change ouput voltage of SMPS?

[tempus]

Hey all;

I've been experimenting with a laptop power supply to power my pedalboard. It's clean, but it outputs 19v. Using a voltage regulator like an LM7809 gets the voltage down, but man does it get hot! Using either this or a 317 will surely require a heatsink, and part of the reason is that I'm supplying the regulator with too high a voltage. It's not unsafe or anything, but there's a lot of inefficiency because of the energy lost to heat. I suppose I could use a series resistor to drop some volts, but the efficiency problem is still there with too much wasted energy.

Does anyone know of a way to lower the output voltage of the laptop power supply? That way I could attack the problem from the source, and if I can get it down to 9v, well hey, I just might not need the regulators and I'd save myself a bunch of parts, real estate and work.
Thanks

[Seljer]

These switch mode supplys are usually implement some kind of PWM control chip, which usually has its internal reference voltage (often 1.25V because its easy to implement with semiconductors) which is used in the voltage control feedback loop (thick of it like feedback on an opamp).

So on the circuit, find the chip, get the data sheet for it, figure out which pin is for the voltage feedback then find the two resistors which form the voltage divider (connected from the output of the power supply to ground, with the feedback going to the controller). Get the values as they are now then lower appropriately to get a ratio which will put out 1.25V on the divider for chosen output voltage.

Ends results may vary, as the inductors and filter capacitors are chosen for each specific requirements.


And yeah, use a heatsink with those regulator chips! The thermal resistance of the case of the regulator without a heatsink is 65°C/W (on the datasheet ). So lets say you don't want it to get hotter than 80°C, and the hottest enviroment you're going to be using it in is 30°C  That means, the maxmum temperature difference is 50°.

50°C/(65°C/W) = 0.77watts

If you're using a 7809 regulator, with a 9volt output voltage, and 19volt input voltage, that means a 10 volt drop over the regulator. 0.77watts/10volts = 77milliamperes will already heat up your regulator to 80°C! So much for that 1.5A maximum current

[tempus]

Thanks for that Seljer. I opened it up, and most of the components are covered with some type of hard silicone gluey type crap, so not only can I not trace the resistors, I can't see most of them and it feels like I might start breaking things if I try to peel the glue off.

Looks like I'm back to regulators and heat sinks...

[R.G.]

It is just as dangerous to pet a rattlesnake as it is to tinker inside an AC powered switching power supply.

No matter how easy it might be, it takes an expert to do it safely.

[tempus]

Thanks for your concern RG. While I am aware of the dangers of SMPSs, I'm not even close to being an expert, which is one of the reasons I'm not going to screw around with it. If I end up using it (which is tempting because it's so small, powerful, and free) I'll have to go the external voltage regulator route.

As an aside, would there be any way to dump some of the extra voltage through a high current transistor like this http://hep.fi.infn.it/PAMELA/pdf/MJE3055.pdf? Or maybe hit the voltage regulator first, then use the transistor to deal with the high current/heat issue?

Thanks

[Seljer]

Thanks for your concern RG. While I am aware of the dangers of SMPSs, I'm not even close to being an expert, which is one of the reasons I'm not going to screw around with it. If I end up using it (which is tempting because it's so small, powerful, and free) I'll have to go the external voltage regulator route.

As an aside, would there be any way to dump some of the extra voltage through a high current transistor like this http://hep.fi.infn.it/PAMELA/pdf/MJE3055.pdf? Or maybe hit the voltage regulator first, then use the transistor to deal with the high current/heat issue?

Thanks

With a linear regulator you're turning the excess voltage into heat (voltage*current = power), be it with a single chip solution or using discrete transistors. The transistor you linked has the same TO220 package so the thermal characteristics probably aren't too different. If you're just using the transistor as a switch you might get away without a heatsink, but when running in the linear region a heatsink is a neccesity.

http://www.kpsec.freeuk.com/components/heatsink.htm

[Cliff Schecht]

Another factor to consider (other than the dangers of AC line voltage as RG brings up) is that the compensation scheme might not be optimized for the big reduction in voltage you are trying to achieve. If the loop response is too slow then you will have lousy regulation and if it's too fast, you can run into oscillation/instability issues.


In tube amps I use resistors often to drop voltage a bit in the HV line (and on tube heaters sometimes). The whole trick to doing this is characterizing how much drop you will get under minimum and maximum load conditions. With the tube heaters it's easy because they always draw a nominal current, with the HV line it's trickier to balance the size vs. voltage drop (at least the first few times, it gets easier to calculate and/or guess after that).

Now whether the resistor will work for you depends on how much current you are trying to provide. If you are trying to be able to handle up to an amp of current, for example, that's a 10 Ohm 20W (well a 10W resistor but you have to factor in some overhead here!) resistor you have to fit in somewhere. But if you are supplying 100mA of current, that's only maybe a 2-3W 100 Ohm resistor.

Another solution would be two sets of regulators to spread the heat out more evenly across two devices. Regulate down to say 15V (7815) and bring that down to 9V (7809). Not the most elegant solution but since you admit to being new to this, I hesitate to provide the switching power supply answer to your solution (the highest efficiency, smallest size and most complicated solution).

Or you can just go buy a 1Spot adapter and be done with this.

[juan0069]

I know this is very late advice  , but for future searchers...

A cheap and easy way to shave off a couple of volts is to add silicon rectifier diodes in series with your load, each diode will shave off 0.6-0.7 (0.3-0.5 volts for Schottky) volts independent of current load.  Just make sure they can handle the current.

Juan