T&L Publications

I've built a power supply, but I decided to keep it modular so that I can change it often.

So far it's like this.

In box 1:
[AC 125V]--[fuse ¿what size? ¿slow blow or fast?]--[switch]--[transformer, hammond 165v18, 18v 20a, new from digikey]--[banana plugs]

Box 2:
[terminals, wires to box 1]--[4xdiodes bridge module on aluminum heat sink]--[caps 20 mili F, that's 10k microF + 10 microF, common black can, ¿is that too big/small?]--[bananas]

Box 3
[a 12vdc regulator I already have, 25A max]--[bananas]--[car cigarette lighter socket]

kept very simple.
Anybody see any problems?

So, what kind of fuse?
When switching ON, it randomly blows 3A, 4A fuses with no load and with nothing connected to the transformer's secondary.

Capacitor? When I put a dummy load ( a ptc heator ) at 8A, I measured the current at the capacitors at 4A. How long will those caps last? Am I using a good size? The transformer is rated 18VAC and 20A. So, let's use up to 80 or 90% of it's capacity.

Everything is hypothetical. I don't have a purpose for this.

[caps 20 mili F, that's 10k microF + 10 microF, common black can

Did skip "k" on second 10 by accident?

I'm surprised you don't blow the 3A (or 4A) fuse every time it is turned on.
(unless you're not waiting for caps to bleed off sometimes)

With caps that large the in-rush of current is HUGE! If you happen to switch on at AC peak ... then really huge.

Suggestion - Between rectifier and caps put a series power resistor to limit current at 18VRMS from transformer to 0V at completely discharged caps to like 5A. After a bit of time short the resistor with a switch or relay. That limits the in-rush but allows full run current.

Cheers,

Oh, yeah, two caps 10000 uF + 10000 uF.

Like I said, the transformer alone blows the fuse sometimes.

Like I said, the transformer alone blows the fuse sometimes.

Yeap, you said that.

Been thinking ... could be an inductive kick with zero load on that big transformer.
Try 1.5KΩ to 2.2KΩ, 1/2W or 1W resistor load on transformer secondary. (permanent)

Rough calculation gives 3A primary current at 20A output load. A 4A slo blo fuse should NEVER blow unless there is another problem.

(the in-rush current limit still makes sense too)

Cheers,

Use the fast blow fuse at the DC output and sized to protect the load. Slow blow can also be used if the load occasionally has short surges that the cap can keep up with. If you can find the datasheet, then you can discover the time to blow at any given overcurrent.

Use the slow blow at the AC input to allow for short surges, size it to prevent overstress of the diodes or caps if the output should become shorted or if any components should short. its job is to prevent fire, it won't prevent damage, presumably that already happened if the just blew.

The Filter caps job is to eliminate ripple at the load current. For an exact value, you need to know about the load. If you check the voltage in AC and in DC mode, its easy to evaluate the ripple voltage. Search for a filter cap calculator, it is explained on many of these pages.

Who was your electronics teacher, Bozo or Ronald McD.?

Purchased from digikey couple of months ago.

From my invoice, here's the actual transformer.
https://www.digikey.com/en/products/det ... V18/455436

Was there something else?

well, anyway, everything is working out fine.
thanks all.
Been drawing 14 amps. works well.