Foot Pedal Adaptation

[LukeKnepp]

Hello Everyone,

Here is what I am doing...
I have a 27V motor that I want to speed control with a foot pedal...
Input voltage 15-20V
I also have a board with an LTC3780 buck-boost that is adjustable with a 5k ohm potentiometer.

My foot pedal is a 5k ohm...
Everything is great!

Until... You back-kick the foot pedal, and it goes to 10k...
Then, zoom the voltage shoots to like 57V, the MOSFETs, capacitors, and diodes blow, and you have a mess....

Also, when the foot pedal is completely released, it is like 2-4Megaohms resistance....
So then the output tries to go to like 1000s of volts....

So, getting to my question, is there a disconnection switch or a way to make so when you back-kick or release the pedal, that it will just shut off???

Or is there a way to take apart the pedal and disengage those to parts of it?

 

[Nigel Goodwin]

Take your pedal to pieces and have a look - unless it's just a simple mechanically linked pot you're probably going to struggle though.

 

[LukeKnepp]

It looks like a little circuit board mounted on it's side, and the pedal pushes and pulls a piece over it, and makes it connect to the circuit board at different places along the path....

 

[Nigel Goodwin]

Can you post a decent photo of it?

 

[gophert]

Is this a foot pedal for
- a guitar effect,
- a racing video game accelerator
- a sewing machine
- a pneumatic hammer
- a ...?

 

[LukeKnepp]

a sewing machine

Yup

 

[LukeKnepp]

 

[gophert]

Calling MaxHeadRoom78 , calling Max. Help needed with sewing machine motor speed controller!

 

[Nigel Goodwin]

Well it's obviously not a pot

It sounds like it's working as it's supposed to, and it's designed to turn the sewing machine on and off as well as alter the speed.

 

[gophert]

Well it's obviously not a pot

It sounds like it's working as it's supposed to, and it's designed to turn the sewing machine on and off as well as alter the speed.

I'm not completely sure. The card with printed pattern looks like it is a resistive ink or strip to make a linear pot (actually a linear rheostat).

LukeKnepp, does the white plastic piece that moves have electrical connectors to connect the two black vertical lines?

do you have a volt meter to measure the resistance along one length of a vertical line?

 

[BobW]

Rather than mess around with the foot pedal, I would change the voltage controller circuit so that erratic resistance values won't cause problems. Can you post your schematic?

 

[Nigel Goodwin]

I'm not completely sure. The card with printed pattern looks like it is a resistive ink or strip to make a linear pot (actually a linear rheostat).

There are two strips, and two other patterns, which is why I suspected switching as well - and as I recall from my wifes sewing machine it stops when you take your foot off the pedal completely.

 

[LukeKnepp]

does the white plastic piece that moves have electrical connectors to connect the two black vertical lines?

Yes, it has two spring loaded pins, but I do not know if they are connected or not.

Rather than mess around with the foot pedal, I would change the voltage controller circuit so that erratic resistance values won't cause problems.

This is exactly what I want to do!
As for the schematic, I do not have one, but it appears to be this one, except the output voltage setting divider is a 50k ohm Trimpot over a 1.5k ohm resistor...

So the output is adjustable from about 1-28V, which is what I want, and I was just planning to change the "bottom" resistor to a 150 ohm, and using my 5k ohm foot pedal...

 

[LukeKnepp]

do you have a volt meter to measure the resistance along one length of a vertical line?

I can, but is this all you need? I put the pedal back together already...

 

[gophert]

and as I recall from my wifes sewing machine it stops when you take your foot off the pedal completely.

Certainly a safer design than what is happening with our OP!

 

[BobW]

What is the resistance when the pedal is completely released?
What is the resistance when the pedal is pressed all the way down?
I'm assuming that you are just working with two wires. Or is it a 3 wire potentiometer setup?
Where are you connecting it into the circuit that you've shown?

 

[Nigel Goodwin]

What is the resistance when the pedal is completely released?
What is the resistance when the pedal is pressed all the way down?
I'm assuming that you are just working with two wires. Or is it a 3 wire potentiometer setup?
Where are you connecting it into the circuit that you've shown?

Looks like just two wires, red and white, in the picture.

 

[LukeKnepp]

What is the resistance when the pedal is completely released?

About 1.25 Megaohms

What is the resistance when the pedal is pressed all the way down?

About 4 ohms

When I press the foot pedal a little bit, it jumps from about 1Meg to about 4.5k ohms, as the turn-on switch, I think...

I'm assuming that you are just working with two wires.

Correct

Where are you connecting it into the circuit that you've shown?

It would go where the 105k top feedback resistor is, and I would just change the bottom one as well...

 

[BobW]

Something doesn't make sense. If you install the pedal in place of the 105k resistor, then output voltage will be inversely proportional to the pedal resistance. If the pedal resistance goes infinite or very high, the regulator output should drop to near zero. There must be something else going on.

 

[Nigel Goodwin]

Something doesn't make sense. If you install the pedal in place of the 105k resistor, then output voltage will be inversely proportional to the pedal resistance. If the pedal resistance goes infinite or very high, the regulator output should drop to near zero. There must be something else going on.

Wrong way round, it's directly proportional, so will go massively high when it goes OC - he needs to be replacing the 7.5K with a pot, and placing a resistor in series with it to set the maximum voltage.

But the sewing machine pedal is completely unsuitable for this purpose, or any other apart from the original sewing machine.

What he wants is an old WahWah pedal, where you have a pedal mechanically linked (via gearing) to a standard potentiometer - simply chuck the electronics out, replace the pot with one of a suitable value (remember you need to be replacing the 7.5K, and putting a resistor in series), and jobs done.