Help needed with low voltage PSU - UK Vintage Radio Repair and Restoration Discussion Forum

Radio_Dave · 3rd Dec 2006, 11:57 am

Hi,

I have all the parts to make a little, LM317 based, variable power supply to use when I'm working with transistor radios.

There's plenty of circuits on the internet but the only thing I'm missing is a transformer big enough to give me the minimum output of 18V that I need.

I have got two identical 12V 1.67A transformers but I'm not sure what's the best way to combine them to give me the highest voltage and power

.

Should I, simply, connect both secondaries in series, then rectify or rectify each transformer first

Thanks
David

ppppenguin · 3rd Dec 2006, 12:04 pm

Connect the secondaries in series. Remember that if you want a low output voltage at a high current the regulator will get very hot. Might be worth having 2 ranges, one with both secondaries in series, the other with just one of them.

Paul Stenning · 3rd Dec 2006, 1:01 pm

Quote:

Originally Posted by ppppenguin

Connect the secondaries in series.

Then measure the total voltage with a meter. If they are connected with the wrong phasing the two voltages will be subtracted, resulting in close to zero. If this happens, reverse the polarity of one of them.

Radio_Dave · 3rd Dec 2006, 2:38 pm

Thanks guys, I'll do exactly as you suggest, including a single 12V ac input for low voltages

6AL5W-Martin · 3rd Dec 2006, 6:55 pm

I want to give you another suggestion.

Get the following Chip:

LM 4975 And as Transformer any 24 upt to 30V / 3-4Amps.

This Chip is a 5A SMPS Regulator, it will use the "restvoltage" as additional useful power for your radio, not as a heating. The maximum rating is 5Amp.

Example: when output = 12V, Input = 24V, load = 5A, Input will only load 2.5A from transformer.
That`s SMPS. There is only one toroid coil to put, very easy !

You will have Enjoy with this, sure

Martin

evingar · 3rd Dec 2006, 7:29 pm

I'm not familiar with that particular device

...but there are a selection of simple to use one chip switching regulators available from National Semiconductor, complete with application circuits. There is a handy selection tool here, it will even design the circuit for you in some instances :-

http://www.national.com/appinfo/power/

I'm sure some of the devices can be bought from RS or Farnell.

One problem may be interference, but the use of ferrite's on the I/O leads and the use of an earthed metal box will probably get around this

6AL5W-Martin · 3rd Dec 2006, 7:57 pm

excuse a Mistake, i means L4975 ( by ST Micro, also fine = L298 )
i will post apllication. This is L298, pimped with a transistor to output more then 10 Amps

http://www.budoclub-biedenkopf.de/51j4/l298.jpg

to make it adjustable put a pot there where is the both resistors in end. The minimum output voltage is 5.1V As inductivity use a ferrite ring from defect PC Power supply, 1" or like that, and make 25 turns (thick for 10A). thats all, the rest is R C

3rd Dec 2006, 11:57 am	#1
Radio_Dave Nonode Join Date: Jun 2004 Location: Norwich, Norfolk, UK. Posts: 2,543	Help needed with low voltage PSU Hi, I have all the parts to make a little, LM317 based, variable power supply to use when I'm working with transistor radios. There's plenty of circuits on the internet but the only thing I'm missing is a transformer big enough to give me the minimum output of 18V that I need. I have got two identical 12V 1.67A transformers but I'm not sure what's the best way to combine them to give me the highest voltage and power . Should I, simply, connect both secondaries in series, then rectify or rectify each transformer first Thanks David

3rd Dec 2006, 12:04 pm	#2
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Help needed with low voltage PSU Connect the secondaries in series. Remember that if you want a low output voltage at a high current the regulator will get very hot. Might be worth having 2 ranges, one with both secondaries in series, the other with just one of them.

3rd Dec 2006, 2:38 pm	#4
Radio_Dave Nonode Join Date: Jun 2004 Location: Norwich, Norfolk, UK. Posts: 2,543	Re: Help needed with low voltage PSU Thanks guys, I'll do exactly as you suggest, including a single 12V ac input for low voltages

3rd Dec 2006, 6:55 pm	#5
6AL5W-Martin Hexode Join Date: Nov 2006 Location: Biedenkopf, [Hessen], Germany. Posts: 425	Re: Help needed with low voltage PSU I want to give you another suggestion. Get the following Chip: LM 4975 And as Transformer any 24 upt to 30V / 3-4Amps. This Chip is a 5A SMPS Regulator, it will use the "restvoltage" as additional useful power for your radio, not as a heating. The maximum rating is 5Amp. Example: when output = 12V, Input = 24V, load = 5A, Input will only load 2.5A from transformer. That`s SMPS. There is only one toroid coil to put, very easy ! You will have Enjoy with this, sure Martin

3rd Dec 2006, 7:29 pm	#6
evingar Octode Join Date: Oct 2003 Location: Newbury, Berkshire, UK. Posts: 1,770	Re: Help needed with low voltage PSU I'm not familiar with that particular device ...but there are a selection of simple to use one chip switching regulators available from National Semiconductor, complete with application circuits. There is a handy selection tool here, it will even design the circuit for you in some instances :- http://www.national.com/appinfo/power/ I'm sure some of the devices can be bought from RS or Farnell. One problem may be interference, but the use of ferrite's on the I/O leads and the use of an earthed metal box will probably get around this __________________ Chris

3rd Dec 2006, 7:57 pm	#7
6AL5W-Martin Hexode Join Date: Nov 2006 Location: Biedenkopf, [Hessen], Germany. Posts: 425	Re: Help needed with low voltage PSU excuse a Mistake, i means L4975 ( by ST Micro, also fine = L298 ) i will post apllication. This is L298, pimped with a transistor to output more then 10 Amps http://www.budoclub-biedenkopf.de/51j4/l298.jpg to make it adjustable put a pot there where is the both resistors in end. The minimum output voltage is 5.1V As inductivity use a ferrite ring from defect PC Power supply, 1" or like that, and make 25 turns (thick for 10A). thats all, the rest is R C