Aniol1349

Low Voltage, high current DC motor turntable PSU
« on: April 12, 2018, 06:54:06 AM »
Hello guys,

sorry for the slightly odd topic, perhaps not really related to pro audio but I do need some advice!

I would like to run a DC motor with an LM317T. (turntable PSU)

The voltage range I need is 1.25V-2.5V but I need constant current at around 500ma.

Could anyone advice on how could this be achived?

Thanks for your time!


JohnRoberts

Re: Low Voltage, high current DC motor turntable PSU
« Reply #1 on: April 12, 2018, 09:55:38 AM »
Hello guys,

sorry for the slightly odd topic, perhaps not really related to pro audio but I do need some advice!

I would like to run a DC motor with an LM317T. (turntable PSU)

The voltage range I need is 1.25V-2.5V but I need constant current at around 500ma.

Could anyone advice on how could this be achived?

Thanks for your time!
Do you want a PS that is an adjustable voltage source or constant current source? Or a voltage source with current limiting?

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Aniol1349

Re: Low Voltage, high current DC motor turntable PSU
« Reply #2 on: April 12, 2018, 12:40:06 PM »
@JohnRoberts

Hi John,

Well I'm not sure what the best solution for me would be, I know the motor is running at the correct speed at around 1.5V however setting the LM317 to that output only gives me around 150ma of current which is to little to drive the plater and the motor stalls.
When I set it so I have enough current  then the voltage and speed is way to high.




JohnRoberts

Re: Low Voltage, high current DC motor turntable PSU
« Reply #3 on: April 12, 2018, 12:47:25 PM »
The LM317 data sheet says it is capable of 1.5A.

If you set it for 1.5V it should put out as much current as the load draws trying to maintain that voltage (assuming the unregulated supply can provide the extra current).

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

PRR

Re: Low Voltage, high current DC motor turntable PSU
« Reply #4 on: April 13, 2018, 01:09:52 AM »
The turntable probably had a controller board with speed feedback which dynamically adjusted the motor power to hit 33 or 45 RPM.

Constant current makes "no" sense for most motors in fixed-RPM use.

Constant voltage "will" make near-constant RPM, but for most small motors at a speed far too high for a turntable.

There are many ways to build a motor controller, none of them dead simple.

Gene Pink

Re: Low Voltage, high current DC motor turntable PSU
« Reply #5 on: April 13, 2018, 03:11:29 AM »
The turntable probably had a controller board with speed feedback which dynamically adjusted the motor power to hit 33 or 45 RPM.

Constant current makes "no" sense for most motors in fixed-RPM use.

Constant voltage "will" make near-constant RPM, but for most small motors at a speed far too high for a turntable.

There are many ways to build a motor controller, none of them dead simple.

Many little cassette deck and turntable motors back in the '70's and '80's had on-board speed regulation, a chip inside the motor, right on the circuit board that also held the brushes. Knowing the voltage and current going in, the chip could figure out in analog what the RPM was, and compensate for varying loads. Armature current feedback.

Much like the 1/4HP variable DC drive on my drill press, it is a tried and true method. 8)

Gone are the 8-track cartridge player days, when the motors had a centrifugal switch on the armature that opened at overspeed, disconnecting one of the three commutator segments from its winding, to slow it down. Barbaric, but it worked, with a rubbery bouncy belt and a 1/2 pound capstan flywheel,  that acted like a physical R-C filter for RPM.

Gene


Re: Low Voltage, high current DC motor turntable PSU
« Reply #6 on: April 13, 2018, 04:04:34 AM »
I'm by no means an expert on the subject but 1,5V sounds awfully low for a turntable motor (if it's direct drive, that is)

A standard Technics SL-1200mk2 for example has a torque of 1,5kgf/cm which is relatively low for a direct drive motor (but enough) as most professional turntables in production today are in the 2kgf/cm mark...

Most turntables also have a dot marking in the side of the platter to calibrate the pitch range..  the dots should stop moving at 0 and -/+ 3,3%, my Reloop RP-4000m3ds have a dot marking for +7,2% as well but they have a -/+10% pitch range (can be adjusted to -/+20%) whereas the Technics are -/+8% (some later models have an option for -/+16%)

EDIT : that is, for DJ use.. I have little to no experience with audiophile turntables. I've seen some people modify a Technics SL-1200mk2 with a Rega tonearm though.
« Last Edit: April 13, 2018, 09:43:10 AM by efinque »

Aniol1349

Re: Low Voltage, high current DC motor turntable PSU
« Reply #7 on: April 13, 2018, 10:05:03 AM »
Thanks for your input everyone,

Yes the turntable is Pink Triangle Anniversary(belt drive) with Battery powered servo drive. I have been trying to repair the PSU but I'm unable to locate the fault.

The was no schematics available for the unit so I have attepmt to create them myself ( see attachment) I'm at a point that I considered making a smimple PSU for it but as I can see now it won't be simple.

Basically the fault is: the motor goes into runaway. The output voltage is 8V, the AC tacho signal feeds back, its rectified and compared to Vref IC.

I have replaced basically every active component and recapped the whole thing. For some reason non-inverting and inverting input signal on the last stage of the feedback ic is the same, but there is no short, the incoming AC tacho signal has no effect on that input.

If someone could give me a hint to why is that I would be the most gratefull.

btw. it was my first attempt at making a schematic diagram from a PCB so excuse the overcomplicated layout




JohnRoberts

Re: Low Voltage, high current DC motor turntable PSU
« Reply #8 on: April 13, 2018, 10:45:50 AM »
That schematic hurts my head...   can I ASSume the motor is connected to CN1?

Over the decades I've worked with lots a small dc motors and a common controller uses a simple negative impedance convertor (to respond to changing motor current draw). These simple controllers are often on a small PCB inside the motor casing.

Back in the 70's I experiments with trying to detect motor brush switching to extract a free tachometer signal but I was unsuccessful (it isn't easy because when the motor is coasting no current pulse to detect). 

Sorry for the non-answer.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Aniol1349

Re: Low Voltage, high current DC motor turntable PSU
« Reply #9 on: April 13, 2018, 12:05:54 PM »
John,

my apologies - I will reimburse you for painkillers!

Yes CN1 is the motor output and AC tacho input.
I didn't intended to share this with anyone hence the poor labeling etc, some parts are simplified without passive components.

I guess my main question would be what could cause the motor to go into runaway?

I assume the motor itself is fine, I can apply external DC voltage and it works good, AC tacho produces output so I'm focusing on the PSU.

The PSU output is 8V however I did read somewhere on another forum that someone did measure the output to be around 1.5V for 33rpm, this is also the voltage I get when powering the motor and the platter of an external supply for 33rpm.





JohnRoberts

Re: Low Voltage, high current DC motor turntable PSU
« Reply #10 on: April 13, 2018, 04:11:26 PM »
John,

my apologies - I will reimburse you for painkillers!
I'm already taking 220 mG of Naproxen Sodium, two times a day, and currently icing my arthritic knee after several hours of yard work.  :(   So I don't know that more would help (of course more painkiller would help something but nah).   8)
Quote
Yes CN1 is the motor output and AC tacho input.
OK tachometer-servo 101... if the tachometer is not reporting speed properly, the servo will say go faster, so check the tachometer-servo loop.

Do you have a scope?  If not, even a VOM can help troubleshoot the tachometer output...  that should be maxed out in runaway conditions.
Quote
I didn't intended to share this with anyone hence the poor labeling etc, some parts are simplified without passive components.

I guess my main question would be what could cause the motor to go into runaway?

I assume the motor itself is fine, I can apply external DC voltage and it works good, AC tacho produces output so I'm focusing on the PSU.

The PSU output is 8V however I did read somewhere on another forum that someone did measure the output to be around 1.5V for 33rpm, this is also the voltage I get when powering the motor and the platter of an external supply for 33rpm.
Focus on the servo loop...

If you vary the motor speed open loop does the tachometer follow linearly?

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Aniol1349

Re: Low Voltage, high current DC motor turntable PSU
« Reply #11 on: April 14, 2018, 07:14:36 AM »
John,

Sorry to hear about your knee!

Out of circuit the motor behaves correctly - increasing Vdc input linearly increases the AC output from the tachometer.

Half of the sinewave(other half is grounded) gets into U7, gets rectifed by the two diodes but on the last stage U7.4 the pin 2 and 3 are sitting at the same voltage no matter what happens to the AC tacho input. The output of that stage is also sitting at a fixed voltage.

I can't find anything that is shorted, I have replaced that IC,  All the caps also.

The only way I was able to drop the speed was to increase resistance going into pin 3 on U7 - there are two trimpots for fine speed adjustment. Thats unusable tho, not enough torque to maintain speed. 

EDIT: just to add to my observations -  both V supply(pin 4 on U7) and Vref (pin 3 on U7) are going through the same rotary switch. This is a 4 position switch OFF - 33-OFF-45. I'm measuring resistance between pin 4 Vsuply and pin 3 Vref and its sitting at around 3kohm. But I can vary it by using one of the trimpots. Could I suspect that somehow there is a short between the supply rail and that input which can confuse the chip?




« Last Edit: April 14, 2018, 07:35:19 AM by Aniol1349 »

Aniol1349

Re: Low Voltage, high current DC motor turntable PSU
« Reply #12 on: April 14, 2018, 10:36:34 AM »
https://youtu.be/OZyJIw7OBh0 

I made a short clip of the tacho circuit behaviour. Maybe that would give a clue to whats going on.

When the PSU is ON there is 130Vdc on the AC tacho input pin. 0VDC when in OFF position

JohnRoberts

Re: Low Voltage, high current DC motor turntable PSU
« Reply #13 on: April 14, 2018, 10:48:29 AM »
John,

Sorry to hear about your knee!

Out of circuit the motor behaves correctly - increasing Vdc input linearly increases the AC output from the tachometer.

Half of the sinewave(other half is grounded) gets into U7, gets rectifed by the two diodes but on the last stage U7.4 the pin 2 and 3 are sitting at the same voltage no matter what happens to the AC tacho input.
That is valid for opamp operating with negative feedback that forces the - input (2) to follow + (3).   Pin three is connected to SW1 where it gets the speed reference DC voltage.
Quote

The output of that stage is also sitting at a fixed voltage.
The output of U7.4 will be the reference voltage at pin 3 , plus or minus current into/out of pin 2 times  R9
Quote
I can't find anything that is shorted, I have replaced that IC,  All the caps also.

The only way I was able to drop the speed was to increase resistance going into pin 3 on U7 - there are two trimpots for fine speed adjustment. Thats unusable tho, not enough torque to maintain speed. 

EDIT: just to add to my observations -  both V supply(pin 4 on U7) and Vref (pin 3 on U7) are going through the same rotary switch. This is a 4 position switch OFF - 33-OFF-45. I'm measuring resistance between pin 4 Vsuply and pin 3 Vref and its sitting at around 3kohm. But I can vary it by using one of the trimpots. Could I suspect that somehow there is a short between the supply rail and that input which can confuse the chip?

Lets pursue the tachometer output.

With no tachometer U7.3 output will be 0V commanding full speed. Making r10 larger is in effect defeating the low tachometer output telling it to speed up.

The output of  U7.3 pin 14 should increase with motor speed in response to pulses (?) at output U 7.2 but as drawn that is unclear how that could happen. The negative feedback will work to hold pin 9 at 0V just like pin 10.  As drawn the output of U7.3 will always be 0V commanding full speed from servo loop.

Are you confident about schematic around U7.2?   Or maybe I'm wrong?

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

JohnRoberts

Re: Low Voltage, high current DC motor turntable PSU
« Reply #14 on: April 14, 2018, 10:51:43 AM »
https://youtu.be/OZyJIw7OBh0 

I made a short clip of the tacho circuit behaviour. Maybe that would give a clue to whats going on.

When the PSU is ON there is 130Vdc on the AC tacho input pin. 0VDC when in OFF position
where exactly was the scope probe  reading?

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

JohnRoberts

Re: Low Voltage, high current DC motor turntable PSU
« Reply #15 on: April 14, 2018, 10:52:44 AM »
where exactly was the scope probe  reading?

The sinewave looks good, but is not changing voltage at U7.3 (my speculation).

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

JohnRoberts

Re: Low Voltage, high current DC motor turntable PSU
« Reply #16 on: April 14, 2018, 11:48:38 AM »
The sinewave is taken from AC tacho input, the second probe is reading from pin 1 - output.
There is no node labelled AC tacho input on your schematic. (that I can find).
Quote
I get 200mvDC and 1.59VAC on inverting input of U7.3
since pin 13 is grounded, NF should keep pin 12 at nominally 0V.

what is pin 14 doing while it's - input is swinging volts? Output of op amp should be pegged from rail to rail whenever there is more than a few mV of difference between inputs. 

Something is wrong about U7.3 behavior.

JR

PS: It is not reliable to probe op amp - inputs as scope capacitance can cause errors.
Quote
when I hand turn the patter everything seems to be working nicley, I can see the voltages rising, sinewave rising in amplitude and frequency. When I switch it to ON it immidietely goes to a fixed value
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Aniol1349

Re: Low Voltage, high current DC motor turntable PSU
« Reply #17 on: April 14, 2018, 12:18:03 PM »
Quote
There is no node labelled AC tacho input on your schematic. (that I can find). since pin 13 is grounded, NF should keep pin 12 at nominally 0V.

AC input is Pin 3 on CN1.

With AC tacho input disconnected pin 13 sits on 70mVdc slowly dropping as platter gains speed.
With AC tacho input connected pin 13 sits on 200mVdc going up and down (change doens't seem to be dependand on platter speed)

Quote
what is pin 14 doing while it's - input is swinging volts? Output of op amp should be pegged from rail to rail whenever there is more than a few mV of difference between inputs.
Something is wrong about U7.3 behavior. 

Pin 14 sits at 0.840mv with AC tacho input connected slowly increasing by few mV as the platter spins , without it sits fixed 0.7Vdc, on both speeds.


Quote
PS: It is not reliable to probe op amp - inputs as scope capacitance can cause errors.

Thank you for the tip.

JohnRoberts

Re: Low Voltage, high current DC motor turntable PSU
« Reply #18 on: April 14, 2018, 03:35:21 PM »
AC input is Pin 3 on CN1.
gotcha
Quote
With AC tacho input disconnected pin 13 sits on 70mVdc slowly dropping as platter gains speed.
Since I don't see a negative supply I need to rethink the LM324 behavior with - rail grounded.

The LP324 with - rail grounded will accept as low as -0.3V at inputs and still work (due to darlington connected PNP input devices). Output will only pull down to +0.8V or so. 

So the tach circuit U7.2 will look like a low impedance for inputs (through R11) that turn on D2 and LM324 will pull as low as it can (maybe 0.8V).  When the tach signal at R11 swings negative and pull pin 9 lower than pin 10 the LM324 turns on again clamping  so output of U7.2 is clamped in both directions.
Quote

With AC tacho input connected pin 13 sits on 200mVdc going up and down (change doens't seem to be dependand on platter speed)
Pin 13 at +200mV will peg the LM324 low which is what you measure.  But loop is saturated off thus the max speed.
Quote
Pin 14 sits at 0.840mv with AC tacho input connected slowly increasing by few mV as the platter spins , without it sits fixed 0.7Vdc, on both speeds.

I just looked at data sheet and similar to old (cheap) LM324...   0.7-0.8V on output sounds like pegged low with single supply operation.

logically for this servo loop to work pin 13 should spend some time closer to 0V for U7.3 to modulate loop.

This design is hurting my head again...   :o

JR
Quote

Thank you for the tip.
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.


 

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