Q:

5v relay to be driven from a limited supply of 20mA which requires 200mA of latching current

Possible Duplicate:
Pulse-powering heavy loads with a coin cell 

I have to operate a latching relay at a nominal voltage of 12 V or 5 V. Right now, i have not chosen the relay so lets consider I have a relay to operate at 5V and the coil resistence is 24 Ohm. The hold current of the relay will be 200mA and we need to give a 5V pulse upto 50mSecs for the relay to latch properly. The problem is I have a limited supply of 20mA from my power source, so I need to employ a charge pump sort of thing which can store some energy and give it to relays when they operate. But is there any specific design which I can follow, and arrive at proper design.

A:

I miss an important parameter: "must-activate voltage". This will (and must!) be less than the nominal 5 V, as it will determine how much the voltage may droop during the 50 ms to activate.  
You won't need a charge pump; a large capacitor will probably do. If you switch on the relay the capacitor voltage will droop exponentially, and the must activate voltage will be reached later for larger capacitors.  
Let's assume must-activate voltage is 4 V. Then
\$ 4 V = e^{\left(\dfrac{-t}{R C}\right)} \cdot 5 V \$
If \$t\$ = 50 mS, and \$R\$ = 24 Ω we find \$C\$ by rearranging the equation:
\$ C = \dfrac{- 50 ms}{24 \Omega \cdot ln\left(\dfrac{4 V}{5 V}\right)} = 10000 \mu F \$

