How do you calculate the REQUIRED voltage from a transformer secondary

This one runs $18 at 15A, looks protected. Doesn't get any cheaper.

There will be voltage drop in the transformer coil, so this also needs to be factored in. The other issue is that the transformer is likely rated at voltage RMS which is not directly a useful number in this case. Multiply by 1.4 to get the peak voltage.

Fuse Diode o----o==o---+ +--+--|>|-----+----+----o | | | | | C||C +--||--|--+ | | | | | | o----o==o---+ +--+--|

The circuit you describe is unstabilised. V_out varies due to load & mains voltage. Not really good for connecting to the battery long term, whih would be the sensible option here.

That is such a common mistake. Diodes do not have 0.65v or 0.7v drop when you hit them with their rated current. 1v is closer, though >2v drop diodes are also to be found.

Transformer V_outs are for full load. At no load V_out rises due to copper loss no longer happening (much). So 12v ac with 10% regulation would deliver 12x1.414x1.10 = 18.66v off load. Diodes at low current will steal 1.3v giving you 17.35v.

15A 1v drop 50/60Hz is a whole lot of uFs. Regulated supplies can permit greater droop and thus smaller reservoirs.

quite!

NT

MEH !! why mess around with a tranny, as said before you can get SWP supplies for peanuts and with no great chunk of hardware to put somewhere. Also for what difference it makes the output is adjustable on the switch mode jobs.

** The boring old troll has another hair brained ide he wants us to waste time on.

** Power supplies are not battery chargers. ** It very much does, actually.

** The capacitor is no use at all, batteries act like giant caps already.

Such a crude unit will over charge the battery and ruin it

** FFS - the AC supply often voltage varies by over 10% during any 24 hour period.

The max voltage must be limited by some means to a safe value.

** Must be an ever bigger idiot than you. ** Don't tempt me.....

..... Phil

Yup. These days, buying a surplus 12V supply is probably cheaper than buying a suitable transformer, much less all the other bits. Just get one that's rated for 15A or more.

I'd try

first, although eBay may be a good source. and come to mind, too -- there's a bunch of decent surplus Internet sources out there. I'd trust MPJA more than some anonymous eBay seller -- you're less likely to get nameless Chinese junk (although I've gotten nameless Chinese junk from MPJA -- buying cheap is a crap shoot).

--
Tim Wescott 
Control systems, embedded software and circuit design 
I'm looking for work!  See my website if you're interested 
http://www.wescottdesign.com

well since you know everything, why are you asking us?

--
This email has not been checked by half-arsed antivirus software

PLONK

That MPJA site sure has a lot of stuff. I never knew that existed... Thanks for the link.

I would still like to find out how to calculate the transformer voltage though. That info would be handy for other things as well....

Perhaps a better reply would have been, "Why do you think it doesn't need to be regulated?"

And "What do you mean by regulation in this context"?

Why not simply dispense with the battery and solar panel?

--
Mike Perkins 
Video Solutions Ltd 
www.videosolutions.ltd.uk

2 people have told you how already. Now a third.

NT

I think this sort of thing still shows up in the ARRL Handbook. My most recent version is 2012, though.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com 

I'm looking for work -- see my website!

Everybody else pretty much said why this is not a good idea. Just one thing to add:

On Sunday, April 2, 2017 at 11:08:25 PM UTC+3, snipped-for-privacy@tubes.com wrote: [del]

Remember these are chinese amps, not US. If you wonder what the difference is, just divide by ten :) Same goes with "chinese steel", "chinese NPN transistor" and other such fine products.

Oh, and a rectifier rated at 20A with a heat sink would cost $5 to ship at absolute minimum from China. Then again, Chinese heatsinks also obey the laws of different physics than US heatsinks.

:) Often that's true. OTOH I got a remote temperature thingy from China, delivery included, for about the cost of a postage stamp.

NT

I was going to suggest the same thing..

+1 m

Then you have a *NEW* version. :)

I only have the 1941 AARL Handbook. It was a free PDF download on

I'm not a Ham, so I never saw a need for the AARL Hb, but after looking over that oldie, I see they do cover a lot. (But not solid state bridge rectifiers). Whenever I get to a WIFI with high speed internet, I download a few of them old (and good) books.

I just looked to see if there are any websites that have more recent PDF downloads of the AARL handbooks, even one from the 90's would suffice, but I am not finding anything. There are few sites that promised a free download PDF of a newer version, but all the ones I tried flooded me with ads and never produced a PDF book. Just more of those fake and wortthless websites. But I suppose the copyright may still be in effect too on those books. Everything on tubebooks.org is supposed to be public domain.

Of course I was not going to attempt to download it on dialup anyhow, I just thought I'd locate a site that actually has it, then go to the library to DL it. (You only get 2 hours per day at the library, so I locate what I want to download at home first).

Ok, so then a 12v transformer would work. I thought it would need to be higher. If I had a small 12v transformer sitting around, I'd give it a try, and just power something small from it. but I dont have one.

So you're saying LEDs are much more sensitive than I thought. I've seen and worked on radios and amplifiers that only have a transformer, rectifier and cap, with no regulation. I would have thought that a radio circuit would be a lot more sensitrive than LED lights.

I'll suggest this to the guy who needs it. This is not my project, I just told this friend that I'd help him build a power supply. That was after I told him that "If this was my project, I'd just use 120V LED bulbs and forget all the solar stuff". He thinks the solar power will save him money, but he's already looking at over $1000 for solar panels and several hundred more for batteries. Not to mention this power supply and other stuff.

I could pay a lot of electric bills with that kind of money.....

What is driving the LEDs? They aren't connected directly to the 12 volt battery are they? There should be something to regulate the current in each string. LEDs are current controlled devices and the corresponding voltage across the LEDs varies with production tolerances. More importantly, the voltage only goes up slightly for a large increase in current. So if there is nothing to limit the current in your LEDs and the voltage rises from 12 volts to 13 volts you will see a very large increase in current!

I believe the units I have seen that run directly from 12 volts (not a

12 volt battery) use a small resistor in series to limit the current, but they are expecting to see a regulated 12 volt power source, not a widely variable battery.

If, on the other hand, there is a PSU between the 12 volt battery and the LEDs, I'm sure that PSU is controlling the LED just fine and will tolerate the variable voltage your PSU provides even with poor regulation.

I've been looking into a solar powered LED light, but because I don't want to run power some 200 feet down near the water. At least not yet. A solar charged battery will do the job nicely. In fact, I fired up a

100 watt LED module today. I don't know if it will be reliable in the end as it gets *very* hot when driven at full power. I have it mounted

may require a fan blowing directly on the LED. The module is a poor design with steel instead of aluminum but still, 200 freakin' degrees! It needs the same level of cooling as an Intel CPU I guess, 100 watts... I wonder how much of that comes off as light and how much is heat. I'll need to look that up. But these units are pretty crappy. I expect they are not using quality LEDs and have much poorer efficiency than quality units.

--

Rick C

Actually, that '41 book should be quite informative on the subject. It should have information on designing supplies with mercury rectifiers, which look a lot like semiconductor diodes but with higher drops and lower currents -- just scale the voltages down and the currents up, and you should be good to go.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com 

I'm looking for work -- see my website!