Ssr Advice Please

[Thirsty Boy]

I'm looking to build a couple of PID temp controllers for my system and general temp control duty. I'm comfortable with the PID side of things and have the units already - but so far i have been running my PIDs via manual relays not SSRs.

I need to buy an SSR or two. I plan to run a maximum of 10A through them in general, but its possible i might one day want them to handle 20A. So i'm thinking i will get a 40A SSR and be 100% sure i am always way insode the limits. i am also hoping that buy being so far inside the capacity of the unit, i might not necessarily need a big heat sink on it and will be able to dissipate the heat through direct contact with the wall of the enclosure.

I would just like the opinion of someone who knows for sure - is there any particular disadvantage to running a 40A ssr when i really inly "need" a 10A? And will i be able to do without heat sink?

This is the sort of unit i am looking to buy


http://www.ebay.com.au/itm/250V-40A-Temper...7#ht_2573wt_962

Opinions??

Thanks TB

 

[roller997]

TB,
There is no practical disadvantage to using a 40Amp SSR instead of a 10Amp SSR.
I have gone the same way to control 9 Amp elements.
In the past there was a pricing difference between the higher rated SSR's and the lower rated ones but these days with all these items being produced so cheaply in China, you may as well go to a larger SSR.
What does save them from burning out is to attach a heat sink to dissipate any heat when switching larger loads.
There are some deals going on Ebay where they sell a combo kit of 40Amp SSR with the proper heatsink.

Cheers

Roller

 

[glaab]

it might last a while without a heatsink but generally the bigger and uglier the heatsink the longer the life of the device. use some heat transfer compound too from rs or jaycar etc

 

[dent]

The data sheet for the SSR you are looking at claims it requires a heatsink 50mm long for a 10A load - so I guess matching the size of the metal back of the unit. Really though if you cram the SSR+heatsink in a box it isn't going to be as good as if you bolted the SSR on the inside of a metal enclosure, for instance. The claimed 1.6 volt drop across the device at full load is only 16W at 10 amps anyhow.

 

[Thirsty Boy]

Thanks guys - i think, because i want to keep the controller units quite compact, i might try to get away without the heat sink (Understanding that this will reduce the life of the unit) But given that they are pretty damn cheap in the first place and i am talking about occasional use devices - i dont mind a bit of a lifetime decrease, unless it is going to be spectacular.

Any clues as to how much reduction in working hours I could expect... And whether they fail in some sort of dramatic "burst into flames" type way if allowed to get hotter than optimal?

If i churn through an ssr or two... I will call it a lesson learned and stick on a heat sink, as long as its not just straight out "it'll only last a week" type folly or dangerous in some way.

Cheers

TB

 

[newguy]

For my 1st generation HERMS I bolted the SSR directly to the aluminum enclosure. Although the enclosure got warm, it never got hot. If I had to guess I'd say it would get to ~35C at most. The SSR is rated at 20A/240V and it was switching a 1500W 120V heating element. For a 9A/240V load, your heat sink will get stinking hot if you don't have it bolted to a heat sink.

My 2nd generation HERMS has the SSRs mounted on large heat sinks, with a push/pull pair of fans providing forced air ventilation.

Regarding how/if it will fail, the warmer you let something get, the more you're living on borrowed time. It can fail as an open circuit or it can fail as a dead short, or it can fail as a dead short between the high voltage side and the low voltage (drive) side, which is potentially very dangerous. Anything is really possible and should be accounted for, just in case the worst case does occur. It's safer (literally and figuratively) to start with a large heat sink and move to a smaller one rather than starting with a small (or no) heat sink and moving up in terms of heat sink size.

And flames are a possibility too. As are explosions. Yes, I'm serious: explosions. If a component gets hot enough, it can boil chemicals in the case or gas in the case can pressurize and eventually the case can rupture in spectacular fashion. Electrolytic or tantalum capacitors can fail in this fashion (they actually sound like a gun being fired), and I've personally seen integrated circuits do so as well. The gases released when a component fails in this fashion aren't generally conducive to life.

I also recommend getting your SSRs from a reputable supplier, just in case. This is the SSR I personally use and I can vouch for its suitability in a HERMS.

 

[MaltyHops]

...
The SSR is rated at 20A/240V and it was switching a 1500W 120V heating element. For a 9A/240V load,
your heat sink will get stinking hot if you don't have it bolted to a heat sink.
...

Wouldn't how hot an ssr gets depend on mainly the load it's driving? Your 1500W
element would need 12.5A at 120V (Power=Current x Voltage) whereas for the
same rated element at 240V, it will be running 6.25A. Thanks for the heads up
warnings though.

I've just put together a PID box containing a 20A SSR with matching heatsink
from Auberins (all enclosed) and am hoping it wont get too hot to need exposing
the heatsink to the outside - am trying to maintain the double insulation levels
- yet to be tested.

I've also wired the output (for a 2200W heat element) to get power via the SSR
(and PID control) and also via a manual switch in parallel.

I figure that I could then do the bulk of the initial heating up of water from cold to
strike temperature without going through the SSR. And once the PID/SSR is needed
to control the heating for reaching/maintaining mash , mashout and boil temperatures
the amount of heating will be much lower relatively so the SSR would be used less
overall. Will wait to see how this turns out.

T.

 

[dent]

The temperature of the SSR comes down to heat coming in and heat coming out. The heat coming in is proportional to the current flow through the SSR (1.6V drop times current = watts). It only dissipates heat only when it is turned on. The heat coming out is proportional to the temperature of the device (since the heat dissipation increases with the temperature differential), and the thermal resistance of whatever the SSR is mounted on. The thermal resistance of a heatsink is often quoted in terms of degrees per watt - this rating is only in open air, which can convect, so it will be much degraded if it is enclosed. You can calculate the theoretical temperature of the device with X heatsink in open air using the degrees per watt figure of the heatsink with the expected 16W of max disspation of the SSR.

Really though the 16W of maximum dissipation you are expecting isn't that much.

 

[Thirsty Boy]

Thanks gents - still inclined to go without the heat sink initially and see how it goes - but will take the warnings on board and most certainly keep a close eye on it for a few trial runs. If things get hotter than I can comfortably hold in my hands, I will just ditch the SSR paradigm and stick with mechanical relays that aren't so finicky.

 

[newguy]

Actually SSRs are more robust/less finicky than mechanical relays in my experience. A mechanical relay has a fixed lifetime expressed in the number of switching operations it can perform. This number is a ballpark estimate of the actual number of switching operations it can endure. Each switching of the contacts will degrade their coating. Over time, the contacts can get oxidised in which case they cease being a good conductor altogether. Conversely, they can also fuse together so that the relay becomes a dead short. The same thing can happen with an ordinary light switch too.

Really though the 16W of maximum dissipation you are expecting isn't that much.

Depends on the size of the object dissipating that amount of power. I've burned the perfect mirror image of an 8 pin SOIC op-amp (in the plastic package) into the tip of my finger, and it was only consuming about a watt. One of the things you learn quickly as an electrical engineer is not to try touching things on a board when you smell something hot/burning.

TB: Even if you mount the SSR to the side of your enclosure (best if it's aluminum but steel is better than nothing), it will help to keep the thing cool. Smear thermal paste on the back of the SSR to aid the heat transfer. Less is better with that stuff.

 

[zxhoon]

What type of element are you using? The resistance shouldn't change, unless its silicone carbide or moly or something like that (been a while since I did heating elements) so if you double the voltage on it, you will double the current, not halve it... power output will go way up...

or is it too early for me on a sunday?

 

[raven19]

TB, I run a larger than needed SSR but use a heatsink also in my RIMS Control Box. Its been working a treat with minimal heat generated even during hot brew days.

Can't comment on the finder details though, but some great readings above from others imo.

 

[kirem]

my opinion is to go with a heatsink. It'll be a shit brewday if the SSR fails when needed most

 

[Thirsty Boy]

For your peace of mind, this is not primarily for my brewhouse. My brewhouse runs on mehanical relays for a couple of specific reasons due to my power availabilty. These controllers will be for running the heating elements I use in my fermentation control and for occasional use as sous-vide/smoker/koji growing controllers and will spend the vast majority of their lives switching loads of less than 500W. Occasionally they will venture up to 1200W and they might conceivably get use at 2400W every now and again. i shall give away the notion of them every switching higher current loads and accept that I will need to build enclosures with a chunky heat sink should I every require that sort of capacity.

I shall (carefully) test them without heat sinking - aware that it might well not work out - and if it doesn't go to plan, I shall just run with mechanical relays as i do with my other PIDs.

Thanks for all your help.

TB

 

[sluggerdog]

How did you go with this? I am thinking about something similar myself. For sous vide.

 

[Zizzle]

Only just noticed this thread.

Dent is on the money.

Silicon junctions tend to fail at temperatures up around 150 degrees.

But don't forget there is thermal resistance between the actual junction and where your hand is.

Look here, the 40A and 25A versions drop the same amount of voltage:

http://www.multice.com/bp/Solid%20State%20.../SSR-40DA-H.htm

If you are switching 10A then 16W of dissipation is needed no matter which one.

So the 40A version wouldn't last any longer than the 25A version if the heat sink isn't big enough to keep the junction temperature reasonable.

I have a mate who is an industrial sparky. He has sent me pictures of the results of these things exploding. Not pretty.

Consider using a computer CPU heatsink with fan. They are compact, cheap, easy to get and designed for devices that produce up over 100W.