Hall sensor - lower the bottom limit without affecting the top

Thread Starter

Miller434

Joined Apr 18, 2020
12
Thank you all for the suggestions.

I did some testing on the supply and I might be at the limit of what it can output. I don't notice any voltage sag with the pedal alone, but that might change with the circuit in place. How do you test a power supply? Is it acceptable to put a variable resistor to ground, and slowly lower the resistance until you see a voltage drop? And how much is an acceptable drop, 10mV, 1mV?

Also sparky1, my environment is really not that harsh, it's a clean home workshop. The welder is inverter based (no heavy transformer), and sits on a cart. I will build a custom enclosure around the pedal, but that's about as far as I'll go. It will be no worse than a commercial pedal, except for the spill-proof aspect. The benefits far outweigh any potential shortcomings though. Being a car throttle, the internals are engineered to give perfect modulation, no commercial TIG pedal will come near it.
 
Last edited:

MisterBill2

Joined Jan 23, 2018
9,236
To know how much current the welder control input requires you could simply put a milliammeter in series with the control lead of the present control and observe the current as the weld power is varied. That will tell you exactly what the substitute must provide. One question is about the connection of the present control. Is it a two wire hookup with the vartiable resistror simply altering the resist5ance, or is it a three wire hookup, with a constant voltage applied across the potentiometer and a variable voltage fed back to the welder package. AND what is that supply voltage? Will the new pedal be able to work from that, or is another power source required?
 

Hymie

Joined Mar 30, 2018
991
It would be a simple matter to add a transistor to the output of the circuit to deliver a higher current if necessary.

Virtually all op-amps are inherently protected from output overload, so there should be no issue with wiring the circuit in the proposed application, to see how it performs with the available current from the op-amp output.
 

sparky 1

Joined Nov 3, 2018
576
Great responses here. Good teamwork. The thumb control and wire feed is another circuit. There are environments that the hand controls are more important. Safety is the most important and reliability is serious part of design even if it is just for an individual it needs to be said that what works for one can introduce risk in other cases.
 

Reloadron

Joined Jan 15, 2015
6,058
The TPS (Throttle Position Sensor) is a potentiometer designed in such a way it outputs about 0.5 volta at idle and about 5.0 volts at WOT (Wide Open Throttle). They have 5.0 volts applied from the engine control module. That should vary a little from make and model to make and model. I am surprised with a 6.0 volt input you are not getting more than 5.0 volts out. You may want to open yours up and look inside. See if there is a stop on the low end. Additionally reverse the power and ground leads and see if the center output changes between idle and WOT.

Ron
 

Thread Starter

Miller434

Joined Apr 18, 2020
12
Ok, I finally have some numbers. The original circuit with the simple potentiometer draws 7mA. My circuit draws 18mA (6mA op amp + 12mA Hall effect sensor on the pedal).

The original potentiometer - once connected, doesn't produce any voltage sag. With my circuit in place, I get about 120mV sag. I take it that's an indication I'm slightly overloading the 6.8V supply. How much of an issue is this?
 

MisterBill2

Joined Jan 23, 2018
9,236
Ok, I finally have some numbers. The original circuit with the simple potentiometer draws 7mA. My circuit draws 18mA (6mA op amp + 12mA Hall effect sensor on the pedal).

The original potentiometer - once connected, doesn't produce any voltage sag. With my circuit in place, I get about 120mV sag. I take it that's an indication I'm slightly overloading the 6.8V supply. How much of an issue is this?
Is there any way that you can look at the welder circuit board and see what the power source for the control is. It may be a resistor and zener diode, in which case you have no problem at all. If it is a 3-terminal regulator you are still probably OK. Probably that small an increase and small voltage change are within tolerances and it would not be an issue at all.
 

Thread Starter

Miller434

Joined Apr 18, 2020
12
Is there any way that you can look at the welder circuit board and see what the power source for the control is. It may be a resistor and zener diode, in which case you have no problem at all. If it is a 3-terminal regulator you are still probably OK. Probably that small an increase and small voltage change are within tolerances and it would not be an issue at all.
That's exactly right, 15V coming down to 6.8V through a zener/resistor divider. Great stuff!
 

Thread Starter

Miller434

Joined Apr 18, 2020
12
Some final thought on this project.

I went ahead and created a basic layout in Diptrace, then ordered 5 boards from JLCPCB at the total cost of £15 inc. postage. That is pretty cheap for what you're getting.

And works perfectly, I can dial in the limits to perfection. Thanks very much for the help!
 

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