Is there a good reason to use an SSR instead of a relay on a PID? Sure, an SSR will not wear out with normal use, a relay will wear out over time, but it will take a long time to do.
A relay generates pretty well no heat, an SSR generates heat that you need a heat sink to dissipate depending on the size of the pot it is powering.
Is there a reason i have missed why i should use an SSR over a relay? i have 240v AC coil relays, easily hooked up to the relay outputs on the PID.
Relays will burn out in short order. They aren't designed to take the cycling that a PID puts out and a SSR can handle. A relay will work but will eventually fail. Personally I don't want to hear them opening and closing all the time as well.
That is what i figured too, the contacts have a life span, an SSR ideally can switch till dooms day.
I'll have to see if my setup will allow for the addition of an SSR, biggest issue is where to put a heat sink..... The case is enclosed, so the heat will have no where to go.
What sort of case? If metal, just bolt the SSR to the case (and drill a few vent holes). If plastic, like mine, I just fitted whatever heat sink bits I had that would fit and drilled some cross-flow ventilation holes in the sides.
I'm not sure I believe this. A regular old relay can handle many many thousands cycles and are used in PID controlled furnaces all the time. Now, they do make noise.
With the Rex C100's I have purchased cheaply from Ebay, it is also a matter of the relay contacts not being able to deliver the current needed to fire up element of a electric casting furnace like the RCBS Pro-Melt.
So changing over to a SSR is an relatively easy solution to this problem, and yes, it is also quiet when switching on and off.
Isn't the Rex c100's relay contacts rated for 5 amps? Too bad, thats almost enough to melt lead.
You have to use a relay 'rated' for the load it controls. Properly sized they will provide service for the life of your pot. If there is a 'short' circuit where many amps rush through they may or may not fail before the circuit breaker trips but they are cheaply replaceable, in that way they provide 'isolated' switching without having a fault current go through your solid state components ruining the entire package. Some relay models have replaceable contacts. Some 20 amp rated 'ice cube' styles are almost completely silent. Don't load your relay more than 80% of its contact rating.
In machine motor control panels I built I used to always use isolation relays between the solid state control and the fractional HP motors having internal thermal protection, heating elements or other heavy current items for the above reasons. We used isolation relays for the actual 'control' circuits on board the machine because of accidental damage of control panels or limit switches etc. resulting in high current shorts. It was my design to keep high fault currents out of the solid state controls because when they melt down it usually involves more than replacing a simple SSR.
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There was a member, now banned that was making PIDs with a relay in them. I did hear of several of these going out.
When my pot or oven is up to temperature the PID cycles about once a second or so. I have cast for 8 hours at a time so I don't know how long the relay would last.
The cheap Chinese SSRs do not last forever either. I have a couple not work from the start and had a batch of 25a SSRs that only last several sessions. The 40a SSRs seem to last longer. I have about 3 years on a couple of them.
The Rex unit is labeled for either a relay or SSR so make sure you match them up as one marked for an SSR will not power a relay and one marked for relay will not power an SSR. I did buy a couple that were mis-marked and had me scratching my head for a bit.
Generally temp. controls have a 'cut in' and 'cut out' setting providing a certain degree of spread there. Say for instance I want it to cut in and heat up to 650 degrees and then cut out. Then it will not cut back in until the temp drops down to say 640 degrees. That way it will not sit there and burn itself out switching in and out trying to stay on an exact temp. I think that's the problem you are experiencing there.Originally Posted by leadman
Air compressors use different controls but the idea is the same. My compressor cuts out at 125 lbs/sq/in, PSI. It will not cut in, restart until it drops down to about 112 PSI or so, that is called a 'differential'. It's adjustable. I think your PID should be the same…of course I might be talking out of my butt as I don't have a fancy control on my pot…yet!
Last edited by OS OK; 02-23-2016 at 05:32 AM.
a m e r i c a n p r a v d a
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It's been mentioned here in other threads - just because the label rates the (PID) internal relay at 5 amps (for example) doesn't mean it can actually handle that load! The common consensus seems to be that those internal relays are probably over-rated somewhat (advertising zeal!). They should only be used to drive an external, heavy duty relay if you want to switch a pot's heating element.
SSR's are generally used because, for most people, they provide simpler wiring requirements. Plus they're cheap enough that it pays to keep a couple of spares around.
About the worst thing that can happen by using a relay is the contacts can weld closed and not break. This would cause it to heat up until the unit either burned out, tripped a breaker, or gets unplugged. Not very likely but it could happen.
Whenever you make ore brake a electrical load with an air gap there is a small arc that arc erodes the contact some that is why relays are rated for booth amps and cycles. A pid will cycle a relay on and off quite a bit and kill it in short order.
PIDs are attempting to maintain to a specific setpoint. There is no cut in and cut out. We aren't talking a well pump here.
You can absolutely use a conventional relay. But it's not a matter of if but when it will fail. They have a lifespan that is a lot shorter than a SSR. I'm still on the original SSR and going on five years. W/ how quickly the PID cycles I would have had to replace a conventional relay by now.
I have to pull the BS cord. Relays will work fine. Anyone here ever run machines? I ran machines for years in a factory and guess what? They had a bank of relays in them and this was before the age of SS relays and logic. Want to talk cycles? I would hate to try to figure out the cycles because the number of cycles times a single years would scare you. The simple plane fact is either will work. Ever wonder what is inside that SS relay? Its solid state which means its internals which I will not bore you with also have a life cycle. Each either a SS relay or a mechanical relay HAVE a known life in cycles. One makes a click and one is silent. The trick is to pick a relay that is capable of the load it controls. And NO a PID will not kill a relay in short order. Stupidity however in design will. I puchased a bullet caster a few years back and its name and model isn't important. The problem I encountered with this unit were staggering. It ran for a whole 4 hours before the PID went kaput. I ended up re-doing the complete control system a complete re-wire on it before it was 100% usable. I also had to install a fan to cool the SS relay in order to obtain workability. I finally set the control unit aside and designed an old fashoned relay controled PID setup and it hasn't missed a beat. This is because I understand electronics and control. Now, I still have the original control box and its workable but I have also proven that the old fashoned relay control system works well also. I have about 20K cycles on the relay and it still functions correctly and no burned contacts to date.
Just to figure a years worth of cycles on one machine It used to run 1,440,000 cycles a year based on production for three shifts and a years production. Since I also kept track of maintenance on the machines in my area I can also tell you that some ran over 5 years at this rate before any relays were changed.
The life cycles are listed w/ the relays and SSRs. Mechanical relay lifespans are small in comparison. There is no doubt they will work. No one is saying otherwise.
i have let out the magic smoke - lots of smoke- from one SSR - the pot was dripping and lead eventually shorted the heating element.
had no circuit breaker either,but PID did not get damaged in any way.
IIRC i read somewhere that SSR's are optically coupled,therefore they cannot damage the controller when they fail.
I agree a relay will work. I work in a pvc compounding plant, the largest in North America as a matter of fact. We don't use relays in our TCU's we use contactors, which is just a big relay with 480 vac running through it. Most or the larger ones are either size 4 Allen Bradley or size 3 Benshaw. If you know much about electrical you'll know what I'm talking about, the smallest ones use Allen Bradley C43's and C60's.
So far in the last 4 years I've had the pleasure of rewiring 3 TCU's due to a stuck contactor. My best advice is if you use a relay go oversize and install some kind of failsafe.
Mechanical relays and SSR's work. Each are different and with the differences comes pro's and con's. SSR's turn on and off when the voltage on the switch leg crosses 0 volts. The relays just cycle but because they do not turn off at the 0 volt level, arcing occurs. The arcing causes contact failure. The SSR is sensitive to spikes and it can be damaged within a fraction of a voltage cycle (60 hertz). With a resistive load (heaters) there is no need to choose a relay that has a contact rating that is greater than the current draw.
If you choose the SSR, it can be mounted to a metal surface but it is advised to use a thermal compound between the back and the metal.
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