PSA: Unlike mechanical relays,

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discussion
(Thomas Sanladerer) #1

PSA: Unlike mechanical relays, most solid state relays are made to switch only AC voltages. Which means that you shouldn’t use one to drive a 12V or 24V (DC) heated bed. A proper MOSFET is a much better choice here.

SSRs are typically designed to minimize switching losses by only switching off (or on) at the point where the switched voltage inverts its polarity - also called zero cross trigger or zero-crossing detection. Since a DC supply never inverts its polarity (or even drops to zero), the behavior of the SSR may become unpredictable, ranging from the SSR never turning on or off or the entire SSR melting down.

Also, SSRs have a comparatively high voltage drop across their output (Fotek states 1.6V), which causes high losses when switching large currents and reduces the available power at your heater.

Further reading:




http://www.fotek.com.hk/solid/SSR-1.htm

(Neil Darlow) #2

Not quite true. A SSR employing a triac will exhibit the behaviour of turning off when load current approaches zero but, unless it has a zero-crossing detector to control switch-on (which a lot do not have), it may be switched on at any part of the AC cycle. This can have issues for e.g. inductive loads which object to the application of a DC bias. Here you must ensure the switch-on occurs at the same point of each AC half-cycle.

(Eugene Lee) #3

@Mark_Rehorst that’s a good price, can you link me to your supplier? Do they ship internationally?

(Scott Lahteine) #4

Any good articles comparing the performance of MOSFET vs SSR vs other types of current switches? I’d like to see how they perform in terms of efficiency, heat, etc. I have a RAMPS 1.4 board and love it, but it seems like a good idea to move power off that board where possible, and SSRs seem to be an economical option. But is the original post correct? Are SSRs a “hack” when used with DC current?

(Eugene Lee) #5

If anyone is looking, here are two switches that use mosfets: work from 12-24VDC and controlled by the 0-5V:

http://reprap.org/wiki/SevenSwitch_1.2
http://reprap.me/heatbed/power-expander.html

Has anyone else had experience using the above switches? Can you recommend an easier way to use a mosfet instead of an SSR?

(Mike Kelly) #6

I don’t really agree with this PSA. There are plenty of DC to DC SSR’s out there and they work just fine.

I’ve been using this for months with great success. Though I’m using 24v on a 1.4ohm bed, so a voltage drop or 2 will be unnoticed because of the massive power available.

MOSFETs also tend to introduce noise into the circuit and you’ll need a low pass filter on the line to keep it quite. The SSR handles most of this for you.