Converting an AC diagram to DC

Thread Starter


Joined Oct 11, 2011
I am familiar with designing and creating AC controls. But when it comes to DC, it is something I have not been involved with very much.
Let's suppose I have a control circuit that uses 120vac for power and consists of a directional control valve that operates a hydraulic cylinder and has limit switches and/or proximity switches and time delay relays. Now suppose I want to take it and set up something similar in a 12vdc circuit. Other than making sure all my coils are 12vdc coils and my swithc ratings are correct, are there any fundamentals that need to be observed and followed with a DC circuit that is different from an AC circuit?

The reason behind my question is because I want to set up a control on a portable, engine driven cart using it's 12vdc electrical system for my power source. It will need to operate some control valves to control hydraulic cylinders etc. I want to make sure I get things started off on the right foot.
Thanks in advance for your response?\!


Joined Jul 17, 2007
Hello AF,
One of the things we've been discussing lately is the back-EMF from relays/solenoids when the current flow is stopped. That probably wasn't much of a concern with AC if you were using TRIACs/SCRs to control relays/solenoids/other inductors with; as the thyristor would automatically turn off when current fell below a certain point; with 60Hz AC, the zero crossings occur 120 times per second, or every 8.333mS

If you simply interrupt DC flow through an inductor by turning off a transistor, MOSFET, etc, the current will try to keep going, and a voltage "spike" occurs in reverse polarity across the coil that may reach very high peak values.

One cheap way to avoid this spike is to use a diode connected in parallel with the coil, cathode towards the more positive end. This is sometimes called a flyback diode (sometimes called a snubber diode, flywheel diode, freewheeling diode, suppressor diode, or catch diode). This diode provides a path for the inductor current back through the inductor, rather than have it build to a high voltage spike.

However, using just a simple diode will cause the current to decay rather slowly. Using a resistor or Zener diode in series with the diode will help the current to decay much more rapidly. Care needs to be taken to ensure that the voltage rating of the switch controlling the relay/solenoid/inductor is not exceeded.


Joined Aug 30, 2007
Another thing to watch for, capacitors used for ac are not polarized.
Capacitors for dc usually are polarized.