Best method of controlling a resistive heater

Hey all, 

I'm creating a resistive heating system (I don't have a choice on that, the resistors are already provided), and I'm looking for the best method of being able to control this. 

Resistors are 50W 4 Ohm Panel Mount Units, of which I've got about 32, but I'll likely need more going forward. 

Currently it uses 4 zones, each of which use a custom MOSFET PCB which is PWM pulsed by an Arduino which is all fed from 4 24V DC power supplies. 

This has worked OK thus far, but it's not the most reliable and I was wondering if there's something COTS which would suit this better that provides a better method of control to be able to vary the temperature of the panel, as the current ones have no sense feedback, so they're unable to temperature compensate. 

I was wondering if an inverter could sort something like this? 

I know I'm limited by the supply of the room (which is just 230V, there's no 415V supply etc.), but I'm curious to see what people recommend in terms of whether to look at motor controllers as off the shelf PWM units, current stabilised sources etc. 

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  • Hey Andy, it's a thermal shroud within a vacuum chamber, we're given a specification for what the temperature ramp up should be over a number of hrs/days, then how long that temperature should be held for, before following a ramp down profile. 

    I was thinking PID controller myself, we've got a number of arduino boards sat around which should be able to do that pretty easily, it's the routing of the resistors I'm struggling with. 

    I was considering solid state relays and using mains voltage on them (230V) but over a single resistor that's like 13kw, and it only drops to what a mains socket can kick out at around 4-5 series resistors. 

    I didn't know whether this was the right way to do it or whether it's worth having multiple combinations of series and parallel strings for dual redundancy sake, and the question is open on whether to use 230V or something like a 24V dc supply. 

    There's 4 x 24V dc supplies down there at the minute from the old setup but we have a raft more resistors now, 16 positions per panel we have provision for and 4 panels total. 

  • Arcol HS50   resistors are very good as heaters at well beyond the nominal rating for de-rating. I have used them in the past as the basis for a soldering press to attach PCBs to brass plate.

    I'd only consider them single insulated though and I'd be  wary of mains derived voltage on the element without considering how ADS would behave with a mid-element to case short.

    They will have a heating time constant of seconds to minutes so 'PWM' is probably a bit optimistic at anything much beyond the tens of seconds. By the time they are screwed to something solid, even less nimble - which is good, a slow loop is easy to organise - if you want PWM at all it can be software think 'burn for  x out of ten seconds, check temp and repeat' .


    Given what you have, and the fact it sounds more like a lab-like set-up that may change, I'd have a quick play with  ADC input to the arduinos and consider thermistor as being a lot more robust for EMC than thermo couples if you have pumps, valves and things clattering away. Do you also have a need for plasma in the vacuum firing up as well ? EMC can be a many headed monster in some settings.

    A typical  ten K at 25C device and a fixed 2.2k resistor will give you a pretty linear  voltage division from  50-100C the funny curve of the thermistor and the A/A+B voltage division more or less linearise each other, and after few hundred nF  of filter capacitance you should have a voltage that you can reda on the arduino and make on-off or raise lower the duty cycle decisions on. (and with the arduino drive a temperature display as well or at least 'up down ready' LEDs)

    I'd also add a thermal fuse somewhere so that if it ever goes to run-away something drops the mains, or if there are too many amps, then drops a contactor that drops the mains.  Maybe not this exact model but something like it.

    IF you really do need that many KW, how is the facility cooled ?  I'd not like to share a room with a 3kW fan heater in summer let alone 6kW of static heat.  Less heater and some lagging to keep it in may be preferred though  if you have to cool in a specified time too, then you may need fans or even cooling water.

    Mike.

  • Hey Mike, 

    Yeah PWM as you say may be a bit long. I was reading a technical paper on here about using PWM for resistive heading and having the frequency at around 0.1 of mains frequency or thereabouts (basically keep the switching SLOW). 

    Yeah I can do any of that in the arduino, it's between that and what roger mentioned about using a eurotherm temperature PID controller  (which could be nice as it allows for analogue and digital IO input for sensor inputs. 

    We've got thermistors at the ready so that's no issue, I can use them as preference. 

    We have got two pumps going to maintain the vacuum, one roots pump and a turbomolecular pump. 

    Plasma, no not really, we're using this for the outgassing of materials, so the idea is to remove atmosphere from the inside of the chamber and keep pulling for a few weeks until the outgassing process is 'complete' 

    Thermal fuse is a good shout. 

    I'd not got an idea of how much power to throw at this to be completely honest as the previous setup was never documented, so it's all been done on the fly previously. 

    I've been doing some number calcs and I was thinking if I use 6 resistors in series, with 2 parallel strings, that gives me just over 2.2kw per panel. 

    Or I could lower it to 1.6-1.9 kw by changing to 8x2 or 7x2. 

    I'm tempted to play it safe and suggest the 1.9kw which uses 14 out of the 16 positions by using two parallel strings of 7 resistors.