diodes

evesautomotive · 03-01-2009, 07:15:02 PM

Hi:
I have wired in a fan and want to control my feedback. I know a lot of people will tell me to wire in a relay which I could do easily but want to learn a little about diodes hence this blog.

I have a 120W fan that operates off of 12V. What rating of diode should I get? I was just at frys (local electronic store) and bought one that had a V rating of 40, forgot the amp rating but it was plenty. Anyways, I monitored diode and did work but noticed that it did get very hot to the touch. I continued to monitor it for about 30-45 minutes. Never burnt up but did get very hot to the touch.

How do I determine what size I need? Again my load is 120W with a push of 12V making the load (I think) of 10A.

onovakind67 · 03-01-2009, 07:33:28 PM

If you want to control the feedback, just use a relay that disconnects the fan from the circuit and put the diode reverse biased across the fan. If you don't want the relay to kick back, install the diode across the coil so it is normally reverse biased.

evesautomotive · 03-01-2009, 09:30:20 PM

Dam I can't find the diode that I had to try your idea out, guess I will go back to frys and get another one.

I like your idea about putting it parallel to the load, just like an a/c diode.

Here is what I have then you will understand what I am trying to do:

I put in a fan, found a key on voltage source and wala, fan goes. Thing is that when I turn key off the engine doesn't die immediately...takes about 4-5 seconds. Why? Because the key on voltage source I used was for the ignition system and now when power is cut off the centrifugal force of the fan now turns into a generator feeding voltage.

Experimenting I wanted to try a diode but went in series on positive leg before load.

Btw, the # in the diagram is for a rectifier. I have been researching these things online

I reeeeeealy didn't want to write all of this again but did since you didn't know why I went w/ a diode and was nice enough to reply to my question.

Hey where did you get the schematic for the fan control? I would love an adjustable one but want to find one cheap.

onovakind67 · 03-01-2009, 09:38:49 PM

Voila!

When you mentioned that you needed a 10A diode I knew you were putting it in series. It gets hot because you're dropping 0.7v across it at your operating current, so that's about 7W. The relay and snubber diode will do the trick.

evesautomotive · 03-01-2009, 10:33:19 PM

sooo will the diode work if I reverse parallel with load? If so what size type of diode do I have to get?