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Help, Please with hot wire foam cutter

OCD

Feb 17, 2018
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Hey gang,

Like to say hello first and foremost.

I need some help here.

I recently built a custom designed hot wire foam cutter and am wanting to reconfigure the electrical configuration to incorporate a voltage ammeter into the mix.

The PWM only shows percentage of max output and not the actual voltage & amperage being applied which is why I want to add a voltage ammeter to the configuration.

Components
PS is an old computer transformer (90w / 19v / 4.74A) dc output

PWM - Diymore DC 6-30V 12V 24V MAX 8A Motor PWM Speed Controller with Digital Display and Switch

DROK Voltmeter Ammeter DC4.5-30V

I've looked on YT and there isn't a single video that illustrates how to wire these up together for a HW cutter or a PWM + VM together.

I'm wanting to use my DC voltage from the transformer to power the Drok so an independent PS (battery) isn't needed if possible.

A doodle of the proper wiring schematic of these components along with which wires from where to the HW cutter would be deeply appreciated.

Thank you

OCD
 

davenn

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Sep 5, 2009
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hi
welcome

The PWM only shows percentage of max output and not the actual voltage & amperage being applied which is why I want to add a voltage ammeter to the configuration

no such thing as a voltage ammeter ;)

so you want a voltmeter and an ammeter
But the problem is, because it is a PWM signal, a normal meter may not respond correctly, not sure how to overcome that

lets see if you get some other responses


Dave
 

Bluejets

Oct 5, 2014
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I built one here for model aircraft wings which was just a 240v to 40v/200va transformer controlled by a dimmer on the primary.

Many would say not a good match but it worked fine and I afterwards built a copy unit for Jabiru aircraft mob which is still used as far as I know.

Didn't find it necessary to have any metering, just simply marked the best position on a scale attached to the dimmer knob for each type of cutter and the nichrome wire size.
 

Kiwi

Jan 28, 2013
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Sorry, I am a little confused as to what you want to measure with the combination voltmeter/ammeter.
Is it the voltage applied to the heating element and the current passing through it?

The duty cycle(0 - 100%) is the important measurement with PWM.

The voltage will be basically the same as the power supply(19V). There will possibly be a small loss through the PWM controller.

The current will be the current the element draws when connected directly to the power supply multiplied by the duty cycle.
For example, a 4A element will draw 4A at 100%, 2A at 50%, 1A at 25%, 0.4A at 10%, etc.

As Dave advised, trying to connect the meter to the PWM output will cause issues with the meter.
Different meters will give different results as they try to average the rapidly changing PWM output.
 
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Alec_t

Jul 7, 2015
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You can use a resistor-capacitor low-pass filter to smooth the voltage reading.
 

KJ6EAD

Aug 13, 2011
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Does the DROK meter not have an instantaneous wattage indication? Mine does.
 

OCD

Feb 17, 2018
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Sorry, I am a little confused as to what you want to measure with the combination voltmeter/ammeter.
Is it the voltage applied to the heating element and the current passing through it?

Yes

The duty cycle(0 - 100%) is the important measurement with PWM.

Agreed

The voltage will be basically the same as the power supply(19V). There will possibly be a small loss through the PWM controller.


Does not the PWM regulate how many volts & amps are allowed to pass through it to the motor/hot wire?
OR,
Does the PWM only regulate the amount of amps allowed to be applied to operation?


The current will be the current the element draws when connected directly to the power supply multiplied by the duty cycle.
For example, a 4A element will draw 4A at 100%, 2A at 50%, 1A at 25%, 0.4A at 10%, etc.

Understood

As Dave advised, trying to connect the meter to the PWM output will cause issues with the meter.
Different meters will give different results as they try to average the rapidly changing PWM output.

I thought that once the level of settings was initiated & stabilized I would achieve an accurate stabilized reading(s).


Thanks to all for the speedy replies.

The reason I wanted to add the voltage / ammeter digital dual display unit is so I could record and document a reference chart of various nichrome wire sizes, operating temperatures & their voltage / amp requirements.

Was planning on connecting a thermocouple wire to the nichrome (hot wire) and record the temperatures and document the corresponding numbers of volts / amps per achieved temperature levels.

This reference chart would provide me documentation which would indicate which wire would be best for cutting any given type of material.

Here's the volt / ammeter I have and referred to above.
It has a built in shunt.

https://www.ebay.com/itm/DC-100V-10A-Digital-LED-Panel-Voltmeter-Ammeter-Current-Amp-Volt-Voltage-Meter/152686191520?_trkparms=aid=888007&algo=DISC.MBE&ao=1&asc=48419&meid=5d001fedd2a642b8a7509e733df71396&pid=100009&rk=1&rkt=1&sd=122957253752&itm=152686191520&_trksid=p2047675.c100009.m1982

As mentioned in my original Post, and bear with me here,
I not only want(ed) to incorporate the VM unit but also eliminate the need of an independent power supply such as a battery to the VM.

In a nut shell, configure all wiring and use the PSU to power all displays.

Here's the hot wire cutter as it sits.

EwHbg2L.jpg


TuT8OMh.jpg


B8bdVMM.jpg


Add here's a link with the rest of the build.

https://imgur.com/a/wmtlo

Electronics isn't one of my everyday things but I do have a somewhat understanding of the components involved.
It's been a few :rolleyes: years since I've built anything electronically, like since high school from the stone ages.
We didn't even have cell phones then, just tin cans and a wire. :D
 
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Bluejets

Oct 5, 2014
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If you use analog meters for ammeter voltmeter then no power supply is required for them.
If that is indeed what you were asking. Bit difficult to know without your drawing of what you have.

Still think all this is really just "bling" for reason I outlined in #3.

Btw, you may need to put a flex across the spring and isolate one end to act as a shunt to keep heater current away from the spring.
It may tend to change shape and defy what you are trying to achieve.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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A PWM controller is like a very fast switch. It gives either full power to the load or switches it off. It does this very fast and with a controlled ratio between the time that power is applied and when it is not.

Ignoring resistance in the controller itself, the current through the load (and for simplicity I'm assuming the load is purely resistive) will either be a value determined by the supply voltage divided by the load resistance, or zero.

If you placed an analog voltmeter across the load, it would measure a voltage that varied from zero to your supply voltage as the ratio between the on and off times (the duty cycle) varies. The reading would vary, not because the voltage was actually varying in a linear manner, but because the meter could not swing from zero to the supply voltage and back to zero again as the PWM controller switches it on and off. The voltage read on the voltmeter will be some sort of average determined by the physical characteristics of the meter. Incidentally, if the switching frequency just happens to be resonant with something in the meter, the meter can destroy itself.

An analog ammeter placed in series with the load will behave similarly.

Often the readings given by analog meters will be close to the arithmetic mean of the voltage or current. Let's assume that your load is operating from 19V, at which voltage it draws 4.5A and is being operated at a 50% duty cycle (it is on half of the time). The voltmeter will probably read close to 9.5V and the ammeter close to 2.25A. So, what is the power?

Normally, to determine the power, you multiply the voltage by the current. So, is 9.5 * 2.25 correct? Is the device dissipating 21.375W?

Thinking about it, when the load is on, 19V is across the load, and 4.5A is flowing through it. The power being dissipated is 85.5W. When the load is off, it dissipates 0W. At a 50% duty cycle it is thus dissipating 85.5W half of the time, and 0W the other half of the time. Because power is defined as Joules per second, it is fairly simple to show that the average dissipation is power x duty cycle, or 85.5W x 50%. Thus the calculated power is 42.75W.

So, measuring the average voltage and current and multiplying them gives an erroneous value!

But wait, if we took either the measured average voltage or current and multiplied it by the peak current or voltage (the average voltage 9.5V x the peak current 4.5A, or the peak voltage 19V x the average current 2.25A) you get the correct figure. That allows you to mark a voltmeter or an ammeter as a power meter!

Is the use of a meter like this to measure power useful? Actually it's not. Firstly, if the voltage supplied changes, any markings on the meter (power or % of power) will no longer be correct. And secondly, if you have a single turn rotary potentiometer controlling the duty cycle, markings on that will be just as good.

If you use a digital meter for either voltage or current, things may be more complex. You may get an average reading, you may get an RMS reading (actually the same in this case), you may get a reading that varies all over the place, or you might get something else. Unless you know, or can reliably test, you won't know. There best case is that you have a situation no worse than with analog meters.

If you are convinced that you want meters, then measuring the supply voltage (before the PWM controller) and the current through the load (and assuming it to be the average current) is your best bet.

There only reason I would consider a meter (and it would be a ammeter) is if I was designing my hot wire cutter to use different types and lengths of wire and I wanted to monitor the average current (because failure due to overheating will be related to current).
 

OCD

Feb 17, 2018
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WOW, ask and you shall receive.

Thank you Steve for the very in depth reply.

I went ahead and started trying different configurations of connection of the components and came to the conclusion that since a hot wire setup is nothing more than creating a dead short it would be impossible to acquire a steady consistent reading of the amps being consumed during operation as it was constantly fluctuating due to the heating and expansion of the nichrome wire.

Am I correct in this assessment?

I also tried powering the voltage/ammeter dual display unit directly from my PSU to no avail.
I finally incorporated an independent (battery) power supply through a switch and now have an accurate reading being displayed of the passive voltage.

At one point I did have the ammeter displaying amps but it was erratic and as soon as it got to 9 amps the meter went into safety shut down mode.

So, in conclusion, considering all things, 2 out of 3 isn't bad and I can live with that.

Hangernades and Horseshoes. :D

Again, I'ld like to say thank you to all whom replied and was willing to help out a knucklehead.

Your a bunch of good guys.

Thanks again

uYSOThk.jpg
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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came to the conclusion that since a hot wire setup is nothing more than creating a dead short it would be impossible to acquire a steady consistent reading of the amps being consumed during operation as it was constantly fluctuating due to the heating and expansion of the nichrome wire.

It's not a dead short, but it does present a low (and somewhat variable) resistance.
 

OCD

Feb 17, 2018
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Correct, a dead short would blow it.
Bad choice of wording on my part.
 
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