12v transformer voltage multiplying

Thread Starter

arthur92710

Joined Jun 25, 2007
307
Ok I have a 12vdc(now ac after I fix'd it:D) with 1200mA output. I need to get either 24vdc or +12 and -12dc from it, whatever makes a fan spin faster.

I have tried to divide the 12 into +6 and -6 then multiply by two.
I have also tried to just multiply the 12 into 24 then but that also does seem to work.
 

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Thread Starter

arthur92710

Joined Jun 25, 2007
307
What kind of caps are those?
Im using plain electrolytic caps is that fine?

Also is there a formula or some thing for choosing the caps and resistors? If I doubled the caps in parallel the fan slowed down.
 

JoeJester

Joined Apr 26, 2005
4,310
You would use electrolytics, so observe the proper polarities. What are the specs of your fan? x Volts @ x Amps.

Did you notice the errors in your schematics?

Doubling the capacitors increase the charge time ...so I would suspect the fan would slow down till the capacitors were properly charged. One meg is probably way too high [discharge time when power is removed is quite a long time] ... 1100 seconds for five time constants ... almost 10 minutes. I certainly made a bad choice for that. I don' like waiting more than 90 to 120 seconds.

Anyway ... talk about your fan's specs or load specs.
 

Thread Starter

arthur92710

Joined Jun 25, 2007
307
The fan is 12vdc .2A
I wanted to control it with a PWM, the UC2525AN, but I could not get it to work. So I used a 555. The 555 does not output enough current, only 100mA, so I got a NPN transistor and connected the base to the 555 output. It worked well but my 9volt battery did not give enough power, so I looked for a transformer. The best one has 12dc and 1200mA.

I tried using the 555 with this but the ones I had on hand some how died when connected to my circuit. So I have to look for some more.
 

JoeJester

Joined Apr 26, 2005
4,310
Ok ...

If you wish to return to the 555 designs we certainly can do that.

Let me know what you want to do ... back to the 555? run the fan with the wall wart itself? what?
 

JoeJester

Joined Apr 26, 2005
4,310
Authur,

I wouldn't run the fan at 24 volts if its a 12 volt fan. Tempting fate is not good.

So, we know you want to use the 555 for pwm and a device to activate the load (fan 12V 0.2 amperes).

You mentioned the wall wart as being 12V 1200 mA. Can I assume that's 12V dc at 1200 mA? What is the unloaded voltage output of the wall wart?

Do we assume you wish to have a variable pwm for the fan?
 

Thread Starter

arthur92710

Joined Jun 25, 2007
307
It says 12vdc but I needed ac so I took out the board that was in side it. It had 4 diodes and capacitor. Now my meter shows 12vac. If 24v is to much for the fan I can use less, like 20v. and yes I need the PWM to be adjustable.
 

SgtWookie

Joined Jul 17, 2007
22,210
Arthur,
I posted a "simple PWM circuit" on "The Projects Collection" forum.
You can use that. It's really quite easy to build, and really just what you need.

Run the 4093 part from the 12v portion of the supply - run the fan from the higher-voltage supply.

You must not try to run the 4093 from more than 16 volts, or you will kill it.

However, the power MOSFET can take a fair amount of abuse.

If you have more questions, ask again. I'll look in tomorrow.
 

JoeJester

Joined Apr 26, 2005
4,310
What was the value on the diodes and the capacitor?

It just seems like a waste of time for you to recreate the wheel by building another bridge rectifier and filter cap to recreate the DC needed.

Here's a pwm using a 555 timer.
 

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