Voltage DC Meter to B+

[cannikin]

I have a few Collins DC Voltage meters rated 500V, I might as well put them in action. I thought the hook-up would be easy -- DC meter, high Voltage rating, no problem ..... nope... fried one of them (my B+ was only 280Vdc).

the pointer now does a 90 degree curve :thumb: :thumb:

All I want them do is meter the B+ voltage.

Anyone have a circuit or could tell me the correct way to connect it to my B+. I thought it was as easy and the same as measuring the voltage with a DVM = Meter plus to B+ and Meter minus to GRND.

wrong!

Who's got the answer for me?

 

[bcarso]

Meters are usually current-driven devices---the deflection is proportional to the current, and the internal resistance directly at the meter coil is usually pretty low. What the faceplate is engraved with doesn't tell you if internal to the enclosure there is a resistor to mostly determine that current.

So you need to find out what you need to put in series with the meter movement to give a full deflection with 500 volts---as you've clearly established that the meter movement resistance isn't near enough! I'm guessing the meter movement is probably in the vicinity of a 100uA one, so in that case the answer is 5 Megohms, less the probably very small actual meter coil resistance.

Make sure that the resistor used is rated for the maximum voltage as well as the associated power. You may want to put a few metal film 1/4W parts in series to assure a reasonably small shift due to the resistor voltage coefficient.

EDIT PS: sometimes the meter movement full scale current is shown in very fine print in a region typically near the bottom of the faceplate.

 

[PRR]

> Collins DC Voltage meters rated 500V

No, they are marked for a 500V reading.

If you look at the plan of the item they came out of, they surely have a resistor that you don't have.

Agre with bcarso. 100uA (0.1mA) is a good first bet. And hey: try on a 9V battery first! 9V at 0.1mA is 90K ohms. Put 100K in series, poke a 9V battery at it. If swings 9/10 of full scale, it is indeed a 0.1mA movement. If it swings not-quite 1/10 full scale it is a 1mA meter, which was a dime cheaper than 0.1mA so very likely.

It might be a 50uA movement. These cost more and are more delicate yet are reasonably common. Then 9V on 90K will "bang" but not "BANG!". Try 220K.

If you determine it is 100uA/0.1mA, and you want 500V full-scale (because that is how its marked), it is indeed 5Meg. Since little resistors are rated 250V and this could be a full-time strain, you want several 1Meg-2Meg in series.

Or if you abandon tubes and want to meter transistor supplies, taking 0V-50V so the original scale can be used, one 500K does the job.

> less the probably very small actual meter coil resistance.

This can also be specified as the voltage-drop at full current, and often called "burden". For most meters likely found in electronics the burden is 0.1V or less. So at 500V or even 50V it is a non-issue. At 5V a perfect 50,000 ohm resistor and perfect 0.1mA 0.1V burden meter would read 2% low, which is marginally repeatable on this class of meter. IAC, once he has the resistor in the ballpark, he'll fudge it so the meter agrees with his best DVM, not try to work it out from assumed specs on an old/tired meter.

 

[cannikin]

thanks guys

So to hook it up to the B+ it is in fact minus on the meter going to ground? and plus on the meter through resistor 5M to the B+? Anyone have an actually schematic with a voltage meter in the circuit... I can't visualize it.

I'll give it a go tomorrow and report back

 

[PRR]

> to hook it up to the B+ it is in fact minus on the meter going to ground? and plus on the meter through resistor 5M to the B+? Anyone have an actually schematic with a voltage meter in the circuit... I can't visualize it.

It's 3 parts! {sigh}

Here's a picture:

> minus on the meter going to ground?

Yes, one meter terminal really should be "near ground"; in systems with grounded negative and hot positive (most vacuum tube work) that will be the negative terminal.

Look at the left, "bad", schematic. The meter reads correctly, BUT the guts of the meter are at 500V above your grounded panel. Now, some meters have 1/8 inch of virgin Bakelite between guts and panel, or they may have old cracks full of damp salty grime, modern meters use cheezy plastics.

Look at the right schematic. Now the meter guts are at zero V to 0.1V relative to panel. (Sez 0V on the meter "hot" terminal because SPICE meters are perfect; real meter will be under 1V here.) You do have to insulate the top of the resistor, of course, since it ties to nasty B+. But the rest of the path is now quite safe.

 

[emrr]

Ouch. those are worth a good bit too. Every one I've seen is a decade type measuring 5V, 50V, and 500V. So I think 5V with no resistive adjust, and corresponding types for the other decades. Look at the meter in the 26W manual; that should spell it out.

Good news is you can probably retrofit another same 5V type into the case with the meter scale and be fine.