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Grogster
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Posted: 07 January 2018 at 12:15pm | IP Logged Quote Grogster

Hello all.

I have a 3mm aluminium plate here that is used for egg incubation. It works, but the temperature regulation is not really what I want, so I am going to change the design.

The current design simply has six 1-ohm 10W wire-wound resistors connected in series, and clamped to the underside of the plate with some heatsink goo. I was thinking more along the lines of four or six TO220 transistors set so they do nothing but heat up, and then bolting those to the underside of the plate.

There is an NTC coupled to the plate, and once it gets up to a certain programmable heat, the current is switched off. This circuit cycles around keeping the plate at the same temperature give or take a degree or two. The plate and eggs are all inside an insulated container, and the lid is not really lifted at all, so as not to let the heat out. Only to retrieve the eggs.

As I am doing some MOSFET research and read that they are a voltage-controlled device, I am thinking perhaps a few MOSFET's would be ideal here. Taking an N-MOSFET such as an IRF540 as a starter, if the gate is 0v, the S-D path will be off(well, very high resistance), and if the gate voltage is 3-4v(gate threshold voltage), the S-D path will be on(start to conduct), but the device will dissipate heat. 10v or so on the gate, according to the datasheet, will switch the device fully on with a very low S-D resistance.

Reading about MOSFETS, the key thing seems to be to switch them as fast as you can from one state to the other, as even in the fully ON state, they dissipate very little heat - which would be right looking at the S-D on resistance of just about any datasheet I pick up.

The same MOSFET info tells me that if you have a gate voltage less then the fully on gate voltage, but still higher then the gate threshold voltage, the device acts like a resistor and will dissipate heat - perhaps a great deal of heat if you are not careful, and if not controlled, you will cook the MOSFET depending on what current can flow in the S-D path.

OK. All that makes sense to me. Now, if I was to use a MM I/O pin to drive the gate with 3.3v or so, this would force the transistor into a state where it would actually dissipate heat, which is what I want.

Now, there would need to be some current-limited testing involved here, but I was thinking four or six(or maybe eight?) MOSFETS, all in parallel, and centred evenly around the aluminium plate so they would all heat up the plate, and share some of that load. Another MOSFET could be used to switch all those on and off if I understand things correctly. There would most likely be a better choice of MOSFET. The IRF540 datasheet is just the first TO220 one I came across.

This is probably where someone will say that bipolar transistors would be a better choice here, having just got me interested in MOSFET's and talked me out of bipolars in the other thread.......

Something else I am also considering is a Peltier-effect device, and putting the plate on the hot side, and a cold-sink on the other side in a standard arrangement for those, and they are very easy to find on eBay or Aliexpress.(unknown quality, but I could try some and see)

All thoughts and comments welcome at this point.

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palcal
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Posted: 07 January 2018 at 12:20pm | IP Logged Quote palcal

I have a couple of Peltier devices, they seem very reiable and easy to use but draw a bit of juice.
Mine are 12v. 4amps.
Paul.

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Posted: 07 January 2018 at 12:25pm | IP Logged Quote Grogster

Juice does not REALLY worry me. The current resistor design draws a few amps when it is "ON". I just had a quick look on eBay, and you can get five Peltier effect devices for about US$14, and if I used four of those centered on the plate perhaps?

The only REAL problem I have with Peltier's is that in this case, you have to find a way to get rid of the cold with a cold-sink.

However, I am thinking if I used a 5mm thick plate for the cold-sink, and a 3mm plate for the heat sink, with the hot-side up, the heat from the hot side would help to warm the cold-sink and keep it from getting frosty?

Never played with Peltier's either. Perhaps I should get some just to play with!

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Azure
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Posted: 07 January 2018 at 12:27pm | IP Logged Quote Azure

I support the Peltier vote. That is what they are made for (heating/cooling/controlling humidity). Cheap unit in different sizes available on ebay.
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Posted: 07 January 2018 at 12:30pm | IP Logged Quote Azure

Fan on cold side (with temp sensing control) could be used to stop it frosting and push cold away by keeping ambient air temp flowing past it.

My wine fridges use peltiers mounted inside the sides to control internal humidity. Do not have access to them so cannot see how they are mounted.

Edited by Azure on 07 January 2018 at 12:34pm
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Posted: 07 January 2018 at 12:43pm | IP Logged Quote Warpspeed

I would use one or more of those "gold" metal clad resistors that can be bolted direct onto your plate.





These can then be switched on or off with a mosfet.

Any one of the switched mode power supply control chips could pwm those mosfets directly. And the error amplifier in the controller connected to a temperature sensor.

The pwm switching frequency need not be high, 100 Hz switching rate would be fine for something like this.

Edited by Warpspeed on 07 January 2018 at 12:45pm


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George65
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Posted: 07 January 2018 at 1:21pm | IP Logged Quote George65


Peltiers are fun and simple.

You really want a fan on the cold side in this case so the cold is removed and the things just don't ice up which will limit heating performance.

2 days ago I was going round to the relos to water the garden etc while they are away and saw the house that just sold down the road with a pile of stuff out the front.
I spoted a Waeco car fridge which I picked up on the way back.

The thing is in perfect condition, looks hardly used and had all the cords. It uses a peltier and the thing works a treat. Sitting out the back now in 40o+ heat it's still keeping the cans I put in it under 5o.
It says it will cool to 30 below ambient but it's doing way better than that.

I have another cheaper car fridge of the same type, the little fan on that died about 10 years ago and I couldn't find one the same size. I hacked a bigger hole in the housing and put in a larger computer fan. the thing works WAY better now than it did before.

Moving the heat or cold from the opposite side of these thing makes a real big difference to their efficiency and effectiveness.
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Grogster
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Posted: 07 January 2018 at 1:59pm | IP Logged Quote Grogster

I'm now looking at these things on eBay - ceramic heater elements. Perhaps these could be a good solution? 12v operation, cheap, hot only, no cold-sink required. Anyone used or played with these things?

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Posted: 07 January 2018 at 2:08pm | IP Logged Quote Warpspeed

They are a good solution where there is a fan, and you don't want the heating element to burn out if the fan fails. Unless your aluminium plate goes above 80 Celsius they will just act like a normal resistor of the specified power.

If your eggs ever get to 80C, they are not likely to hatch.....!

So I cannot see the point really.
A simple basic resistor would work just as well as a heater as anything else.
Bonding the resistor to the metal plate is the only complication, and those gold resistors solve that problem very simply.

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Posted: 07 January 2018 at 2:24pm | IP Logged Quote Grogster

Those gold resistors are expensive . NZ$4 and up each PLUS another $2 for freight for each one! I would need four or five(or more) to get even heat distribution. Using just one(or two) would be pointless, as you would end up with hot-spots where the resistors are, and the rest of the plate gets cold - this is the problem I have now with the resistors clamped to the plate. So I would need to use 6-8(maybe ten) of those gold resistors, and that is starting to get rather expensive compared to other options. You're looking at $40 or so just for the gold resistors, plus another $20 in freight - we're up to sixty bucks just for the resistors so far.

I will have another search on those. Perhaps I was unlucky to strike expensive ones or something, but the other choices just below that listing show similar ratings, and similar prices too!

EDIT: OK, this is interesting. HIGHER power ones are cheaper! I found these when I searched for '50W resistor' - fully expecting them to be almost twice the price of the 30W ones. Weather they are any good is another thing, but perhaps......

Edited by Grogster on 07 January 2018 at 2:28pm


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Posted: 07 January 2018 at 2:34pm | IP Logged Quote Warpspeed

You want cheap even heat, use a much thicker aluminium plate, or use a single resistor to heat up a suitably shaped water storage tank in the base of your incubator.
This could probably be some plastic "Tupperware thing" as the temperature is not going to be high.

Edited by Warpspeed on 07 January 2018 at 2:38pm


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Posted: 07 January 2018 at 2:44pm | IP Logged Quote Warpspeed

If you can scrounge one of those huge heatsinks used on most grid tie inverters, mount that fins up, with a single metal clad resistor bolted underneath.

Eggs on top.... Job done.

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