As the subject says, what relay are you using?

Mark

I am setting up an appleseed GL and will be using a solid state relay for the heating element. Mainly because I already have one from a previous project. You do have to remove the snubbing resistor across the output of your pic, as sometimes these will pass enough current to trigger the SSR.

A simpler item is a mercury relay. These use a slug to displace mercury to cover a set of contacts. They are pretty much immune to welding the contacts, and are sealed so there is no arc.
I have use mercury relays to control my lab furnace at work. The furnace currently uses a SSR with great success.

Thank you. What other considerations are there?

Most people I"ve seen use PID-controlled methanol recovery systems use whatever relay they've had lying around. I'm designing one from the ground up- ie, new parts, supposed to be easily replicated. What considerations should I take into account on the relay side of things?

Mark

Mark,

I am currently using a 40A contactor with my PID. This is after cooking my SSR relay due to too small of a heat sink. The factory heatsinking for an SSR is rather large and can be difficult to fit in the box. It seems the heat sinking is less of an issue if you never use the auto tune feature on the PID. In auto tune the the SSR is on for a conciderable length of time and even at less than rated amperage they over heat easily. If using a contactor be aware that ther is a setting on most PID's for contact duration. If set for SSR it seems to be about 1 or 2 seconds but on mine can be set up to 99 seconds, if using a mechanical contactor a setting of about 5 to 10 seconds seems to solve the contact welding issue.

Looking forward to the Jan. class in Tampa!

Tony

Thanks.

From an energy use perspective, how does a contactor versus an SSR versus an HVAC relay versus whatever other options there are all stack up? lifespan? Sparks and safety? Any other things I'm missing here?

I guess I forgot costs?

With your normal open-contaact relay (usually called a "contactor" when used with motors) you are going to get sparks at the contacts, this could be a problem if there is flammable vapor in the area, I would rather not use these around alcohol do to the possibility of igniting the vapor, not likely, but definatly a possibility.

Contactors are expensive if bought localy from your home improvement store, definitely expensive if bought from a commerciel supplier. There are almost always a huge selection of both new and used contactors on ebay, even the new ones go for a reasonable price, most don't even draw any bids. these will still cost $5 - $10 bucks plus another 7-10 for shipping.

Solid State Relay -- These don't make any sparks, and switch silently, and very fast, but they DO make a fair amount of heat when switching higher amps for an extended time (hours). These will drop about 1 1/2 to 2 volts across the solid state relay, heat is expressed as "watts", watts is calculated by multiplying the voltage drop across the relay multiplied by the amps flowing through it, with a 1 1/2 volt drop across the SSR, and flowing 15 amps, it will create just under 25 watts of heat, that is a LOT of heat that must be continuously removed from the solid state relay so a pretty big heat sink must be used, mount the SSR to a foot square piece of 1/8 inch thick aluminum, plus a fan, type thing, even then it will likly still run pretty warm. If it gets too hot to hold on to for more than a couple seconds, make the heat sink bigger.

What ever type power switch you use the PID will likely turn it on and keep it on for however many hours it takes to get your tank of oil up to the preset temp, then it will start to cycle. You have to design the system for these "continuous-on" situations.

Mercury contactors - These all make sparks at the contacts but some have the contact area totally sealed, some don't. the sealed contact type keep any vapor from getting to the sparks, unfortunatly, these are EXPENSIVE.

The less expensive "Wetted contact" type mercury relays are like dry contact contactors but have small cups of mercury around the contacts, this keeps the contacts from welding together but they are not totaly sealed so vapor can get to the sparks, they also must be mounted in usually only one orientation. If buying used ones of these you are not gauranteed that the mercury has not drained from the contact area over time.

Mercury relays were used extensivly to switch power on airplanes in the past (before maybe mid 1960's) so they can usually be found at aircraft salvage yards (mojavy desert or Davis monthan AFB, Tucson AZ type military salvage yard places). These are fun places to shop even if you don't find a relay. These usually need 24 volts DC to switch them.

Different type SSR's -- They all look like 2 inch by 2 inch lumps with 4 places to connect wires but the cheaper ones of these do NOT do what is known as a "zero-crossing" type of switching, these non-zero switching units turn on as soon as the PID tells them to, this may be during any portion of the 60 cycle AC power cycle, if they turn on when the AC voltage is high they will create a bit of radio frequency static ( radiated energy, not a spark) these pulses of static can be heard in a radio or tv, they also can cause electronic equipment to crash, may cause the PID to twitch a bit , or cause your laptop to crash, not likely, but possible, switching more amps will create more static. The other type of SSR's have "zero crossing" circuitry built in. If they use "zero switching" they will say it in there data sheet (do a web search of the manufacturers web site, you should be able to download the data sheet for free). These look the same as the somewhat cheaper ones but the PID control signal is more of a "suggestion" to turn on rather than an instantanious command, the pid says "turn on" but the SSR waits for up to 1/120 th of a second until the AC voltage drops to zero during its normal AC cycle, THEN it turns on, this eliminates the static because the switching only happens when the voltage is at it's lowest point of the AC cycle. These will cost maybe another $10.00 over the cost of the "non-zero" type SSR's.

Either of these are usually available from equipment salvage type places, they are used in a LOT of commercial equipment that ends up being salvaged, used SSR's will cost $15.00 verses $40.00 new. There are many neat electronic or electrical salvage places around the SF bay area, or most larger cities, do a web search for electronic salvage plus your local area name, someplace will likely show up. These are also on Ebay or from surplus places on the net, C & H sales (here) ,in Pasadena Calif, has several to select from. Mouser.com or digikey.com have them new. allelectronics.com have used ones that might work, they don't have the specs for them so are selling them cheap at $3.50 each, the picture they show looks like the normal 10-25 amp 110-220 volt units but it is a gamble, order one and see if it works, problem is they may be out of stock when you try to re-order, surplus is always a gamble.

Thank you thank you thank you thank you!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

I was just flipping through a Jameco electronic supply catalog, they indicate they stock new SSR's on there web sales page, jameco.com. Jameco is sort of half way between a "commercial-only" seller and a "hobby" seller, there prices are usually a bit cheaper than the "commercial only" sellers, they also wave there additional "under $25.00 small order" fee if ordered over the web.

Tim,
Do you know what the surface temp requirement is for those SSR's?

I'm thinking a 128 deg steel biodiesel tank would pull away the heat fairly well.

Or do they need to operate lower than that?

This links to a data sheet for a Crydom 25 amp AC SSR, looking at the current derating curves seems to indicate the absolute max temp allowed for baseplate of the SSR is 120 deg C (248 F), to use the entire 25 amps the max temp is listed as 100 deg c (212 F).

Max "full amps" temp for the 50 amp unit is also aroung 100 deg c, 90 amp unit is about 90 deg c.

The baseplate of these SSR's is electrically isolated from any of the electrical connections so can be mounted to anything that is capable of drawing away the heat. The chart indicates that for this specific SSR the max heat developed when flowing 25 amps is only 25 watts, should not be a big problem to remove this much heat, mounting it to about anything big, like a tank, should work fine, as long as the tanks temp is cool enough to create a transfer of the heat.

Round tanks may not allow a 100% surface contact due to there radius so this must be considered, mounting the SSR flat against a flat plate that is welded to the tank still does not give all that good of a heat transfer to the surfac of the tank, have to try it and see? The SSR won't overheat instantly so you will have a few minutes to test the heat transfer situation before the heat will damage anything.

Worth noting that SSRs don't tolerate short circuits of the output well - they will be instantly destroyed if connected to a dead-shorted output. The only way to protect them is with a very fast fuse designed for semiconductor use, which is likely to cost the same as the SSR. Personally, I am taking the risk, but if my heating element goes up in smoke and shorts, so will my SSR. I'm using an Omron G3NA-220B with heatsink to drive a 3KW heater at 230V.

All the best

Pete

Heat sink info -- Just to get a comparison of the size of heat sink required for different amount of amps. This links to a data sheet sold as a companion for the Crydom SSR's, one is sized for the SSR's up to 25 amps, one is for up to 50 amps. The 50 amp unit is about 6 inches square with a LOT of fins.

I gotta ask this question,

What is it that you folks see in SSR's that make them so attractive you spend all that extra cash for them over a standard relay?

I honestly don't get it.

No arks or sparks to light off flammable vapors.

No mechanical contacts to slowly burn up due to arking.

No contacts to possibly become welded together.

Buy them from surplus venders and they are cheaper than relays or contactors.

With the right temp controller you can switch them on for as little as one 120th of a second as often as is needed.

No clicking.

They will turn on using any DC voltage between 3 and 32 volts.

They only need about 2/1000 of an amp from the control circuitry to turn them on.

The input control voltage is totaly electrically isolated from the output power circuitry using optical couplers inside the SSR.

Thank you guys again for hashing these pros and cons out.

Mark

quote:

Originally posted by Tim c cook:
No arks or sparks to light off flammable vapors.

No mechanical contacts to slowly burn up due to arking.

No contacts to possibly become welded together.

Buy them from surplus venders and they are cheaper than relays or contactors.

With the right temp controller you can switch them on for as little as one 120th of a second as often as is needed.

No clicking.

They will turn on using any DC voltage between 3 and 32 volts.

They only need about 2/1000 of an amp from the control circuitry to turn them on.

The input control voltage is totaly electrically isolated from the output power circuitry using optical couplers inside the SSR.

While I understand the features of the SSR's, most of what you listed is non-applicable to use in an appleseed processor.

Worried about arcing contacts? Isnt that a bit like eating a Big Mac and drinking a diet coke because you're concerned about calories?

In my humble opnion, the SSR's have a purpose for specific applications but making biodiesel with an appleseed isnt one of them.

But when you're intentionally boiling up a bunch of explosive gas and your condensor doesn't work as planned (ie R&D/tinkering phase of getting the still going, especially people's tendency to collect methanol in plastic carboys that aren't really easy to make fully sealed to a vent line), then having sparky things hanging out mingling with oxygen in your reactor room doesnt' seem like a good idea. Especially when you are tempted to leave it alone for hours on end, which a PID system allows you to do.

Then there's a certain engineer on this forum whose sparky relay is UNDERNEATH his reactor in his garage while he does unattended methanol recovery. OK, he's got it ventilated so it shouldn't have methanol/air vapor under the reactor, but still...

I understand your point of view but it goes back to the Big Mac/diet coke thing.

There are so many sources of arcing in your shop that a pair of relays isnt going to make a difference either way.

The engineering equivelent of using an umbrella in a hurricaine. It wont make any difference.

Everytime you flip a light switch, the air conditioner, the furnace or boiler, the coke machine, the very door you enter and exit the shop, the pump, and every time you plug or unplug something from the wall.... All these and more are sources of arcing. Eliminate all of them, or design so you don't have to waste your time with any of them.

I guess my point is that an extra arc in your shop isnt going to make a beans worth of difference.

I weld next to my biodiesel processor all the time. Yup.. Sparks flying everywhere while the thing is running.
The reactor is completely fumeless so I'm not worried about that. However, if something else is going wrong, I'd rather have a welder spark make it go "poof" than to let the entire building fill with fumes and have it go "KaBOOM".
(In my humble opinion, Poof is always better than KaBOOM) But that's just my opinion.

Anyhow, I understand your reasoning and normally agree, but I think the logic is flawed in this particular instance.

Murphy,
One less spark can't possibly hurt.

Mark,
In an environment where we strive to keep our health intact (minimizing fumes) will there be an explosive concentration of fumes?

I have used both the SSR and the contactor and I can tell you that when an SSR goes south there is an impressive amount of sparks. Controls should be in an enclosure and this should minimize their exposure in and of itself. My processor is fumeless (GL) and I use a contactor, I don't think endangers me in any way.

An SSR , judging by the size of heat sink, generates alot of heat. Putting one or two of these in an enclosure and fan cooling may be necessary. Mount them to the outside solves this but in the case of a failure still leaves you with sparks.

So it all comes down to which flavor you like IMHO.

Tony