| Cable Testers and Harness Testers Made Easy!
When you need to test a cable, harness, or PCB assembly that contains components you may wonder if a low voltage cable tester will damage those components. (For high voltage testing you should see High Voltage Testing Components). In most cases the tester can't damage the components. In fact we've never heard of a low voltage test (by a Cirris tester) damaging components. Still, there are a few (very specialized) cases where damage might occur. To understand when it's safe and when it's not safe to test components with low voltage you'll need to understand a little about how the tester works.
Modern Cirris testers use a current source and a voltage sensing circuit to do the work of testing connections (see Figure 1). This arrangement is very similar to a common ohm meter. The current source drives a known current through a connection and the voltage sense circuit measures the voltage that develops across the connection.
Figure 1. The current source drives a current (Ic) through a wire's resistance (R) and a voltage sense circuit measures the voltage that develops (Vm).
There are a few important points to note about the current source. The current source only works up to a maximum voltage. That maximum voltage limits the power the current source can deliver. Also, the current source is programmable. The tester can reduce the current to keep the voltage from getting too large. The specifications for each tester show the highest current and the highest voltage the current source can develop (See Table 1). The highest current and highest voltage can be used to calculate the highest power a tester can deliver to a component.
|Tester||Maximum Current||Maximum Voltage||Power|
|Signature 1100R+, 1100H+, Touch1||6mA||5V||30mW|
|easy-wire CR||6.5 mA||10V||65mW|
Table 1. The highest current, voltage, and power each tester can deliver.
In addition to limiting the current and the voltage the tester limits the test time. The tester works quickly to make a measurement. The current is applied to a given component for a very brief amount of time. The current is typically applied for less than 10ms (often less than 2ms). Limiting the duration of the test makes it even less likely that damage will be done.
Most components have maximum current, voltage, and power specifications so by comparing the tester's specifications to the component's specifications we can tell if damage might occur (see Table 2).
|Component||Maximum Current||Maximum Voltage||Power||Notes|
|Resistor (through-hole)||NA||300-400V||250mW||Can handle short duration overload at 2x rated power.|
|Resistor (smt 0805)||NA||100-200V||100-125mW||Can handle short duration overload at 2x rated power.|
|Resistor (smt 0402)||NA||50-75V||50-75mW||Can handle short duration overload at 2x rated power.|
|Diode small signal||20-100 mA (forward current)||75-100 V (reverse)||250-500 mW||Forward voltage is about 0.65V.|
|Capacitor (smt 0402-0805 ceramic)||NA||10-100V||NA|
|Capacitor (smt polar)||NA||10V-32V||NA||Can handle brief reverse polarity for voltages that don't exceed 15% (tantalum) or 60% (aluminum) of rated voltage.|
|Transistor (Bipolar)||20-100mA (Ibe)||40-100V||100-500mW|
|Transistor (MOSFET)||100mA-10A (Ids)||10V-16V (Vgs)||500mW-10W|
|Integrated Circuit (CMOS)||20mA (clamp diode current)||7V||300-500mW||IC is not powered which eliminates "latch-up" issues. Clamp diodes limit pin voltages to 0.7V or 1.4V|
Table 2. The highest current, voltage, and power for some common components.
In most cases the tester doesn't develop enough voltage, current, or power to damage the components. There are a few special cases that need further explanation: polar capacitors and integrated circuits.
For polar capacitors it is possible that the tester will apply a small amount of "reverse charge" to the capacitor. In general the tester will apply it's maximum current for less than 10ms then the tester will discharge the capacitor. In typical applications polar capacitors have diodes, IC's, or resistors connected in parallel with them. These parallel components keep the capacitor from receiving much reverse charge. Still, it's possible that the capacitor may see a few tenths of a volt of reverse polarity. This brief reverse charging of the capacitor is so fast it generally doesn't degrade the capacitor's performance. (Tantalum capacitors can generally tolerate 15% of rated voltage applied in the reverse direction, aluminum electrolytic capacitors can tolerate 60%). In some cases you can reduce the chance of reverse-charging a polar capacitor by adding a capacitor test (with the correct polarity) to the test program.
Most Integrated Circuits, whether based on bipolar transistor or MOS technology, have some form of protection circuitry (usually diodes) built onto each pin. The protection circuitry helps to make the IC more robust. These protection diodes will have a maximum current rating that is typically 20mA or more so the tester will not damage them. You can use an ohm meter (set to "diode check") to measure from pin to pin on an (unpowered) IC. The meter will give you a feel for what the tester will see as it tests the IC pins. You will typically see one or two diodes (in series) connecting each pin to every other pin so the voltage of the test is typically 0.7 or 1.4V.
The tester's low current and voltages will not harm most IC's. It's unlikely (but possible) that some low voltage MOS parts might not have protection circuitry. You should check the part specifications and avoid testing parts that indicate they do not have any protection circuitry.
In general, using a low current, low voltage, low power source eliminates the chance of damaging components in your device under test. If you have a concern about your test application call us (1-800-441-9910) and we can discuss the ways you can adapt your test to reduce the chance of damaging components.
© Cirris Systems Corp.