I wanted to control the power to a bridge rectifier. I had two ideas in my mind:

  1. Using a PCB mounted power relay driven by a transistor (is isolation maintained?)
  2. Using an SSR, i.e, a power triac driven by a optotriac, as shown below

In both cases, I would have to place a NTC thermistor to limit the inrush. The idea I chose was the SSR, which I haven't tested yet. But, someone told me:

Zero crossing will work for the first 1/4 cycle, but then:

  1. There will be no current flow to the "zero-volt switch" IC for it to know when the zero-cross happens, because "2200uF" is charged to 170V & all bridge diodes are reverse biased.
  2. Even if a firing pulse is given to triac @ zero-cross, it will not latch-on, because the available current is below "hold-on" current for Triac*

I would like to hear your viewpoints.

enter image description here

  • \$\begingroup\$ why not use a regular transistor between the bridge and the capacitor? or use two SSRs in the bridge? (you'd still need isolation but it seems simpler) \$\endgroup\$ Aug 5, 2018 at 21:28

1 Answer 1


I would like to hear your viewpoints

It's true what you heard but is it that big of a deal? If the load is very light it won't need to draw much current but, the voltage at the output of the bridge will still be 170 volts because the capacitor has charged to that voltage.

As load current consumes energy from the capacitor, the DC voltage will drop and the next positive or negative AC cycle will replenish the charge in the capacitor should the holding current be exceeded. If not that cycle then the next or the one after that.

If the load is "normal" i.e. sufficient to discharge the capacitor between charging peaks then it's highly likely that the triac holding current will be exceeded in the next cycle.

However, if you hope to remove the voltage from a very light load in order to deactivate it, this could take several seconds to discharge the capacitor down to 0 volts.

  • \$\begingroup\$ Thank you. What I still don't get is if the circuit actually presents a potential problem. I want to implement it in practice to see if it really works or not. \$\endgroup\$
    – user115094
    Aug 3, 2018 at 13:19
  • 1
    \$\begingroup\$ I've tried to describe the what-if scenarios and if you can live with voltage still across the capacitor when you turn off the triac (very light loads) then this might potentially be a problem? BTW I noticed that you'd asked about 17 questions but you hadn't formally accepted any of them. You might want to take the short tour so you ensure support in the future. \$\endgroup\$
    – Andy aka
    Aug 3, 2018 at 13:45
  • \$\begingroup\$ Thank you. And sorry for not formally accept the questions. I will take it into account. \$\endgroup\$
    – user115094
    Aug 3, 2018 at 14:11
  • 2
    \$\begingroup\$ @XavierPachecoPaulino you only need to formally accept those that have been good but, for the future you should consider raising a comment if you can't accept an answer based on something you can't understand. \$\endgroup\$
    – Andy aka
    Aug 3, 2018 at 14:15

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