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DIY I.H. basic Wiring Clarification request

Mono Loco

Legalization Pending!
Southern New England
Hi, all.

VAS is some sort of complex exponential phenomenon. Currently, my addiction is centering around I.H.s. I have an SKJ Dental Heater and it works pretty well, albeit a bit slow and cumbersome ... but gives good results. I'm on Pipes' list - wanna' get both his PSM and his Caldron ... just to have them in my collection, as they are classics that, IMHO, deserve a place in any worth I.H. "collection". I also will be getting his B.B. module ... might as well! I also have an Alpine 2020 on its way (well ... we hope!) because it looks so sleek and the price was right (with the Pre-order and coupon back-to-back). I do intend to get a Flix, also ... at some point. BUT ... I also want to make my own. I can solder and I can screw things together (I can also screw thing up!). I have a couple of Questions and would like some basic answers, if possible (trying to avoid overly-technical, explanatory, instructional essays). All the questions pertain to desktop units, not interested in portable units at this time.

Q1: From what I've read, MOSFET boards are only required to give some protection to low-amperage-rated switches, such as the mini tactile switches that are used in the bottom of the heating chamber ... if a larger, higher-Amperage-rated momentary switch that is mounted on the chassis/enclosure is used, a MOSFET is not required. As long as the switch is rated for more Amps than the power supply, there is no need to protect the switch and therefore no need for a MOSFET, right? If I am using an 8A power supply, a 10A switch should suffice, right? (someone posted that a 20A switch should be used if no MOSFET is in-line). Sooooo ... even if a MOSFET is not "required" when using a capable switch, would a MOSFET still be "recommended" (advised) to give any additional potential longevity to the device?

Q2: If a MOSFET's main purpose is to protect the switch, it seems logical, to me, that the MOSFET would be placed between the Power Supply and the Switch ...flow of power would be from Power Supply>MOSFET>Switch>Heater. If so, the Power would go into the Vin of the MOSFET and the switch would be powered from the Vout of the MOSFET. Indeed, this is what I see in most on-line schematics that are posted by various users on Reddit's I.H. group.

However, the schematic provided by VapOven (attached here) Mains-IH-with-MOSFET_VapOven.jpg)

feeds power to the switch from the Vin not from the Vout. To me, this would seem to defeat the purpose the the MOSFET as the switch will now be getting full direct power. Obviously, they feel this is the proper way. And, one particularly neat/tidy schematic posted on Reddit, by user "u/thisismytressact" (photo posted below), also has the power to the switch coming from the Vin side of the MOSFET ... in his "updated" schematic anyway. In his original schematic (also postedbelow), he had the power to the switch coming from the Vout of the MOSFET ... but he changed it. I am confused by his decision to make this change. I wonder if the author of this schematic (u/thisismytressact) is also a member here, on FC (?). I did ask him about his change, but I did not understand his reply (my fault, not his). I am hoping that some one here can clarify for me the differences between his two schematics. Both still have the power to the heater coming from the Vout of the MOSFET, but it's just the power to the switch that's different. Both he and VapOven seems to do if differently than the many other schematics I've seen shared by other builders (most are hand-sketched, not as nicely "drawn".


Man, if I'm having this much trouble with a simple, basic desktop unit, I wonder how much hair pulling I'll be doing if I ever decide to tackle a battery-powered unit!?!?!?

Thanks, all!
Mono Loco,


Well-known Member
First of all, the MOSFET switch is redundant. Second of all, the positive trace on the MOSFET switch is a solid trace from the input to the output. Therefore your last two diagrams are identical. You will notice that a lot of these circuits 'switch' the negative side with FETs. Now I'll read your epic story. Looks like something I'd write with painful detail :cheers:

And you first question, yes, I can be fairly confident that you won't draw over 10 amps so you need a switch rated for 10 amps DC. The direct current does carry weight on switches.
The reason FET are rated higher is so they have less heat-rise from operation. This is normally done to eliminate the expense of passive or active cooling devices.

My only recommendation on the switch is make it momentary. Leaving power applied to the IH will make it hotter than hell. It runs a constant 10-15 watts when "idling".

If you want to use a simple switch as a fire trigger you can make a small change on the board to do this. Bascially, the positive power lead will remain on the IH directly. The circuit is isolated from negative through the FET, the same as the MOSFET switch does. You apply power to the positive input, and the FETs close (alternately), and the circuit is active. The only power through the physical switch will be to drive the FETS, which requires no current [FETs are voltage activated devices].

Happy to help with whatever circuit you wish to implement @Mono Loco . I've got it down to 'pretty damn simple' by now :clap:

In the USA and throughout time for us tinkerers and development engineers switch positive on DC circuits. Age old. Negative common and fused positive in. These IH circuits reverse that. Likely due to the nature of MOSFETs.
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