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PMX 45 (nonxp) voltage divider question

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PMX 45 (nonxp) voltage divider question

Postby robertspark » Wed Nov 07, 2018 3:36 pm

I know these sorts of questions have been posted before, but I was wondering if I could get a better (clearer) answer than a home brew guess

The PMX 45 (nonxp) has a 50:1 voltage divider output, and from the schematics within the service manual the voltage divider is created (as would be expected) from two resistors

What I would not have expected was that the resistors are listed as 100k and 2k.

While the ratio is 50:1 the voltage divider output is not 50:1, because the total resistance is 102k, so therefore say for simplicity 1000V / 102000 ohm = ~9.8mA
9.8mA X 2,000 = ~ 19.608V

So the actual voltage divider ratio is 1000 : 19.608, or 51:1..... Is it not?

The reason why I ask is because it like to change my PMX 45 from a 50:1 voltage divider to 20:1 voltage divider for a different THC

The question is for a 20:1 divider, do I go with:
100k & 5k (2 X 10k resistors in parallel)?
Which is actually a voltage divider ratio of 21:1 (eg for simplicity 1000v across the voltage divider would give ~ 47.61v across the smaller resistor, so 1000 / 47.61 = 21:1

Or should I be looking for (simplicity) 1000 v across the divider and 50v across the smaller of the two resistors so the smaller resistor (retaining the existing 100k ) should be closer to 5,263 ohms (4700+560 ohm in series)

Yes I know that the voltage is just a number (setpoint) but I'd like to be close to the book numbers for voltage setpoints thanks

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Re: PMX 45 (nonxp) voltage divider question

Postby tcaudle » Wed Nov 07, 2018 7:38 pm

The correct formula is
Vout = VIn * R2/R1+R2 where Ri is the top resistor connected to the voltage to be divided and R2 is in series and to the voltage ground. The Vout is at the junction of the R1 R2 connection and the same ground
This all works perfectly until you throw in other components or you hand any kind of load across the Vout side
Rather than take a chance of blowing up an expensive control board in your 45 I think I would consider building my own 20:1 divider and use the raw volts (Hypertherm Field service document shows where to hook external voltage divider.) You will also find its not a pure DC signal and it has lots of switching noises that need some filtering to be able to use. Be mindful of the voltages involved (they are lethal ) when the torch is fired. Be careful of power dissipation. E^2/R. Raw volts can reach 175 to 180 volts on open circuit 45 . So the lower the voltage ratio the lower the resistance and the wattage goes up at the square of the numbers. You take the voltage drop across each resistor and divide it by it reistance to get watts of power

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Re: PMX 45 (nonxp) voltage divider question

Postby robertspark » Thu Nov 08, 2018 11:16 am

Thanks Tom for taking the time to reply.

Vout = Vin * R2 / (R1 + R2) for Hypertherms Resistances of 100K and 2K still provide a 51:1 voltage divider and not 50:1 as listed in the book.

Am I missing something?

Again, correct me if I'm wrong but a 50:1 voltage divider, I'd put 50V between the input and should expect to get 1V between the output pins.
Likewise, for a 20:1, if you put 20V on the input of the divider, you'd expect to measure 1V between the output of the divider?
2018-11-08 15_42_03-Powermax45 Service Manual.png

2018-11-08 15_44_00-Powermax45 Service Manual.png

The open circuit voltage from the manual is 275V which aligns with the varistor on the LHS of the screenshot attached.

The brunt of the voltage drop is taken by the 100K resistor to provide the major voltage drop.

current through the 100K resistor at 51:1 (100K+2K) at OCV (275v) is 2.696mA, which gives a power dissipation through the resistor of 0.73W

changing the circuit to ~20:1 (100K+5260 {4700+560}) at OCV puts the current at 2.6125mA. which gives a lower power dissipation through the 100K resistor of 0.68W

As can be seen on the service manual screenshot the 100K resistor is rated at 3W so no issue there.

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Re: PMX 45 (nonxp) voltage divider question

Postby tcaudle » Thu Nov 08, 2018 3:09 pm

Open circuit voltage is not 275V.
You need to run the numbers for 20:1 if that is you goal. You will raise the voltage drop across either resistor you elect to change so the wattage will go up for that one.

I suspect any schematic is not really the exact values that are in the circuit. They will be 1% values. all you have to do to adjust the ratio is to slightly change the lower R2 value.

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Re: PMX 45 (nonxp) voltage divider question

Postby jimcolt » Wed Nov 14, 2018 12:48 pm

The idea of the voltage divider is to provide an analog feedback voltage to control torch to work distance, as well as to limit the highest voltage output to protect the THC electronics in the event that open circuit voltage happens. (this only will occur if there is an issue with the consumables in the torch) The difference in actual torch height that would occur if the voltage divider had a ratio of 50:1 as compared to 51.1:1 is inconsequential to torch performance in terms of cut quality or consumable life. I agree with Tom Caudle....if you have the need for exactly a 50:1 divider ratio.....access the raw arc voltage and build a more accurate voltage divider. Keep in mind that consumable wear in the torch as well as air pressure and cut speed variations all affect the voltage / cut height relationship far more than a small error in the voltage divider output ratio does! Jim Colt Hypertherm

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Re: PMX 45 (nonxp) voltage divider question

Postby robertspark » Wed Nov 14, 2018 2:48 pm

It's just interesting that something quoted as 50:1 may not be, when it connects to another device that requires a quoted input of 50:1 .

Whilst the effect may be realativly minimal, it would be nice for someone to admit that it's actually 51:1, and the manual quoted values are taken as such from the CPC.

Say I intend to cut at 140v, I set the 50:1 THC at 140v, and the output voltage at the CPC will be 2.8v..... but the raw volts will actually be 142.8V..... ok, only 2.8v out but it will mean that for the first cuts the torch will be too high and I'll need to reduce the setpoint to ~ 137v to align with the book values.

I am fine with raw arc as I've been using it for a while, I was just trying to make better use of the onboard voltage divider and was curious about the math because 1:50 seemed more like 51:1 so I was wondering if the same applied to 20:1 and it should be 21:1

It's like when Tom says that ocv is 175 to 180v on the raw arc, and you print an extract of the service manual that says ocv 275v, and CPA 225v..... And still comes back and says the ocv is not 275v

But when you look at the circuit it's clamped with a varistor rated at.... 275v....

I ain't making this up its from the service manual link and extract posted earlier.

Thanks all but I've not gleened or corrected any info that I didn't have when I posted this thread and posed the question.... Is the pmx 45 50:1 or 51:1 with a 100k 3w resistor and a 2k resistor

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