L7810 voltage regulator question

[Jolly 7]

I have a 13V 500 mA transformer and an L7810 regulator. I have successfully built a full wave rectifier but now want it to give me three outputs, 3V, 5V and 9V at the maximum possible current. Is this something that can be easily achieved by using a three way switch and suitable resistors to drop the voltage or is there a better way ?

[evingar]

I think that's a fixed 10V reg. You would be better off with a variable reg like an LM317
http://www.ti.com/lit/ds/symlink/lm317.pdf


Looking at the diagram on page 1, you can vary the output by varying R2 (omit resistor Rs). There is equation for the value elsewhere in the data sheet.

[Boater Sam]

L7810 is a nominally 1 amp 10 volt regulator, not a variable output regulator, and quite noisy too.
LM317 as Chris said is a much easier option, be careful with the heat sink, your 13 volt transformer and rectifier will produce about 18 volts, on the 3 volt setting the chip is going to dissipate some serious heat.

[TrevorG3VLF]

Raise the 10V a little by adding a resistor in the 7810 reference lead. Feed into a chain of 9V, 5V, 3V regulators. The dissipation will be divided between the regulators and the outputs can be selected with a switch. I would not put a switch in the regulator circuitry for reliability reasons.

[Boater Sam]

Do they make a 7803 regulator?

Be a very noisy supply with a chain of regulators. What's it for?

[turretslug]

Slight diversion, but at one time I found that 7802 regulators inside the sealed rotary encoders used on rather expensive sound mixing consoles were an Achilles heel, I suspect they were running too hot in their cramped enclosed setting. I joked that perhaps they were 7805s that failed to meet spec., though 78-just-about-anything seems to have been available if the order was big enough.

[Terry_VK5TM]

Originally Posted by Boater Sam

Do they make a 7803 regulator?

Yes, but they are switching regulators (and 3.3V), not linear such as the 7805.

Digikey has a selection of them.

[Jolly 7]

Originally Posted by Boater Sam

Do they make a 7803 regulator?

Be a very noisy supply with a chain of regulators. What's it for?

I am hoping to use my supply mostly for radio experiments because I have all the parts and even a dedicated metal case. More of a fun project really.

One of my older multivolt Maplin wallwarts has just packed up and I am not sure if I can retrieve the transformer from within without breaking it. No screws. It was the older heavier type, not the featherweight noisy ?switch mode ones that are everywhere nowadays. I suspect the electronics inside must have 'gone bad'.

[bikerhifinut]

If it's for an experimental PSU, why not make it fully variable. Just use a 3 legged variable regulator, I'd advise a 5 amp one for a bit extra ruggedness on a decent heatsink and a potentiometer instead of the bottom resistor in the adjustment leg.
You can then simply use a multimeter for the voltage readout or one of the cheap LED Voltmeter modules available for washers off eBay.
Heck you could even incorporate an Ammeter from the same source which could be very handy when prototyping/testing.
Simples!

A.

[Biggles]

I think an LM317 on a decent heatsink and switched resistor combinations is the way to go. If you just wanted 5, 9, and 12V I would probably use an 7805, with the appropriate zener diodes switched into the common lead to artificially raise the output voltage. It would depend on what I had lying around to use. I'm sure there are more "professional" solutions but I would avoid switched mode regulators. I am a bit old school when it comes to design and I tend to stick with what I know works.
Alan.

[G6Tanuki]

78xx regulators are really rather old-world. They're 'first generation' parts - noisy, and they need a rather high difference-between-input-and-output-voltage to work.

The 317 is a 'second generation' variable-voltage beastie; easier to tame, quieter... the TO-220 cased versions can have issues if dissipating significant power - use the LM317HVK [TO-3 case] if you're working it hard. I still like the LM350K in its TO-3 case

https://uk.rs-online.com/web/p/linea...ators/6633246/

which can handle 3A [though they're getting harder/more-expensive to come by]

There are also a whole slow of modern Low-drop-out [LDO] regulators that are vastly better then the old 78xx series - and cheaper too!

[bikerhifinut]

depending on your pocket this wee board will do everything you need to do for under £6, you might find it cheaper on Ebay or somewhere. Considering it includes a ready made PCB I think its reasonable value for a One off project. Just add a standard 4.7k pot in place of the preset on the board to use as a panel control, I think that is even mentioned in the instruction manual.
I've made use of these boards, I got myself a small stash when maplins went bust and have adapted them, using a 5A regulator in place of the LM317 even though I was only drawing an Amp, I felt happier with a bit of headroom and a good size heatsink. I also bunged a 4700uF reservoir cap in but that's not really vital.

http://www.velleman.co.uk/contents/e...198_k1823.html

Andy.

[G6Tanuki]

That Vellemann board/kit is what I used in this little project a few years back:

https://www.vintage-radio.net/forum/...d.php?t=129371

It's seen a few thousand hours of service and has so far done everything I've asked it to do.

[bikerhifinut]

What do you think of using something like a 3A or better reg instead of the 317 when drawing close or equal to the 317s 1.5A? I'm not advocating bunging a bigger reg in so that you can draw a bigger current, merely to give it an easier time? By the same token I used rectifiers that could handle a larger current. Mainly because I had a stash of them. I appreciate that the PCB tracks will limit the total current you can draw so its not for that reason I put an uprated regulator in. Of course if you are only pulling half an amp then no worries.
Andy.

[G6Tanuki]

Originally Posted by bikerhifinut

What do you think of using something like a 3A or better reg instead of the 317 when drawing close or equal to the 317s 1.5A? I'm not advocating bunging a bigger reg in so that you can draw a bigger current, merely to give it an easier time?

Up to a point, I'd agree with this: over-engineering will give the components an easier life.

There's a rule-of-thumb that every 10 Centigrade increase in temperature doubles the speed of chemical reactions [and hence the degradation/inexorable-path-to-failure] of components.

So running-cool is always good; but equally you need to consider whether it's worth spending money on over-engineering-for-reliability if the probability-of-failure is already rather-low and the cost-of-failure likewise.

Spend an extra £10 to reduce the likelihood-of-failure from 1-in-50,000-hours to 1-in-100,000-hours?

And if the cost-of-things-destroyed-in-the-event-of-that-failure is only £20?

My business-sense is to go for the cheaper option and spend the money saved on another couple of pints this evening at the pub!

I only use the 3A-rated "steel" TO-3 cased regulators because I still have a tray-full acquired when I was prototyping stuff for .gov.uk on 'cost-plus' contracts.

[Biggles]

The original transformer spec was 500mA (post#1) so there probably is no need to go over the top with high current regulators. I would agree the Velleman kit may be the easy way to go. I have used Velleman kits for years now and found the designs good and ultra reliable.
Alan.

[bikerhifinut]

Originally Posted by G6Tanuki

Up to a point, I'd agree with this: over-engineering will give the components an easier life.

There's a rule-of-thumb that every 10 Centigrade increase in temperature doubles the speed of chemical reactions [and hence the degradation/inexorable-path-to-failure] of components.

So running-cool is always good; but equally you need to consider whether it's worth spending money on over-engineering-for-reliability if the probability-of-failure is already rather-low and the cost-of-failure likewise.

Spend an extra £10 to reduce the likelihood-of-failure from 1-in-50,000-hours to 1-in-100,000-hours?

And if the cost-of-things-destroyed-in-the-event-of-that-failure is only £20?

My business-sense is to go for the cheaper option and spend the money saved on another couple of pints this evening at the pub!

I only use the 3A-rated "steel" TO-3 cased regulators because I still have a tray-full acquired when I was prototyping stuff for .gov.uk on 'cost-plus' contracts.

Fair comment G6Tanuki, and I'll readily admit I used a bigger reg etc because I also had some in and I was pushing the board to its design limit. I can't off the top of my head remember but i think the reg was an LM337?

Biggles, I concur and for the price (not the cheapest) I have found the velleman boards to be very good and the manuals that come with them are jolly useful too.

A.

[Jolly 7]

Thanks everyone for all the ideas. The LM317T looks similar to TIP31C transistors, of which I have many. After some searching I have found one in my parts box. In addition I also found a salvaged heatsink. Here are some pictures. The unearthed mains wiring probably won't meet UK safety standards as the case is metal and was bought abroad. The mains wire has been fed through a rubber grommet and will be fitted with cable strain relief too. Next I need to decide where the regulator IC and resistors are going to go.

[bikerhifinut]

On a bit of veroboard, along with the reservoir capacitor, rectifier etc. Bolted to the chassis on some stand offs nice and secure. Theres room there i think for you to fit an IEC mains chassis plug (like a kettles) and you can do the job all proper like and take mains earth to the steel case. Take no chances, do it right. and theres lots of info in internet land on how to build a regulated supply including why you need certain values/types of resistor and capacitor in a couple of critical locations. If you are careful you could mount the 317 on the steel box as a heat sink, remembering to use a proper insulated spacer and washer to prevent the body shorting against the case, this is a quite normal procedure.
You should be able to get a rather nice variable PSU in that box.
A.

[Diabolical Artificer]

Jolly 7 I have a shed full of selectable small wall wart PSU's with those voltages on if you want one for the postage, no doubt containing a 317.

There are several 317 R calculators on line but a 5k pot and 220r will give you a variable PSU, this is sometimes better than a SW and various R's as the OP voltage changes with load. If you make a PSU don't forget to put protection diodes on.

Andy.