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From the Puckerbrush...

Editor's Column

Build a Resistive Digital Voltmeter Adapter

Expanded scale loaded voltmeters have become the accepted means of field checking a receiver battery pack to see if it has the capacity for another flight. $10 to $15 analog meters are available for this purpose but are otherwise of very limited use. With the dramatic drop in price of practical and flexible digital multimeters, many R/C modelers now find them ``necessary'' support equipment. However, these meters have an extremely high impedance (resistance) and therefore display the ``unloaded'' open circuit voltage of a Ni-Cd battery pack which does not reliably indicate the pack's state of charge. Therefore, a better method of checking a Ni-Cd battery pack is with a "loaded" digital voltmeter.

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One problem, of course, is that no one makes such a device. But this is a simple fix! The simplest trick is to connect the appropriate plug to the receiver charge jack, across which is placed a resistor which ``loads down'' the receiver nicad battery. the digital voltmeter then reads the more indicative voltage of the pack under load.

For a 250 mAH pack Use one 50 tex2html_wrap_inline129 , 1/2-watt resistor
...Or two 25 tex2html_wrap_inline129 , 1/4-watt resistors in series
...Or five 10 tex2html_wrap_inline129 , 1/8-watt resistors in series
For a 500 mAH pack Use one 25 tex2html_wrap_inline129 , 1-watt resistor
...Or three 8 tex2html_wrap_inline129 , 1/2 watt resistors in series
...Or five 5 tex2html_wrap_inline129 , 1/4-watt resistors in series
For a 1200 mAH pack Use one 10 tex2html_wrap_inline129 , 3+ watt resistor
...Or five 2 tex2html_wrap_inline129 , 1/2-watt resistors in series

This is functional and simple, but a more aesthetic and professional project might look like the following. Start with two pin jacks to match the leads or probes on the digital multimeter; one red for the positive lead and a black for the negative lead. Instead of a resistor providing the load as above, a light bulb can substituted, which clearly demonstrates that it is providing a load for the battery. After experimentation, a # 44 bulb (6.3V, 250mA) was found to load the pack with 220 mA. This is fairly close to the load imposed by a typical R/C system. If a higher load is needed, a PR12 or PR13 bulb provides a drain of about a 500 mA. Finally, some type of enclosure will be needed to house the components; a small clear plastic box about the size of a small matchbox makes a neat compact unit. One of the leads can be soldered directly to the tip of the light bulb base. The side of the bulb base might not accept solder, in which case a small brass clamp can be made from 1/32'' thick stock by 3/16'' wide and secured to the bulb. The clamp can be made so that it can be soldered directly to one of the jacks providing a secure mount for the bulb. To measure the receiver voltage, plug the meter into the red and black jacks on the load box and the battery lead into the charge connector on the plane.

figure57

If the loaded digital meter indicates 4.8 volts or greater, it has sufficient charge for additional usage. The following procedure can be used to calibrate the minimum ``safe'' voltage for your setup: On a day when the equipment is not being used, it should be allowed to discharge until the meter reads 4.8 volts. Then the system should be turned on and range checked every 5 to 10 minutes. The servos should be actuated to determine how long it takes before they start to twitch due to insufficient voltage. This will give a good notion of the minimum safe voltage.


next up previous
Next: Build Your Own Flight Up: Front Page Previous: At the Last Meeting...

Craig Kulesa
Thu Nov 4 17:28:36 MST 1999