I have a Pulse PLA900 AB amplifier and I would like to use a non-latching relay switch to switch it on/off from a digital platform. Unfortunately, the technical data sheet of the amplifier lacks crucial information for my project, such as standby power (i.e. when there is no input signal to be amplified) or power consumption on full load, or peak power when the amplifier is switched on, therefore I can just guess them. The RMS is 760 W so with the assumption the efficiency of the amplifier as well as of the transformer is 80% then the maximum power consumption of the amplifier from mains is 10/8 * 10/8 * 760 W * buffer = 1187.5 W * buffer = 1500 W. I have no equipment to measure the peak power that the amplifier picks up when it is switched on but it has a 10 A fuse on the mains - which means nothing but this is what I have.

This is my simplified diagram, the starting point - I do apologise for its simplicity and imperfect nature, hope it is good enough for visualisation:


simulate this circuit – Schematic created using CircuitLab

I have tried a switching relay with no load on the amplifier, that was certified for AC 250V 10A, and the guess based on similar types its maximum switching power was around 200W, but it probably got welded because I was just able to switch it on/off like three times then it never switched off again.


simulate this circuit

Someone then suggested to use two relays in parallel with a small power resistor (10 ohm 5 W) in series of the first, then for switching ON follow the below steps:

  1. switch ON the first with the power resistor
  2. wait 0.5 sec
  3. switch ON the second without resistor

and for switching OFF follow the same steps in reversed order:

  1. switch OFF the second without resistor
  2. wait 0.5 sec
  3. switch OFF the first with resistor


simulate this circuit

My questions as follow:

  • Is this proposed method standard and safe?
  • If this is standard and safe, then
    • What would be the best way to calculate R and W of the resistor with the limited available of information?
    • Which type of power resistor would suit best for this problem: cement or wire wound (aluminium case)?
    • Would a relay with max switching power 2770VA 210W be sufficient? I have a 2 relay module handy, that data sheet could be this but unfortunately not sure https://www.generationrobots.com/media/JQC-3FF-v1.pdf enter image description here

1 Answer 1


Is this proposed method standard and safe?

I guess it's safe in that it isn't going to burn but I'd just go for a meatier solution like a solid state relay rated at 40 amps: -

enter image description here

The surge current for this relay is 500 amps if you read the data sheet.

  • \$\begingroup\$ This answer is exactly what I would suggest. \$\endgroup\$ Nov 17, 2020 at 17:39
  • \$\begingroup\$ @andy-aka thanks for the suggestion. I am afraid a heat sink would not fit in because of its height with the SSR on the top. Is it safe w/o heat sink for a load, like <7A? I believe the heat would at most 1.15V*7A=8.05W; So, I found this SSR that could be controlled by the 3V3 that my SBC can produce, but not sure whether the 3V3 16mA is sufficient to drive the relay and if a resistor in series is required. I cannot find 'switching current'/'coil resistance' equivalent on its data sheet that would give me some input for Ohm's law. \$\endgroup\$
    – ben
    Nov 18, 2020 at 22:00

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