Do these potentiometer numbers mean anything to anyone?

[whydontu]

The 1000uf are power supply filtering, way too big for PWM filtering. Looking at the MC2100 TM PWM controller, they’re not using any filtering. Seems there are differing opinions on whether it’s needed with PWM.

MC2100_rev_engrd.pdf

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My guess is don’t bother way a cap, they’re cheap enough that the designers would have added them if there was a benefit.

 

[Tecnico]

Just reminding that my electronic skills are limited so my questions will probably show it....

I believe the schematic shows U5 (which I think is an MIC4427 P/N not 4227) driving a MOSFET, presumably with a PWM input from the micro/Pin 8. I think that's roughly the logic of the board sitting beside my keyboard as well. My initial plan is to replace the input from the micro on my board with a stand alone PWM generator from AliEx, mainly because I want to ditch the TM display but also because the display and main board aren't talking right now.

AliEx PWM Module, XY-KPWM

That said if that doesn't work out then my fallback is an SCR module like THIS which Daze says needs the choke. As far as building a power module to drive the TM motors, that would be the simplest but more expensive and with the filtering concern that started this conversation for me.

What I'm not clear on is whether the SCR based unit will have an output signal as good as the PWM/MOSFET type or if it needs to be cleaned up either by a choke or whether a capacitor would be a suitable alternative.

Thanks!

D

 

[whydontu]

The SCR module you identified works best with a choke. I experimented with one just before I got my MC60 TM motor and controller.

Quick-n-dirty:

An SCR motor control switches a pulsating DC supply voltage on and off at 120Hz full-wave DC derived for the 60 Hz line frequency. So the frequency fed to the motor is chopped but the fundamental frequency is still 120Hz, just with the on/off ratio adjusted. An inductor has low resistance at DC, increasing resistance as frequency increases. Easy enough to select an inductor that works at 120Hz but can still handle the high motor amperage.

Inductor resistance in ohms = 6.28 x frequency x inductance in Henrys. 1H at 120Hz = 754 ohms. This is in series with the motor windings. Keep in mind this is reactance, so the DC component of the signal goes right through the inductor relatively unchanged.

PWM uses a much higher pulse frequency, so any inductor that could work would have to be very small to allow the required motor current without being very inefficient. Assuming the controller PWM frequency is 20KHz, our same 1H inductor would be 120k ohms. So we’d have to use a very low inductance value, which means it won’t do anything. In fact, the motor internal inductance would have far greater impact than any practical external inductor.

Conversely, using a capacitor on the PWM drive is also pointless. Any capacitor that would smooth out the spikes is going to have very low impedance at the high PWM frequency. Capacitor resistance in ohms is a divided by 6.28 x frequency x capacitance in Farads. 1500uf at 20KHz = 0.0053 ohms. And this resistance is parallel to the motor windings. LOTS of current being shunted to ground. So any useable capacitance value would have minimal effect on noise. Maybe something like a 0.01uF would be practical, but long motor leads would swamp out any installed capacitance.

I’m sure they’re someone on the board with many more letters after their name who could explain it better, but if you’re using a PWM controller you don’t always need some filtering. SCR controllers need filtering and an inductor is the most cost-efficient.

 

[jcdammeyer]

My 2 cents.
The SCR in the above circuit is for safety if the micro-processor goes bye-bye. The SCR is switched on by the charge pump signal and effectively the SCR ensures that the voltage across the capacitor remains at about 1.414 * AC line voltage or about 162VDC. The capacitor filters that to make it a reasonable steady DC up from 115VDC. This voltage is connected to the plus terminal of the motor.

The actual speed control is done by the FET on the minus terminal of the motor. Now it's just simple PWM with a 60uS period with a minimum on period of 4uS up to full on in 0.25uS steps or essentially a range of 16 to 240.

The inductance of the motor and back EMF effectively turns that into varying DC voltage with the advantage that the high voltage pulses can make it turn much slower than if it was just a varying DC voltage.

The micro-processor also monitors motor current on Pin 4. Likely to detect a locked rotor condition. I suspect this type of drive will have the typical requirement to turn the speed up until the motor starts turning to overcome static friction and then one can turn the speed control down to reduce it from the initial starting speed. Although it might be clever enough to look at that motor current and provide an initial higher pulse width to get the motor turning before reducing the pulse with to match PWM to a comparable speed control setting.

 

[Susquatch]

Just looking at the board beside me I think the caps are downstream of the rectifier bridge but I think the switching device (driven by the PWM) is downstream of the caps so it may be cleaning up the power supply to the switching device but not the output of the switching section. This brings me back to wondering if capacitors at the output would be applicable. Your thoughts?

I don't know if this will make sense to you but I put it out there just to help you visualize what you are trying to do.

The difference between capacitive filters and inductive filters IN TYPICAL POWER SUPPLIES, is that all the current flows through the inductor, but only the excess current gets stored (and therefore filtered) in a capacitor. So if your input current capacity and rectifier output current is much bigger than your load current, the capacitors can store the excess between cycles and thereby smooth the load current / voltage between cycles. If not, the output will simply draw down the caps and you are left with dirty output.

This need is dictated by the design strategy (power supply type).

Bottom line here is that size matters and so does the choice of inductors vs capacitors ..... Hope that helps a wee bit.

Great time to drag out your scope and experiment!

 

[Tecnico]

Thanks gents that's good info, appreciate the effort that went into it!

If I get this correctly, the SCR type controls speed by voltage amplitude so it is less likely to maintain the desired speed.

@jcdammeyer leads me well into the next question, I have a board which uses PWM and I have pondered adding a stand alone PWM generator to act as the controller. Noting that pin 4 of the example schematic monitors motor current and there are probably other conditions monitored, how big a risk is there to driving the MOSFET directly with the aux board (and a driver chip) and bypassing the original controller circuit.

On a related note, I scooped an MC1200 EDIT MC2100 board today and it appears that it will accept a PWM signal at one of the input connectors so there may be a simple way to drive that board and perhaps looking for a similar arrangement on my other boards may be worthwhile. Would it be unusual for the (upper) dispay board to be the home for the PWM circuit? Any further thoughts there?

Thanks,

D

 

[jcdammeyer]

Thanks gents that's good info, appreciate the effort that went into it!

If I get this correctly, the SCR type controls speed by voltage amplitude so it is less likely to maintain the desired speed.

@jcdammeyer leads me well into the next question, I have a board which uses PWM and I have pondered adding a stand alone PWM generator to act as the controller. Noting that pin 4 of the example schematic monitors motor current and there are probably other conditions monitored, how big a risk is there to driving the MOSFET directly with the aux board (and a driver chip) and bypassing the original controller circuit.

On a related note, I scooped an MC1200 board today and it appears that it will accept a PWM signal at one of the input connectors so there may be a simple way to drive that board and perhaps looking for a similar arrangement on my other boards may be worthwhile. Would it be unusual for the (upper) dispay board to be the home for the PWM circuit? Any further thoughts there?

Thanks,

D

No I think in that above circuit the SCR is used as the safety on/off control since the drive is called a charge pump. As long as it's there the system functions. The actual PWM is directed to the FET and the motor is wired for only one direction.

Way back in the late 90's I started on my own PWM DC treadmill motor controller. I don't remember why I stopped. I think I discovered that it was cheaper to buy something. And the particular controller chip really wasn't good for position control. Now I'd never use a chip like that but instead use some sort of digital signal processor with the appropriate motor H-Bridge drive capabilities.

 

[Tecnico]

No I think in that above circuit the SCR is used as the safety on/off control since the drive is called a charge pump. As long as it's there the system functions. The actual PWM is directed to the FET and the motor is wired for only one direction.

I think there's a minor confusion, I was not referring to the schematic from a few posts up but rather the SCR based motor driver alternative to a PWM approach. The PWM would use a constant voltage but would vary the duty cycle of the on time (pulse width) whereas I was assuming the SCR approach would control the amplitude of the voltage (Pulse Amplitude?).

D

 

[jcdammeyer]

I think there's a minor confusion, I was not referring to the schematic from a few posts up but rather the SCR based motor driver alternative to a PWM approach. The PWM would use a constant voltage but would vary the duty cycle of the on time (pulse width) whereas I was assuming the SCR approach would control the amplitude of the voltage (Pulse Amplitude?).

D

Ah. The SCR type also creates a varying voltage. The filtering is done by the motor inductance.
Points 3 and 4 in this doc https://www.nutsvolts.com/magazine/article/scr_principles_and_circuits describe how the SCR is switched on and how it switches off. Again the timing is such that the SCR is switched on some time after the start of the waveform.

What the above points mean is that the SCR is latched on some time after the signal goes positive. Then stays on until it reaches 0 and current through the SCR drops below the hold point. The fastest if can do that if there is a bridge rectifier in front of it is every 8.3333mS. You can't filter that with a capacitor because then the current through the SCR may never reach 0.