Super-cheap DC-SSR

[dirknerkle]

I was noodling some ideas about making a super-cheap DC SSR and came up with this idea...

uln_dc_ssr.png

The concept uses an ULN2804A as the driver. It can handle up to 50volts which seems to take care of most things we do with DC and LEDs -- at least single LEDs and some strips. Each of the eight output pins can handle 500ma continuous current; peaks up to 600ma. And you can use two or more outputs together to multiply the current capability.

Also, using header pins and Dupont connection wires would provide selectivity with the input channels to the ULN2804A, and on the output side of the chip, the assignment of the desired channels to the terminal blocks (lights). Selectivity might be helpful in cases where one output was overloaded or shorted, killing that output on the chip; simply set the Dupont wire to a different input on the ULN and the terminal block to the same respective output. In a pinch, the ULN2804 could be replaced with a ULN2803, ULN2802 or ULN2801 if need be, but the 2804 can carry more juice.

The RJ45 jack would be pinned for normal DIY 4-channel SSR use with +5v on pin 1, pins 2-4-6-8 for the channels and pin 7 ground, back to the controller. Since the ULN2804 inverts the polarity (+v input results in -V output), you'd want to take that into consideration in your wiring.

Tayda's current pricing for this design:
ULN2804 $.81
RJ45 jack $1.60
2-conductor terminal blocks are either $.12 or $.15 each (total $.72 - $.90)
1 40-pin single row of breakapart headers is $.15
18-pin DIP socket is $.06
A pack of 40 200mm jumper wires is $.90

It looks like you could build this thing for under $5 and you'd have parts left over. This could be super-compact, too and if you wired it directly instead of using header pins, jumper wires, and terminal blocks, it could be like an extension of the cat5 cable and be cheaper yet!

And if you're wondering, no, I don't plan on having PCBs made for this. The postage to send it out would be about 40x more than the cost of the PCB. You could easily put this together on a piece of cardboard or tagboard - punch holes with a nail, fit the parts in, solder it on the bottom side and you're good to go.

[P. Short]

Beware of power dissipation limits on the devices...saturation voltage is over 1V at 350 mA, so the package would be dissipating above 2.8W if all eight channels are driving current at 350 mA each. The temperature rise (junction to ambient) is 55C/W, which would drive the junction temperature to 154C above ambient...and the junction temperature limit is 125C. They are good parts, but I'd be leery about pushing the per/channel current without taking the junction temperature limits into account.

[dirknerkle]

Yes, of course. The example was a 4-channel unit, doubling the outputs for the same current draw would lighten the load though, and the benefit is that it's a single chip. One could also glue a heatsink on the flat top of the chip, too. In applications that I use a few LEDs, typically I only need one or two channels and it's a pretty simple way to create a DC SSR. It also think it would work well with DirkCheapStrobes, which use a simple 20ms blip of current to "strobe" the 1W star LEDs. In that kind of usage, it likely would never get hot.

In making this, I was also trying to demonstrate that there are many ways to light an LED and one shouldn't necessarily get locked into one SSR design.

[P. Short]

It is a nice concept and design. I wish more people were doing this sort of thinking, rather than just following the herd and using completely off-the-shelf components. That sort of interaction and ingenuity was what was fun back before 2010 or so.

[dirknerkle]

Yes, it was. And I'm still trying to keep that spirit alive. It's why I started up the DIGWDF Store and made all those crazy inventions -- to get people thinking and inventing. I wish there was more of that kind of activity too. (.....sigh.....)

[n1ist]

I would grow the traces between the power input connector and the ground on the chip, and to the output connectors. They are quite thin now. You can also loop the trace from pin 10 to the V+ input over the top of the connectors to make this a single side design.

/mike

[dirknerkle]

I would grow the traces between the power input connector and the ground on the chip, and to the output connectors. They are quite thin now. You can also loop the trace from pin 10 to the V+ input over the top of the connectors to make this a single side design.

/mike

Yes, certainly. This layout was just a sort of visual aid to demonstrate the connections and not intended to be the actual PCB. I've found that posting the schematic of a circuit often loses many people who are new and perhaps don't know how to read a schematic. So when I post a simple visual example like this, most every diy'er can read, understand and actually make the connections. Schematics can come later.