Would this work? (Micrcontroller and heating up 0805 SMD resistors) - Page 1
Forum section: electronics / projects

author: wosser
author_rank: Contributor
timestamp: May 31, 2014, 04:08:27 pm
content: Hi all,I'd like to use a 5V microcontroller (probably an AVR, powered by a PC's USB port) to make some very small resistors (0805 size SMD chip resistors) heat up quickly.Can anyone recommend a good way to do this? I suspect that using a MOSFET to connect the resistor between the 5V and GND rails will simply cause the microcontroller to brown out and reset instantly (and screw up the USB port on the PC too right?).So how about if I used a 3.3V voltage regulator running off the 5V input to provide the power rail for the resistors only? That should let the 5V rails stay healthy shouldn't it?I want the resistors to get hot (around 80 Celcius) as fast as possible. I figured that I can use 10KHz 100% duty cycle at 3.3V to allow the temperature to ramp up quickly and then drop down to some sensible duty cycle to maintain that temperature for a moment before shutting off again. I need 8 resistors to all be heating up independently at different times. I realise this will be a considerable load on the regulator.Is this a feasible approach? I don't want to damage the regulator by doing this of course, nor do I want the USB to be adversely affected.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 31, 2014, 04:49:03 pm
content: All the current comes from the 5V rail, so you are just outright limited in the power you can use. All the 3.3 V regulator is going to do is waste some of your power, provide a current limit, and filter the load a bit. Choosing the proper resistor value will limit your current just as well. Make sure you have plenty of capacitance on your +5V line and you should be fine with your original idea of the resistor connected 5V to ground with a MOSFET.

author: wosser
author_rank: Contributor
timestamp: May 31, 2014, 05:54:47 pm
content: I guess that does let me keep my circuit nice and simple after all. I should be able to judge how much capacitance I need on the 5V rail by using an oscilloscope to see the rail sag when the MOSFET is turned on - as long as I assume the worst case, which would be all resistors on 100% at the same time .Thanks for the advice, I'll write here again when I've got it working!

author: rob77
author_rank: Super Contributor
timestamp: May 31, 2014, 06:20:07 pm
content: would you mind to tell us what's the application ? 0805 size resistor is a rather tiny heating element

author: larry42
author_rank: Regular Contributor
timestamp: May 31, 2014, 07:32:07 pm
content: The OP seems to have a number of misconceptions. What is the resistor value? How quick should the heating be ( say to 75•C). 10ms? 1s?Sent from my One S using Tapatalk

author: wosser
author_rank: Contributor
timestamp: June 01, 2014, 12:18:05 pm
content: I should admit that this is a fairly odd project I'm working on.I'll be using some thin thermochromic film as a primitive display.The resistors will be arranged in a tight circle (8mm diameter perhaps).The film is glued on top of the resistors. When the resistors heat up theycause the film to change colour. This is how I hope to get away with such tinyheating elements.The absolute temperature I need is not yet known (I'll have to dial it in oncethe device is built and I can play around to tune it just right).Ditto the heat-up-time. I simply won't know the thermal characteristics of mydevice until it's built so I'm still basing most of this on guesswork and wishful thinking.I will probably have to get creative with cut-outs in the PCB and put in a few craftycopper shapes to get the heat away quickly when the resistors are cooling down.My gut tells me that something between 75ms and 100ms would be a good amountof time to heat up to the right temperature. I will have to dig through some datasheetsto figure out what a good resistance value would be for this weird application.I seem to remember an old EEVBlog video where Dave was melting fishing line with a hotresistor. I'll have to take a look at that again.

author: rob77
author_rank: Super Contributor
timestamp: June 01, 2014, 01:07:40 pm
content: i would definitely go for a separate supply for the resistors to avoid issues with the dropping voltage for the processor.personally i would go for a 12V supply and place 5V linear regulator for the processor and power the resistors from the 12V directly - using low side switching transistors.btw.. the cooling of individual resistors will be quite tricky (cooling one resistor while the adjacent ones needs to be kept hot), i wouldn't expect great results. if you'll use a common heatsink for them, the temperature will even out. and to be honest, i can't image how would you manufacture individual heatsinks for each resistor (and even with that, they will heat each other by heat radiation).

author: Jeroen3
author_rank: Super Contributor
timestamp: June 01, 2014, 02:13:19 pm
content: Maybe you need some miniature peltier elements. You can heat and cool selected surfaces by flipping the current direction.

author: MrsR
author_rank: Regular Contributor
timestamp: June 01, 2014, 03:14:01 pm
content: Why not use some resistance wire, A IR. 103amp Mosfet would supply enough Amps without getting to hot. Pulse width controls the temp. a kink in the wire away from the Heat sensitive plastic To give a variable heat time.As rob77 says run main Voltage supply to the Mosfets and regulate the Voltage supply to your mcu.You could use a Comparator or OP Amp As a sensor to switch on a fan to cool the wire faster when you want it to cool down.

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 01, 2014, 04:10:29 pm
content: As rob77 already said, the power supply for the load (the resistor heating elements) is the KEY to the success or failure of this scheme.If you have a proper (and ISOLATED) supply for the load, it could be 100s of amps with no ill effect to the controller or USB communication. It seems extraordinarily doubtful that this scheme will work from the USB 5V supply. Maybe with an isolation diode and a big reservoir capacitor if the average heater load is low enough(?)

author: babysitter
author_rank: Frequent Contributor
timestamp: June 01, 2014, 05:28:14 pm
content: Unfinished idea: Look at the heater elements of thermoprinters.

author: rob77
author_rank: Super Contributor
timestamp: June 01, 2014, 05:35:36 pm
content: Quote from: babysitter on June 01, 2014, 05:28:14 pmwell.. have you ever seen a 0805 resistor sized pixel on those printers ? those printers have microscopic heating elements - much less thermal inertia and radiation (orders of magnitude less).and probably those are using a single heating element printing a single row of pixels each pass - but i'm not sure about this.

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 01, 2014, 05:52:51 pm
content: Many "inkjet" printer heads also use thermal devices to "boil off" a droplett of ink for depositing on the paper.But, as rob77 says, those thermal elements make 0805 SMD resistors look humongous by comparison.If you can get SMD resistors to work in this scheme, I fear the dynamic response will be pretty slow.

author: mariush
author_rank: Super Contributor
timestamp: June 01, 2014, 10:14:55 pm
content: Why not some 0603 or 0805 leds with a very sharp angle... both heat and some light that may shine through the material a bit. Maybe if you pulse them for a few us with some high current you heat up material fast.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 01, 2014, 10:26:54 pm
content: If you could find some SMD PTCs in the size you're looking for, they would be much easier to control. Those can literally just be turned on and the material itself controls the temperature.

author: rob77
author_rank: Super Contributor
timestamp: June 02, 2014, 09:15:27 am
content: Quote from: mariush on June 01, 2014, 10:14:55 pmi like this ideadefinitely less thermal inertia than a 0805 resistor.

author: wosser
author_rank: Contributor
timestamp: June 02, 2014, 07:39:10 pm
content: Quote from: mariush on June 01, 2014, 10:14:55 pmThat is an intriguing idea. I'll look into that.