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I have an AC voltage source which runs at about 6V to maximum 70V (usually around about 14V @ maximum 8W) which is then rectified in a diode bridge rectifier and later regulated to stable 5V. From this 5V I run a microcontroller.

Depending on the AC voltage I want to either include more or less capacitance (C1, C2, C3) before the bridge rectifier. To do this I use SPDT relays which are switched from the 5V rail using the microcontroller.

Image 1 - current circuit

Now what I would like to do - mainly due to size (or rather height) constraints - is to get rid of the relays since they are just way too large. My idea was to use two back to back MOSFETs (with the sources connected) to replace every relay. I could drive them with my 5V rail through an optoisolator (like the VOM1271). But this way I'd have a normally open switch.

How would I get this to be normally closed?

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  • \$\begingroup\$ But you don't need to use power MOSFET to DIY a SSR. You can buy cheapy assembled SSRs. \$\endgroup\$
    – tlfong01
    Commented Nov 8, 2020 at 18:25
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    \$\begingroup\$ you can just leave the smallest capacitor always connected and switch in extra when needed. However a capacitive drop for that power (8W) and input range (1:12) can't possibly be the way to do it. I'd rather consider something like this EC7BW18-72S05 (50€ on Mouser) \$\endgroup\$
    – carloc
    Commented Nov 8, 2020 at 18:31
  • \$\begingroup\$ @tlfong01 are there any with high enough amp rating that are NC? I couldn't find any that weren't also really big so far \$\endgroup\$
    – lhjs
    Commented Nov 8, 2020 at 19:47
  • \$\begingroup\$ I guess so. You might like to check out this catalog. aliexpress.com/popular/solid-state-relay.html for the smallest size SSR or SSR breakout module that fits you. \$\endgroup\$
    – tlfong01
    Commented Nov 9, 2020 at 1:42

1 Answer 1

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Build your own solid-state relays (SSR) utilizing optocouplers and Triacs. The tallest part of this build would be the heatsink(s), but you could custom this (i.e. longer over taller) to the space you have available to you. Each SSR can switch up to 12A and based on a 5VDC rail opto supply.

SSR schematic

The BT138s can get hot, so you MUST use heatsinks. However, that being said, as you are only running 6-70VAC through them @ an average of 14VAC/8W/1.75A, you will not get as much thermal dissipation from them.

Input Voltage: 5V - 15VDC; Load Voltage: 6V - 230VAC; Max Load Current: 12A; Max Load Power: 2.7Kw

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    \$\begingroup\$ Check your triac connections. IC1, etc., should be connecting 3 to 1, not 3 to 2, I think. \$\endgroup\$
    – Transistor
    Commented Nov 8, 2020 at 22:57
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    \$\begingroup\$ The schematic component library threw me the symbol and although I noticed the 'arrows' are the opposite way around, I do not have time to edit them at present. Pin layout and connections are correct for operation though :) \$\endgroup\$
    – Titch2K6
    Commented Nov 8, 2020 at 23:22
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    \$\begingroup\$ I was mistaken. You have drawn it upside-down to normal. \$\endgroup\$
    – Transistor
    Commented Nov 8, 2020 at 23:28
  • \$\begingroup\$ thanks for your reply - I'll have a look into it. gotta read up on triacs first, though ;) \$\endgroup\$
    – lhjs
    Commented Nov 10, 2020 at 17:40

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