Dropping voltage
 

I've got what seems to be an embarrassingly simple problem, but thought I'd post it here to get some collective wisdom, as what I initially thought was a solution doesn't seem so good after a little thought and research!

In a nutshell - I need to drop from around 30-33 V to about 12 V, at anything from 30 mA to 80 mA.

I have what I think is a late 70s or early 80s Pye "Pro-Lab" receiver that I use in the workshop. Pro-Lab was a brand they used for pre-configured systems; dad had an Akai Pro-Lab made up of Pye-assembled parts in the early 80s, while this one was Pye branded and lower end.

Earlier this year I realised that it was time to replace my Lumia 950 with a new phone (thanks to Microsoft for giving up...), and with some of the options having no headphone jack, I decided to add Bluetooth so I could listen to music out in the workshop. In the end I bought a Nokia 7 Plus which has the jack, but it was always a pain with phone cases and the like plugging things in, so kept going with the Bluetooth idea.

Last week I received the module I'd ordered on eBay - one that was advertised as for car/truck use and supporting 12-24 V. As far as I can tell it has a small regulator on board, and has an unpopulated +5 V header as well as the "V.in" which happily runs on around 8 V or more from my bench supply.

So, today I wired the thing in - it switches audio, so I've just placed it between the output of the phono amp (I don't listen to records in the workshop!) and the input switching, and pinched what appeared to be regulated +15 V from the 4558 opamp in the phono stage.

Everything looked good, until I powered it on - small flash from the on-board LED, but that was it. The rail had collapsed to around 5 V. If I'd bothered to check before that effort, I'd have noticed the resistors, Zener diodes and TO92 transistors feeding the preamp and tone controls.

Damn.

So, rather than trying to reverse engineer the tuner section in hope that it's powered from something around the same voltage but a little beefier, I've decided the best option will be just to provide my own power supply. The rails look to be around +/- 30 V, but unregulated so were heading up to 33 V.

The current drawn by the Bluetooth module seems to range from 30 mA to 70 mA depending on what it's doing, so dropping with a resistor isn't really an option. My other thought was a 7812 - I've got a pile in stock - but 33 V is getting a little close to absolute maximum ratings (35 V for the ST ones I last bought from element14), and there's still the issue of dissipating over a watt.

Next step would be those cheap little DC-DC converters on eBay and Aliexpress, but this is where I'm out of my comfort zone and just not sure how well they perform.

What is the best option here?

Re: Dropping voltage
 

Simple maybe best. Use a power resistor to drop to say 15v at 80mA, then the 7812. Should work fine.

Ken

Re: Dropping voltage
 

Could use a 7824 or 15 to drop the 30 odd volts first or use a zener and resistor.

A.

Re: Dropping voltage
 

Hi, a simple beefy zener and low value resistor to stabilise the input supply at say 24v for your 7812.

Ed

Re: Dropping voltage
 

With an MP3 player and all the digital noise it makes a switched mode 3 terminal regulator will not make the situation any worse.
That is what I would go for.

Re: Dropping voltage
 

These modules usually have a 7805 or other regulator in the power line. Do you just want to take power from the main 30V supply? A resistor will drop enough volts to stop the reg failing. I must be missing something.

I have an MP3 module in a small wooden box hooked up to the hifi system. I power it from a 20V wall wart from the junk box.

Re: Dropping voltage
 

The modules I have seen have a 5 pin SMD chip and a choke with the FET built into the chip.
There is just the choke and a couple of caps on the board.

Re: Dropping voltage
 

A barefoot TO220 7812 should just about cope with a watt or so. Add a little twisted fin heatsink and it'll be fine. An added series resistor as suggested would give it an easier time.

Or you could do it with something like a TIP41x with a 12v zener and a series 1N4148 etc up its tail. Component count about the same..... output a bit soggier!

Re: Dropping voltage
 

Thanks everyone for the input!

This is looking like the direction I'll go. If my calculation is correct (ohms law!) a simple 220R resistor should give me a drop of 6.6 - 17.6 V, so back within the comfort zone of the 7812. I've got a pile of scrounged small TO220 heatsinks.

It's a Bluetooth module - I'm hoping the audio side isn't too noisy.

Dropping that many volts in a little SMD package with no heatsinking just didn't seem wise - I was wanting to hit it with something a little more controlled, but with the current varying as much as it does I didn't want it to be too soggy.

Anyway, I'll grab a pack of suitable 220R resistors from Jaycar this week (I don't generally have much useful stock over 0.5 W!) and get back into it in the weekend!

Re: Dropping voltage
 

Just noticed that you have several 7812s but no suitable power resistors. How about two 7812s in series ? The first configured with a couple of resistors a la LM317 to give say 23v, then the 12v one. Splits the dissipation and the degraded regulation of the first one is of no consequence. Saves buying wirewounds .

Ken

Re: Dropping voltage
 

You could use a couple of 0.5Ws in parallel. The values aren't critical as you just need to dump a few volts.

Re: Dropping voltage
 

A power transistor like a TIP31C in emitter follower mode with the base controlled from a potential divider would be another way to drop an intermediate number of volts before the regulator?

Re: Dropping voltage
 

I've never really liked the "resistor-ahead-of-a-classic-78xx-3-terminal-regulator" thing - it doesn't do anything to protect the regulator from initial peak-input-voltages at switch-on.

OK, you can possibly avoid this by fitting a *big* electrolytic between the resistor and the input of the 3-terminal regulator.

These days there are plenty of much-better-than-78XX regulators that can happily handle more-aggressive input voltages.

Re: Dropping voltage
 

I've used a DC-DC converter based on a LM2596 I got from an auction site (I can't recall which). It cost a couple of pounds and seemed fine for powering the little Bluetooth module I was using. I didn't measure for noise but it was "good enough".

I suspect for HiFi, some filtering and a metal enclosure might be in order...

Re: Dropping voltage
 

Look for Automotive ones.

Re: Dropping voltage
 

A 2W 10V zener in series with the 78xx ?

Re: Dropping voltage
 

Why can't you use a LM317 ?.
http://www.ti.com/lit/ds/symlink/lm317.pdf.
John

Re: Dropping voltage
 

I could, I was just trying to get a handle on best practice these days, seeing I don't have an LM317 in stock at the moment!

I've got a lot of options here to think about, was hoping to get back to it today but was doing a bit of tidying in the garden instead (a few days of no rain and north-westerlies - Invercargill is sheltered by the Southern Alps - means a rare opportunity in mid winter).

Will report back once I've worked out what I'm doing :D

Re: Dropping voltage
 

Ok, I've finally finished with this!

I did try an LM317 - had a couple in stock, pulls from old gear, but neither would play the game. Both tended to just stubbornly drift all over the place, tracking input voltage into a load. I've never really gotten on with the LM317, and once again this hasn't endeared them to me.

So I've taken the 7812 with resistors approach. I've used four 180R 2W resistors in a series/parallel arrangement to drop volts and spread the load - overkill, but they were cheap. The 7812 gets a little warm, so have added a recycled heatsink.

It's working reasonably well - the module is adequate for the purpose given the quality of the amp and speakers it's feeding, the only annoyance is a little bit of RF buzz. That's not just on the input the module is using, so it's actually getting into the amp itself. It's like the buzz you get from a phone inadequately shielded gear, but more constant like an old CDMA phone, not periodic like GSM (that may not mean much to UK people, but I'm sure Australians understand it - Telstra used to run CDMA too).

Pics attached - yes, I know it's a little rough ;)

Re: Dropping voltage
 

Just realised I forgot the before shots....