What Precautions should I take when using external DC power with my sound level meter?
The Model 831 sound level meter can be powered several different ways including
A) Internal AA batteries
C) External 12V DC supply
The Model 831 includes many input power protections to help avoid damaging the instrument.
Input Power Protections
A) When using internal AA batteries – The Model 831 is protected from reverse polarity and includes a 0.75 ampere fuse in the positive supply
B) When using USB current limit protection is provided by the USB port supplying power, i.e. PC, hub or PSA029.
C) When using the 12V DC input – There is a 0.5 ampere fuse in the positive supply and reverse polarity protection
The Model 831 ground or negative supply is available in multiple locations and can be found at:
- The preamplifier, typically PRM831, and the interconnecting cables
- The metal nose on the 831 body
- The USB connector and interconnecting cables
- The AC/DC output connector
- The connector for 12V DC input
- The negative terminal of the internal batteries
Internal to the Model 831 all grounds are electrically connected and there are no fuses or other protection against externally induced ground current. It is therefore important that all 831 ground points remain at the same voltage. Failure to keep the ground voltage consistent can result in electrical currents that can permanently damage the Model 831.
A voltage differential can be created by a variety of situations and listed below are a few.
1) The exterior of the preamplifier, microphone or cable connector comes in contact with an unprotected power supply such as the positive terminal of a battery.
2) The shield of a USB cable that is connected to the 831 comes in contact with an unprotected external power supply.
How to Protect Against Destructive Short Circuits
A) Make sure all power connections are covered by an electrically insulating material. This includes the terminals of any external 12V batteries which should have terminal covers securely installed that cover all conductive portions of the terminal and connector.
B) Disconnect power when servicing the Model 831 or system components by disconnecting the negative wire from any external batteries
C) Use a 4 ampere fast blow fuse in both the positive and negative terminals of any external power cables.
D) When shipping, use the following precautions
a. Ship systems without the battery installed to prevent possible short circuiting while in transit
b. Alternatively, disconnect and secure the negative wire during transit
c. Review and follow instructions provided by the shipper for shipping batteries
d. Review and follow and instructions specific for the system being shipped
E) Store cables securely to avoid accidental movement and an inadvertent short circuit