Here are the basics for linear psu design:

http://www.zen22142.zen.co.uk/Design/dcpsu.htm

If you add a regulator, just make sure that the troughs of the ripple waveform can't make the regulator's input voltage minus its output voltage become less than its dropout voltage rating. Also, account for +/- 10% mains voltage, and transformer regulation factor, to get worst cases.

Cheers,

Tom

 

Hi guys from a newbie/lurker --

Shipping heavy transformers:
Fortunately, here in the US, heavy transformers (up to 70 pounds) can be shiped for a flat rate of about $13 by priority mail. They just have to fit in the box supplied (free) by the post office.

Some thoughts on hi-current transformer supplies:
I have not had much experience with switch-mode transistor power supplies, so if I start yamering about them, just ignore me.

But, I have had some experience with other 12v/24v/variable-voltage/regulated supplies so here are some suggestions:

Variac:
As has been mentioned, the easiest way to get variable voltage and reasonable regulation but is probably the most expensive way is to buy yourself a variac (autotransformer) and just run your fixed-voltage supply off the variac. A variac is a real handy bench tool anyway so it is worth the investment.

Choke in series with the primary:
A choke in the primary side of the transformer will reduce the current available to the supply output, but at the cost of very poor regulation. If you are going to use it to charge car batteries, run a constant-torque motor, or as a welder, this is a good thing. But if you are going to run radios/amplifiers with it, this is a very BAD thing.

Choke input filter:
OTOH, a secondary-side choke input filter (choke between the diodes and filter capacitor) and a heavy bleeder resistor accross the output will give you very GOOD regulation. Downside: Hi-current, Hi-inductance chokes are difficult to come by and expensive, and the bleeder resistor puts you back near the "space heater" class. (Google "critical inductance")

Linear transistor regulator:
A conventional linear regulator is probably the best scheme for best dynamic regulation (voltage stays steady even with changing load current). And as Potato Pudding pointed out, it is also the worst for efficiency. If you can pick up a big heatsink with 8 big power transistors on it and a large fan from your local recycle guy for a buck and a half -- then go for it! But, if you have to buy new parts and machine large heat sinks, or your house is already warm enough, it's probably not a good Idea.

SCR regulator:
Secondary-side switch-mode SCR regulators using discrete components are simple, efficient, and give excellent static regulation. Two rectifiers of the bridge are replaced with SCR's. An error amplifier controls the timing of a UJT pulse generator that triggers the SCR's. I have built a couple of these. Unfortunately, there seems to be little info on SCR regulators on the web. Look for GE, RCA, and IR "SCR" or "power devices" manuals for circuits. Downside: Static regulation is very good; dynamic regulation; not so much. This is because linear regulators can adjust ouput voltage almost instantly, but power-line frequency regulators can only make adjustments at 1/120 second intervals. If it over-adjusts, then it must compensate, but not until the next 1/120 second comes around. As a result, it may take tens or even hundreds of milliseconds to bring the voltage back to where it should be after a load change. Another bummer: an oscilloscope is almost a necessity for trouble-shooting. Still, I have had good luck with them, so I recommend them.

Triac / SCR primary control:
Primary-side Triac or dual-SCR or controllers need to supply less current so less-than-heavy-duty components can be used. They can be used with manual control (light dimmer style) or with pulse transformers / opto-isolaters, they can be used in a regulator scheme. I have tried light dimmer scheme with a triac with a "snubber" across the transformer primary with some success. A 100 watt light bulb across the snubber/primary improved the "smoothness" of the circuit.

Pre-regulator - linear regulator:
Beenthere's suggestion of a secondary-side SCR preregulator followed by a linear regulator seems like a really good idea, but I haven't tried it (yet). Note that there are major problems with the schematic pointed to by beenthere. First, the pass transistor has a direct short across its collector-emitter. Second, these is no low-resistance path from the pwr diodes output to the filter cap or pass transistor collector (not even through the SCR). There are probably other not-so-obvious errors. It would be nice to know the original source of the schematic's writeup -- the basic idea is sound. This one: <http://www.epanorama.rackhost.net/schematicsforfree/Power%20Electronics/Thyristors-SCRs_and_Triacs/SCR_Switching_Pre-Regulator.pdf> looks awfully simple to be effective, but might be worth a try.

Transistor switchmode:
Hi-frequency switchmode FET or bipolar transistor regulator technology has greatly improved over the years so now it will likely out-perform the older SCR technology. But again, I have little expertese here so I will shut up now. <g>

Food for thought,

Teri

 

Here are the basics for linear psu design:

http://www.zen22142.zen.co.uk/Design/dcpsu.htm

If you add a regulator, just make sure that the troughs of the ripple waveform can't make the regulator's input voltage minus its output voltage become less than its dropout voltage rating. Also, account for +/- 10% mains voltage, and transformer regulation factor, to get worst cases.

Cheers,

Tom

That same web site has a bunch of power supply schematics:

http://www.zen22142.zen.co.uk/Circuits/Power/power.html

including a 12 volt, 30 amp supply:

http://www.zen22142.zen.co.uk/Circuits/Power/1230psu.htm

which wouldn't be any problem at all to change to a 24 volt supply.

 

I am mucking around with DC motors so as they can be hungry it seemed like a good buy.

How much current are your DC motors actually going to draw? The transformer you've got could supply a one horse motor. Are your motors that big?

 

.

http://datasheets.maxim-ic.com/en/ds/MAX15046-MAX15046B.pdf

Hi there, this must be a delteled item. I cannot find it on Farnell's website.

Thanks

 

Hi there, this must be a delteled item. I cannot find it on Farnell's website.

Thanks

Maxim's website shows it as an active "in production" device. They're sampling the A & B versions but they're surface mount. Maxim makes a ton of this type IC that can be adapted into the same circuit I posted just as an illustration of one way to do it.

They mention a 25A limit but that was back when the on resistance of inexpensive MOSFETs was much higher than it is nowadays.

 

What I do in these situations is wind down the secondary of the transformer. This way you still get your 30amps of current, nothing is wasted in heat and you can set the voltage right where you need it. I do this on many of our tube amp transformers.

 

Hi there, I certainly do! Sometimes you can be really lucky. However, sometimes it pays to shop at the real price for the guarantee.

Back to the power supply. I am a bit at sixes and sevens now with all the options. I like the variac, and, have been looking for an "auction" one. Need a min of 240 volts with 5 amps input. Not an elegant solution but practical.

I have started to look at the switched mode power supplies. Lots of work here for me. The circuit from "Beenthere" looks very clever and I wonder if it will give me the power control that I might want, especially as woltage regulator chips hit the high price once they get over a few amps. And I still have to add that to the design.

Regards

Just get a cheep buck converter, put your output of your setup to the in of the buck then adjust the output to what you want.
dps5020 is a 20 amp buck convert, but you can use a cheaper one with less amps.

 

And yet another necropost by a newbie.
This thread is nearly 8 years old and the TS was last seen here in 2014.

 

MOD: Thread Closed, Very old Thread.