# Connecting capacity modules to Weidmueller power supplies

In weidmueller power supply catalogue, There is a "Capacity Module" as an expansion module for the power supply to ensure a secure power supply even at peak loads (for example when the motor starts) and the specific triggering of circuit breakers(as the catalogue says). But I cant understand what exactly capacity modules are? are they a solution for bridging short-term power supply failures? How can they be used in triggering specific circuit breaker?

this is the link to the weidmueller capacity module for power supply catalogue. http://catalog.weidmueller.com/procat/Product.jsp;jsessionid=86A7E09EBD413FB06A36C80BE6551A5C?productId=(%5b1222240010%5d)&page=Product

The capacitor module provides a very short-term fill-in power supply to cater for transient peak current demands. In an application that requires a high peak current this unit saves having to fit a power supply capable of handling the peak current. This may be a desireable feature as the smaller power supply will have a lower short-circuit current limit and lower current rated wiring could be used, for example.

Figure 1. Extract from datasheet.

Some of these datasheets have a high "tease-factor" in that they don't disclose the innards and this can make it difficult to predict performance in non-standard operating conditions.

At its simplest this unit contains a large capacitor (probably several to fit the case style) and a few other components. We can estimate the capacitor value from $i = C \frac {dV}{dt}$. Rearranging and assuming that the voltage might drop, say, 10% over the rate 1 ms at 40 A we can calculate

$$C = i \frac {dt}{dV} = 40 \frac {1m}{2.4} = 16,600~\mu F$$

This seems quite feasible.

The recovery time is 1 s so there is a resistor or current limiter in series to limit the current draw from the power supply it is supplementing.

Figure 2. The internal wiring showing the charging resistor, the discharging diode (to prevent rapid recharge) and voltage monitoring circuit and relay. Source: CP M CAP Operating Instructions.

In use the capacitor module is simply wired in parallel with the PSU. (See the operating instructions.)

are they a solution for bridging short-term power supply failures?

Probably not. See answer by Transistor for capacitance estimate. The module has only a small amount of stored energy.

How can they be used in triggering specific circuit breaker?

The relay included in the module switches when the voltage drops below about 20 volts to indicate the power supply has failed. If you wish to shut off some other equipment when the connected power supply fails, the relay contact can be used to trip a circuit breaker using a shunt trip mechanism. The instructions mention a specific circuit breaker that is compatible.