Overload wirewound resistors

[ashok.das81]

I need to add some wirewound resistor in series with a voltmeter to make it high range. I am converting the voltmeter from 300v FSD to 900v FSD.
As per my calculation the wattage should be 20W, but I am getting only 10W resistors in market.

So my question is if I use the 10W resistors in place of 20W resistor, can I run it for a short duration?
If yes how much duration will be safe to run them ?
If I place a cooling FAN directly over the resistor will it help to extend the operating time ?

[moffy]

Why not use two 10W resistors in series to get 20W? Overloading depends on the type of wirewound, the vitreous enamel coated ones you used to be able to run red hot, literally, other types not so much. You had to be careful with the vitreous enamel ones as they had a tendancy if they got too hot to desolder themselves from the PCB. When mounting power resistors give clearance between them and the board as at their power rating they will be hot.

[Gyro]

At 900V, you should check the voltage rating of the resistors, a single resistor may suffer insulation breakdown regardless of wattage. I would actually put four in series - running a 10W resistor at 5W (50% derating) is a good recipe for long and trouble-free life, especially in high ambient temperature. I would go for a minimum of three regardless, running a resistor at 100% of its power rating is never a good idea (let alone 200%!).

[jzx]

For a voltmeter, if the resistor is too hot you can have measurement errors due to the temperature coeficient.

[Wallace Gasiewicz]

20 Watts through a VOLT Meter?

[TimFox]

The highest current through an analog passive (no powered devices) voltmeter I have ever seen was 1 mA.
50 to 100 uA is more typical.
At 900 V, the power dissipation is 0.9 W at 1 mA.
I once discovered the hard way how hot a vitreous-enamel wirewound resistor gets when operated at full rated power (I think it was a 20 W unit tubular unit, supported off the chassis by brackets into the tube by about 2 cm).

[Gyro]

The highest current through an analog passive (no powered devices) voltmeter I have ever seen was 1 mA.
50 to 100 uA is more typical.
At 900 V, the power dissipation is 0.9 W at 1 mA.
I once discovered the hard way how hot a vitreous-enamel wirewound resistor gets when operated at full rated power (I think it was a 20 W unit tubular unit, supported off the chassis by brackets into the tube by about 2 cm).

It's possible that it's an old moving Iron meter, especially if it is an AC one (not specified), they are still around in some installations. Either that or the OP has his sums wrong.

[TimFox]

20 W at 900 V is 22.2 mA -- that's huge.
I remember 1 mA moving vane meters.
In high-school and college math, we were taught to "show our work".

[TizianoHV]

Old school meters had some enormous burdens, in particular if they are optimized to measure lower voltages.
I modified my 0.25% Weston S.67 wattmeter to measure voltages up to 300V and it draws 33mA! (but still bang on after 70 years)



I used 20 220ohms 50ppm 0.6W resistors in series plus a few for trimming. At nominal voltage they work at 0.25W each.
If you don't need high accuracy you can load them more, but I would just solder a bunch of small resistors (for example 27*1.5k 1W or 40*680R 0.5W) in series on a protoboard.





What circuit are you measuring? 22mA could be enough to drag down the voltage you are measuring

[themadhippy]

20 Watts through a VOLT Meter?

Try an  old wiggy tester,that thing could take out a 300ma rcd without breaking a sweat,but on the plus side you didnt need to worry about false readings from ghost voltages.

[ashok.das81]

Hi all,

I have attached the picture of my meter and the calculation page from my notepad. Please check if I have done any mistake in calculation.

Looks like I have got what @TizianoHV have. But my meter is Voltmeter. Its almost same looking and same size as well.

[ashok.das81]

@TizianoHV. I am measuring the voltage O/P of a PSU I am designing for Vacuum tube RF power amplifier. The PSU is under construction. The PSU is rated at 500W 750V. So maximum voltage the meter will face is approx 850V.

[T3sl4co1l]

Related backlink: https://electronics.stackexchange.com/questions/717692/overloading-wirewound-resistors

Tim

[TizianoHV]

Looks like I have got what @TizianoHV have. But my meter is Voltmeter. Its almost same looking and same size as well.

The two 100ohm resistors don't need to be that big since they will dissipate only 100ohm*0.0297^2 = 0.09W each!
I would remove them and put a 500ohm (>1W) potentiometer for calibration.

Do you already have these resistors or will you buy them?

Before planning too much it would be good to open it and see if inside there's enough space to fit everything. with 900V 17W you can't just throw these resistors inside, you should leave some free space around.

Does the case have air vents? Dumping 26W in a closed wood enclosure it's not ideal.

[ashok.das81]

Hi  TizianoHV, No I will buy the resistors. Yours idea is good. I will use a 500ohm 1w pot in place if 2 100 ohm resistor. There is enough space inside. I need to make a vent on the cabinet. But that is not mandatory. I can make a separate standalone resistor and then connect it to series with meter. Today I will open it UP and post a pic of inside.

[Gyro]

Hi all,

I have attached the picture of my meter and the calculation page from my notepad. Please check if I have done any mistake in calculation.

Looks like I have got what @TizianoHV have. But my meter is Voltmeter. Its almost same looking and same size as well.

Looking at the small markings on the meter, it is a moving-Iron AC one. They've done a pretty good design on it though as the scale markings look fairly linear, apart from the ends of the scale. Irrc, they read approximately RMS, but for DC you should verify on one of the lower ranges that the reading on DC are accurate.

Hopefully you don't need to be told that contact with a 750V 500W capable supply rail will cause severe burns and possibly fatal - it is the same voltage as is used on train third rail traction systems. Insulation and voltage withstand ratings of your resistors is essential.

[Wallace Gasiewicz]

I have two old meters I use for HV in Tube Amps. They have special sets of resistors that are used. I am including a link to the Simpson 635 Manual which includes the HV "Multiplier" Unfortunately I cannot attach portions of it because this site tells me it is too big.   

https://www.scribd.com/document/644926904/Simpson-635-Manual   


I think this is something that may help a lot:
Here is a quite simple setup for measuring voltage with an analog meter, but you need to know the mAmp requirement of the meter itself and any added resistance in the box. It computes the value of the "Multiplier"   
Unfortunately I am Ignorant of the mAmp (or uAmp) rating of your meter and the inherent added resistance. Perhaps you could share it.. Or maybe someone else knows..    Or maybe you could measure the resistance?


https://sites.ntc.doe.gov/partners/tr/Training%20Textbooks/11-Electrical%20Science/14-Test%20Instruments%20and%20Measuring%20Devices.pdf   


When I measure HV in RF Tube Amps. I connect the meter with the proper HV wire leads FIRST and then turn on the Amp (or Power Supply) and watch the meter closely and am ready to PULL THE PLUG if necessary.

[ashok.das81]

@Gyro,  yes i have check that by connecting a 36 V DC source to 75 V range and to my surprise, it measured accurately. This meter can measure both DC and AC. For high voltage safety, thanks for reminding me, though I have done extensive study on high voltage safety and done in depth study of various international standards on how to deal with high voltage. I am an engineer and amateur radio operator so already familiar with high voltage.

[Wallace Gasiewicz]

When you look inside and get the value of the resistor for the 150 volt scale, you can just increase this value to a total of 10X and you will have a 1500 V scale

[CaptDon]

With that kind of voltage it would be foolish to have all kinds of kludged together wires and resistors and if you lost the ground connection everything would be live to the high voltage with a low enough resistance to kill you. If you are building this amplifier you will have to package it so why not just buy a correct panel mounted meter like 0 to 1KV? You will probably need other metering like screen voltage, plate current etc. It looks like you will be buying meters anyway. Using that old dinosaur of a meter with a bunch of resistors is just bad karma looking for a way to kill you or your helpers.

[ashok.das81]

@CaptDon, Thanks for reminding me about dangers of high voltage. I am compelled to use this meter as it has a big dial and i can see the voltage from a distance. I do not have to go near to test equipment when I am measuring. I have planned all safety measures of high voltage and there will be no high voltage part exposed to operator. All contacts will covered with poly-carbonate and/or glass plates when measurement will be done. This meter is not for permanent use inside cabinet. It is just for occasional measurement. I will not use it in place of panel meter. I will have to buy panel meters, but they are low resolution and 1KV DC panel meter are not available in my country easily. If i try to import which is the standard way, the cost is extremely high that not feasible for a home-brew project. Anyway I am posting internal of this meter as i mentioned earlier. can someone tell me what is the the wire-wound thing on the meter ? Looks like wirewound resistors but the shape is like flat.

[TizianoHV]

Yes, they are wirewound. They are flat to reduce inductance. The small resistors soldered to the binding posts are there only for trimming (most of the power is dissipated by the wirewounds).

There's enough free space inside, but I think that building an external box would be easier and safer.

[Wallace Gasiewicz]

Yes those are wire wound resistors that are commonly found in high quality instruments instruments that are far younger than yours. They were used for a long time in high quality resistance boxes and Wheatstone Bridges and such...... I do not think they are used much any more but are very good, very expensive resistors.   
The connections on the box are one COMMON and then three progressively higher resistance connections for the three scales. I think you already know which connection is for which scale.   
Looking at the non linear scale, I have changed my opinion. If you wish to measure about 1 K Volt I would increase the highest resistance (300 Volt Scale) to ten times this resistance (total resistance) with an external series resistor.  This would give you a scale that would put 1 K Volts about in the middle of the needle indicator range and be easier to read. 
If you do this use a resistor rated for high volts. The total scale reading would be 3 KV and your approx 1 KV reading would be around the 50 mark,close to center.   
 Depending on the mA requirement for your meter, the power thru the setup could be maybe ONE WATT. So I would use a 5 Watt resistor. We do not know the mA requirement of your meter but we can determine it if you measure the resistors in the box. We can be more exact about the resistor Watt rating if we knew the mA full scale requirement of your meter.   

Of course, be careful and make your connections solid with good insulation and distance them from any possible short. 
For anyone following here is a simple summary for both moving coil and moving Iron Meters that includes calculations for series resistors for different ranges:   
   
https://electricala2z.com/electrical-instrumentation/moving-coil-moving-iron-meters/

[Wallace Gasiewicz]

In addition, I would not put anything inside the meter box or attempt to solder anything to the resistors. I do not know how the resistance wire is attached. Sometimes it is welded or maybe silver soldered. If it comes off during any soldering, it is hard to put back on it's connector. The resistance wire is more prone to break than regular copper wire also. So I would use an External Series Resistor to the connectors on the outside of the box and leave the antique laboratory instrument entirely intact. It really looks nice.   
Also, if you use an external series resistor, in my suggested setup 90% of the power in the circuit will be dissipated in the external series resistor, so you should not have to worry about the wire wound resistors in the box.

[ashok.das81]

Hi all,

I would like to thank everyone in this thread to enlighten me with all the information that i needed. Yes I will use an external HV proof BOX to keep the series resistances. I will not disturb the original meter. I already planned to use the 300V range to extend to 900V, but it will be better I think extend it to 3000V and then my desired voltage will be approx at the center of scale. Next step is to make an enclosure for series resistance. Also going on the safety matter, I am working on a plan to insulate all contact points from operator using thick acrylic plates and putting fuses where necessary. All switches are actually implemented with high voltage relays. All relays are remotely activated using an ESP32 based wifi control system. I do not even touch anything during testing. Multiple reminders on safety on this forum has compelled me to use this remotely operated system. Thank you again.