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 Post subject: Power supply for farm radio
PostPosted: Nov Wed 23, 2016 5:05 pm 
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I have designed a power supply for a 90v dc/1,5v dc farm radio. It is mounted in the case.

I am using an 18 volt laptop power adapter. The brick will be mounted at the top of the radio. Its output cable will be attached to a pre-built regulator to supply 1.5 volts DC to the A side. It will have a three conductor cord to a surge protector which has a 15 amp circuit breaker.

There will be a 1 amp fast acting fuse between the brick tap and the "B" side rectifier diodes.

There will also be a 1 amp fast acting fuse between the brick output and the regulator.

There is no switch planned at this point but 2 neon lamps at the dial will indicate power on.

Comments regarding the circuit protection/safety would be welcome.

The 120 volts at the brick will feed the supply for the B side as in the schematic except it will have a potentiometer instead of a fixed resistor to adjust the output to supply 90 volts DC to the B side. Schematic below.

Note the 18v dc brick w/3 prong plug. The pre-built regulator ("A") and "B"s rectifier diodes, pot, and capacitors are in the white ABS box. Both outputs will be hard wired to the radio.
Attachment:
PB230039[1].JPG
PB230039[1].JPG [ 91.72 KiB | Viewed 6570 times ]



The pre-built LM317 variable DC regulator has AC input of 5-35 volts. The output is 1.25-30 volts DC. It is not a switching type (thyristor) EDIT: power transistor. Cheaper ones are available but this one has numerous features like circuit protection etc. It was $4.10 after shipping from Asia. The 18v brick was $1.50 from Goodwill. The ABS box was $7.42 from Digi-Key. Capacitors, and potentiometer are from Ebay/Asia $2.60 (total). Diodes from Ebay/USA 32 cents (total). Fuse holders and fuses ,50 total Ebay/Asia. $16.44 total not counting wire and solder. Could have been done for about $10 with a free box but I had the box so...

Attachment:
B supply.jpg
B supply.jpg [ 17.79 KiB | Viewed 6127 times ]

Attachment:
Variable DC Regulator.jpg
Variable DC Regulator.jpg [ 111.41 KiB | Viewed 6570 times ]


Last edited by Ozo on Dec Thu 15, 2016 10:41 pm, edited 4 times in total.

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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Wed 23, 2016 5:35 pm 
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Hi

A couple comments. There is no isolation from the AC line for your 90 volt supply. It's better to use an isolation transformer. Commercial radios are built directly off the line but they are designed that way. You will have extra components mounted somewhere in the radio.

Is your 18 volt supply a switching type? If so there will be noise generated by this supply especially mounted anywhere rear the 1H5 tube. Dropping 18 to 1.5 volts will leave a lot of wasted power, generating heat.

18 - 1.5 = 16.5 volts X .25 amps = 4.125 watts.

These radios draw a lot less than 1 amp so fuse size could be smaller, depending on turn on surge.

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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Wed 23, 2016 5:57 pm 
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Agree with Norm's observations, this B+ supply has to be isolated from the AC power line for safety reasons since this radio was never designed for operation from the power line. Your power supply for 18 volts also needs to be verified that it is fully isolated from the power line. I would seek a lower voltage brick, something on the order of 5 to 9 volts DC output, in order to minimize wasted power and create less heat in the regulator.

Many farm radios were designed to have the chassis connected directly to earth ground, if yours is one of them it will be necessary to do that to get the best reception. You couldn't do that with the power supply the way you have shown.

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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Wed 23, 2016 6:09 pm 
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Mr. Detrola wrote:
Agree with Norm's observations, this B+ supply has to be isolated from the AC power line for safety reasons since this radio was never designed for operation from the power line. Your power supply for 18 volts also needs to be verified that it is fully isolated from the power line. I would seek a lower voltage brick, something on the order of 5 to 9 volts DC output, in order to minimize wasted power and create less heat in the regulator.

Many farm radios were designed to have the chassis connected directly to earth ground, if yours is one of them it will be necessary to do that to get the best reception. You couldn't do that with the power supply the way you have shown.

Ok, it will not work "for safety reasons". I have 1.5 volts DC output and 90 volts DC output so do not see a problem. What will the result be if I foolishly ignore your advise and plug this in. Will it blow up? Will it catch on fire. It will be some time before I can actually test it because I still have to refurb the chassis so I am still in the planning stage.

EDIT: How does my design eliminate the possibility of a chassis to ground connection. It is just dc in, there is no ground connection to the chassis.


Last edited by Ozo on Nov Wed 23, 2016 7:43 pm, edited 3 times in total.

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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Wed 23, 2016 6:18 pm 
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Location: Santa Rosa, CA
Quote:
Ok, it will not work "for safety reasons". What will the result be if I foolishly ignore your advise and plug this in.


--you risk an electrical shock

--if you have any problems with the radio, you could apply the full energy of the power line to the circuit and cause some "spontaneous dis-assembly with flame."

A 1 amp "fast" fuse will allow 2 amps for up to 5 seconds... long enough to cause some mayhem.
http://www.littelfuse.com/~/media/elect ... et.pdf.pdf

That "brick" is an RF noise source. Not a good choice near or, especially in, a radio.

A better choice:
Image

This eliminates the noisy "brick" and give you isolation.

Rich

PS:
Quote:
It is not a switching type (thyristor).

Switch-mode regulators use transistors, not thyristors. You will find thyristors in light dimmer.


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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Wed 23, 2016 7:03 pm 
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Norm Leal wrote:
Hi

A couple comments. There is no isolation from the AC line for your 90 volt supply. It's better to use an isolation transformer. Commercial radios are built directly off the line but they are designed that way. You will have extra components mounted somewhere in the radio.

Is your 18 volt supply a switching type? If so there will be noise generated by this supply especially mounted anywhere rear the 1H5 tube. Dropping 18 to 1.5 volts will leave a lot of wasted power, generating heat.

18 - 1.5 = 16.5 volts X .25 amps = 4.125 watts.

These radios draw a lot less than 1 amp so fuse size could be smaller, depending on turn on surge.

The 18 volt brick is an old one so I assume is non-switching. The variable regulator also is assumed to be non-switching.


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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Wed 23, 2016 7:13 pm 
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OOPS, duplicate.


Last edited by Ozo on Nov Wed 23, 2016 7:35 pm, edited 1 time in total.

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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Wed 23, 2016 7:16 pm 
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Rich, W3HWJ wrote:
Quote:
Ok, it will not work "for safety reasons". What will the result be if I foolishly ignore your advise and plug this in.


--you risk an electrical shock

--if you have any problems with the radio, you could apply the full energy of the power line to the circuit and cause some "spontaneous dis-assembly with flame."

A 1 amp "fast" fuse will allow 2 amps for up to 5 seconds... long enough to cause some mayhem.
http://www.littelfuse.com/~/media/elect ... et.pdf.pdf

That "brick" is an RF noise source. Not a good choice near or, especially in, a radio.
Rich


PS:
Quote:
It is not a switching type (thyristor).

Switch-mode regulators use transistors, not thyristors. You will find thyristors in light dimmer.

OK, thanks for the info regarding the thyristor. However; I see the variable dc regulator above has a 2N5551 NPN high-voltage transistor in its circuit. Will that make noise? If so then one more reason not to use my design.

You are saying any electronic device could be destroyed if powered by a wall wart or power adapter. Did you mean to say that or did you mean that it is not safe to have an AC line inside the radio case.? I have many delicate devices powered that way.

I will have the power supply outside the radio and the two dc output cables will be plugged into receptacles on the radio. Now ac is isolated from the radio. I will measure the output for ac . If AC is somehow able to get on the dc because of an incident inside the remote unit which is unlikely then I will have a problem so I will always power this with my isolation transformer. Is that satisfactory?


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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Wed 23, 2016 11:22 pm 
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The power supply that Rich showed a diagram of is the same one I am building for a customer of ours. The transformer is a nifty little one available from digikey for about 8 or 9 dollars. I'm waiting on diodes, of all things! I know I ordered several dozen but for the life of me can't find them! But I know I will find them after the new order comes in!


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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Thu 24, 2016 2:45 am 
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Location: Santa Rosa, CA
Quote:
You are saying any electronic device could be destroyed if powered by a wall wart or power adapter


NO!

You want a wallwart or adapter that contains an isolating transformer. Most conventional wall warts have a small iron core isolating transformer inside. A switch-mode "brick" is light weight and may have a ferrite core isolation transformer. The brick usually generates a lot of RF noise and hash due to the high frequency switching of its power transistor.

You seem to be resisting the simple obvious answer. Use a small transformer as described in the circuit above.

Rich

Typical switch-mode wall wart:

Image

Typical non-switching wall wart:

Image


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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Thu 24, 2016 2:39 pm 
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Rich, W3HWJ wrote:
Quote:
You are saying any electronic device could be destroyed if powered by a wall wart or power adapter


NO!

You want a wallwart or adapter that contains an isolating transformer. Most conventional wall warts have a small iron core isolating transformer inside. A switch-mode "brick" is light weight and may have a ferrite core isolation transformer. The brick usually generates a lot of RF noise and hash due to the high frequency switching of its power transistor.

You seem to be resisting the simple obvious answer. Use a small transformer as described in the circuit above.

Rich

Typical switch-mode wall wart:

Image

Typical non-switching wall wart:

Image


Yes, I am resisting that option because I want to try the less expensive and simpler option of using a wall wart. That is why I got an older non-switching wall wart and variable dc regulator to supply the A supply of 1.5v dc. I personally think that would work just fine and be safe as well as silent (if outside the box).

The thing that seems to be the insurmountable problem for members above is the 120v ac directly to a rectifier. They are being negative and "resist" this idea. I was hoping for a positive response with perhaps a solution to making this "safe" other than using the transformer suggested.

The Tamura transformer in the schematic above is listed at Digi-Key as a "power transformer". They list "Isolation transformers" too which are more expensive. I have both types of transformers on hand but want to design a less expensive alternative. I suggested using an isolation transformer (which I have and use on the bench anyway) to plug it into but was "ignored".

I opened a couple of older wall warts and they have metal shielding surrounding the electronics even though they do not have the transistors. Is that to block other types of interference or is it a heat sink?


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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Thu 24, 2016 6:16 pm 
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Location: Santa Rosa, CA
Quote:
The thing that seems to be the insurmountable problem for members above is the 120v ac directly to a rectifier. They are being negative and "resist" this idea. I was hoping for a positive response with perhaps a solution to making this "safe" other than using the transformer suggested.


Sorry, you will not get a positive response from anyone with real experience.

Quote:
The Tamura transformer in the schematic above is listed at Digi-Key as a "power transformer"


True, this is not rated as an isolation transformer, but it is much better than NO transformer at all. Several of us have used that design and it works. The power transformers inside older tube radios are also not called "isolation" transformers, even though they provide that function.

Quote:
I opened a couple of older wall warts and they have metal shielding surrounding the electronics even though they do not have the transistors.


If the metal plate has a transistor bolted/riveted to it, then it's a heatsink. Non-switching (linear) warts usually have no shielding. If your wart has a heavy iron core transformer, then it is linear (non-switching).

If you are determined to use wallwarts, get two identical linear wallwarts and harvest the transformers. Connect them in a "chain" arrangement.

Image

Disregard the voltages shown in the example that I snipped from the Internet. To make a good isolation transformer, use two identical transformers with the secondaries tied together.

Rich


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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Thu 24, 2016 6:56 pm 
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[/quote] If you are determined to use wallwarts, get two identical linear wallwarts and harvest the transformers. Connect them in a "chain" arrangement.

Disregard the voltages shown in the example that I snipped from the Internet. To make a good isolation transformer, use two identical transformers with the secondaries tied together.

Rich[/quote]

That is a positive response and just what I was looking for. What do you think of my idea to use an existing isolation transformer to supply the 120v ac to the rectifier in the B supply.

Attachment:
cascaded_transformers.jpg
cascaded_transformers.jpg [ 18.15 KiB | Viewed 6463 times ]


Last edited by Ozo on Nov Thu 24, 2016 11:48 pm, edited 1 time in total.

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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Thu 24, 2016 11:48 pm 
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You don't need identical wall warts, driving a 12v with a 9v is still going to produce more than enough B+... Under no circumstance try to drive a 9v with a 12v, or any other with higher voltage than one being driven...

The beauty part is you can use the secondary of first xformer(well assuming at least 6v) to power a LM317 regulator, so both voltage requirements can met with this arrangement... The driving transformer should have probably have at least 500-600ma output, a 750ma would no doubt be better...


Tom


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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Thu 24, 2016 11:54 pm 
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35Z5 wrote:
You don't need identical wall warts, driving a 12v with a 9v is still going to produce more than enough B+... Under no circumstance try to drive a 9v with a 12v, or any other with higher voltage than one being driven...

The beauty part is you can use the secondary of first xformer(well assuming at least 6v) to power a LM317 regulator, so both voltage requirements can met with this arrangement... The driving transformer should have probably have at least 500-600ma output, a 750ma would no doubt be better...
Tom


This may sound stupid but by "driven" I assume you mean the one to the right. The first one (to the left) is the one doing the driving. Sometimes when I am going down the road with my wife I am not sure who is doing the driving.


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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Fri 25, 2016 12:19 am 
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EDIT: duplicate.


Last edited by Ozo on Nov Fri 25, 2016 12:28 am, edited 1 time in total.

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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Fri 25, 2016 12:26 am 
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Yes, a couple of transformer wall worts would yield what the Tamura transformer does on one core. Some transformer wall worts are regulated, a little ingenuity could use those parts too. You already have a good "A" regulator.
Chas

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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Fri 25, 2016 12:40 am 
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Mr. Detrola wrote:
I would seek a lower voltage brick, something on the order of 5 to 9 volts DC output, in order to minimize wasted power and create less heat in the regulator.


The reason I chose the larger brick was because it has a 3 prong plug. The smaller warts do not. I may have to go smaller if heat is a problem. I will cross that bridge if needed but will be watching for heat in the meantime. Thanks.


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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Fri 25, 2016 12:43 am 
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Ozo wrote:
This may sound stupid but by "driven" I assume you mean the one to the right. The first one (to the left) is the one doing the driving. Sometimes when I am going down the road with my wife I am not sure who is doing the driving.

Yep...

Here's enough of the schematic that it should be understandable... Use the schematic already posted for remainder of circuitry...

Note you must use AC supplies or cut them open and use xformers only, most are DC...

Image

download/file.php?id=160693


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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Fri 25, 2016 1:07 am 
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ANNOUNCEMENT
Having weighed all the advise above, I am going to go ahead and use the Tamura transformer ($8.83) in my power supply after all.

I am going to modify the design from Solar Orbit submitted by Rick above. The modified design will use the variable ac/dc regulator ($3.90) I proposed earlier for the A supplies 1.5v DC. The B side 90v DC will be as per the schematic with the exception of a 20 cent potentiometer to adjust the voltage instead of the fixed resistor.

The components will be mounted on a 4"x5" PC board (438-1019-ND0 $4.68 from Digi-Key) except the 16 cent fuse holder and 99 cent switch is mounted through the box for easy access. Further, I am not going to install the box inside the radio as Solar Orbit advises. This means I will have 2 cables (A and B) from the supply to the radio. Also I am not going to attach the ground to the radio chassis but to the transformer frame only. Light weight three prong power cord was 99 cents from Goodwill. I think I have addressed all the issues brought up in this thread. Here is a link to the Solar Orbit project which is the basis for my project. My modified designs schematic is below.

EDIT: I am placing the project in a small wood box I got at Goodwill for $2. No need for the ABS box. Also an additional cable will power a single very bright LED/resistor which doubles as power on indication and dial light. Project cost was appx. $30 but some minor items were salvaged.

http://solorb.com/elect/hamcirc/farmradiops1/index.html

Attachment:
farm radio PS (Modified).jpg
farm radio PS (Modified).jpg [ 46.84 KiB | Viewed 6379 times ]


Last edited by Ozo on Nov Sun 27, 2016 4:50 pm, edited 12 times in total.

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