Suitcase - 24v to 9v step down for effects

[craig]

Hey all - I want to add some diy effects to my suitcase behind the rail and was thinking about powering them by tapping into the 24v going into the preamp. I tried a 9v regulator (7809) and while the voltage was 9v coming out, it got too hot so I took it out. Would one of these work well? Would I also need to re-regulate the 24v going into the preamp? Any other suggestions? Thanks!

https://www.ebay.com/itm/LED-Display-DC-DC-Buck-Step-Down-Adjustable-Converter-4V-40V-to-5V-9V-12V-24V-3A/322912382620?hash=item4b2f13ee9c:g:0e0AAOSwi8VZORv9

[pnoboy]

It's got plenty of power, but being a switched regulator, it may add some audible noise.  It's so cheap, why not give it a try.

[sean]

I would not want to use an electronic device with zero documentation and zero warrranty and zero support.
I would not install this inside of an irreplaceable piano.

You can get thousands of DC to DC converters from Digikey or Mouser.  Fully documented, with warranty and support.

The 7809 regulator would indeed require a heat sink, and would run hot.  Modern switch-mode replacements are much more efficient, and can be quite tiny and inexpensive.

I personally like the tiny units from Recom, like R-78C9.0-1.0 (and R-78E9.0-0.5 if you only need 500mA).
https://www.digikey.com/product-detail/en/recom-power/R-78C9.0-1.0/945-1396-5-ND/2652149
You can comfortably touch this unit while in operation (I can't detect any temperature rise with my fingers.)

These work great for audio, as long as you make sure you have at least 10mA flowing.  With less current, the protection circuitry kicks in and you start getting audible hiccups... well, it is a steady buzzsaw sound.  So all you have to do is put a resistor across the output that will draw 10mA and create a tiny bit of waste heat (not anywhere near as hot and wasteful as the old analog/linear 7809 regulators).  If you put an 820Ω resistor across the output terminals, it will silence the output, and only waste 11mA.  In theory, you only need a tiny resistor (1/10th Watt), but you should use a big fat one to allow it to run cool (choose whatever you have on hand - 1/2W, 1W, or 2W).

I use a 10uF tiny little ceramic filter capacitor across the output, like https://www.digikey.com/product-detail/en/murata-electronics-north-america/RDEC71H106K3K1H03B/490-14508-ND/4906199

Sean

[craig]

Sean,

Thanks so much for your reply. I'm going to do just that, with the 1a Recom. Just to clarify, the 820r goes across the output only (not from v+ output to ground like the 10uf caps in the datasheet (which I'm going to adhere to)), as well as the 10uf ceramic? BTW - here's a picture of my 76 suitcase. I've had it now for (gulp) 20 years, and did the sparkle tolex about 15 years ago. Thanks again!

[pianotuner steveo]

Why so many knobs?

[sean]

The 820Ω resistor goes from the +9V output to ground.  In parallel with the 10uF capacitor (C2).

This resistor simply creates an 11mA drain on the regulator output, so that the regulator never starts sputtering.  I think that when the output current gets close to zero, the comparator in the over-voltage protection circuitry freaks out.  You won't hurt the regulator if the output current goes to zero, but the output voltage will get an ugly dose of ripple and buzz. 

The spec sheet for the regulator clearly and completely explains this with Note 1 on page 1:  "Note1: No load operation will not damage these devices, however they may not meet all specifictions.  A minimum load of 10mA is required."  Okay, not clear and not complete, but the shunt resistor solves the problem by keeping the output current above 10mA.

The capacitor that I linked in my previous post is a fine choice for C1 and C2 shown in the spec sheet circuit that you show.  Buy ten of them, they come in handy.


Sean

[craig]

Sean,

Just wanted to say thanks. I made the reg. board and installed it last week and it works great. Excellent suggestion.