currentsolid-state-relay
A solid state relay specifies a trigger current of "7.5mA/12V". How much current does it draw at 5V?
This is the relay. I'm controlling it with an Arduino which has a maximum current draw of 40 mA per pin. Am I correct in calculating it multiplying 7.5 by 12 then dividing by 5 (the new voltage)? That gives me 18 mA, well within the limits of the Arduino pin – is it really that simple, or am I missing something important?
You seem to be mixing up the ratings on the coils and on the switching contacts. Notice the ratings you quoted come from two different tables in the datasheet:
If your power supply is being used to drive the coils, then you need to consider the required coil current to switch the relay. As your item #1 says, a 10 A supply could potentially supply 100 100-mA coils simultaneously.
If your power supply is being used to drive the load (switched by the relay) then you need to worry about what current your load draws when powered by 12 V. If it does indeed draw 3A, then you could only power 3 such loads with a 10 A supply.
If one relay is being used to switch current to the coils of a bunch of other relays, then you get the limit of your item #2. The 3 A allowed through the contact of the first relay is enough to power the coils of 30 100-mA coils that are its load.
Also, if you want your system to be reliable for a long time, you'd probably want to de-rate all of these specs rather than operate the power supply or the relays at their maximum current limits. (The coils will operate at roughly the spec'ed currents due to the coil resistance and Ohm's Law)
So,is there any specific reason for using an SCR power controller over a higher current relay/contactor? [From comments: What I meant to ask is if a simple SCR triggered controller (ignoring the above example) is better than a relay?]
Yes, there are several advantages:
Figure 1. With non zero-cross SSRs dimming is possible.
Figure 2. Proportional on-off time control. Note that step size is one half-cycle minimum. This can make the response seem coarse if the repeat time is short.
Also is an SSR almost same as SCR triggered controller?
simulate this circuit – Schematic created using CircuitLab
Figure 3. A very simple SSR (a), a thyristor (b) and a triac (c).
An SSR (a) will have electrical isolation between the trigger circuit and the actual SCR. An SCR controller might not and, at its simplest, might just be a thyristor or triac. Note that in Figure 3a I have shown a constant current source to represent the internal circuitry that allows the SSR to work over a wide range of input voltages - 4 to 32 V in your example.
For more information see:
Best Answer
I have a comparable "hockey puck" SSR handy. It also has the 3V to 32V input, although it's a different model (ESR5102401000Z). I made some quick measurements.
I was curious how these solid state relays (SSR) manage to accept a control voltage with a fairly wide range: from 3V to 32V. Of course, the datasheets for these "hockey puck" SSR don't provide the details about the input side. I can think of 3 schemes. Different models of SSRs may use different schemes.
Current limiting resistor
The LED and resistor are such that the current is low enough at 32V not to destroy the LED, but still high enough at +3V to activate the relay. In this arrangement, the input current will increase linearly with input voltage.
Series current limiter (active)
The input current shouldn't vary much with input voltage. The constant current source may be more or less stiff. The plot in Spehro's answer suggests a series constant current regulator in his SSR.
Shunt current limiter (active)
The input current will increase linearly with input voltage. Excess current shunted around the LED, but still drawn from the input. Here's an example of what a shunt current limiter may look like (source).