DE1113748B - Direct reading ohmmeter - Google Patents

Description

Direct Reading Ohmmeter The invention relates to a direct reading ohmmeter Ohmmeter with several measuring ranges.

The invention is based on a direct reading ohmmeter a voltage divider circuit fed by a constant voltage with several Range resistances, the different ones, with the same scale division of a voltmeter assigned readable measuring ranges and via two leads to two parallel to Terminals located next to the voltmeter and used to connect the test object can be connected are.

The invention is intended to eliminate errors that occur in the Measurement of very high resistances through the shunt resistance of the display instrument caused.

A direct reading ohmmeter of the type specified above is in accordance with of the invention characterized by additional, located in the voltage divider and by a range switch in series with the part generating the measuring voltage of the voltage divider switchable compensation resistors, the size of which is dimensioned is that the same voltage drop occurs across them as across the voltage divider and the series resistance lying in the test terminals due to the caused by the measuring device open test terminals with current flowing.

The invention will now be explained in more detail with reference to the drawing, a circuit diagram of an embodiment of an ohmmeter according to the invention shows.

The ohmmeter contains a voltage source 18, for example a glow stabilizer 21 with a series resistor 19 and an adjustable one Resistor 23 may contain. The ones with a plus sign and a minus sign A suitable DC voltage source is connected to the supply lines provided. Of the The positive pole of the voltage source 18 is connected to the negative terminal 27 via a voltage divider from the series-connected resistors 29, 31, 33, 35, 37 and 39 are connected. The resistor 41 lying parallel to the resistors 29, 31, 33, 35 and 37 can remain out of consideration here.

The connection point 43 between the resistors 37 and 39 is direct connected to a fixed contact of a measuring range switch 45. This Contact corresponds to the lowest resistance range of the ohmmeter.

The connection point 47 of the resistors 35 and 37 is direct connected to the switch contact for the next higher decade. The connection point 49 of the resistors 33 and 35 is directly connected to the switch contact for the next higher Decade (for example 100 ohms) and via the resistors 51, 53, 55 and 57 with connected to the contacts for the next four decades of resistance. The connection point 59 of the resistors 31 and 33 is connected to the switch contact of the via a resistor 61 next higher measuring range and the connection point 63 of the resistors 29 and 31 finally via a resistor 65 on the switch contact for the highest decade of resistance, for example 100 MOhm.

The movable switching arm 67 of the switch 45 is via a supply line 69 with a test terminal 73 and the other test terminal 79 with a lead 75 the negative terminal 27 of the voltage source 18 is connected. With high measuring ranges The lead resistances shown as concentrated resistors 71 and 77 can be used be disregarded.

Between terminals 83 and 85, which is practically with the test terminals 73 and 79 are identical, there is a voltmeter 81. A resistance to be measured 87 is connected to test terminals 73 and 79.

In the lower measuring ranges the shunt resistance of the voltmeter 81 can be neglected.

If the resistance of the measuring device is 200 MOhm, it is through errors in the l-MOhm measuring range resulting from the shunt are not more than 50 / o.

The internal resistance of the measuring device 81 however, they can no longer be neglected. It should be assumed that the full scale of the voltmeter is 1.0 V and the internal resistance is 200 MOhm, which can easily be achieved with tube voltmeters.

The current flowing through the voltmeter 81 is generated at the resistor 61 a voltage drop and thus deceives a lower resistance value of the test object 87 before. A compensation is now effected according to the invention in that one the voltage drop across resistor 61 caused by the voltmeter current, added to the voltage that drops between 27 and 49 at the voltage divider.

This can be achieved by going into the voltage divider a small resistor 33 turns on, which is dimensioned so that on him by the Voltage divider current creates a voltage drop that is caused by the voltage drop the voltmeter current at resistor 61 just corresponds.

So if the switch 45 is set to the measuring range 10 MOhm and the test terminals 73, 75 are open, they must have a voltage of exactly 1.00 V occur with an effective internal resistance of 10 MOhm. As the voltmeter resistance of 200 MOhm is always parallel to the test terminals 73, 79, you can determine the size of the resistor 61 can be calculated as follows: R61 R81 10.0 MOhm R61 + Rs or R6l 200 200 10o l06 = 10.0 MOhm (l) 106 R61 t (200.106) = 106) MOhm (1) or R61 = 10.525 MOhm.

Since a current of 1/200 10-6 A flows through the resistor 61 when the voltage on voltmeter 81 is 1.00 V, the voltage drop becomes at resistor 61: E61 = (1/2) 10-8. R61 = 0.5 10-8. 10.525 10-6 = 0.051125 V; (2) thus the voltage at point 59 must have the following magnitude: E59 = 1.00 + E6l = 1.051125 V. (3) In order to obtain this voltage, the resistor 33 must have the following value: E59 0.051125 R33 = = = 5.1125 ohms. (4) I 0.010 The same measures are also used at hit the next decade, when the switching arm 67 on the 100 MOhm contact stands. In this case, R65 has a value of 200 MOhm, and the voltage at the point 63 is 2.00 V. Resistor 31 to compensate for the voltage drop across the resistor 65 is then for the voltage divider current 1 = 10.00mA, exactly 94.8875 Ohm. in the above is the influence of the series-connected resistors 31, 33, 35, 37 and 39 neglected, since their total value of approximately 200 ohms is only one vanishing caused small bugs.

Another expansion of the measuring range upwards in another Decade level at which the measuring display in the middle of the scale would be 1000 MOhm, is only possible if the internal resistance of the voltmeter is increased to a value which is greater than 1000 MOhm.

With the ohmmeter described for example according to the invention can perform precise measurements in a range from 0.02 Ohm to 5000 MOhm guided will. The circuit can easily be integrated into a tube voltmeter with a direct display or milliammeter, since most of the required switching elements are in such a device already exist.

Claims (1)

PATENT CLAIM: Direct reading ohmmeter with one of a constant Voltage-fed voltage divider circuit with several range resistors, the different measuring ranges that can be read off with the same scale division of a voltmeter assigned and via two leads to two parallel to the voltmeter, Terminals used for connecting the test object can be switched on, with measuring ranges, which also include high resistances where the internal resistance of the voltmeter is no longer negligible, characterized by additional, in the voltage divider located and by a range switch (45) in series to which the measuring voltage generating part (39, 37, 35) of the voltage divider switchable compensation resistors (31, 33), the size of which is such that the same voltage drop occurs across them like the series resistance between the voltage divider and the test terminals (61, 65) as a result of the flowing through the measuring device (81) when the test terminals (73, 79) are open Current.