Many times an existing wire run is undersized for the current draw of the load device.  This could either be due to an unexpected change in the device being powered, a miscalculation at the planning stage of the job, or a retrofit situation where the existing wire size is not able to be changed.  This undersized wire results in a large voltage drop, leading to improper or erratic operation of the load device.  Even if the voltage at the device is at the low end of the acceptable range, the voltage will quickly drop to unacceptable levels when on battery power.

What is Voltage Drop?

Wire has a specific resistance per foot of length.  The smaller the wire gauge, the higher the resistance (given equivalent wire types).  As the length of the wire increases, the total resistance between the power source and the load device increases.  When current is now drawn through this wire resistance, some of the voltage is "burned off" in the wiring as voltage drop, as defined by Ohm's Law (V=IxR).

Per Ohm's Law, two main factors affect the amount of voltage dropped within the wire run:  the wire resistance, and the current drawn through the wire.  This is why a lower-current device can get away with a smaller wire gauge. 

This is also one of the main reasons the lifesafety industry has, and continues to, switch from 12V to 24V.  A given device will use a certain number of Watts.  If that device is designed to use a 24V input rather than 12V, the current required will be halved (per Ohm's Law I=P/E), which in turn will halve the voltage drop.

How can the B100 help?                         

If powering a 12V device, a B100 can be used to give an adjustable output voltage greater than the 12.5V nominal setting of an FPO power supply.  The FPO will need to be set for a 24V output, and the B100 placed into the adjustable range by moving JP3 to position 2.  The output can then be set by adjusting VR2 to a level giving an acceptable voltage at the load device.  Since this voltage is run off of a 24V supply with a 24V battery set, this voltage will remain constant until the battery set drains to well below 20V.

Please note that if the device being powered has varying current levels during normal operation, the voltage at the device will change with this current, possibly with damaging results.  As an example:

A B100 is set to a level of 16.5V to overcome the voltage drop through 500 feet of 18AWG wire powering a 12V edge device and a 12V maglock at a door.  The draw of the lock is 400mA and the edge device is 100mA, giving a total draw of .5A when the lock is powered.  When the lock is powered, the voltage drop will be 3.24V, leaving 13.26V at the door.  However, when the lock is released, and the current draw drops to 100mA, the voltage drop will decrease to 0.65V, giving 15.85V at the door, which is likely too high for the 12V edge device. 

In the above example you could decrease the B100 voltage to compensate, but you must take the whole operating current range into account.  If the operating current range is too large, there may not be an acceptable voltage to cover all load conditions.

For more information on the B100, see the B100 manual and Application Note AN-07.  Also, be sure to download our FlexCalculator Suite for quick voltage drop calculations.  And remember, our This e-mail address is being protected from spambots. You need JavaScript enabled to view it department is always here to help.

In the previous two parts of this series, we covered the very basics and the inputs of the C8 board.  If you missed these parts, you can find them here:

Part 1 - Power and LEDs
Part 2 - Inputs

This week we will go in-depth on the outputs of the C8, including the wiring, operation, and jumper configuration.

The Anatomy of the C8 Output

Like the inputs, each output of the C8 has two terminals, labeled A and B.  Each output may be individually configured for a voltage output or a relay output and contains a reverse diode to dissipate and reverse EMF from a locking device or other inductive load.  See our White Paper and Application note on reverse EMF for more information.

When configured as a voltage output, the B terminal is the positive and the A terminal is the DC Common (ground).

C8 Voltage Output

When configured as a relay output, due to the reverse diode, the current must be fed through the contact in the proper direction.  Regardless of what the relay output is activating, there will be a current present.  If the relay output of the C8 is connected backwards, the reverse diode will always be conducting and will not change with the relay.

C8 Relay Output

Each output has four configuration jumpers, detailed below.

Black Jumpers (C and E)

The black jumpers for each zone, labeled C and E, configure the output for either a voltage output or a relay output.  BOTH jumpers must always be set in the same position (by the jumpers' markings) for proper operation.  Check the position markings carefully on the PC board, as position 1 and 2 for each of the black jumpers is different.

By setting the black jumpers in position 1, the output will be configured for a relay output.  By setting these jumpers to position 2, the output will be configured as a voltage output.

White Jumper (F)

The white jumper selects between the NO and NC of the output relay.  When set for a relay output, this selects a NO or NC output.  When set for a voltage output, this selects whether the output is normally powered or not powered (maglock or doorstrike). 

By setting the white jumper in position 1, the output will be NO or will normally have no voltage on the output until the input is activated (flashing green LED).  This is the typical setting for a fail-secure door strike or electrified handleset.

By setting the white jumper in position 2, the output will be NC or will normally have voltage present on the output until the input is activated.  This is the typical setting for a fail-safe maglock.

Again, there is no need to memorize all of the settings - once the blue jumper for the input is set properly (steady when locked, flashing when unlocked - see Part 2), the white jumper can be changed until the output operates correctly.

Yellow Jumper (D)

The yellow jumper selects between the buss 1 and buss 2 voltage supplied to the C8 for each output (See Part 1).  If only a single voltage source is connected to the C8, then this jumper should remain in position 1.  If two power sources are connected to the C8, then setting this jumper in position 1 will select the voltage source connected to B1 and position 2 will select the B2 power source.

Note that this jumper has no effect when the output is set as a relay output.

Voltage Output

The most common output configuration for a C8 is a voltage output.  When connecting a device to the terminals when set as a voltage output, the positive connection goes to the B terminal and the negative connection goes to the A terminal.

C8 Voltage Output Wiring

Relay Output

When configured as a relay output, the current through the relay must flow from the B terminal to the A terminal (the more positive side of the voltage on the B terminal).  Note that ONLY a DC voltage may be switched through the C8 relay due to the reverse polarity diode.

C8 Relay Output Wiring

Next Week

Next week we will go into detail on the jumper settings of the C8, including the usage of our Excel-based C8 Jumper Configuration Tool.  Until then, if you need any assistance, our This e-mail address is being protected from spambots. You need JavaScript enabled to view it department is here to help.