dc power supply

[electros6]

Hi guys,
 I replicate a HP power supply for 30v 5A . when I tested the power supply the output ripple was around 20mvP-P , but when I connect a dc motor to the power supply the ripple gone to around 500mvP-P but for the other load it works fine. can sombody tell me what is the reason. I also used diode for reverse polarity protection.
                                                regards
                                             N.Baskaran

[jeroen79]

Which power supply? Did you replicate it exactly?
Which motor?
Likely this is spikes from the motor's coils when it switches polarity.
Does the riple's frequency match the motor's RPM?
Does a resistor drawing the same current have a lower ripple?

[calexanian]

As you load an unregulated DC supply, and in some cases even a regulated supply, ripple will increase. Its a function of the size and ESR of the filter capacitors. Look up the calculation of ripple in dc power supplies and i am sure you will find information. Generally as the load current increases the ripple will increase and it may not be in a linear fashion due to the dynamic characteristics of traditional transformers/inductors and capacitors.