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 Post subject: Speed control circuit mod
PostPosted: Jan Mon 08, 2018 8:09 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18579
Location: Warner Robins, GA
Here's a speed control circuit I made with help from a forum member for a VCR video head motor I re-purposed for my RP-190.

Attachment:
Speed control final.jpg
Speed control final.jpg [ 45.82 KiB | Viewed 460 times ]


The problem is I have to use my DMM to measure the tach frequency so I can adjust the speed properly and it isn't always reliable due to the lower voltage of the tach output plus I want to use it with a digital frequency display I have which will not work with that low of a voltage from the tach.

I have an extra section of the TL-082 OP-AMP.

How would I configure it to where I get the full output the OP-AMP can do which would be a square wave so that it can properly drive a frequency counter?

I'm limited to the 5Vdc B+ so I figure I'll be able to get a nearly 5Vpp square wave. I know that I will need to bias the + pin at 1/2 the B+ which can be done with a 1 meg resistor from B+ to the + input pin and a 1 meg resistor from the + input pin to ground to keep the input impedance high, but I don't know what I will need to do to the - input pin in order to get the correct gain.

Figure the square wave at a higher voltage will be better for the frequency counters.

The 15uF cap on the tach output is going to be made smaller when connected to the TL-082 input.


EDIT:

Here's what I figured up.

Forgot the pin numbers on the other half of the TL-082

Will the values of resistors I chose for the feedback and bias resistors work ok? I had used two 1 meg resistors for the bias resistors then realized no I have gain those won't work. I did some calculating and figured I needed a 498mVdc input to make the output 2.5Vdc.

Attachment:
Speed control final 2.jpg
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 Post subject: Re: Speed control circuit mod
PostPosted: Mar Thu 22, 2018 3:21 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18579
Location: Warner Robins, GA
I tried that and it didn't work good at all so I ditched that idea.

When wanting to use the 45 player a couple days ago I had used the 12 volt switvh mode power supply for something else and found a 12 volt unregulated linear supply that originally went to a cable modem.

It's output was around 15Vdc so I added a 12 volt regulator. I also plan on adding a switch to the rear of the cabinet so I can switch the record player on and off instead of having to unplug it every time.

Attachment:
Speed control final 3.jpg
Speed control final 3.jpg [ 58.07 KiB | Viewed 208 times ]


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 Post subject: Re: Speed control circuit mod
PostPosted: Mar Tue 27, 2018 8:59 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18579
Location: Warner Robins, GA
When I added the regulator I used a 100uF 30 volt cap as the input filter cap right off the DC input jack.

B+ under load (me slowing the motor down by hand would drop low enough to where the regulated 12 volts would drop as well.

I had to order some parts for another project so I ordered a 2200uF 35 volt cap.

Installed that today and the 12 Vdc B+ no longer drops under load with the input B+ no longer dropping below 14Vdc.

Also shortly after installing the power switch on the rear the thought came to me that I could have found the original switch, reinstalled it and used that.


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