[HeathKit] SB-200 Plate Voltage reading...

[Jordan Arndt]

Hi Alex.. yes I do understand all that, but what I was wondering is why Heath
used this particular string of resistors... it is inaccurate from the
beginning... the meter specs are in the manual, 200uA @ 1400 ohms... I replaced
the series string of 4.7M's with 5.1M Metal film 2 watt resistors... that is
about 300K too high and the meter reads 1950 Vdc Measured with a 5Kv VOM shows
2060 Vdc which is rated at 20K Ohms/Vdc which is a 100Meg impedance and should
be very close.... I'm just trying to get the string right.. I couldn't get any
4.7M R's at 5PM and used these instead... I may go for a different string, and
measure to within 1k or so... that should solve the problem... Maybe sub out one
of the 5.1's for a 4.7 to bring the series R to 14.9 Meg.... that should just
about do it... should be within about .6% which is pretty close... Parallax
would cause a larger error....73 de Jordan...

AI2Q Alex wrote:

> Let Mr. Ohm and his Laws be your guide. You really should measure the
> internal resistance of the meter (its movement) before applying your
> calculations.
>
> Also, instead of high-value meter multipliers, it's better to measure a
> portion of the total B+, across the cold end of the bleeder, and apply your
> own multiplier there, and then calibrate your meter accordingly.
>
> That's what I typically do when I'm homebrewing and select a small
> low-current meter
> movement from the junkbox. I measure its internal resistance. Once I
> know that, I determine the voltage drop across the meter at its full-scale
> rating.
>
> You can measure internal resistance with a DMM (never a VOM!), or consult
> the handbook for a simple substitution method using two pots and a battery
> or small low voltage power supply. But, I digress.
>
> Select a small resistor to go in series with the load (in the rig; whatever
> stage that may be). The current that's being drawn at that point determines
> the approximate value of the resistor. Once I know the voltage drop across
> that resistor, I then make a meter multiplier for my milliammeter movement.
>
> To find the value of the multiplier, I take the total voltage across the
> "shunt" resistor that's in series with the load, and subtract the full-scale
> (F.S.) voltage that I calculated would be across the movement (I know the
> F.S. current, and the internal resistance, so E = IR).
>
> Then I find the value of the multiplier by taking the difference voltage and
> dividing it by the F.S. rating of my meter movement. Usually I locate a
> small pot that's a bit higher in value, put it in series with the meter, and
> then calibrate it for a F.S, reading--in the final circuit application, or
> using a "standard" benchtop instrument of your choice.
>
> As an example, I just added a little 0-1 mA meter to the HBX-2Q SSB
> transceiver I'm building. The meter has an internal resistance of 153 ohms.
> The voltage across it at that current would be E=IR, or 0.001 (153) = 0.153
> V or 153 mV. Seems inconsequential, eh?
>
> Well, I need to measure 500 mA in the drain circuit of my VN66AF power
> amplifier. If I put a 1-ohm resistor in that line, at 500 mA the voltage
> across that series resistor would be E=IR, or 0.5 (1) = 0.5 V. Gee, a half
> volt drop is something I can live with out of 13.8 V from a typical supply.
> :-)
>
> So, if I subtract 0.153 V from 0.5 V, I get 0.347 V or 347 mV. To find the
> value of the meter multiplier pot, I divide 0.347 by 1 mA, or 0.347/0.001 to
> get 347 ohms. Reaching for a 500 ohm pot, I solder it in series with my
> meter and proceed to calibrate it, where the final setting will be 347 ohms
> (but I don't really care, as at this point I'm simply setting my new meter
> to full scale visually).
>
> This is usually far easier to do than to try and wind a meter shunt of very
> low value. If you do that, it's hard to control the resistance at the
> terminals, soldered connections, etc. This method lets you precisely set the
> meter reading to what you desire. It does put a small "burden" in the form
> of an IR-drop on the circuit you're monitoring, but usually an ohm or two is
> tolerable. In my example, I lose a half volt.
>
> To gild the lily, you can sometimes carefully open the meter, remove the
> scale dial plate, and repaint the scale to reflect the units you wish (you
> can add color warning bands and your callsign in there too; always a nice
> touch if you can open the meter movement).
>
> GL es vy 73, AI2Q, Alex in Kennebunk, Maine  .-.-.
>
> -----Original Message-----
> From: owner-heathkit at qth.net [mailto:owner-heathkit at qth.net]On Behalf Of
> Jordan Arndt
> Sent: Thursday, May 04, 2000 8:51 PM
> To: boatanchors; Heathkit reflector; HEATH-request at LISTSERV.TEMPE.GOV
> Subject: [HeathKit] SB-200 Plate Voltage reading...
>
> Hi seeing as the meter is a 200uA movement , shouldn't the dropping
> string total 15 Megs instead of 14.1 Megs with the three 4.7 Meg
> resistors..?? Seems it would make the Voltage read about 7% too high...
> What are you SB-200 owners reading for Plate voltage with your stock
> meter strings...??? 73 de Jordan...
>
> ---
> Submissions heathkit at qth.net
>
> ---
> Submissions heathkit at qth.net

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