperature feedback. It's quite likely that the connectors to the PCB take up most of the space.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 07, 2022, 11:32:11 am
content: Quote from: Doctorandus_P on July 07, 2022, 11:29:42 amThat would be obvious. Modern 1A 5V USB supplies are so small, not much bigger than just a passive plug. +1 for everybody having them around. Hence, the product could use just a USB connector for power. Is there some reason why this obvious solution was dismissed that we don't know about?

author: ricko_uk
author_rank: Super Contributor
timestamp: July 12, 2022, 11:12:03 pm
content: Thank you allThe external power supply is not an option because it needs to be plug-type enclosure.I have been working on the CAD model and various solutions trying using the heater configured in both mains voltage and 5V.This is the enclosurehttps://evatron.com/wp-content/uploads/evatron_tech_FE4UK.pngthat has to fit:- the PCB with micro,- infrared and light sensors and related components,- SMPS module/components including mains filter,- a 30x30x7 mm fan (required by the system)- a 15mm OD x 20mm long cylindrical heater (which I am trying to see if it is acceptable to let protrude a bit to gain a bit more space inside)As you can see the space is not much and I'd rather not make multiple PCBs because of cost (both PCBs and assembly). But if that is the only solution then ok.Driving both fan and heater from low voltage would require 1.8A @ 5V (rather than the initial 1A - after today's meeting), so almost 9W offline power solution with universal input i.e. 80-260V).I cannot find a AC-DC PCB-mount module not a Linkswitch solution that fits in. Especially by the time the front end MOV/EMI filter are added (which I am thinking of perhaps leaving out - I have to check if that still passes the certification or not).I have never design offline solutions of this kind... I think a C-dropper might provide a smaller solution? But not sure if 2A @ 5V is realistic?Any additional feedback is always welcome!Thank you

author: langwadt
author_rank: Super Contributor
timestamp: July 12, 2022, 11:41:36 pm
content: Quote from: ricko_uk on July 12, 2022, 11:12:03 pmhttps://dk.rs-online.com/web/p/switch-mode-stromforsyninger-smps/1358929?

author: ricko_uk
author_rank: Super Contributor
timestamp: July 13, 2022, 12:00:18 am
content: Thank you LangwadtI did see that part previously but did not notice that it is EMI class B without adding any external filters. So without external filters components it might just fit if I also add a airflow guide for the heater and fan.I will now add it in the 3D model to check all clearences. Fingers crossed...Thank you and everybody else as always!