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Re: 12VDC to 18.5VDC

Subject: Re: 12VDC to 18.5VDC
From: "Paul E. Schoen"
Date: Wed, 9 Aug 2006 15:07:51 -0400
Newsgroups: sci.electronics.basics, sci.electronics.design
<mrdarrett@xxxxxxxxx> wrote in message 
news:1155100783.237344.201930@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>
> mrdarr...@xxxxxxxxx wrote:
>> Paul E. Schoen wrote:
>>
>> ...
>>
>> > You will probably need to "roll your own", but it should really be 
>> > nothing
>> > much more than a small toroid about 1" diameter, wound with a couple 
>> > dozen
>> > turns of wire, if you use about 50 kHz PWM. #22 to #26 AWG wire should 
>> > be
>> > OK for 3 amps or so. Otherwise, you could probably use parts of an old
>> > computer power supply. Of course, you can just buy a DC-DC converter, 
>> > but
>> > 18 watts will probably cost at least $50.
>> >
>> > Paul
>>
>>
>> My PWM controller is basically this:
>>
>
> silly, forgot the link:
> http://www.cpemma.co.uk/555pwm.html
>
> (sleep deprived - woke up 4:30am this morning to fly out to LA for a
> meeting.)
>
>> Probably a silly question to ask, but just making sure: should I make
>> C1 equal to 200 (or the more commonly available 220) pF to get 50 kHz?
>> Any special type of cap, or would a simple ceramic do?
>>
>> Thanks,
>>
>> Michael
>

The 555 will handle 50 kHz, but 200 pF is marginal. It would probably be 
best to change the pot to 10k, and then use a 2 nF capacitor for C1. 
Ceramic or film types are OK.

You cannot just put a transformer in place of the motor, because you will 
have DC which will saturate it. The easiest method is a push-pull design 
with a center tapped primary, and the best IC is a regulating PWM 
controller such as SG3526, UC3526, or TL494, which is designed to drive two 
MOSFETs or bipolar transistors. It has provisions for soft start, 
overcurrent shutdown, dead time, and voltage regulation. They are used in 
many switching power supplies.

For wire size, you can use my spreadsheet at: 
http://www.smart.net/~pstech/WireSize.xls

It shows that #22 AWG should handle 3.8 amps. #20 allows 5.3 and #18 will 
handle 7.6 amps. These values are for a conductor in free air with 
something like 30 C temperature rise. Use the heaviest wire that will 
easily fit the core, for best efficiency.

Good luck,

Paul 



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