Solid-state relay interface with 08M2

abenn

Senior Member
I plan to use ASSR-1611 solid-state relays with a PICAXE 08M2.

Do I need a 1k (or any other value) resistor between the 08M2 output and the ASSR-1611 input, as per the standard transistor interface in the PICAXE manual; or can I make a direct connection as per the standard Darlington driver interface in the manual?
 

Jeremy Harris

Senior Member
This particular SSR needs a minimum of 5mA, so the series resistor needs to be a bit lower than 1k, to allow for the internal LED Vf of around 1.3 V. As a Picaxe pin can deliver around 20 mA, which is right at the upper limit for the SSR input, I'd fit a series resistor of around 470 R, which should allow a reasonable margin for safe operation.
 

abenn

Senior Member
Thanks for the prompt reply Jeremy. It helps me see how I should be using the device's data sheet to calculate the requirements.

Now that you've explained it to me, I see from the spec. sheet that the ASSR-1611 has a "recommended" input current (for ON state) of 20mA, but it also says that "Input Current Average (IF)" = 25mA, "Surge" = 50mA, and "Transient" = 1000mA. I understand the significance of surge and transient figures. 470R is going to give me an input current of around 10mA, isn't it? Presumably, so long as that's enough to light up the LED inside the package, that's all it needs, and it will be less stressful on the PICAXE? At the same time as driving the SSR, my 08M2 will be driving a transistor via a 1K resistor.

Just out of interest, could I use a 20mA or 10mA LED driver instead of a resistor?
 

Goeytex

Senior Member
Now that you've explained it to me, I see from the spec. sheet that the ASSR-1611 has a "recommended" input current (for ON state) of 20mA...
You seem to be misreading the datasheet. The recommended operating input current is a 'range' from 5mA to 20mA. 20mA is no more recommended than 5mA. Jeremy provided a resistance of 470R that will place it in the middle of this recommended range. I have used these successfully with a 680R resistor but 470R is probably a better choice.

... but it also says that "Input Current Average (IF)" = 25mA, "Surge" = 50mA, and "Transient" = 1000mA.
This is in the Absolute Maximum Ratings section and should not be confused or equated with recommended operating conditions.

If you really want to reduce stress on the Picaxe, then consider using a general purpose NPN transistor to sink current through the SSR input LED. In that case consider a 330R current limit resistor for a current of ~ 15mA. Use a 4K7 resistor from the Picaxe to the base of the transistor. An LED driver? Not necessary at all.
 

abenn

Senior Member
Thanks for your input Goeytex. You're right, I overlooked the minimum of 5mA :rolleyes: Since you've had success with a 680R resistor (giving about 7mA) I'll probably start with that

My thinking about the LED driver was, since the purpose of the resistor is to limit the current through the SSR input, why not use an appropriately-rated LED driver instead -- I just happen to have some 10mA and 20mA ones in my components box.
 

Jeremy Harris

Senior Member
The voltage from a Picaxe pin is essentially constant, so a constant current LED driver is completely pointless, as a resistor will do the job very easily, simply, cheaply, reliably and effectively.

470R allows enough headroom for the worst case at either end of the Picaxe pin spec and the SSR spec, which is why I suggested it. There's no need for precision here, you just need to ensure that, with the lowest possible Picaxe output pin voltage and the highest possible LED Vf, the current is greater than 5mA. You also need to ensure that with the highest possible Picaxe pin voltage, and the lowest possible LED Vf, the current doesn't exceed 20 mA. Anything inside that range will work perfectly in this application.
 

Jeremy Harris

Senior Member
If you ever want to work tolerances out like this, then look at the max and min figures on the data sheets and see what the range is, then pick something inside that range. In this case, the Picaxe pin output "high" voltage (assuming a 5V supply) could range from close to the supply voltage (5V in this case) to 4.3 V, depending on the load. The Vf of the LED in the SSR varies from a min of 1.1 V to a max of 1.7 V.

Assuming the worst case to get at least 5 mA through the LED, this means taking the lowest Picaxe output voltage of 4.3V and the highest LED Vf of 1.7 V, which is a voltage difference of 4.3 - 1.7 = 2.6 V. With a 470 R resistor, the current flowing to the LED for this condition would be 2.6 / 470 = 5.53 mA, just over the 5 mA minimum required for the LED to operate (with a 680 R resistor the current drops to 3.82 mA, not enough to meet the LED specification).

Checking the worst case in the other direction, with the maximum Picaxe output voltage and the lowest LED Vf, gives 5 - 1.1 = 3.9 V. The maximum current flowing into the LED with a 470 R resistor would then be 3.9 / 470 = 8.3 mA, well within the 20 mA maximum allowable.

These two quick calcs show that 470 R is about optimum in terms of minimum power consumption and heat dissipation, combined with guaranteed operation of the SSR over the full range of the data sheet specifications.
 
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Circuit

Senior Member
If you look at the datasheet in more detail you will see figures given for the Switching Specifications; the turn-on and turn-off times are given for 5mA and for 10mA. If you don't need slightly faster switching then operation at 5mA is perfectly acceptable; it is notable also that the maximum diode current before switch-off occurs is stated as 0.8mA. Also the specified Output On-Resistance of 0.065 ohms is stated for a diode drive of 5mA. Several of the figures show identical performance at 5mA and 10mA - most notably the On-Resistance (Figs. 10 & 11). Also the manufacturers' own test circuit in Figure 18 drives the diode at 5mA. All this supports the idea that operation at 5mA is considered the "normal" level. I routinely drive these devices from PICAXE at 5mA, usually because I want to limit the overall current through the PICAXE because of other connected devices.
 

abenn

Senior Member
Thanks Circuit. My application doesn't require fast switching, so looks like I could be aiming for 5mA. My 08M2 will be driving two of these devices plus a couple of ZTX1051A transistors and an RC servo.
 
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