A different PID question (i hope)

I have a "MyPin" PID off of e-bay. The thermocouple thermometer was from E-bay from China, it came with a Celsius only readout. It's a bead type which is the best for the hotplate IMHO. The 40A SSR/heatsink combo I have two of now since I'm also planning on PIDding my Espresso machine on my Christmas break (but I need a second PID).

Why 40A and not 25A? It's about $2 more and the Chinese tend to overrate their stuff. It's cheap insurance for $2 that the SSR won't likely fail since you are switching at most 15A through it.

I'm not seeing the thermometers on today for $4.. but here is the probe only for about the same price.. ($4 shipped)

http://www.ebay.com/itm/K-Thermocoup...item3a6821c761

So, yeah this is "common" industrial control stuff and is pretty cheap. The guys here packaging these up and selling them are doing a few hours of work and putting them into $25 boxes, so I'd cut them a little slack. If you decide not to do that, you are paying them for their time at a pretty low rate, actually.

But for this stuff.. e-bay, China, and be prepared to wait 20-30 days for it. All of it has worked well for me so far, tho. And I've never been screwed by a Chinese, Taiwanese, Thai seller. I wish I could say the same for the American ones...

Alright, it's rather obvious y'all have no clue about heatsinking a part.

First. the wattage in the part has very little to do with much. that is the heat in the junction of the silicon.
Getting that heat out of the case and into the air is what you need to worry about.

For that you use the manufacturers tables for that device.

Attachment 87299

Above is a portion of the datasheet for a 25 amp SSR that is very much like what you would get from Auber Instruments.

The chart has 2 curves - lets work with the 'free air' one first - That is the black section.
Notice that Only at 20º C is the full current available as the device package gets warmer it will allow less and less current to flow before it burns up.

This is what you get if you don't have a heatsink. It is worse if you put the part inside a case and don't vent the box! The heat generated by the part causes the temp to raise - raising the temp farther.
You get a positive feedback loop - and things soon burn up.

Now look at the 2ºC /Watt line. It's better at 20ºC but it quickly degrades as the temp rises.

Attachment 87300

Here is what a given heatsink is capable of.
We have graphs for free air, 1ºC/W, and 2ºC/watt sinks. The ºC/Watt is the amount a sink rises above ambient temp. The lower the number the better the sink.

Now a 12"x12"x1/8" (about 300 sq in) sheet of aluminum standing vertically in free air is approximately a 2ºC/watt sink.
A 18" square (about 650 sq in ) x 1/8" thick sheet mounted the same way is a 1ºC/watt sink.

I hope this helps.

In case any would like a complete spec sheet for a SSR of 'standard' design go Here

One other comment... you need to verify that any PID you buy is the SSR driver type and not relay type for this or any other application. I also use my casting hotplate as a reflow soldering plate.. so I've been doing this for awhile. It's good stuff. Having a PID hot plate was really useful for tumble lube warm up when I still did that (I'm PCing now)... and the PID hotplate in general is one of the best tools I have. I use it for a LOT of stuff since I can literally adjust it accurately from 100 dgrees to 850 degrees F...

2WheelDuke: That should work fine. I did mine in pieces, so the 40A SSR was not that much more $$, like I said. The thermocouple that comes with that is a pretty big thermal mass, but it might be fine with a real casting pot. My cheap a$$ method of using a hotplate to cast really didn't work well with the thermocouple like that (I have a couple of different types). I use a bead like I posted before and put some hi-temp thermally conductive insulation on it and have it attached to the "big mass" cast iron piece on the hotplate. So.. if you can't find a place to properly mount that stud-type thermocouple, try a bead instead... but otherwise.. yup, that's pretty much exactly what you need. Set it up, let it autotune and you will be good to go.

I get a lot of things off dealextreme, including some thermocouples recently: http://dx.com/p/k-type-thermocouple-...er-grey-213591

Quote:

---

Originally Posted by bhop

el34- so as long as i can keep the ssr cool i dont need a heatsink? so if i put a small computer fan blowing across it would that be enough?

---

The purpose of any heatsink is to speed up the transfer of heat from the thing you don't want hot to somewhere else, usually the surrounding air. It basically adds lots of square inches of surface area to contact the air. The next thing is to prevent that surrounding air from building up too much heat itself so a fan or vent holes are used to pull in new air.

If your SSR will stay cool by itself due to its location/mounting (somewhat possible) there is no need for a heatsink.

Felix is right- heat is electronic's worst enemy. There is a big chunk in engineering that deals with heat flow and there's no simple rule that works or is necessary in all cases. For the SSRs used with PID controllers for casting pots, I don't think it's very critical as mentioned earlier. When I first wired up mine it was all just sitting on a countertop so I could check it before building it into a box. I had no heatsink on the SSR, it just sat there. When it was all powered up and heating the pot I couldn't really feel any temp rise on the SSR. But I still provided it a small heatsink when I boxed it up and put a small (1") Radio Shack fan near it that slowly blows air out of the box.

The SSR is producing most of its heat in the several minutes while the pot is initially heating up. After that it pulses off and on as required to maintain pot temp. There's a light on the controller that indicates when the SSR is being turned on, that's the only time it produces heat.

A few months ago I posted my project, it has a few pics-
http://castboolits.gunloads.com/show...D-4-20-project

Quote:

---

Originally Posted by Frozone

Alright, it's rather obvious y'all have no clue about heatsinking a part.

First. the wattage in the part has very little to do with much. that is the heat in the junction of the silicon.
Getting that heat out of the case and into the air is what you need to worry about.

For that you use the manufacturers tables for that device.

Attachment 87299

Above is a portion of the datasheet for a 25 amp SSR that is very much like what you would get from Auber Instruments.

The chart has 2 curves - lets work with the 'free air' one first - That is the black section.
Notice that Only at 20º C is the full current available as the device package gets warmer it will allow less and less current to flow before it burns up.

This is what you get if you don't have a heatsink. It is worse if you put the part inside a case and don't vent the box! The heat generated by the part causes the temp to raise - raising the temp farther.
You get a positive feedback loop - and things soon burn up.

Now look at the 2ºC /Watt line. It's better at 20ºC but it quickly degrades as the temp rises.

Attachment 87300

Here is what a given heatsink is capable of.
We have graphs for free air, 1ºC/W, and 2ºC/watt sinks. The ºC/Watt is the amount a sink rises above ambient temp. The lower the number the better the sink.

Now a 12"x12"x1/8" (about 300 sq in) sheet of aluminum standing vertically in free air is approximately a 2ºC/watt sink.
A 18" square (about 650 sq in ) x 1/8" thick sheet mounted the same way is a 1ºC/watt sink.

I hope this helps.

In case any would like a complete spec sheet for a SSR of 'standard' design go Here

---

Great post!

Couple of small points-

1/ More than likely at least a few members other than you have greater than no clue about heatsinking.
2/ The IsquaredR wattage dissipated matters because it's the very cause of rising internal junction temperatures that then need to be dealt with externally.
3/ The info you provided is exactly the stuff needed to actually know what to do, if it's understandable and representative of the SSR in hand. I would point out that the full current rating of the SSR probably doesn't represent the current required for a casting pot. A commonly used pot on this forum is the Lee 20 pounder that pulls about 5.5 amps when the element is being energized. The operating current and its less than 100% duty cycle might identify different areas of the curve to be concerned with.

Casting boolits is tricky stuff.

Ok guys, the SSR V drop is 2-3 volts x 15 amps = >30 watts of heat to dissipate. My Lee pot draws 750 W, toaster oven is 600 W, hot plate is 450 W. Remember to use HS compound under the HS & SSR. I just use a computer processor HS! 15-20W.

The Lee 4-20 pot claims 700 watts, =6.1A @ 115VAC.

The 25A SSR I used (Crydom CSD2425) has a max drop of 1.6v and that's with all 25A flowing through it, probably much less at 6A. If the drop vs current is close to linear mine would dissipate 2.4watts which wasn't anything that captured my attention when feeling the external mounting surface while it was sitting naked on a countertop. And once the pot is up to temp the SSR is only on a small fraction of the time.

I'm sensing this isn't fun anymore. The purpose was to address SSR heat in the context of controlling a casting pot but not to beat it to death. :smile:

ok so you guys are brain prostitutes because your blowing my mind

Mine is bolted to a rather large plate of Aluminum with Arctic Silver heat sink compound behind it (its a high performance heat sink grease used for computer CPU's).

http://i885.photobucket.com/albums/a...ps9ddd4bf4.jpg

I've been following this thread with interest. What sort of temp. rating does the PID have to have? Some that I've looked at seem to be a bit low.

Most e-bay pids will go up to 1200F.. it totally depends on the type of thermocouple. K-types are the most common and are probably the "right ones" for this application. The only ones I know of that would be wrong are RTD's (Resistive Temperature Device) because they can't handle much more than 600F.

The e-bay heatsinks/SSR combos for $10 will work as is (with heatsink paste under them) not attached to anything else. My PID and hotplate is mounted to an aluminum plate but for a long time was all sitting loose on the bench. Not to sweat. If you don't want to understand how much heatsink just get the e-bay "equipment rail" one that everyone sells and be happy.

And yes, I *am* an Electrical Engineer... ;)

Thanks for that, I don't really need a PID controller, just curious. DealeXtreme, mentioned in an earlier post, appears to have the necessary bits at reasonable prices.

Me, I'm just a lowly ex-Telecommunication Techie-type person who likes to know these things [smilie=s:.

Yesterday or the day before, I got that eBay kit. The thermocouple probe is really short, but thermocouples are cheap. I might be able to use that one on a smoker. I'm impressed with their shipping speed.

just my experience.
I dabble with desktop computers fixing and robbing enough parts out of the old junk ones to keep to make others running. As a consequence I have over time acquired a few odd parts and pieces.

I have built so far, 3 PID controllers, I use old computer power supply boxes, keeping the plugin, switch and cord. A little time with a Dremel tool and file will fit most everything together.
I purchased most of the other parts off Fleabay. The last one cost (Mypin PID, Thermocouple. and SSR 40A) all of 48 bucks. The heat sink was off and old cpu processor chip that I modified a little to fit the case. I used a little of the Artic Silver heat sink paste to the SSR and heat sink for conductivity and haven't had any problems at all. I did add a 110V plugin to the case so I can plug the pot directly to my PID and also a plug in for the termocouple as I want to be able to interchange any of them to either my lubesizer or lead pot. My biggest problem was the one PID that I ordered was C only with no feature to change it to F.

I have been casting more uniform bullets with the pot controlled by PID than ever before.

Actually thanks for the idea for using the metal boxes from computer supplies.. I have a few that should have gone to the landfill a long time ago, and I might finally box my PID up on one of those.. great idea.

Three catches on looking these up for people on PM's who have asked so far that have not been obvious so far. The "REX" pids are deg C only and that can be an issue for people (including me personally).. those are the cheapest e-bay pids at $13-14... it's becoming clear they should be avoided for the C read out only and the fact that a lot of places claim up to 15% or so DOA. Also, a lot of the "kits" on e-bay that use "Mypin" controllers come with the TD4-SNR and not TA4-SNR.. the TD4's won't autotune, which means set up of the PID completely manually.. .. it can be done.. but *I* wouldn't want to. The third issue is relay outputs.. They can be "undone" for SSR outputs.. but that's not desirable either. Not at all worth the effort. Vital that the output is SSR. "Alarm" outputs don't need to be because you won't use them.. but the "process outputs" need to be SSR.

The TA4-SNR's are $25+...and should be the minimum standard for these boxes...

So -- again.. since I just got snipped at by Frozone... and he's at least not wrong to do that to me... not really right either, IMHO.. the $200 units are nicely packaged and there is no way I would sell a packaged up one at that price.. so if you desire to not to learn about pids, by all means go that way...I'll recommend his units for those who have no desire to learn about pids or take 2 or more hours to properly box one up. (I should myself but have not.) They seem pretty nice...

If you want to roll your own.. I'm going to say the autotuning Mypins off of e-bay are the lowest quality ones that are okay.. and.. the 40A SSR (overdoing the SSR is usually a good thing) and bead "K" thermocouples are they way I'd go.. you need to be careful to avoid the three issues I mentioned above... Even the Mypins need to have "D" turned on in the menus before the autotune. That's all done for you in the built up units for a quite reasonable amount of dollars for the time spent on the Frozone units...