<p>According to the instructions I have read, you use the same output on the printers mainboard to control a 110&nbsp;V heated bed with a solid state relay, as you do to power the 12/24&nbsp;V heated bed that comes with the printer. </p>

<p>The relay's datasheet states that its max input current is 25&nbsp;mA, obviously a 12/24&nbsp;V heated bed would draw a lot more than that.</p>

<p>How does Marlin know that the heated bed pins are controlling a relay now instead of a bed directly, and therefore should limit their current output?</p>

<p>In other words: I am worried that if I just drop in the relay, it will burn up since the board still thinks it needs to supply high current to the bed.</p>

[{'answer_id': 11315, 'author': 'Pᴀᴜʟsᴛᴇʀ2', 'author_id': 11242, 'author_profile': 'https://3dprinting.stackexchange.com/users/11242', 'pm_score': 0, 'selected': False, 'text': '<p>There are two current parameters which you are looking at in your instructions for the solid state relay. The first one (the 25mA) is for the <em>control</em> circuit. This is how much amperage the device itself will draw when in operation. The second is for the <em>load</em> circuit. This is the max amperage which can pass through the device.</p>\n\n<p>Your <a href="https://pickercomponents.com/pdf/Relays/PCS15.pdf" rel="nofollow noreferrer">instruction sheet</a> shows the device having the ability to work at 10A, 15A, 20A, 25A, or 40A. These are different ratings for the same style of device. When you purchase the device, you\'d need to specify which amperage rating you\'d want your device to be at. They use the same spec sheet for all five flavors, because they are basically the same thing with the one exception, which is the amp rating.</p>\n'}, {'answer_id': 11317, 'author': 'Tom van der Zanden', 'author_id': 26, 'author_profile': 'https://3dprinting.stackexchange.com/users/26', 'pm_score': 1, 'selected': False, 'text': '<blockquote>\n  <p>How does Marlin know that the heated bed pins are controlling a relay now instead of a bed directly, and therefore should limit their current output?</p>\n</blockquote>\n\n<p>Marlin does not (need to) know this. Simply put, all Marlin does is switch on/off the <em>voltage</em> at the heated bed output. The amount of current that will flow is a function of the voltage and the device connected. If you connect a heated bed to a 12&nbsp;V voltage, a high current will flow. If you connect your relay to 12&nbsp;V, only a small current will flow.</p>\n\n<blockquote>\n  <p>In other words- I am worried that if I just drop in the relay, it will burn up since the board still thinks it needs to supply high current to the bed.</p>\n</blockquote>\n\n<p>You do not need to worry about this. Your question is essentially equivalent to this: "<em>if I replace my 100W incandescent lightbulb with a 1W LED one, do I need to replace the light switch in the wall?</em>" The switch (i.e., your printer board/Marlin) does not care what load is connected to it, so long as it does not exceed the <em>maximum</em> rated current.</p>\n'}]
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