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.

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

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

Dennis

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).

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

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

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

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

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.

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

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.

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

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-

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.