A switching alternative to 7905 regulator

[protek]

I'm wondering whether there is a switching alternative to the 7905 regulator? Or is there differences between 7905s?

I have a Meanwell PT65 power supply and I'm using a 7905 to generate the -5V from the -12V line for my Apple IIGS. It's working otherwise but I suspect it to be the source of the faint static coming out of the PSU, when the Apple is running.

Any suggestions?

 

[Deleted member 14264]

I'm wondering whether there is a switching alternative to the 7905 regulator? Or is there differences between 7905s?

I have a Meanwell PT65 power supply and I'm using a 7905 to generate the -5V from the -12V line for my Apple IIGS. It's working otherwise but I suspect it to be the source of the faint static coming out of the PSU, when the Apple is running.

Any suggestions?

The 7905 regulator is linear device which means it simply wastes off as heat the drop from -12 to -5 volts - if you use 1A from your new -5V source, your regulator will dissipate (12-5)*1=7 Watts of heat. Because of its linear mode of operation it generates lots of heat but absolutely no electrical noise.

There are many alternative to a linear regulator and the more power you need, the more expensive and complex they are so in order to make any suggestion, you need to tell us how much current you'd like the replacement solution to provide.

 

[protek]

The -5V pin in the original PSU is specified to output 0.25 Amps so the IIGS can't draw any more than that.

 

[rootboy]

You must have the PT-65B...

Your Apple is going to try to get .25 amps from the Meanwell's .5 amp source. So, not much wiggle room there...

Even linear regulators can benefit from noise reduction methods:

http://cr4.globalspec.com/thread/8637/Power-Supply-Noise-Reduction-Methods


And since you are using a switching supply to provide the input to the 7905, it's going to look like a very noisy mains to the 7905. Try cleaning it up at that end first.


Not much help in noise reduction, but I found the write-up interesting:

http://www.righto.com/2014/09/reverse-engineering-counterfeit-7805.html

 

[protek]

I've used this project as reference:
https://drewbiegs.wordpress.com/category/iigs-power-supply/

 

[Bryce]

A 7905 won't be the source of the "static" (I assume you mean humming or buzzing). However, the project you referenced isn't particularly well designed. You should add a 100nf ceramic capacitor in parallel to the output and input capacitors to clamp the high frequencies from the switchmode supply.

Bryce.

 

[protek]

Well, it's more like hissing, like in an old amplifier.

Thanks, Bryce! I got the caps and will add those. One cap each, right? At least I got two.

- - - Updated - - -

I got a bit of heat from an Applefritter user for butchering the US PSU and putting the MeanWell in its place. It turns out that the original could've been modified for 220V by desolering two jumpers and probably changing some caps. It's just that until then I couldn't find any info about the mod. Well, I rationalized it that a new PSU is better than a thirty year old.

 

[Deleted member 14264]

A 7905 won't be the source of the "static" (I assume you mean humming or buzzing). However, the project you referenced isn't particularly well designed. You should add a 100nf ceramic capacitor in parallel to the output and input capacitors to clamp the high frequencies from the switchmode supply.

Same thing here...

 

[protek]

I decided to take an another approach with this. I ordered the Mean Well RQ-65B PSU, which has by default the needed outputs.

I'll use the old PT-65B for making a common PSU for my Amiga, CPC and Spectrum +3.