Why use an SSR instead of a relay on a PID?

[Mike W1]

Those old telephone relays were for VERY low current and ~70vdc.....and would not work on a 10-15A lead pot. Contacts did not usually corrode, but could not handle any current either. And the coils would not work with the outputs of those cheap controllers everybody uses.

banger

Actually when I worked in the switchroom the voltage was 48 VDC and the contacts were valuable enough that we clipped the ends off the old relays and they were recycled. Think my spare SSR was about $3 shipped so that's a no-brainer as to what to use.

[dikman]

True, most of those relays weren't designed to handle more than probably 3 amps, but as Mike said many had precious metal contacts and also used "wetting" ccts. to reduce arcing. Point is that if a relay is designed properly they are very reliable. But I still wouldn't bother with one.

Tazza, it's easy enough to change the PID over (at least, my Rex C100 was). I opened it up, worked out which relay was the controlling one, desoldered it and installed two links to bypass it. Simple).

[Tazza]

I think i will have a stab at it on the weekend, see if i can avoid letting out the magic smoke inside the PID

[6bg6ga]

If I understand you correctly you wish to parallel two 6A contacts. I wouldn't count on that working. Instead source a relay with with higher amp rating. Most PID's have relay contact that either work all the time or can be programed to work. Use the 3-4 A PID relay contact and run control voltage thru these contacts to activate an external relay that is rated 15A. To recap for you... You could use a 24Volt supply and run this thru the PID relay contacts and from there to your external relay that is rated for the load. The relay points of the external relay will switch the 120 or 220 or what you have in the land down under. Across the external relay coil you will use a 1N4007 diode to take care of the back EMF cause by the coil deactivation. It doesn't have to be a zenier in conjunction with a diode to work just a simple 1N4007 diode and this diode is rated 1A@1000v which is enough to take care of the back EMF pulse.Stay away from the ebay relays and use a control voltage that will work well. A lot of control circuitry is done with 24VDC and it should be simple for you to either build or purchase a 24VDC supply. If you need help PM me and I will send you diagrams.

[6bg6ga]

If you wish a visual indication you can easily run a LED or multiple LED's with a properly sized voltage dropping resistors to give yourself a display that shows cycle on or cycle on and cycle off. As long as you use a good quality relay you will get years of service from a single relay. As mentioned in posts above relays will work. Every control design can have its drawbacks. Some feel SSR's are industructable and granted they can have a long life but if not sinked correctly their life can be short. I used the cut a hole in the box method on a new piece of equipment and found that dispite the fact that I had air moving in the box it wasn't enough to properly cool the SSR. It ended up taking a heat sink 3 time normal size that was fan cooled to work in the enclosed box so as I have said design is everything. I had tried to make an original control box work when I should have scrapped the box and purchased a new one that was bigger that would have allowed one to either heat sink and fan cool an internally mounted sink or externally mount the sink with a fan. I finally did get the control box working without incident but opted to build yet another control box with a relay and use it.

[Tazza]

The paralleled contacts is just an example. The relay i used in my other control box was rated at a suitable current, i just joined them up as i only needed to switch one pole and the other was un-used, i figured it would make the contacts last longer. I didn't do it so i could use a 6a relay instead of a 12a.

[jsizemore]

I think i will have a stab at it on the weekend, see if i can avoid letting out the magic smoke inside the PID

I found a relay with 120 vac control.

[Tazza]

The SSR has been mounted and hooked up. The heat issue is not an issue, when running, it only gets a little hotter than ambient temperature.

After the auto tune, and when not adding ingots, it holds the temperature very steady. I set it to 340c and it stayed there.

I'll get pictures off my phone when i get a few minutes spare.

[Tazza]

It fitted pretty well up in the right corner

[dikman]

Just curious, what else have you got running in that box?

[Mike W1]

Just curious, what else have you got running in that box?

Was wondering that also!

[Tazza]

Sorry, i didn't mention that it was for an automated master caster.

[dudel]

Tazza, if you use a 40 amp SSR it will barely get warm when used with a typical lead pot. I used a 25 amp and have had no problems with minimal heat sinking.
On the subject of relays, prior to digital circuitry telephone exchanges consisted of relay switching - untold thousands and thousands of them, switching almost continuously. Failure rates were extremely low.

Been in one of those. The noise was amazing! I remember the conversion to digital. Once the wire frames were removed, the COs were almost empty.

[el34]

Just curious- has anyone ever felt the SSR to get an idea of heat generated? Theoretically the worst case would be just before it's starting to cycle on-off as it nears the set point.

I have an SSR rated for 25A. Its datasheet says at that current it would drop 1V across it. The pot only pulls about 7A so the SSR shouldn't ever dissipate more than 7 watts (1V x 7A), about the same as an old-fashioned night light. Being nitpicky I put a small fan in my controller box between SSR and the outside but the barely-detectable air flow feels the same temp as the surrounding air.

Once the pot is up to temp the on-off cycling dissipates even less heat since it's off a good part of the time.

[JonB_in_Glencoe]

Just curious- has anyone ever felt the SSR to get an idea of heat generated? Theoretically the worst case would be just before it's starting to cycle on-off as it nears the set point.

I have an SSR rated for 25A. Its datasheet says at that current it would drop 1V across it. The pot only pulls about 7A so the SSR shouldn't ever dissipate more than 7 watts (1V x 7A), about the same as an old-fashioned night light. Being nitpicky I put a small fan in my controller box between SSR and the outside but the barely-detectable air flow feels the same temp as the surrounding air.

Once the pot is up to temp the on-off cycling dissipates even less heat since it's off a good part of the time.

The aluminum enclosure I used, is a two piece. One piece is perforated and makes up 3 sides. The other piece is solid and makes up both ends and the top. I mounted the SSR (25A) to the top (inside). So I can feel the heat generated, by feeling the top (outside). It barely gets warm to the touch during and at the end of the initial 20 minute ramp up to temp...but I've noticed it "IS" warm at the end of a hour long casting. session, so the heat does build.

[Tazza]

I think mine is a 40a one, during initial heat up it just gets warm. I installed a very small heat sink, not sure it was really even needed. When the pot is hot, the heat sink is pretty well room temperature, even with it's cycling on and off to maintain temperature.

From what i read, they dissipate about 1 watt per amp, so really bugger all for the melting pot we are running them on. I had expected quit a bit more loss to heat.

[Mike W1]

I install heat sinks in my units so I don't receive a "Gottcha!". I worked the graveyard shift at the Palm Desert Central Office in 1979, and you could always tell when some radio station was having a dial in contest. Sounded like a company of Tigers was rolling through the Ardennes.

OB

Line Load Control ? used to have to test that once a month but far as I know nobody ever had to activate it.

[dikman]

I only have a 25amp SSR and it barely gets warm.

When we installed a "relay" exchange (Swedish Crossbar system) we had to load test them prior to putting into service and we would listen to the rhythm of the relays as it would often give an indication where to look if there was a problem. The earlier Step by Step switching systems were even noisier!

And yes, dudel, when these were replaced with digital stuff the buildings looked almost empty!!!!

El, always good to hear from you.

[Tazza]

I wonder what the power savings were when they changed over too. The old stuff that took up masses of space with mechanical switches vs digital switching. Also, the cost to repair the newer stuff, i can imagine that the new stuff isn't cheap and you can't just replace one component, you replace a card. Old school, a bad relay, you replaced just the relay.

Gotta love the throw away society we live in now.

[Vann]

I wonder what the power savings were when they changed over too. The old stuff that took up masses of space with mechanical switches vs digital switching. Also, the cost to repair the newer stuff, i can imagine that the new stuff isn't cheap and you can't just replace one component, you replace a card. Old school, a bad relay, you replaced just the relay.

Gotta love the throw away society we live in now.

I can't tell you how much replacing a whole board cost vs just replacing a single relay, I know that in the maintenance field a PLC board cost several hundreds of dollars. The flip side is down time, it can take several hours trying to find a bad relay verses jumping on a computer with Modicon, Allen Bradley, or one of the other PLC programs and being able to find the problem in a matter of minutes. I'd say down time saved by not having to trouble shoot a switch board verses the price of a PLC board that can be sent off and repaired, you'd come out a head with the PLC.