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Simple PCB modification

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RobertLemon3

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I'm just a mechanical guy, so any help would be appreciated.

I have a Makita Router and a Router Speed control to vary the rpms. My challenge is, I am trying to limit the upper range of the router to not exceed 15000 rpm (standard speed is 22000). The speed controller board is very basic, a general pot, a trim pot, a few resistors and other small components with markings like 104K 400V.

1. What is the easiest way to modify the board to limit the upper speed and maintain the low speeds?

2. What are those components called with the 104K 400V markings (I thought they were capacitors)?

Thanks,
 
1) I can't be sure but it's probably as simple as connecting a fixed resistor in series with the pot. but we need to know the value of the pot.

2) They are 100nF 400V capacitors. The value is in pF, the last digit gives the number of zeros after the first two, i.e 104 = 10 0000pF = 10,000×10^-12 = 100×10^-9, if it said 223 it would be 22nF.
 
Thanks for the response. The value of the pot:

1. My multi-meter was set to 2000K
2. I read the resistance across the outer to legs on my pot
3. The screen settled in at 980.
4. Does this mean it is a 980K (or 1M-ohm) resistor?

Question:
Since I want to reduce the speed by 1/3, would I need to add a 300K ohm resistor in series?

rl
 
I take it you measured the resistance with the pot. disconnected from the controller?

If not the controller might interfere with the measurement because there will be other resistors you won't be able to see?

Anyway, 1M sounds reasonable.

You need a 500k resistor, the nearest standard value is 470k.

If you connect the resistor to the wrong end, then the speed will range from 1/3rd to full instead of 0 to 2/3rd of the maximum speed.

This might not work anyway, I'm just making an educated guess at how the circuit works. :D
 
Thanks for the response.

1. Yes, I disconnected the pot from the circuit. The value should be accurate.

2. Without knowing the guts of the circuit, if I connect it to the wrong end, I can at least swap it out. This is good to know.

3. An educated guess is a whole lot better than a shot in the dark.

Your comments have been educational so I can make informed decisions. Thanks again.

rl
 
Thanks for the reply, please come back and tell us how well it works or otherwise.
 
Oh, just one more thing: reducing the speed might also reduce the power because you'll have the same torque, if not slightly less, at a lower speed so you need to overrate the motor; you're better off with gears.
 
I was able to find the 470K resistor at RadioShack and connected it to the pot. All worked well. I can now run from 100 rpm up to the 14500 range which is what I needed. Thanks for your help. I plan to do a load test this week and check out the performance for torque and speed combined. This was a great find.

Thanks,
rl
 
Thanks for replying.

Good, I'm glad it works, the circuit works as expected. I hope the load test works out.
 
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