DC Brushed Motor Circuit for Coffee Grinder - Page 1
Forum section: electronics / beginners

author: logscool
author_rank: Contributor
timestamp: January 31, 2021, 02:06:21 am
content: Hi everyone,I've got a coffee grinder from china that has a built in timer but it seems to fault out and stop grinding sometimes and act like someone interrupted it by pressing the controls.I'm thinking some sort of interference from the brushed DC motor (EMI, BEMF, RF,) is causing issues when it starts grinding.It often acts as though it is receiving inputs to the controls and stops grinding however it seems to work fine if the grinder is empty and there is no load on the motor which makes me think it is an interference issue of some sort.I don't know the specifics of the controller circuitry but it does have capacitive touch buttons which might make it more susceptible to interference.As for powering the motor it seems like a fairly simple design, there is a relay that clicks on and then a full bridge rectifier (KBPC1010) which is then wired right into the DC motor. I'm wondering if some additional filtering or protection circuitry should be used after the full bridge?Also the full bridge seems to receive ~6 VAC even when the relay and motor are turned off, I don't know if this is an issue or expected as part of the design.

author: amyk
author_rank: Super Contributor
timestamp: January 31, 2021, 02:24:38 am
content: Schematic please... it's much easier to see what's going on that way.

author: beanflying
author_rank: Super Contributor
timestamp: January 31, 2021, 02:35:51 am
content: At least a model number or a picture would be a startTypically DC based grinders use a Mabuchi 540 sized can which the brushes will handle 10-12 Amps without to many problems so what id the current rating of the relay? Is there any sort of current shunt that indicates it might have a current limiting feature as part of the circuit?Also is it a new grinder or is it one that has been used before? Coffee Oils and gunge (technical term) get stuck on the burrs and body and may be causing it to get stuck or do you have it screwed down so tight that the burrs may actually be touching?

author: logscool
author_rank: Contributor
timestamp: January 31, 2021, 02:58:20 am
content: It is nearly identical to this grinder listed on aliexpress.https://www.aliexpress.com/item/4000784206551.htmlThe grinder is new and the burrs are not touching and it runs fine without any beans loaded in.It does have an overcurrent protection/circuit breaker button on the back. I don't know what it's rated for but it hasn't gone off.It's kind of tricky to get pictures of the boards inside as they are glued in place but I gave it a shot.Also to be clear the grinder does run and will grind just fine but then it sporadically stops and acts as though someone pressed a button on the control panel.

author: beanflying
author_rank: Super Contributor
timestamp: January 31, 2021, 03:22:26 am
content: Bit more hardcore than I was expectingMy 3 group Lever Espresso machine sits on my Electronics bench along with my K10 Compak grinder and the other end sits my 1kg Roaster so welcome fellow nerdInitial thoughts the Power Supply is pathetic and is likely getting pulled down to a point where the controller and relay is cutting out. At a rough guess that motor it going to be circa 200-250W so 20'ish Amp rated at 12V. That transformer is more like 30W on the power supply board. The 10A relay is also similarly under rated for the motor.So what I would be doing is putting a multimeter across the motor supply wiring or much better if you own one a Storage Oscilloscope and looking at the voltage and if you have the gear do that on the output of the transformer too. Throw some beans in it and watch.EDIT Having a second look over a coffee are you sure it is a DC motor?Drop a multimeter on it and check in which case some of what I wrote above is wrong as the supply is only needed to provide power to the relay and controller.

author: logscool
author_rank: Contributor
timestamp: January 31, 2021, 03:48:23 am
content: Nice that sounds like a sweet setup! A lever machine is one of the things I'm looking at diving into next. Right now I've got a single group Laurentis heat exchanger machine and a breville dual boiler along with a mazzer super jolly and a bunzilla grinder.So I think you are kind of right with both of your statements...I think the power supply is just for the controller and relay as you said but the motor is still DC.Maybe it wasn't really clear from my all over the place pictures but the wires running from the board to the motor are 120 VAC there is then that bridge rectifier (KBPC1010) kind of bodged inline and 120 V DC goes straight into the motor.I don't really know much about power supply design or driving brushed motors but I'm thinking the lack of any circuitry beyond the bridge rectifier could be causing issues. Seems like more should be needed.

author: james_s
author_rank: Super Contributor
timestamp: January 31, 2021, 03:51:37 am
content: You could try adding a pair of 0.01-0.1uF or so ceramic capacitors from each motor terminal to the motor case, make sure to use sufficiently high voltage rated parts, 1kV is probably a good idea to be safe.I suspect the problem is more likely to be faulty electrolytic capacitors in the low voltage power supply though.

author: beanflying
author_rank: Super Contributor
timestamp: January 31, 2021, 04:08:06 am
content: Ok so from that the motor is fairly high voltage DC but fed from the Relay straight to a Bridge. If as it sounds like there is NO filter capacitors after the Bridge and NO filter capacitors on the motor itself then yes maybe it is noise generated by the motor.If you cant see a Capacitor of a decent size after the bridge then the motor is seeing something like the first waveform. If there is a Cap after then like pic 2So in either case at best there is a large or smaller ripple to the motor. No Capacitor as a supply is a complete No No so you will need to look at something like this series of calculations with an appropriate voltage rating as wellhttps://makingcircuits.com/blog/calculate-filter-capacitor-smoothing-ripple/Edit as per what James wrote above. and as follows nextWhen you get that sorted there is additional steps you can take to reduce the spikes from the brushes we used to do this a lot on the earlier Brushed DC motors we used for R/C aircraft. So something similar to this sort of solution. It is possible the motor may even have them fitted but as the manufacturer could save $0.02 I doubt it. Background reading for a start this is 'ok'https://www.pololu.com/docs/0J15/9same deal watch the voltage ratings!

author: WattsThat
author_rank: Frequent Contributor
timestamp: January 31, 2021, 04:24:26 am
content: Yes Yes Yes. Those DC motors are fine with rectified AC. Modern DC variable speed drives are the same concept and those are built to power motors 10,000 HP and larger. Same as the older DC based treadmills. No DC smoothing caps. In some extreme situations, where it would affect proper commutation in the motor, you’ll find inductors, not capacitors.I suspect all the timing and smarts are in the timer module, that pc board appears to be just the power supply, relay and it’s switching transistor.I agree with ceramic caps on the motor and if that doesn’t fix it, some photos of the timer module may help with placing some additional noise suppression. It appears there is MOV on the pc board, it’s that blue disc. That is a voltage spike suppressor and based on where it is on the board, it is probably across the black wires that are the ac supply to the rectifier. Have a good look at the bottom of the pc board, it may be as simple as a bad solder joint.

author: logscool
author_rank: Contributor
timestamp: January 31, 2021, 04:49:08 am
content: Thanks for the tips everyone, I will try and get some caps to put on the motor and maybe try twisting the motor supply wires and moving them away from the controller. Would it also help to put some sort of shielding around these wires, would it need to be grounded?Yeah I wish I could easily pull out the boards have a look at the solder joints or really see anything on the controller board but it's all just glued in place. Might have to start breaking the glue and digging in if these don't work.

author: james_s
author_rank: Super Contributor
timestamp: January 31, 2021, 05:47:34 am
content: The older electric weed eaters and lawn mowers didn't even have a rectifier, they used a single diode to give the motor half wave pulsating DC.Assuming this thing worked properly at some point you shouldn't need to add shielding or anything like that. Something that was ok has deteriorated and is now causing problems.

author: floobydust
author_rank: Super Contributor
timestamp: January 31, 2021, 09:11:18 am
content: Where is the chassis ground wire? The power cord earth-ground should connect there.The PC boards just glued in place on a piece of fish paper?I have a Baratza Virtuoso+ conical grinder and every morning I appreciate spending the extra money on it.

author: logscool
author_rank: Contributor
timestamp: January 31, 2021, 07:20:39 pm
content: There is a ground wire connected to the motor casing even though it's disconnected in the pic I took. It's the only ground connection though, I'm thinking I should add a direct case ground as well.Yeah the electronics quality are well not exactly great hahah. The mechanical side of the grinder seems pretty good though. I figure worst case I will completely scrap the built in controller and just put in a switch for motor on/off. For my use I don't really need a timer but if I can get everything to work as "stock" it might look a bit better.Edit:Also I've found that if I set it up with the case placed next to the rest of the grinder and one of it's metal panels in between the motor and the electronics it seems to work pretty well. So this would seem to suggest it's some sort of induced noise issue or something like this? Would adding the capacitors to motor ground help with this noise?

author: james_s
author_rank: Super Contributor
timestamp: February 01, 2021, 01:10:08 am
content: Quote from: logscool on January 31, 2021, 07:20:39 pmIt certainly won't hurt. Most equipment that uses motors like that has capacitors to prevent RF noise.

author: floobydust
author_rank: Super Contributor
timestamp: February 01, 2021, 08:45:17 pm
content: It's hard to know how the EMI gets into the timer board.First ensure both the enclosure and motor housing have a safety ground. Nobody wants to die making coffee.I would first add a big X-cap across the motor terminals, close as possible. That might loook like a 0.1-0.22uF 275V cap at the bridge rectifier DC output.You can add a clamp on ferrite bead with a loop on those motor wires (red,blue).Beyond this, adding two Y-caps to the motor and next is looking at the timer board grounding. The ribbon cable likely picks up EMI.Is it a conical burr grinder? The mechanical looks good it's just the electronics are Wun Hung Lo and no metal standoffs to properly mount the boards.Coffee shop commercial grade grinders are silly expensive $3,000-$5,000.

author: logscool
author_rank: Contributor
timestamp: February 02, 2021, 05:56:27 am
content: Thanks for all the suggestions I've got some 0.1uF 1kV ceramic capacitors coming to add between motor terminals and to ground. I'll also definitely be adding case grounding.Also the manufacturer is actually going to send me a replacement main board and controller board as they think there is a fault with the controller board, but they may take a while to arrive. Also it turns out the controller board actually can be removed with screws I just didn't notice how it was screwed in to the plastic glued to the casing. The main board is still definitely just glued straight on though.Yes the grinder uses 83mm conical burrs which are pretty much the biggest available in a conical style grinder. Yes the mechanical portion of the grinder while maybe not as refined as some of the very high endfully custom CNC hand made grinders is quite good. It should hopefully allow the grinding burrset to do a good job

author: floobydust
author_rank: Super Contributor
timestamp: February 02, 2021, 11:54:10 pm
content: Also note when you assemble the thing, the mains and motor wiring should not push up/be as far away as possible from the backside of the controller and the ribbon cable. Maybe sticky tie-wrap clamps could help the lead dress.For USD $1,000 it looks strong if the burrs are decent metal. A bit weird the motor is DC, not universal.

author: beanflying
author_rank: Super Contributor
timestamp: February 03, 2021, 12:00:01 am
content: Quote from: floobydust on February 02, 2021, 11:54:10 pmDone for Torque reasons I suspect. Able to crunch concrete, sand and other defects easier

author: james_s
author_rank: Super Contributor
timestamp: February 03, 2021, 02:49:07 am
content: Quote from: floobydust on February 02, 2021, 11:54:10 pmThe motor looks an awful lot like an automotive windshield wiper or power liftgate motor. It wouldn't surprise me if they used an off the shelf design and just wound the armature for line voltage. Starting with a frame designed as a permanent magnet motor it wouldn't be very practical to convert to a wound field universal motor. Lots of small appliances have PM DC motors in them, paper shredders, string trimmers and electric mowers come to mind, though the latter two are quickly transitioning to brushless motors and battery power.