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Registered Member #3883
Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87
Being home sick I've had time to think about a project I've had on the shelf for a while: I need a simple, rugged adjustable constant DC current supply for anodizing capable of delivering 3A from 0-30V. The transformer available is rated at 24V, 75VA.
I know there are a lot of designs out there, but few match my needs "straight out of the box" and they are usually beyond my level of understanding. In addition they are usually based on PWM-regulators not available around here and with no Spice-model available. So I'd be working pretty much in the blind on those, and I really don't like that.
So I would like to build it around a LM350 voltage regulator. These are cheap, available and pretty darn rugged. Only problem is they can't dissipate anything close to the 75VA needed. They can deliver 4,5A as long as the voltage drop is below 10V, but I need 3A at up to 30V, at this load they can only deliver 1A. Sure I could use 3 in parallel, but that isn't really an elegant solution. The obvious solution is going switchmode, but something simple I can design myself. So here's my idea: I keep the LM350, put a transistor across this to produce a current proportional to the voltage drop over it. This is fed into an inverted schmitt trigger that controls a mosfet at the input of the LM350. Adding an inductor and a capacitor this should produce a sawtooth that can be kept within say 5-10V of the output, right? My brain isn't up to the actual design yet, I just wanted to check if my idea was generally sound. It doesn't have to be elegant or very efficient as long as it works with simple components. Am I on to something or is the fever playing with my brain?
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Schmitt trigger for a switchmode psu?
