I wanted to use a 74hc595 shift register to control 7 SSRs on an 8-channel board.
Most of these relay boards indicate a switching current of 20 mA at 5 V (datasheet for SSR only is below). I think that is control current and not supplied through the separate 5 V power these boards need(?). Initially, I thought the 74hc595 could handle this since it lists 35 mA per output, but now I think that the maximum draw for the whole chip is only 70 mA. So at 20 mA per SSR x 7, it would be exactly double the rated power!
I am not sure I am reading the SSR datasheet correctly. Does it indicate a maximum rating of 20 mA and that a minimum of 7 mA is needed to activate the SSR?
Can I limit the total switching current to below 70 mA using resistors? I tested a single SSR with a 510 Ohm resistor and it still works.... so at 5 V / 510 Ohms = 9.8 mA? How low can I go and are there any problems with this approach?
Yes, you need to limit the current going thru the input LED of the SSR.
Yes, you need to limit the total current going the 74HC595. That is a logic circuitpart, not intended for driving lots of current - sadly it was selected early on and I'm is working at overstress conditions in many a design.
Use a part intended for high current switching. TPIC6C595 (100mA outputs) or TPIC6B595 (150mA outputs).
Here is a board I offer with 8 SSR channels and TPIC6C595
Thanks for the reply. Your boards can switch 1 A at 120 V?
It looks like a lot of these multi-channel SSR boards have a 10K resistor on the trigger input. So that should limit the current, right?
I have some boards very similar to these - here they state that trigger current is only 2 mA, and working current is 12.5 mA per channel (presumably from the 5 V PSU). I tested one with a multimeter and it only draws 0.5 mA to trigger. I guess part of my confusion is that most of the time these boards are described in terms of total current?
The datasheet of the PVT412 (that Crossroads mentions as one of the SSR options) says it can switch this voltage, but no more than 120 mA, and unless that PCB is designed with mains voltage in mind (I do assume you're talking about mains) separation may not be sufficient. I didn't check the specs of the other parts mentioned, I'll leave that to you.
The same accounts for the other boards. They may or may not have been designed with mains voltage in mind. Check the specifications carefully!
