Ideas for driving an electromagnet with high voltage AC current? - Page 1
Forum section: electronics / projects

author: Taako
author_rank: Newbie
timestamp: December 30, 2020, 05:17:43 pm
content: Hello,I have an aquarium pump that is no longer being used and I want to integrate it into a design of mine. I am inexperienced designing for stuff with high voltage AC or AC current in general. i've done many <12V DC electronics designs, even complex ones involving BGA FPGAs, I've never NEVER touched high-voltage or AC current on my PCBs before.Here are some photos of the pump:https://imgur.com/a/iZFpGGLThe pump plugs into mains voltage and seems to have a potentiometer and some passives to control the amplitude, and what i believe is a flyback diode for the inductive load. I'm not really sure the circuit for it but i'd be happy to dissect if further if someone wants to help explain what each passive does. I assume the diode is for sinking the induced negative voltage from the massive inductor when the AC voltage switches polarity, but i'm not sure what the capacitor is doing there. I only plan to use "half" of it since it's essentially two electromagnets each of them pushing two dual-diaphragms. So I only intend to use one electromagnet pushing two diaphragms.Theproduct pagefor the pump shows that the entire unit only uses 6 watts at 120VAC, i only need half the unit so the section I would need would use 3W@120VAC; quick calculation means one half only uses 25mA of AC current.At first I was thinking of plugging mains power directly into my PCB using something like an IEC connector and then using aPCB mounted AC-DC converterto step down to 5 volts for the rest of my design (raspberry pi zero, stepper motors, LCD display, etc) and have the AC side of the PCB feed into a relay that connects to the inductor and have the relay switched with the raspberry pi's logic level output. This seemed like an okay approach but it also has me nervous since i've never used high voltage and in addition the PCB mount switching power supply i'd need to go from 120VAC to 5VDC would be huge and unwieldy to design around. That part alone would take up half of my planned PCB board space.I need at least ~7Amps supplied on the 5VDC side because that side of the design uses 12MG996R servoswhich have a running current of 500mA; truth be told I will only ever be actuating one servo at a time but when the board comes online I don't want the inrush current from all the servos to burn anything out. The other components (rapsberry pi, LCD display, addressable LEDs, etc) all use a relatively small amount of current, the Pi being the largest at no more than 300mA (but probably no more than half that to be honest).I was wondering if a better idea was to use a commercially available power brick to provide 12VDC 10-15A to the board and then make a smaller section of the PCB somehow drive the AC electromagnet. Is believe DC-AC converters are called inverters? Does it seem reasonable to be able to convert 12VDC to AC for something this low power (roughly 25mA by my calculation)?I've also found thesebut the output frequency is too high to drive the electromagnet correctly.What do you all think? Should I use the huge PCB mount AC-DC converter with a relay thrown from the 5V side? Or is there another circuit I could build if instead I supplied 12VDC 15A and used a section of PCB to convert to higher voltage AC?Would an H-Bridge driver work for this? Maybe I could use a DC-DC converter to generate 120DC and use an H-Bridge to alternate the current flow? It would be a square wave so maybe I could use a low pass filter to get a sinusoid. I would need another circuit to do the switching unless I manually controlled the switching in software (non-ideal as it's one more thing to deal with).

author: geggi1
author_rank: Frequent Contributor
timestamp: December 30, 2020, 06:04:38 pm
content: For safety issues I would recommend to keep the I would 120V on a separate PCB for safety issues.If the pump only is going to run as an on/off unit without any kind of regulation I would just use a relay or a solid state relay to control the pump.With a Solid State Relay you can even control the stroking of the Diaphragm Pump. A SSR is basically a beefed up opto-cupler.https://www.digikey.no/en/articles/how-to-safely-and-efficiently-switch-current-or-voltage-using-ssrs?utm_adgroup=Relays&utm_source=google&utm_medium=cpc&utm_campaign=Dynamic%20Search_EN_Product&utm_term=&productid=&gclid=CjwKCAiA57D_BRAZEiwAZcfCxQ4HGZ-7fi1bk_JIPAVlKyXTPuEzwODvG3gvVjFqZOpSQctGH11vdxoCwMoQAvD_BwE

author: Ian.M
author_rank: Super Contributor
timestamp: December 30, 2020, 06:12:23 pm
content: So 12*0.5A is 6A, and the Pi probably doesn't need more than 3A, so worst case you are unlikely to need over 10A @5V, or 50W Bucking from 12V with 90% efficiency, you'll need 55.5W @12V which is under 4.65A in. OTOH, why not use a second hand 19V laptop PSU? That would reduce the input current requirement to under 3A, vastly easing choice of connector for DC in.As to powering the pump, why not rewind its coils for lower voltage and drive it from a H-bridge? 19V supply would give enough headroom to deliver 12V RMS AC by PWMing the H bridge with a sinusoidal variation in duty cycle, and if you used 1/10 the number of turns and 3.2* the existing magnet wire diameter (for *10 CSA) it would give equivalent results. Raspberry Pi SBCs aren't very good at generating complex PWM signals so I'd off-load that job to a small MCU with a full-bridge capable PWM module, fast enough to get the PWM above the audio frequency range so you don't hear it and the only noise from the pump will be due to the 60Hz fundamental frequency of the drive waveform.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 30, 2020, 06:46:07 pm
content: Looks like a pair of electromagnets, and probably permanent magnets on the armatures? So, as the field alternates, the magnets are pushed across the gap, left and right, and presumably there's a diaphragm and valves inside the plastic, thus pumping water with little more than a buzzer.The "easy" way would be to simply unwind all the fine wire on there, and wind 10% of those turns, with 3.16 times thicker wire, and use a 12V oscillator to drive it.Failing that,you might settle for the same basic thing, but using a 12:120V transformer in reverse.For ideas, look up typical mains inverter circuits -- the simple kind is a low voltage CT transformer with two transistors (a chopper oscillator, aka astable multivibrator), and you get something resembling mains on the high voltage side. Just hook it up and off you go. Adjust resistors/capacitors to get close to 60Hz (likely the armatures are resonant near 60Hz, so they'll work best when driven near that frequency), and that should do it.The control board looks to be a phase controller -- little more than a lamp dimmer, just a small one. (Probably the TO-92 part is a TRIAC rated for 400-800V and <= 1A, and the diode is a DIAC. The resistors, pot and capacitor set a time constant, which with the DIAC, sets a turn-on delay relative to mains zero crossing, hence, phase control.) Probably, the pump only needs variable amplitude to control its flow -- again, it probably resonates, plus it's direct mains powered, it's not like you could change the frequency (as you would with a motor driven pump).That said, if it's not too resonant, or you need it controlled down to very low (all the way to zero?) flow, it would probably still work with a direct drive circuit. Wouldn't be able to use a transformer though; it would look more like a stepper motor circuit, but with high voltage supplies. Heh, amusing I suppose, but probably not useful.For the two-transistor chopper circuit, possibly just varying base bias will do the job of varying output. It won't be proportional, but it should have at least a little range, and maybe that's enough. Otherwise, supply voltage can be varied, or PWM controlled with a more formal circuit (say based on a TL494, super easy to make alternating PWM that way).Tim