Meter multiplier suggestions

[KC2TAU]

In a continuation of a previous question I had asked I have two Simpson model 27 meters. One is scaled from 0-10ma and the other is scaled from 0-25vdc. I would like to have the 0-10ma meter display from 0-1000ma and I would like the 0-25vdc meter to display from 0-2500vdc instead. I will be using these meters for an ART-13 power supply and so maximum voltage will be around ~1600vdc. In light of this I plan on using 1 watt resistors for the multipliers. What value of resistor would be good for each application?

[KA2DZT]

Measure the DC resistance of the 0-25 volt meter.  Most likely, it's a 1000 ohms per volt.  So, it should measure about 25000 ohms.  It is probably marked on the lower edge of the face "1000 ohms/volt".  So, to have the meter read 0-2500 volts you need to add multipliers to equal a total resistance of 2.5megs.  This would include the resistance of the meter.  The external multipliers would be 2.5megs minus the 25K of the meter itself.  You need to use a string of 4 or 5 separate resistors to make up the multiplier for this voltage level (2.5 kv).

Making the 10ma meter read 1000ma is a bit harder to do. You need to add a shunt across the meter terminals.  Measure the DC resistance on the meter.  You need an accurate digital ohmmeter.  Once you know the resistance you can figure what shunt resistor you need to have 990 ma flow through the shunt while having 10 ma flow through the meter.  The shunt and the meter form a parallel circuit.  There is another way to do this which is a bit more complicated but easier to do.  I can tell you if you need to know.  Let me know.

Fred

[KC2TAU]

Hello Fred,

With regards to the volt meter, yes, it is 1000 ohms per volt. According to Simpson the approximate resistance of the current meter at 60hz is 10 ohms.

[N2DTS]

I have a LOT of meters on my homebrew stuff, and not one of them was the correct scale at the start.

Here is what I do:

For current, I get a variable power supply with a current meter on it, you can use a seperate meter (DMM, VOM), then use what I have to make a shunt to get the meter to read right.
I use a low ohm resistor, and wind enaameled wire around it, or a ceramic standoff.
I had some small pieces of nichrome wire used in shunts (and heater elements) but its hard to work with, as a small piece is all that is needed, and small pieces (very short lengths) is hard to work with.

Most shunts need a VERY low resistance, and I do not have a lot of sub ohm resistors, so a wire shunt works best.

For high voltage, its best to tap off a center tap on the bleeder resistors if you can, that starts out as a lower voltage.

Resistors that drop a lot of voltage tend to be unstable, and they make special resistors to do that.

At minimum you need 2 watt resistors, and many in series, and I variac up the power supply and measure the voltage and adjust the resistors till the meter reads right.


At 2000 to 3000 volts, the resistors tend to change over time, even if I use 10 in a string.


Using the math way of doing it, if you know the meter spec's, may end up with an odd value you do not have anyway, and the meters need to be calibrated against a known source, so I skip the math and do it my way.

Brett
N2DTS
 

[Jim, W5JO]

When you start building shunts for HV remember that resistors have a max voltage rating.  Be careful to not exceed that value.

[KC2TAU]

With regards to the shunt, if the meter has a resistance of 10 ohms and, using the equations found here (http://www.neatcircuits.com/meter-mult.htm#1) I have come up with the following value:

10ma = .01 amps
Internal resistance = 10 ohms

E = (.01 * 10) = 0.1 volts

R = (0.1 / 1) =  0.1 ohms

(1 represents the desired scale reading of the meter in amps)

If this is correct, what would an appropriate wattage rating for the shunt be if this was used with a power supply operated at a maximum of 2000vdc and providing 300ma of current?

If it helps the schematic for the power supply I have is located here:

http://web.archive.org/web/20041109112600/http://www.kc2kj.net/art-13/art-13powersupply.PDF

The meter in question is on the lower schematic and is labeled "1 amp".

[N2DTS]

Math makes my head hurt.

I thought it was current squared times resistance.

http://en.wikipedia.org/wiki/Copper_loss

Or volts times amps for power, .5 amps (full scale meter reading) and say .1 ohm is .05 watts.
.5 amps times 1 ohm is .5 watts.
.5 amps times 10 ohms is 5 watts.

[KB2WIG]

"Math makes my head hurt."

Well, its easy as P IE.

klc

[KC2TAU]

Okay, that's fine.

Considering my maximum voltage of 2000vdc and maximum current of 300ma, what would be a good setup for creating the ~2.4 meg resistance required for my multiplier resistor for my voltage meter. How much wattage dissipation will I need?

[N2DTS]

Depends on how much current the meter takes to go full scale.
Unlike current shunts, power is an issue with dropping resistors.

Only high voltage resistors seem to be stable, but if you do not want to get them, use a string of as big resistors as you can.
More is better so the drop across each resistor is not real high.

Oddly enough, they tend to go higher over time, making the meter read lower then the voltage is.

[KA2DZT]

Hello Fred,

With regards to the volt meter, yes, it is 1000 ohms per volt. According to Simpson the approximate resistance of the current meter at 60hz is 10 ohms.

The 10maDC meter would not spec the resistance at 60hz.  Sure that is a DC milliamp meter or an AC meter.  I'll have to look at my Simpson catalog.  I have some model 27 meters which I'll look at.

Interestingly, most of the meters are all 1ma movements.  Simpson just puts multipliers or shunts inside the meter case to change the ranges for voltage or current.

I'm thinking that a 10ma meter should read about 5 ohms.  Most 1ma movements measure about 50-60 ohms.  About a 5 ohm shunt would allow it to read 10ma.  I'll check some of mine, maybe later today and I'll get back to you.

In my HB 6146/813 xmtr I use meters from ART-13 xmtrs.  That multi-range meter from that xmtr is a 1ma movement that measures about 60 ohms. I use three of those along with other similar Westinghouse meters all with the exact same case style.  The multi-range meters all work off switches, each measuring a number of currents and voltages.

Note, DO NOT attempt to measure a meter movement directly,  the ohmmeter has 1-2 volts at its terminals which will pin the needle and can damage the movement.  You have to add in series a few Kohm resistor of an exact know value. Measure the resistor with the movement then subtract the value of the resistor yielding the movement's resistance.

Fred

[KC2TAU]

According to the Simpson data sheet the resistance is measured at 60hz. It struck me as a bit odd.

http://www.simpsonelectric.com/images/File/datasheets/rectangular_datasheet.pdf

[KA2DZT]

The confusion is coming from your use of the word "resistance".  Resistance usually refers to the DC resistance.  Simpson's spec @60hz is an impedance which they show as 10 ohms.  The impedance of the coil @60hz probably could be 10 ohms also the shunt is probably a coil of resistance wire yielding some additional reactance.  Get an ohmmeter and measure the DC resistance of the meter at the terminals.  I'm thinking it should be something less than 10 ohms.  It is possible that Simpson is not using a direct shunt across the meter movement as there are other ways to make a 1ma meter read 10 ma.  You would have to open the meter to see what method they are using.  No real need to do that, just measure the DC resistance and work from that DC resistance to figure what shunt resistance is needed to allow the the meter read 1000 ma.

Fred

[w4bfs]

In my HB 6146/813 xmtr I use meters from ART-13 xmtrs.  That multi-range meter from that xmtr is a 1ma movement that measures about 60 ohms. I use three of those along with other similar Westinghouse meters all with the exact same case style.  The multi-range meters all work off switches, each measuring a number of currents and voltages.

If you are function switching a meter to read several currents or voltages you might want to consider a multiple resistor multiplier or shunt ....

Voltage multiplier ... using several megOhms to read hv with a single resistor multiplier in the common arrangement is fb until the meter connection is broken (by either the meter going open internally or the function switch selecting some other function) then full hv appears on the meter terminal or function switch ...not a good thing .... use a 2 resistor solution with the meter in parallel with the second resistor ..... this is not hard to do

Current divider using a low resistance shunt ..... hard values to obtain with very low resistance ....use a second resistor to make the job easier ...same sort of reasoning  

[KA2DZT]

I have many very low value resistors.  I don't use them very often.  I measure current indirectly in my xmtr.  The meters are only reading 0-1 volt.  I measure the cathode current for the 6146 by using a 5ohm cathode resistor.  200 ma through the 5 ohms drops 1 volt.  From the 5 ohms there is a 940ohm resistor which is the meter multiplier for the 1ma movement (ART-13 plate/grid meter).  The multiplier is near the tube circuitry and only a single wire runs to the rotary switch.  All the other voltages or currents are wired the same way.  Each has its own set of multiplier resistors.  Measuring the modulator current is set up the same way as the 6146 but a different meter and switch set.  The meters read other voltages, grid current, screen voltage, screen current, modulator bias, 300 volt line, 650 volt line, 20 volt relay supply line.

In the 813 section of the xmtr I have a straight 300ma meter for the 813 cathode current and one for the 811s cathode current.   A separate 20ma meter for the 813 grid current. Another 1ma ART-13 meter is used to measure the 813 screen voltage, 1500V high voltage supply, The HV at the midpoint of the two series connected bleeder resistors (which will read 1/2 the HV if the bleeders are OK and working.  The multipliers for the HV are mounted in the HV PS deck.

You're right about HV appearing at the switch terminals when the meter is reading some other voltage.

Fred