July 11th: Homemade Solid State Relays...

This week while searching for enclosures for this microcontroller car wiring project (that still needs a name), I had the idea to mount the IPS6011 High Side Intelligent Power Switches inside ISO Micro Relay cases. The micro relays have 5 connections and Delphi makes some great modular enclosures to hold them. Once I arrived home from the beach and the IPS6011s had arrived, I purchased a few micro relays to see if they would fit. Short version - they fit with minimal effort, and can convert an existing micro (coil) relay into an IPS (solid state) relay in about 20 minutes with some practice. You can see the ugly results below. I'm sure there is a better way, but this is the result of my first attempt. So far I've switched and PWM'd a 10 amp inductive load (automotive cooling fan) without any issue - with the top reinstalled. PWM switching of a 10 amp load makes these things HOT. I've had it over 200 degrees Fahrenheit which is still 100 degress below the thermal protection threshold. For now, the only heat sink is the existing coil mounting tab which is also the main power terminal.

Here are some assembly pics...

Some other notes: The diagnostic output works great. I'm using the simple version of the diagnostic circuit which requires no components other than the resistor that is in the relay case (and the internal pull-up resistor in the Arduino circuit).

So far I've run everything off the 12amp bench power supply. The next tests will be off a car battery - I need to test some larger loads, and see if I can hit thermal shutdown. Destructive testing is fun - I'll have the fire extinguisher handy :)

UPDATE:

Full size car battery tests - I can run one cooling fan long term (a 10 amp load) and the temperature stays stable at about 125 degrees. Running both fans (in-rush current over 20 amps, then settles to about 15 amps) causes the temperature to skyrocket (even without PWM) and it hits thermal shutdown (I thought there would be a longer off cycle, it seems to go off, then right back on, generating even more heat - I didn't let it keep going...) and the diagnostic pin DOES pickup the issue. I'm wondering if more heat sink would make the difference...I'm tempted to build my aluminum block relay...

No, I haven't figured out why both fans don't seem to draw 2x the current...something is odd...

In my next Digikey order, I think I'll try some other higher-rated power switches. What is nice about my "modular" concept is that I could use inexpensive switches for smaller loads and more expensive switches for larger loads as needed. The IPS6011 were $4.90 each - and these things go up to almost $11 each. When you plan on 30 of them in a system, that cost adds up fast. I think my next part to try is the Infineon BTS50085-1TMB-ND  - $6.95 each, but rated for 44 amps. It is a 7 pin part, but use 4 pins for the output - which makes a lot of sense. They drop the microcontroller ground, which is interesting - but if it works, its one less connection :)

I'm also tempted to try the Infineon BTS555E3146IN-ND - it is $10.15, but rated for 165 amps. I don't know how that's even possible, but if it could PWM my BIG cooling fans (35amps each...) I'd be very willing to spend the $20 for two of them.

Update - July 12th - those BTS50085's are on backorder - DOH! I'll be getting a few of the BTS555s to play with in the mean time. I also made a billet relay case tonight. I've always wanted to make something out of billet, it was very, very time consuming, but came out...pretty nice. I should have taken some pics but I quickly mounted it to my PCB-mounted IPS6011 as a heatsink to see if it would add amperage capacity at a reasonable heat. Turns out, that in a room with some moving air, it stabilized about 180 degrees with both cooling fans as the load. When I stopped all moving air in the room, it climbed slowly to about 200 and stayed there. I think that is still too hot, but I was encouraged to see that a little bit of aluminum went a long way - before I would run them for about 10 minutes before it would go into thermal shutdown. Tonight I went over 30 minutes before I shut it down to get some sleep. More experimentation to come...