I have a large transformer (see below) that I wanted to switch on/off from a digital platform using a solid state relay. The equipment works without any issue and makes no noise when the solid state relay is bypassed but the transformer makes a buzzing noise when it is in series with the relay. I tested the equipment with the relay and the noise and it seems to be working but the buzzing noise worries me and I fear for the transformer.
This is my SSR: PF240D25R
My qusetions as follows:
- Why is my transformer making an audible buzzing noise with the SSR, while there is no audible noise without the SSR? Is it caused by the approx 1.6V drop (see the datasheet)?
- How can I stop this noise?
- Is it safe for the transformer or should I stop using this SSR?
UPDATE 1: The control circuit is a Raspberry PI Zero (GPIO: 3.3V) and the resistor is 330R that makes the current 0.01A
UPDATE 2: My SSR is a 'Random Turn-On' type
Best Answer
Your SSR may be too powerful (25A) for that current that has to switch. More powerful triac means also higher latching/holding triac current.
In case the SSR is zero-cross type, it would skip semi-periods, so this type of SSR is not adequate for large inductive loads. Your's seems to be random phase type, so the correct one. Your setup misses the snubber circuit, which is usally a must when switching inductive loads. My recommendation is to try adding a snubber, before making any wrong assumptions.
EDIT:
The input signal of SSR is 3-15V @ 15mA. The SSR has an internal circuit to limit the LED current. It is not recommended to add an additional series resistor, rather you could use an external N-MOSFET as a switch if you want to protect the GPIO.
simulate this circuit – Schematic created using CircuitLab