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Please note, this is not a shopping question; this is a what type of component works best for this scenario question.


I have an induction heater circuit and a 5A max. / 24V wall wart, and I'd like to switch off / on the output of the wall wart so that I can control the power output of the induction heater.

About 7 years ago I found something by the name of SSR or SCR (or something) on digikey and that cost a reasonable amount (< 10 USD), and now I can't find it any longer. The SSR's on there that can handle that much power cost around 50 USD, which is too much to design with. The frequency can be low (5-100 Hz) range, since it is a relatively slow heating process.

I plan to switch the relay on/off with a raspberry pi, and will probably need a transistor in between. I don't want a mechanical relay since the thing will probably break before even making real use of the product.

The component I found prior was upright flat-rectangular with 4 thru-hole terminals coming out of the bottom smaller side. I found some that look like that but like I said they were in the 50 USD range. Also, that component if I recall correctly was for switching on/off AC for a different type of heater, so maybe that's why it was cheaper?

What types of alternative components should I be considering based upon these requirements?

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    $\begingroup$ "I don't want a mechanical relay since the thing will probably break before even making real use of the product." What? My car has 210,000 miles on it and I've never had to replace a relay. That means the turn signal and brake light relays which get used all the time and the big 50A starter relay which is used literally every time I start my car. $\endgroup$ – Chuck Oct 23 '15 at 8:54
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    $\begingroup$ Mechanical relays are established, proven and very reliable devices. $\endgroup$ – DLS3141 Oct 23 '15 at 11:57
  • $\begingroup$ @DLS3141, The application is to switch on and off at 5 to 100Hz. A mechanical relay would fail quickly because they are not designed for that high of duty cycles. Best case if the relay was rated for 1M cycles, it will only last 2.8hours at 100Hz. $\endgroup$ – ericnutsch Oct 23 '15 at 21:57
  • $\begingroup$ @Chuck, the number of cycles the relays in your car sees will be significantly less than one attempting to switch at 5 to 100Hz. $\endgroup$ – ericnutsch Oct 23 '15 at 21:59
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    $\begingroup$ What are you even trying to do running any kind of relay at 100Hz?? Why not just put a triac (AC dimmer) on the AC side and be done with it? A 120W heater isn't going to care about whether you're switching at 5Hz or 100Hz thermal mass won't let it heat and cool that quick. If you're trying to do variable voltage/PWM then a relay is not the way to go. $\endgroup$ – Chuck Oct 24 '15 at 2:02
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SSR or Solid State Relay is what you are after. You should be able to trigger this one right off the 3 volt rpi gpio. FOTEK SSR-40DA. Here is a DC one CPC1709J. Use a voltage divider or zener diode so you dont exceed the 1.2v input. You will likely need a heat sink.

You may want to consider using a mosfet instead and running higher frequency. Higher frequency means you will transfer more power for a given coil size. Also reliability and efficiency will likely be higher than an SSR pushed to 100Hz.

This one switches low side (which is safe in your 24v application), but it is nice because it triggers right off of the 3 volt rpi gpio without a driver.RFD3055LE 60V 11A N-Channel Mosfet

Here is a really nice mosfet wiring tutorial. bildr.org mosfet tutorial

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If you want to use an SSR, you'll need to get one designed for switching DC. Like this one Unlike mechanical relays which can switch AC or DC, SSR's are designed for switching either AC or DC loads.

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