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 Farm Radio Power Supply ? UPDATE

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joybird



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PostSubject: Farm Radio Power Supply ? UPDATE   Thu Oct 14, 2010 10:08 am

Looking for input from any member that has built the Farm Supply that Dennis Wesserling was kind enough to supply schematic and parts list for.

I notice the B+ Supply (88-66-44-22 volts) the 4 Zener Diodes 1N4748A 1W 22V run very hot.

WITH NO LOAD

Is this the norm or is something amiss on the one I built?



Thank You
Bob T.


Last edited by joybird on Mon Oct 18, 2010 5:56 am; edited 2 times in total
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N7ZAL

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PostSubject: Re: Farm Radio Power Supply ? UPDATE   Thu Oct 14, 2010 12:21 pm

Don't mean to change the topic but I was interested in the schematic and parts list....where can I find it?
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Dennis Wess

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PostSubject: Re: Farm Radio Power Supply ? UPDATE   Thu Oct 14, 2010 12:27 pm

I can send you a copy but I'll need your email address

Dennis
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Johnnysan



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PostSubject: Re: Farm Radio Power Supply ? UPDATE   Thu Oct 14, 2010 1:10 pm

I haven't seen the schematic, but the only way to heat up zener diodes is to run too much current through them. Zeners should be used to drive transistors that are rated to handle the current; using them as shunt regulators is a total waste. Transistors are cheap, and the extra time it takes to install them is well worth the effort.
Since there will a loss of .6 volt in the transistor, you will need to go one volt higher in the zener diode (23 volts for 22 out).
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N7ZAL

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PostSubject: Re: Farm Radio Power Supply ? UPDATE   Thu Oct 14, 2010 1:41 pm

Thanks Dennis and I sent you a PM here with the email address.
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Dennis Wess

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PostSubject: Re: Farm Radio Power Supply ? UPDATE   Thu Oct 14, 2010 2:01 pm

I've sent schematics etc to those of you who requested one.

Anyone else wanting one please go here to make your request......so as not to unintentionally hijack joybird's thread

http://antiqueradios.forum5.info/antique-radio-discussions-f1/request-copy-of-farm-radio-power-supply-here-please-t116.htm#813

Thanks
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Tom Albrecht



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PostSubject: Re: Farm Radio Power Supply ? UPDATE   Fri Oct 15, 2010 10:24 am

joybird wrote:
I notice the B+ Supply (88-66-44-22 volts) the 4 Zener Diodes 1N4748A 1W 22V run very hot.

WITH NO LOAD

Is this the norm or is something amiss on the one I built?

Hi Bob,

This is normal. The zeners run hottest when there is no load. Zeners regulate by creating an artificial load when there is no load present. Hook up the radio, and the zeners will run cooler.

Ive' built a number of power supplies for antique radios using this type of zener regulation system. Works quite nicely when the needed current is quite low, as it often is for the B+ of battery radios.

I haven't seen the actual schematic for Dennis's design, so my comments above apply generally to low current supplies that are properly designed with zener regulation. It's possible that the design under discussion here puts too much current through the zeners when there is no load. That's easy to check if I see the schematic. Maybe someone can actually paste the schematic into a post here?

Tom
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joybird



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PostSubject: Re: Farm Radio Power Supply ? UPDATE   Fri Oct 15, 2010 11:51 am

Hi Tom

Built this for the Skipper 45 Radio/ Phono

90 V for the radio. Gotta be low current draw.
Still building the 7 pin connector.

The Zener's are too hot to hold a finger on them for long. With No Load.

Had it on for 30 minutes without them self destructing.

Will see when i get the load attached.

Handy supply. I was able to get 1.5 VDC 6VDC for the Motor and 90 VDC With taps for 88-66-44-22.

Thank You

Bob


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Tom Albrecht



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PostSubject: Re: Farm Radio Power Supply ? UPDATE   Fri Oct 15, 2010 1:29 pm

Too hot to touch is normal for a properly functioning 1 watt Zener. Hot enough to boil water is where you should start to get concerned. Heat dissipation is similar to a 1 watt resistor, except that resistors can go to even higher temp without damage.

Tom
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Johnnysan



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PostSubject: Re: Farm Radio Power Supply ? UPDATE   Fri Oct 15, 2010 1:40 pm

Absolutely not!
Zeners that are too hot to touch indicate a serious design flaw, and it's time to go back to the drawing board.
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Tom Albrecht



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PostSubject: Re: Farm Radio Power Supply ? UPDATE   Fri Oct 15, 2010 1:46 pm

Hi Johnnysan,

Can you explain? In my view, it's OK for a 1 watt Zener to dissipate a decent fraction of 1 watt, which unavoidably makes them hot. This is normal, and does not result in failure of the Zener. Check the operating temp specs.

Tom
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Johnnysan



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PostSubject: Re: Farm Radio Power Supply ? UPDATE   Fri Oct 15, 2010 1:55 pm

1 watt zeners were never meant to run hot, or even warm. They are not power transistors (which is what you should be using to deliver current to the load). It is not OK to use zeners as shunt regulators to the point where they can't even be touched. Just because they haven't failed in half an hour does not mean your design is perfect.
Use zeners and resistors as a voltage reference source, and have the power transistors do the work they were designed for.
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Tom Albrecht



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PostSubject: Re: Farm Radio Power Supply ? UPDATE   Fri Oct 15, 2010 2:26 pm

Zeners come in all sizes -- 0.5 watt, 1 watt, 5 watt, and 10 watt. The only reason they would ever make all those different types is because they are in fact designed to dissipate what they are rated for. The larger ones need heat sinks, of course, to stay within their rated temp range, which usually goes to some rather uncomfortably hot temperatures.

As long as circuitry is designed around the published specs of the devices, your circuitry will perform well and reliably, whether warm or not.

A design using pass transistors can also be a good choice, especially for currents higher than the tiny currents we are discussing here. For a very low current supply like this one, your lowest cost approach is zener regulation. If you need 100 mA or more, you're certainly into territory where a pass transistor will be an attractive choice.

Tom
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MarkPalmer



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PostSubject: Re: Farm Radio Power Supply ? UPDATE   Fri Oct 15, 2010 2:36 pm

I tend to agree with Johnny in that I don't like to design or build circuitry where components run excessively hot without a means of dissipating the heat, and those little Zeners can't be heat sink mounted.

But, the 1N4748A zener is a durable component, and has a junction temperature rating of 200 deg C, or 392 deg F, more than hot enough to allow the outside of the zener to burn your finger when it's operating under its normal perimeters. I think the designer of that power supply used these zeners in the spirit of keeping the circuit simple by using reliable zeners for regulation. You can always measure the current under load, and then you'll know for sure if you are exceeding their 1W rating.

-Mark-


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Tom Albrecht



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PostSubject: Re: Farm Radio Power Supply ? UPDATE   Fri Oct 15, 2010 3:05 pm

I took a look at the schematic which Dennis sent, and it looks like the no load power dissipation in each Zener is about 0.8 - 0.9 watt, if the transformer losses are very small. Actual no load dissipation is probably a little lower. Under no load conditions, these zeners will indeed run hot, although within specs if they have reasonable ventilation.

As mentioned above, connecting a load (the radio) reduces the power dissipation in the zeners, so things will run cooler with a load.

If you're worried about the temperature, don't run the power supply without load.
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joybird



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PostSubject: Re: Farm Radio Power Supply ? UPDATE   Fri Oct 15, 2010 3:34 pm

Thank you one and all.

Good discussion on the post.

Now knowing more , I am comfortable with the supply as the original designer intended.

The supply will only be used under a load , which in this case is less than 60ma.

During power up after construction I had noticed the Zeners did run very hot.
Just wondered if this was the norm.

This is for the Skipper RCA Battery operated Radio/45 Record player.
This power supply will replace the missing AC adapter that was once available, but now like hen's teeth to find.

Now I am anxious to build the 7 pin Connector so I can try it out under load condition.

Thanks Very much
Bob T.

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joybird



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PostSubject: Re: Farm Radio Power Supply ? UPDATE   Mon Oct 18, 2010 5:55 am

UPDATE


Got connector built .

Power Supply runs very cool under load.

The Zener's I was worried about run cool as a cucumber. You can hold your finger on them all day long.

Very Happy with this Power Supply. Think I will build another for my Battery sets.

Bob T.
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