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 Post subject: Battery eliminator Zener Resistor
PostPosted: Jun Sat 13, 2020 6:02 pm 
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Location: Fenton, MI 48430
I am building a battery eliminator. Using a string of six 22 volt 5 watt zeners in series, I can get voltages of 22, 44, 66, 88, 110 and 132 volts with about 155 volts into the string. For a maximum current draw of 50ma, what size series resistor would I use from the 150 volts to the zener string input? I believe 100 ohms would work and allow up to 70ma current draw..

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 Post subject: Re: Battery eliminator Zener Resistor
PostPosted: Jun Sat 13, 2020 7:14 pm 
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Joined: Nov Tue 14, 2017 5:09 am
Posts: 3064
Location: Austin, Texas
jimbenedict wrote:
I am building a battery eliminator. Using a string of six 22 volt 5 watt zeners in series, I can get voltages of 22, 44, 66, 88, 110 and 132 volts with about 155 volts into the string. For a maximum current draw of 50ma, what size series resistor would I use from the 150 volts to the zener string input? I believe 100 ohms would work and allow up to 70ma current draw..

A 100 ohm, 5W, will work but the zeners will run fairly hot. You could get by with a larger resistor.

Assuming 150VDC input under load and 132VDC for the total zener string, the voltage difference is 18 volts.
If you want to supply a load current of 70mA then the voltage dropping resistor needs to provide that plus some minimum bias current to the zeners.
Something like 80mA should be OK assuming 10mA bias to the zener string.

18V / 80mA = 225 ohms

Using a 220 ohm resistor, the power dissipation would be (18V x 18V) / 220 = 1.47W
I would probably use two 110 ohm, 2W, resistors in series so each would be running about 0.75W.

Jay


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 Post subject: Re: Battery eliminator Zener Resistor
PostPosted: Jun Sat 13, 2020 7:36 pm 
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Joined: Jan Tue 07, 2020 1:41 am
Posts: 2729
Location: Fenton, MI 48430
JnTX wrote:
jimbenedict wrote:
I am building a battery eliminator. Using a string of six 22 volt 5 watt zeners in series, I can get voltages of 22, 44, 66, 88, 110 and 132 volts with about 155 volts into the string. For a maximum current draw of 50ma, what size series resistor would I use from the 150 volts to the zener string input? I believe 100 ohms would work and allow up to 70ma current draw..

A 100 ohm, 5W, will work but the zeners will run fairly hot. You could get by with a larger resistor.

Assuming 150VDC input under load and 132VDC for the total zener string, the voltage difference is 18 volts.
If you want to supply a load current of 70mA then the voltage dropping resistor needs to provide that plus some minimum bias current to the zeners.
Something like 80mA should be OK assuming 10mA bias to the zener string.

18V / 80mA = 225 ohms

Using a 220 ohm resistor, the power dissipation would be (18V x 18V) / 220 = 1.47W
I would probably use two 110 ohm, 2W, resistors in series so each would be running about 0.75W.

Jay

Thanks. I have a 7 watt 250 ohm resistor. Likely draw only 50ma, but a 112 audio tube replacing a 201A would draw a few more ma. My radio came equipped with either the 201A or 112 tube. It is being shipped to me, will find out which tube installed.

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 Post subject: Re: Battery eliminator Zener Resistor
PostPosted: Jun Sat 13, 2020 9:01 pm 
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Joined: May Tue 30, 2006 4:46 pm
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Location: Santa Rosa, CA
On-line Zener calculators will give you resistance and power. I would double the power rating of the Zener and the series resistor to have both run cooler.

http://www.calculatoredge.com/electronics/zener.htm

Rich


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 Post subject: Re: Battery eliminator Zener Resistor
PostPosted: Jun Sat 13, 2020 11:35 pm 
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Location: Lexington, KY USA
When building, don't forget to provide enough heat sinking for each lead of each zener. Especially when you have a bunch in series. You are not going to get rid of any heat down the lead to the next zener in the string. A small fan can do wonders.

Going to 10W zeners in place of 5W ones may not be very economical. At least one distributor wants almost 100X as much $$ for the 10W parts. 5W is fifty cents and easy to find.

If you use twice as many 11V zeners, you will potentially have twice the power capability. And more different voltage taps.

The eliminator designs that use a series pass transistor to feed the first zener have some advantages, though just a brute force shunt regulator can certainly work.

One simple thing to do is to switch-in different series resistances depending on your load current, so not so much current hits the zeners when the load is light. Sort of a manually operated series pass circuit. A zener current meter might not be a bad feature for your supply.

Ted


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 Post subject: Re: Battery eliminator Zener Resistor
PostPosted: Jun Sun 14, 2020 3:19 am 
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Joined: Nov Tue 14, 2017 5:09 am
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Location: Austin, Texas
Usually Lurking wrote:
If you use twice as many 11V zeners, you will potentially have twice the power capability. And more different voltage taps.

Ted


You could also put two 22V, 5W, zeners in parallel for each 22V step. Since 22V zeners have a positive temperature coefficient, the currents will tend to balance between the two devices if one is getting too hot.

Jay


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 Post subject: Re: Battery eliminator Zener Resistor
PostPosted: Jun Mon 15, 2020 5:09 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 768
Have a look at the circuit of the battery eliminator on page 29 of this article:

http://worldphaco.com/uploads/THE_GREBE_MU-1.pdf

It is much better to run a lower current in the zener chain and take the output off the chain with emitter followers, so that these transistors, rather than the zeners are responsible for the heat dissipation. The base currents are about 1/50th or less of the emitter currents, you can also use convenient screw down transistors rather than trying to attach heat sinks to zeners. It pays to have the series diodes too, in case say you have been powering something and it has stored charge in its B+ capacitors and then you hot plug that into one of the lower output voltage terminals.


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 Post subject: Re: Battery eliminator Zener Resistor
PostPosted: Jun Wed 24, 2020 5:16 pm 
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Location: Fenton, MI 48430
I found out a 500 ohm resistor work well for a 5 tube 201A radio. If drawing over 20ma, I would use 350 ohm.

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 Post subject: Re: Battery eliminator Zener Resistor
PostPosted: Jun Thu 25, 2020 7:35 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 3900
Location: Lexington, KY USA
Paralleling zeners is usually a poor practice. Connecting them in series works much better.

Adding an emitter follower to each output tap makes the unit far more complex. More complex schemes can be better, but the string of zeners can be good enough.

Ted


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 Post subject: Re: Battery eliminator Zener Resistor
PostPosted: Jul Fri 03, 2020 2:26 am 
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If you're only going to use one voltage tap at a time why not make a variable regulator that uses a rotary switch and zeners or resistors to set the voltage?


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 Post subject: Re: Battery eliminator Zener Resistor
PostPosted: Jul Sun 05, 2020 9:11 pm 
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He may be doing what I am doing. Need multi B voltages for old battery sets. The one I am building is with an 84V transformer whose filtered output will be equivalent to the 90V. I am gonna use resistors and a pot to divide that for 45V and a resistor feeding zeners for the 22V of the detector tube. I found a 22V regenerative detector only draws between .5 and less than 1 mA. so I used a decade resistor to find what resistor is putting only about 4 mA at 90V thru my 1 watt 22V zener hookup. I found that the detector load does not make much change and it is not hyper critical that it be exactly 22V. If a detector tube drew alot of current that might not be practical but it is pretty small. Then I use a filament transformer rectified to drive a TO-3 cased, 5V 5A three terminal regulator. My radios are primarily 2 or 3 tube jobs that I want to operate and the B currents are not very large. IT will hardly be the ultimate or ideal but it should work fine and cost my junk box some odds and ends. The only possible case where the B voltages would be of any critical value would probably be if you were using a 200 type tube detector where adjustable B is helpful for optimum detection. If you 90V is coming out to 89 or 95 under load, no issue IMO.

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