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Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
Dur... should read more carefully - a full bridge rectifier is ideal. I shall implement forthwith (obviously not using 1N4148s waits for bang)
I hear what you are saying about the epoxy, sounds like I won't have to go down that route now.
Progress so far has been good, I've butchered the PSU and removed almost everything on the secondary power side except the coupled choke and the 12V inductor. It's nice to see that the circuit is almost identical to the AT schematic I linked to in my first post. This means it has been fairly easy to track down the relevant components.
PSU now works with 12V in isolation and no-preload necessary. I've put a 7805 from the 12V to the 5V rail as it powers some of the ancilliary circuitry.
BEFORE
AFTER
REMOVED PARTS
To do
Upgrade output diodes to withstand correct voltage and current in fullbridge configuration
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
From my experience separating cores, I've found that there are two different kinds of resin used in ferrite transformers - the white stuff (not a chance, too high a melting point) and then some transformers use a kind of opaque brown resin, that is very brittle and chips off. That kind of resin melts easily into a liquid if you drop it into a pan of boiling water, I separated a transformer from an old monitor like this. Dropped it into a pan and was surprised when 2 minutes later the transformer just fell apart!
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
Steve Conner wrote ... If you added another two diodes to make a bridge rectifier, got rid of all the 5V rectifier bits, and disconnected the centre tap then the same winding would want to output 24V.
The key to this sentence is "disconnect the centre tap", otherwise the windings get shorted out to ground when the bottom winding goes negative - see attached image.
The gain over the conventional half wave rectified output appears to be high at light loads, factor of x2.8 at 10W, although I would expect this to come down as the load increases.
More hacking away this lunchtime, but it's looking pretty good
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
The gain over the conventional half wave rectified output appears to be high at light loads, factor of x2.8 at 10W, although I would expect this to come down as the load increases.
Ahem... have you "closed the loop"? Cant expect even a forward converter to regulate properly without negative feedback from the output.
I just recently built another switch mode supply using the old TL494. Its a constant current source for charging NiMH batteries (our robot uses a moster 50x sub-C cell pack). I was happy to see it pumping 38V at 7A with ease. It was actually a rather easy project, the only set back was when i used too small of wire on my ferrite core, and it got rather toasty! The 10awg litz fixed that up .
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
Steve Ward wrote ... Ahem... have you "closed the loop"? Cant expect even a forward converter to regulate properly without negative feedback from the output.
Yes, there is feedback from the output rail. The power supply was operating in burst mode when I made this measurement (buzzing away merrily) so when it is operating properly, this ratio should come down a bit.
Success! Yesterday evening yielded 1.5A @ 48V = 72W into an electronic load - the maximum voltage output. I would hope to be able to get 3A out at this voltage = 144W with the fan cooling running in the chasis (originally a 200W supply). A 100k pot in series with a 1k resistor in the feedback gives me the full adjustment range from 3V to 48V.
However, I still need a regulated 12V supply for the SSTC control electronics, so I've had to install a NatSemi simple switcher to get a 12V 3A rail from the variable output. As its a buck converter, it will only regulate when the input is above 12V. Since the main output can vary lower than this, some kind of "12V good" signal will be required - a zener and bipolar transistor should be good enough.
The intention now is to mount the control cct on the inside of the lid and mount the H-bridge and coil on the top of the case.
Thanks once again for all the assistance in helping me achieve this goal, I will do a full write up on my website before long.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
However, I still need a regulated 12V supply for the SSTC control electronics, so I've had to install a NatSemi simple switcher to get a 12V 3A rail from the variable output. As its a buck converter, it will only regulate when the input is above 12V. Since the main output can vary lower than this, some kind of "12V good" signal will be required - a zener and bipolar transistor should be good enough.
You can use a buck-boost converter on -5V rail so it will correct it to positive, any voltage you want. I used buck boost in power supply for my little class E coil with good results.
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
A buck boost, specifically a SEPIC regulator is not a bad idea. The main reason I've gone for a buck is that I already had the circuit built up.
What controller did you use in your application? Obviously something that would run at 5V... I can't use a UC3843 as it has a max turn on voltage of 9V.
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
Firkragg wrote ...
I used LM2575 since I didn't need anything under 7V, but there are tons of usable regulators around: I often dig my regulators out of old motherboards and computer trash, and they can be sampled too.
Cool thanks, I shall investigate my options.
Incidentally, I've come across some more AT PSU schematics on this page - click on the power supply link.
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PC Power Supply Hacking - 50V Output
waits for bang)
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