I am building a controller based around the PIC16F18877 which will need to control power to a Neon Sign Transformer which draws 6A. To do this, I was looking into using a solid state relay (e.g., [URL='http://chrome-extension%3A//efaidnbmnnnibpcajpcglclefindmkaj/https%3A//www.sensata.com/sites/default/files/a/ez-series-panel-mount-ssr-datasheet.pdf' ][/URL]). Based on the wiring diagram I see that it has internal resistors on the DC side to power the LED. I've never used SSRs before so I have two questions/concerns:

1. As it already has the built-in resistors, I am assuming that I don't need any external resistors on the DC side, correct?
2. Per the datasheet, the various models have an input current of 10-15mA and any standard IO pin on the 16F18877 have a maximum current of 50mA. Therefore, can I run the DC side of the SSR directly from the PIC or would it be better to use a transistor to control the SSR?

TIA

 

Your reference to the SSR is blocked, for some reason.

Yes, you should be able to drive the SSR directly from the micro output, but it will depend upon the input resistance and input voltage input range of the SSR.

 

Hi Carl,
His link is corrupt, this is what I can salvage.
Mod.
https://www.sensata.com/sites/default/files/a/ez-series-panel-mount-ssr-datasheet.pdf

 

So the SSR with the 3-15Vdc input should work when directly connected to your micro.

 

If you're switching highly inductive loads like a transformer use a random cross version of the SSR.

https://www.te.com/commerce/Documen...ion=srchrtrv&DocNm=13C3206_AppNote&DocType=CS

 

If you're switching highly inductive loads like a transformer use a random cross version of the SSR.

https://www.te.com/commerce/Documen...ion=srchrtrv&DocNm=13C3206_AppNote&DocType=CS

Thanks for this tidbit. I review the datasheet and the only thing I could find which was related to this was Turn-on time for random turn-on versions is 0.1 msec (DC Control Models). I am right to assume that "random turn-on" is the same as "random cross" and, if so, since I am using the DC Control version then I should be good?

 

Thanks for this tidbit. I review the datasheet and the only thing I could find which was related to this was Turn-on time for random turn-on versions is 0.1 msec (DC Control Models). I am right to assume that "random turn-on" is the same as "random cross" and, if so, since I am using the DC Control version then I should be good?

What's the full part number? Don't assume anything.


From the posted SSR link.

 

When sourcing or sinking relatively high currents (15 mA), a PIC output pin will not swing all the way between Vcc and GND. Is your PIC powered by 5 V or 3.3 V? If 3.3 V, you might get more reliable performance by driving the SSR with a small transistor as a saturated switch, to get those last few millivolts across the SSR input.

ak

 

What's the full part number? Don't assume anything.
View attachment 308180

From the posted SSR link.

Wow...I didn't even notice that; its been a long day already. Thank you.

When sourcing or sinking relatively high currents (15 mA), a PIC output pin will not swing all the way between Vcc and GND. Is your PIC powered by 5 V or 3.3 V? If 3.3 V, you might get more reliable performance by driving the SSR with a small transistor as a saturated switch, to get those last few millivolts across the SSR input.

The PIC is running on 5V via an LM7805.

 

I review the datasheet and the only thing I could find which was related to this was Turn-on time for random turn-on versions is 0.1 msec

Interestingly, due to the phase-shift between voltage and current, the minimum surge current when energizing an inductive load such as a transformer is generated by a voltage peak-turn-on SSR.

 

Interestingly, due to the phase-shift between voltage and current, the minimum surge current when energizing an inductive load such as a transformer is generated by a voltage peak-turn-on SSR.

. . .and the worst by a zero-crossing type, because it starts with the transformer flux at zero when it should be at maximum, then it saturates approaching the end of the first half cycle

 

Below is the LTspice sim for the two scenarios:
Note that the theoretical peak current for the zero-voltage-start (yellow and green traces) is double that when started at the peak voltage (blue and red traces).
Of course, in practice, a transformer would likely saturate for the zero-start case, as noted by Ian, with the peak current actually going much higher.