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.
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?
Best Answer
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.
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