Wiring a Dual Stir Plate w/o PWM's

[DJTroz]

I just built a box that has space for 2 stir plates side by side. I built a single stir plate a while ago using a typical setup using a potentiometer, LM317, heatsink, on/off switch, 2 caps and 2 resistors as found in several sites including HBT. I took this setup out of the single case and put it into the dual case. I'd like to build a replica, put it next to the other, wire using 1 power source, 2 on/off switches, and 2 pots.

Let me preface by saying my electric knowledge is minimal and far from an electrical engineer. I wired my original setup using posts from others and looking over several schematics. My thought is to simply rewire the power source by removing the leads from the current setup, then splicing and running them parallel. This seems the easiest but I don't know the impacts on the setups when using a single or both at the same time. My current power source is a 12v, 0.8mA; my fans are rated about 200A each.

I have several on/off switches, LM317s, and 2k Ohm Potentiometers, and plenty of resistors and capacitors if I need to rewire anything. Until I move to PWMs, would this setup work and not blow up the house?

 

[TallahasseeBrew]

Assuming you meant the fan is rated at 200mA not 200A.

the LM317 is rated a recommended output not to exceed 1500mA, so assuming you're ok with both fans running the same speed at the same time, easiest way is to just run the 2 fans in parallel, the LM317 can easily take it.

For variable control just put another pot between the 2 fans.

 

[DJTroz]

Assuming you meant the fan is rated at 200mA not 200A.

Yeah, that's a typo (not enough coffee yet). They are around 200mA each.

So if I created another circuit like the first one (same caps, resistors, voltage regulator w/heat sink, and potentiometer) and ran the wiring from the power adapter spliced in parallel, it should work without overheating anything?

 

[TallahasseeBrew]

I would say so, and that's a pretty easy way to do it if you have the parts on hand. The biggest power draw is the fan, even with the circuit, id bet the current doesn't go over 250mA per circuit as the lm317 doesn't really draw any current, so I think you're good to go.

I do fix aircraft electronics for a living, so I do know electronics pretty good. I think you're on the right track.

 

[DJTroz]

Great. Thanks for the input. I work on and install/repair basic electronics around the house (fans, outlets, etc.) and our cars but when it comes to designing or modifying circuitry, I'm always worried about something overheating and catching fire if I'm not around. I have a DMM, but most of it is Greek to me except for some very basic tests I use it for.

 

[TallahasseeBrew]

Well, if you want to be 100% sure, you could temporarily install the DMM in series with the power input from the power supply to the stir plate circuit on the positive side and measure the current. If it's under 400mA which I'm sure it is, then you are good to go with 2 circuits on a 800mA power supply.

Worst case is the power supply will open or enter an overcurrent protection mode, the chance of starting a fire is probably pretty low.

But I can't imagine a LM317 power supply circuit and a 200mA fan drawing over 250 to 300mA