High Power Relays

Tim Hall

Hi all,

I'm just learning about the microcontrollers, and my electronics experience is a little limited. I'd like to control three line-voltage relays, and have been reading up to figure out how to do this, but I'm not sure I'm going the right direction.

I found this Panasonic 75-264VAC, 2A SSR: http://panasonic-denko.co.jp/ac/e_download/control/relay/solid-state/catalog/semi_eng_aqg.pdf

Spec's say the input is 5V with a 0.3k impedance. If I understand what that impedance means (and I'm doing my math right), it appears these SSR's will draw about 17mA each.

Can can anyone tell me if I have this right, and will the BS2p drive three of these relays?

Also can anyone explain what's purpose/difference between "zero-cross" and "random" with regard to relays?

Thanks for any help,

Tim

Mike Green

Yes, those Panasonic SSRs (solid state relay) look about right for your needs. Be sure to get the 2A version since you were talking about that amount of current. Get the zero-crossing version. By detecting when the AC waveform crosses the axis (at 0V) and using that to control switching, the SSR minimizes switching noise. In other words, the zero-crossing version switches when the AC voltage is low (near zero) thus the current through the load when the SSR turns on will be low also and the switching transients will be minimized.

Distribute the SSRs among the groups of 8 I/O pins. Each individual I/O pin can handle about 20mA and groups of 8 I/O pins (P0-P7 and P8-P15) can handle about 50mA. Three SSRs come to about 17mA x 3 = 51mA which is OK, but borderline. It would be better to move one SSR to another group of I/O pins if all three SSRs can be on at one time. Keep that all in mind with other loads as well.

Tim Hall

Mike, thanks again for your help. Now I understand the I/O power capability much better too. Starting to feel more confident about this project.