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iRad 1000/1 High Voltage Multimeter DVM Adapter
Posted: Mon Nov 16, 2020 7:51 pm
Hi from chernobel
I always had a problem with measuring high voltage from my ludlum devices, probes don't work properly, and had to buy something that gives me the real ludlum voltage, so i went and bought iRad 1000/1 High Voltage Multimeter DVM Adapter
For 89$ .
It seems my Multimeters were lying, after reading the true values, everything worked,
So i opened the box just for fun ,what inside. Check the pictures.
Total three resistors
1- 1000 mega ohm
2- 1.3k Ohms resistor
3- trimmer 10 ohms to 2 megohms ( data sheet) link:
https://www.trustedparts.com/en/part/bi ... s/66WR1MEG
Re: iRad 1000/1 High Voltage Multimeter DVM Adapter
Posted: Tue Nov 17, 2020 6:04 am
A common circuit found in old TV 30kv add-on probes to VTVMs of the 50's on. It has about $6.00 worth of components in it. My post here on the 100 megohm input Z conversion of the old $5.00 harbor freight digital voltmeter was similar but limited to 0-2kv using the 20 volt range. Its suggested use was for bias setting readout on neutron and PMT tubes.
Many portable GM counters love to place a series 22 meg resistor in series with the HV output to the GM tube's input circuit. This forces them to suggest using an "electrostatic voltmeter" to make adjustment of their bias pot (effectively infinite input Z)
All such 1000:1 networks absolutely rely on a good stiff, accurately calibrated, usually digital read out, HV supply, (lower Z ), for proper calibration of such a network. I mention the need for this accurate calibration supply in my homemade 100meg input Z HV meter posting.
Most portable GM meters and the GM tubes they use have a 50-100 volt sloppy slope from beginning of GM mode to the end of the Geiger region, (Townsend breakdown). A precise voltage is never called for in such adjustments. Different tubes even of the same type and number have different GM region slope lengths and idealized voltage operating points. Spec sheets on such GM detector tubes usually quote a "suggested" operating voltage. This may or may not be the best operating point voltage, but will usually put you in a suitable voltage range on the Geiger region's working slope.
A lot of folks do not know that under intense source counting, (>>100K cpm), these high Z internal instrument supplies voltage sag a good bit. (The tube is turned on - pulsed - almost continually) this continuous gas break down, heavily loads such supplies. Naturally, this pushes the tube's voltage down the slope. Many times in hyper-high count rates many folks think their tube saturates and stops counting, when in many cases, the weak internal HV supply drops to such a low voltage the tube falls below the Geiger region!! The very reason for a 22meg output Z on a GM supply is that should some idiot adjust the 800 volt GM tube voltage to 1400 volts causing it to arc continuously, the tube cannot draw a current so large that it can kill the tube. Manufacturers of portable instruments do not tune their systems for the ultra high range realizing that assuming your are counting much above 100,000 cpm you are not getting a true count anyway due to resolving time limitations of the tube, itself! (missed counts)
A lot to learn and know, even in a simple GM counter. There is an ideal operating voltage for each and every GM detector tube, near the middle of its slope which, in the best of all worlds, is carefully searched for by a competent technician with decent instrumentation. The factory of the tube maker can't take the time to do this. However, with uniformity and careful control during manufacturing, they can give you a good voltage recommendation to make the counter system they are installed in always work satisfactorily.