I’m not new to electronics but I wish I knew more than I do at times. I haven’t had much luck in forums like this, but here it goes anyway. To get down to it, I had a failure of a pump driver that drives a 12V AC motor pump, like you would see in a desktop “Zen” fountain or something like that. The only problem is there is no AC current in the automated self-contained hydro-garden; it is only fed by a 24V DC 1500mA power supply. I did a lot of mucking around to figure out how they accomplished this and with such little circuitry (see attached schematic). Lots more to learn, but what I gathered, I figure it’s probably a custom H-bridge IC that does the dead-time manipulation of the input square waves to prevent shoot-through (but not well enough!), and some internal filtering to create a sine wave output to drive the pump motor.
So, I’m not looking for a drop-in replacement, but if one exists, I’m all ears!
I was going through all the Half-Full H-Bridge motor drivers, and I need someone to help me filter through what matches best my needs. I see some that have dead-time insertion built in but are super complicated and some are super simple 8-pin soic (similar to mine but the pinout is different), but probably most don’t have dead-time built in. I guess I can learn and build a circuit to manipulate the duty cycle of the input square waves before the h-bridge driver (ugh!). Any help is appreciated and please let me know if more info is needed, as I’m sure there is and hopefully, I can fill in the gaps. I have an ancient O-Scope (Tek 465), but I’m not afraid to use it!
After looking at them, no, because both operate at 5V and all I have available is 3.3V and 12V. the old one seemed to use Vsource (12V) for both logic (if there was any, it would have probably stepped it down internally) and drive voltage. I’m hoping there’s something that operates at CMOS level? 3.3V?
I didn’t realize something like that exists, but ok, that definitely opens up the field a bit, I’d have to test it out. I have an Arduino Uno already to toy with, but maybe I’ll get a Nano (3.3v logic level) to simulate the hydo-garden’s MCU. is the lL298 available as an evaluation board? I actually already ordered the L293 shield for the uno just to test some stuff I’ve been knocking around in my head for proof-of-concept of using a device that it wasn’t exactly designed for what I’m about to do with it. Hopefully I don’t blow it up on the first try. Thanks!
Update: So, this has been a long journey and almost all my spare time to get control of the pump for the hydro-garden back. Using the ST Micro L298 did indeed function as an inverter (mostly) to drive a small shaded-pole ac motor (12v), but it took a lot of control circuit building in order to make it function the way I wanted it to. Because I only get 2 complementary, 60 Hz, 50% duty cycle square waves from the MCU on a “Pump On” command, I basically had to us e Ti 74HC123 dual one-shot to make a 120 Hz square wave to be fed into the enable pin of the L298. I also had to delay the 120 Hz square wave to be approx. 30 degrees out of phase with the 2 60 Hz complementary inputs.by running it into a 74HC14 schmitt trigger hex inverter, then a RC filter then back into the hex inverter. I used both the pulse duration timing feature of the 74123 and the RC filter by using pot resistors to vary duty cycle and phase of the enable signal to make sure there was enough dead-time to prevent shoot-through. Outputs were clamped with schottky diodes. All in all, it did function as a concept but lacked enough punch to start the motor every time. a lot of wave distortion from the motor I think prevented it to spin up properly. My final assesment is that it is perfect for DC motors, but not so much for low-voltage AC motors. I thought of filtering and then amplifying, but then i thought, maybe that’s all the original chip did in the first place!
So…I went off to build a buffer-filter-amplify circuit (x3) with a push-pull amp at the end (for each compliment wave), that would gradually build the 2 MCU-supplied square waves (50% duty-cycle) into two perfect sine waves with enough juice to to consistently spin-up the little 12v ac motor. Much easier said than done! Requires a lot of math and a ton of impedance-matching and way out of my league to design as well as out of my patience range. it spun the motor but lacked enough head to move water around. If that’s how the original IC functioned (now doubtful), I didn’t want to pursue it any further because it required both extreme accuracy of design and a lot more circuitry than I thought.
After going through countless you-tubes on the subject, I came across a fellow who pretty much built an inverter h-bridge circuit from scratch using various control IC’s and MOSFETs in a H configuration. I was impressed enough because it was doing something that maybe my original design lacked: boot-strapping the switching transistors. I don’t think it’s possible with the L298 and while inspirational, I knew h-bridge drivers existed that controlled external transistors shoot-through compensation and internal bootstrapping capabilities. had some old UPS’s hanging around and decided to raid them for parts. Lo and behold, I found one in an old APC offline UPS, an Intersil HIP4082. What’s cool about it is that it has both a user programmable delay circuit pin (determined by resistor) to delay the turn-on of the MOSFETs to prevent shoot-through, and pin provisions for bootstrapping circuits internally (using external BS capacitor and diode), very handy! Once I got the SOIC to PDIP adapter delivered and once mounted, it was a breeze to hook up. I basically just used the MOSFETs that were in the UPS unit. Although way overkill, free trumps everything. Circuitry remained simple and not a whole lot of fuss to get going. Even though the output remained a square wave, the motor had no problem digesting it and in fact, I had to close off the intake gate (pump) because it was pumping too much! The output wave didn’t even distort that much like it did when using the L298. I could probably do a buffer-filter-amplify on the output to get a triangle wave to tone down the pumping action since it saves so much extra circuitry on the back end. We’ll see on my final design! I just had to post one more time because I was elated to get something to work the way that I imagined it would work.
Another observation, at least in my instances, CMOS and TTL logic level seem to be very compatible. A TTL “ON” or “1” states minimum is usually 2.5v minimum which CMOS 3.3v level easily overlaps. Maybe there’s instances where this can be a problem, but as long as you buffer, it won’t make a difference and I didn’t need any conversion between CMOS and TTL.