36v to 12v dc/dc conversion

[khcshadow]

Hello, I am currently working on the electronics for a CNC machine, and am having a problem with supplying 12V for a cooling fan. I need a cheap and easy way to convert 36V from the dc power supply to 12V for the brushless fan (12v, .32A). I already purchased a 12V 1A linear regulator, only to realize that its operation was stable up until 35V. All of the dc-dc converters I am finding are very expensive, as they are designed to handle upwards from 10A (usually automotive devices).

Also, using as little power as possible is very important. The PS is 8.88A, and each stepper motor controller (there are 3) supplies a max of 3A. I will set the controllers to limit the motors to 2.7A, just to make sure I don't destroy my PS. 2.7(steppers) * 3 + .32 (fan) = 8.42A, leaving .46A. In reality, I doubt all three motors will be running simultaneously for any extended period of time, but I just don't want to strain my PS too much. I'm guessing the best solution will be a small transformer, but I really don't have very much experience with electronics, so I was hoping someone could point me in the right direction. Thanks!

 

[(*steve*)]

This is probably way overkill, but it's not expensive and won't be operating near its limits.

 

[khcshadow]

Thanks for the reply, but as you said its a bit overkill. I'm looking for something that would be a couple bucks max and is relatively small. I'd be surprised if there wasn't a component designed for this sort of thing—would a transformer do the job? My understanding is that transformers have essentially no ohmic loss and would be ideal for this situation, but I'm not sure as I have no traditional training in electronics. The only transformers I am finding are designed for very high power or are for mains voltage. Could I just coil some wire around a toroid? Thanks

 

[(*steve*)]

Transformers are for AC. For DC you need a DC-DC converter.

There are many available at the price point you're interested in, but sadly, most top out at around 40V input voltage.

Here is an example. Note that there are similar chips to the one used here that can handle a higher input voltage, but you'd have to build the regulator yourself. It that isn't an issue, go for it. An example is the LM2576HVT-ADJ.

And lookee here -- Available on ebay already built up. Not as cheap as the lower voltage ones, but still not expensive. edit: even cheaper.

 

[CocaCola]

I'm going to assume the 36v DC supply is fed from the mains? Why not just get a 12v wall wart (a few bucks) and just tap of the mains input to your existing 36v supply get to run the 12v wall wart? No worries about starving the 36v supply as you are pulling off the mains and it's a dirt cheap solution...

 

[khcshadow]

Thank you both. Either solution should work well; that DC-DC converter is pretty cheap. I am going to check to see if I already have a 12V wall wart, if not I will go with that. Just out of curiosity though, why would the toroid coil not work? I thought that transformers simply output a voltage based on coil ratio due to induction.

 

[(*steve*)]

transformers only work with AC

 

[khcshadow]

Any issue with wiring two wall warts together? I have a 5.1V @ .7A and a 5.2V @ .46A. I plan on wiring the AC input wires together in parallel, and the DC output in series. Also, does it matter which AC input wire goes where? On one unit it is labeled neutral and live, but the other is simply AC1 and AC2. I wouldn't think that it would matter since the signal is going into a transformer.

Thanks

 

[(*steve*)]

Check to ensure that none of the outputs are earthed. If they are, exercise caution when connecting them in series.

Note that connecting the outputs in series will render some of the over-current protections (for the lower output current device) less than useful and you risk damage to it in fault conditions.

The AC connections can probably go either way around as they should be totally isolated from earth or the outputs.

 

[Sparkchaser]

I can already smell that, oh so familiar smell, burnt electronics....

I think if I were going to use a wall wart, I would find a single 12 volt specimen instead of trying to wire their outputs in series...

 

[khankll]

If you want to the go with your original option then fan use few diods in series to drop few volts.. That way you can get the news volts.. But in ur case if using linear regulator u range wasting 32-12 *. 3 watts... Ii. 3.6 watts as heat.

 

[khcshadow]

Yes, unfortunately I cannot lose couple hundred milliamps. I think the wall wart / buck converter is a better option. I hope nothing gets fried—I may end up using a single 9V since I don't have a 12. The fan will probably run fine at 9 as its just a DC fan. Neither wall wart has a ground pin so I'm covered in that aspect. I will see how easy it is to get the 9V out of the case. Thanks!