LM7815 power supply question

[mikepukmel]

Hi All!

I got some cheap regulated power supply boards: has 4 diodes LM7815, heat sink, and a couple of electrolytic capacitors.

I think I screwed up ordering the transformers.

The LM7815 data sheet specifies absolute max input 35v. I got a couple of Triad F229X 115V primary, 24v secondary. After rectification, about 24 * 1.414 = 33.94v almost right on the limit.

I hate to waste usable current, but should I put a dropping resistor to get the input to the LM7815 down 15% or so to give the LM7815 some breathing room?

Thanks
Mike

[Jazz P Bass]

Either way you have to 'dump' some 10 volts.

One or the other must do it.
The resistor or the device itself.

[mikepukmel]

Thanks Jazz. Wondering if I should exchange the transformers.

[nickb]

It's complicated. Hard to believe for so few components, I know

1) You mustn't have too many volts in i.e < 35V
2) You mustn't overheat it i.e voltage drop x current and is your heatsink big enough?
3) Oh.. but wait, you can't just lower the voltage as can't have too few either i.e >18V

It gets worse...
Under no load conditions expect about 10% more volts out of the secondary. Now add in say 5% for higher than usual line voltage.
Under load, the output will drop due to winding resistance. Naturally the drop will be more than you expect as the current flows in (very) big gulps to recharge the smoothing caps.
To add to it all, the voltage on the caps is not constant but a ripple that depends of the size of the cap and the current draw. You must allow for that too.
Finally allow for 5% line voltage drop.

All this is why we have simulation tools. Go and get Duncan's power supply designer and work it all out using that. Define your inputs i.e. min and max volts and output min & max volts and current.

You might want to consider sending the transformers back before it's too late. Dropping volts just wastes power.

[mikepukmel]

Thanks Nick. The calculation I should have done before (one of them anyway) but screwed up:

(35-17)/2 = about 26 -> 26/1.414 -> about 18.4ac. They sell a 20vac sec, similar price.

Thanks, I got a copy of Duncan's software, looking at it now...

[Enzo]

If that is a transformer rated for 24v, that will be at its rated current. Unloaded or lightly loaded, that 24v can rise too.

[mikepukmel]

Oh man, you flick that power switch on, and stuff really starts to happen before it settles down.

[mikepukmel]

If that is a transformer rated for 24v, that will be at its rated current. Unloaded or lightly loaded, that 24v can rise too.

Thanks Enzo. So, its important to have the post rectifier voltage right in the middle of the regulators min/max input range.

[g1]

It gets worse...

And then it gets even worser!
In addition to the items nick mentioned, the data sheet says 115V primary. Do they mean it? How about at 120V, or 124V as I have?

[mikepukmel]

And then it gets even worser!
In addition to the items nick mentioned, the data sheet says 115V primary. Do they mean it? How about at 120V, or 124V as I have?

Got me thinking, maybe I should give up this hobby and take up whittling. Ain't quite as fun, tho.

[eschertron]

At least I'm not the only one a-fussin' and a-frettin' over power trannys!

[mikepukmel]

And then it gets even worser!
In addition to the items nick mentioned, the data sheet says 115V primary. Do they mean it? How about at 120V, or 124V as I have?

Guess I better break out the meter and measure that as well . . . Well, my 115vac is 122.4 to 122.6 at least at 8:36pm on Monday evening.

The secondary on these 24v transformers measures 27.4vac. Didn't hook up to rectifiers, but should be 38.8v or so. unloaded. Not the worstest, but definitely worser for the LM7815.

2 x 16vac sec transformers on the way. Also, bought some beefier heat sinks. Should I put that white heat sink grease on the connection?

[nickb]

Guess I better break out the meter and measure that as well . . . Well, my 115vac is 122.4 to 122.6 at least at 8:36pm on Monday evening.

The secondary on these 24v transformers measures 27.4vac. Didn't hook up to rectifiers, but should be 38.8v or so. unloaded. Not the worstest, but definitely worser for the LM7815.

2 x 16vac sec transformers on the way. Also, bought some beefier heat sinks. Should I put that white heat sink grease on the connection?

My gut tells be that might be too low in general. But we'll see. I can't answer the heatsink question w/o knowing a bunch of other things.
The first question is what is your required max load current?
Who is your preferred transformer vendor (so I can look up the specs)?

[Mick Bailey]

When I'm sizing up for small PSUs using 78XX regulators I get the closest VA rating (higher rather than lower) to my maximum load and the closest AC voltage to my output voltage. Assuming 10% regulation this usually works out fine for dropout voltage. So for a 15V regulator I'd go for a 16v transformer. Usually the rectified output is a little higher than calculated due to primaries being wound for 2x115v or 1x230v and the mains being higher (mine is 248v today).

[nickb]

The secondary current will be much higher (about 1.7x) than the output current plus the secondary voltage is higher than the output meaning the VA rating needs to be at least twice the output power (as a starting guess). For such a transformer the regulation is be around 25%.

Here are my estimates for a 16V 32VA transformer.



Plugging the numbers into a simulation to be more precise I get,
Vo min = 18.3V this is above the 17V 7815 requirement, OK
Vo max = 28.2V this is below the 35V 7815 requirement, OK
Pd = 7.9 Watts dissipated in 7815
=> Heatsink better than 7.8 C/W, so yes best use thermal compound.
Secondary current= 1.64A rms
Min VA rating - 1.64 x 21.5 = 35.3 VA ( very worst case of continuous max load and max input voltage).