hacking a solid state relay

[Ldanielrosa]

There are some projects that may be easier if I had a few SSRs on hand, but between the price of them and the parts I have laying about I'm considering making my own.  I have SCRs good to 50A and 600V with isolated tabs (in rediculous quantity), a few triac optoisolators, and probably all of the other components to make them.


Here are some of the issues I'm curious about.  Will JB-weld be a good potting compound?  I believe it is electrically non-conductive, but it may contain metal dust to improve thermal properties.

I'm pretty sure I'll want to use screw terminals for durability and repetitive connecting/removal.  Will brass (likely more expensive) be desirable to steel after considering strength, solderability, conductivity?  Is there a specific piece of hardware that lends itself well to this?

Since a lot of optoisolators have compatible pinouts with wildly different internals, I was considering leaving a 6 pin DIP socket exposed.  This would allow me to make the relay in advance of deciding upon the driver (zero-crossing detect or not), but would also have a few drawbacks (more exposed mains level metal, a potentially loose part).

edited for spelling

[ante]

Hi daniel,

The circuits are not too hard to build but if you are going to make complete molded blocks with terminals, it’s cheaper and simpler to buy them!

[Ldanielrosa]

Thanks for the fast reply, Ante.

I haven't looked further than a couple online catalogs for price, but they seem to start at $30 each for the molded block type.  I'll definitely have to look on ebay, but the selection there is not necessarily stable.  Anyway, the application that prompted me to consider this will require four of them that can handle 30A/240V.

I've no doubt that the circuit is easy, even if I'm using two SCRs back to back instead of a triac.  The hard part is making it pretty.  You're probably right in the frustration/cost analysis that improvising a mold and working the component layout and optimizing the mechanical hardware may very well take twelve hours plus (at $10 per hour) giving me at best a break even on buying them.

[ante]

Good hunting on ebay! ;)

[MP]

Do you really need 60A? What are you using them for? I use the OPTO22 type for most projects. These are pretty stout but not very expensive.

http://www.opto22.com/documents/0859_Solid_State_Relays_data_sheet.pdf

Good luck on your parts search.

MP

[Ldanielrosa]

Insert Quote
Do you really need 60A? What are you using them for?

I might.  I have a cabinet mounted oven with a failing thermostat.  The appliance shop tells me that the oven is so far outdated that they can't locate a replacement part for it.  Since it is in otherwise good condition I was thinking about using it as an excuse to build a PID controller.

After seeing the leakage specifications on SSRs, I may use mechanical relays.

I use the OPTO22 type for most projects. These are pretty stout but not very expensive.

Thanks for the lead, MP.  Their specifications are fairly clear.  I haven't found their prices to be too different from other manufacturers', I may need to find a different distributor though.


My searches on JB Weld have turned up some interesting discussions.  There seems to be some disagreement on how electrically insulating it is.  The fact that it _is_ a filled epoxy (steel) came up early.  For an SSR it shouldn't matter that the potting compound is magnetically responsive, but that may complicate things for a power supply.


Please keep the input coming.

[ante]

Hi Daniel,

Can you tell us a little bit more about the “cabinet mounted oven with a sailing thermostat” so we know what you are trying to do here. There might be some brilliant ideas out there! ;D

[Ldanielrosa]

I don't know what more there is to tell about the oven.  The thermostat is off by enough that a cooking thermometer has to hang from one of the racks to give a better indication than the setting on the knob.

By cabinet mounted, I mean that it is not in the same enclosure as the range top- that is mounted on the counter a few meters away.

I figured I'd replace the thermostat with a microcontroller, since I'm looking for a cause.  I'll have to use four SSRs because there are two heating elements, and I don't want to leave one half-energized when it's off.  An LCD and six button keypad for human interface, and a MAX6675 to convert the thermocouple to a digital value for the uC.

[ante]

Hi Daniel,

Maybe you could make a regulating circuit instead of the off / on (relay) switching to get a more even temperature. This will probably save some energy too and generate fewer disturbances for other appliances too.

[Ldanielrosa]

That's a good idea too, Ante.

I've already started thinking on the disturbances part.  I'll use a triac driver with zero-crossing circuitry (I have about thirty on hand) to limit the interference.  I'll also use at least one full cycle at a time to balance the load, but I don't think I'll go so far as counting the half cycles for longer periods.

As for saving energy (and safety too) I'll include a time limit, after which the oven will turn off.  I've left the oven on over night a few times.  I'll also have it shut off if it goes far over temperature.


The circuitry inside the little bricks will be simple enough, but finding the right hardware is becoming frustrating.  Standard nuts are too small/shallow and have no cavities for theepoxy to fill for a good grip.  Brass hardware is even more limited, and expensive.  If I could find predrilled brass cubes in the right size, then tapping them would be no problem.

I'd given some thought to using a base plate.  It might simplify a few things, but then I'll also have to cut aluminum rectangles.

Keep the feedback coming- it's making me think.

[ante]

The simple way is to get a square rod of brass, cut it to suitable bits, drill and thread! This piece of brass can be soldered to a PCBs copper surface thus be fixed inside the moulding, done! 8)