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 Post subject: Speed control circuit
PostPosted: Jun Sat 13, 2020 3:57 pm 
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Location: Warner Robins, GA
I have the following speed control circuit for a VCR video head motor used with my RP-190.

Attachment:
Speed control final 3.jpg
Speed control final 3.jpg [ 86.48 KiB | Viewed 1198 times ]


It was mentioned way back that regulators operate better with a minimum load.

What load should I put on the LM-317?

Also does the 7805 which feeds the TL-082 and LM-317 need an extra load or is the load provided by those two devices enough?

The issue is I have to slightly readjust speed each time I power up the phono which I suppose could be due to the lack of load on the 317.

Other than that are there any other improvements I can do?

I at one time wanted a circuit using chips which would be much better, but having used this circuit it seems to work well enough as in the speed stays constant once initially adjusted to where I think I'll keep it. I'd say this circuit is about as speed stable as the Pioneer PL-530 record player is if not slightly more stable.

The diode I think is a 1N4002 if I remember right. Are there any other diodes that would be a better choice?

Also I may look at adding a bridge rectifier so that I can power it from an AC transformer instead of needing a DC supply.

Isn't that bad of a circuit for it being built from parts on hand.


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 Post subject: Re: Speed control circuit
PostPosted: Jun Sat 13, 2020 4:55 pm 
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Location: Santa Rosa, CA
Most any power diode is better than a 1N4002. Why not a 1N4007? I assume your transformer secondary might be >30 V rms, so the peak is at least 43 V. A 1N4002 is only rated for 100 PRV. I would spend a extra dime to get more voltage margin.

Your incoming DC is pulsed half-wave as you have no filter or reservoir cap after the rectifier. All of your regulator chips will likely drop out during the 8.3 millisecond periods between DC pulses.

The 2200 uF on the output needs to be on the input side with a smaller cap at the output.

Rich


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 Post subject: Re: Speed control circuit
PostPosted: Jun Sat 13, 2020 5:07 pm 
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The transformer takes the tach signal from the motor (has DC on the tach output so the transformer is needed for isolation as the dc doesn't change) and the diode converts the signal to a DC voltage. The only reason that works is because as the motor slows down the tach output reduces in amplitude which reduces the resultant dc voltage.


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 Post subject: Re: Speed control circuit
PostPosted: Jun Sat 13, 2020 5:18 pm 
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So you are using the tach signal as the power source for this controller? Is there some other source of DC power that I don't see?

Seems like a 2200 uF cap would really load down the signal from the tach.

Rich


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 Post subject: Re: Speed control circuit
PostPosted: Jun Sat 13, 2020 5:56 pm 
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The input is the 15-30Vdc.

I think the motor power is taken across the 47uF cap if I recall correctly so that it gets a max of 12.35Vdc since its controller chip is rated +15 max.


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 Post subject: Re: Speed control circuit
PostPosted: Jun Sat 13, 2020 7:33 pm 
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Rich, W3HWJ wrote:
Most any power diode is better than a 1N4002. Why not a 1N4007? I assume your transformer secondary might be >30 V rms, so the peak is at least 43 V. A 1N4002 is only rated for 100 PRV. I would spend a extra dime to get more voltage margin.

Your incoming DC is pulsed half-wave as you have no filter or reservoir cap after the rectifier. All of your regulator chips will likely drop out during the 8.3 millisecond periods between DC pulses.

The 2200 uF on the output needs to be on the input side with a smaller cap at the output.

Rich

Rich, W3HWJ wrote:
So you are using the tach signal as the power source for this controller? Is there some other source of DC power that I don't see?

Seems like a 2200 uF cap would really load down the signal from the tach.

Rich

I am confused by the above quotes and the subsequent responses. Please let me know where my analysis of this circuit is incorrect.

When I first took a look at it I assumed the power supply was 15 to 30V dropped down to 12.35V by one biased 7805 regulator which I now know from Tube Radio's description is being used as the DC supply for the VCR motor. The input voltage is further dropped down to 5V by the other 7805 which supplies the TL082 and the variable LM317 reference regulator.

The LM317 regulator is used as a variable voltage reference for the TL082 (I don't know why the regulated 5V from the TL082 supply couldn't have been used with the variable resistive voltage divider instead of the LM317). The TL082 is being used as an integrator with gain which takes the 1/2 wave input from the tachometer and compares it to the variable voltage reference from the variable LM317 regulator. The difference is then integrated (rectified) and amplified.

The varying output signal from the TL082 controls the speed of the VCR motor.

Tube Radio wrote:
Other than that are there any other improvements I can do?

Removing the LM317 and using the regulated 5V DC from the 7805 with the variable resistive divider as a reference is at the very least a simplification. Because there is no longer the requirement that the LM317 must have at least 10 ma. minimum total load current, the values of the variable resistors can likely be increased by an order of magnitude to reduce the load on the 5V 7805 regulated supply.

In general the data sheets for 78 series voltage regulators suggest a bypass capacitor both on their inputs and outputs.

A small filter capacitor on the non-inverting input Pin 3 of the TL082 might be a benefit.

All 3 suggestions are shown in the second schematic below.


Attachments:
Speed control final 3.jpg
Speed control final 3.jpg [ 86.39 KiB | Viewed 1157 times ]
Speed control final 3 mod.jpg
Speed control final 3 mod.jpg [ 84.51 KiB | Viewed 1114 times ]
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 Post subject: Re: Speed control circuit
PostPosted: Jun Sun 14, 2020 3:28 am 
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The reason for the regulator is that the main adjust control is on the board, but the fine adjust is mounted to the rear of the cabinet and the wire length was affecting the circuit.

The LM-317 solved that issue.


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 Post subject: Re: Speed control circuit
PostPosted: Jun Sun 14, 2020 5:00 am 
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Tube Radio wrote:
The reason for the regulator is that the main adjust control is on the board, but the fine adjust is mounted to the rear of the cabinet and the wire length was affecting the circuit.

The LM-317 solved that issue.

A filter capacitor on the non-inverting input Pin 3 might have also solved the issue.


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 Post subject: Re: Speed control circuit
PostPosted: Jun Sun 14, 2020 5:34 am 
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Has to do with the gain I believe which is real high. The circuit itself has a filter cap which is part of the OP-AMP circuit. Filters the control voltage

Not sure the size of filter cap that would have been needed.

I'm sort of stuck with the LM-317 though. Won't be easy to remove.

I forget but I think it was recommended by a forum member to use it.


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 Post subject: Re: Speed control circuit
PostPosted: Jun Sun 14, 2020 12:22 pm 
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Tube Radio wrote:
Has to do with the gain I believe which is real high.

Just for your information, the circuit has a voltage gain of 10, 1M/100K.
Tube Radio wrote:
The circuit itself has a filter cap which is part of the OP-AMP circuit. Filters the control voltage

But not the variable reference voltage on input Pin 3.


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 Post subject: Re: Speed control circuit
PostPosted: Jun Sun 14, 2020 2:25 pm 
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Oh ok. I see.

What value of cap should I use? Also what resistor to make the 317 draw the minimum current?


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 Post subject: Re: Speed control circuit
PostPosted: Jun Sun 14, 2020 2:57 pm 
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Tube Radio wrote:
What value of cap should I use? Also what resistor to make the 317 draw the minimum current?

You can use a wide range of capacitors likely from 0.1uF to 100uF, the goal being to filter out any noise that might be induced on the reference voltage input. Voltage regulator spec sheets usually advise some type of filter cap on their input and output.

The LM317 apparently requires a minimum of 10ma. load to ensure good regulation. The values presently used are the only load that the LM317 sees and it is less than 10ma. Good regulation is not an issue in this application. The LM317 is being used only as a variable source. I don't think that varying the resistors from their present values will buy you anything.


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 Post subject: Re: Speed control circuit
PostPosted: Jun Sun 14, 2020 3:40 pm 
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I'll try a cap on the output of the 7805 and 317.

Think I have some 1uF caps.


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 Post subject: Re: Speed control circuit
PostPosted: Jul Mon 20, 2020 3:27 pm 
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Haven't done anything further with the circuit.

Will try the caps sometime this week.

Also I may try to make the LM-317 draw minimum current as when I turn the motor on the speed starts out slightly different and after it has run for a minute or two I usually have to tweak the speed slightly.

Ultimately I want to find a small enough thin enough direct drive record player motor and figure out how to install that as that would be the best option.


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