CW-80 Transformer ?

[steve24944]

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I know there has been a lot of discussions about the CW-80.  That Lionel had a problem with a reverse in the polarity on the output posts.  Some say that they are junk, some have said that they are fine and work well for them.   I have a Post War ZW that I use for my layout, works great.   The CW-80 I have I use for testing and stuff when I am working on a locomotive, or testing a signal or something like that.   The one that I have has on the bottom  P/N 6-14198,   I think it is one of the ones with the reversed output posts.

My question is this.  When I hook up my volt meter to the output it will show voltage when there is no voltage.   (  I'm using a Gardner Bender digital multimeter ) I have hooked up a 14 volt lamp, apply voltage and the lamp comes on and my voltmeter shows voltage running through the circuit. Shut down the throttle and the voltage goes to zero.  But, when I take the lamp out of the circuit the voltmeter will show voltage when the throttle is off.  I  apply the meter leads to the posts and switch on the transformer  ( the transformer is on a outlet strip with a on-off switch ).  With no load on the circuit the meter will show anywhere between 18 and 22 volts - then slowly start dropping down until it gets to about 3 volts,  then the drop slows way down.  This is with the throttle off !    When I put the lamp in the circuit the meter will go up with the throttle and go down to zero as the throttle is shut off.   If I cross the bare wires when the throttle is off the circuit breaker does not trip.  But the meter will show voltage.  ????

Anybody got a clue what is going on here ?

Steve

[lionelsoni]

Yes.  The output is connected to the actual transformer inside the box through a triac, which is the electronic switch that opens and closes 120 times per second to control the voltage.  There is also a capacitor in parallel with the triac.  It lets a tiny amount of AC current bypass the triac, which is what is showing up on your high-impedance meter.  Any significant load will drag the output voltage to zero, which is what happens when you connect even one lamp to the CW-80.

By the way, most voltmeters will not read the CW-80's voltage accurately, even with a load.  Unless the meter is described as a "true RMS" meter, it probably measures the average of the absolute value of the instantaneous voltage and, assuming that the waveform is sinusoidal, scales that measurement to show "RMS" volts.  The problem with the CW-80 is that its output is far from sinusoidal; so the meter gets it wrong.  Here is a table that will let you convert such a meter's reading to the actual RMS voltage.  Notice that the meter reading can be off by over 4 volts in the middle of the range.

Meter           RMS 
  0             0
  0.5           1.6
  1             2.7
  1.5           3.7
  2             4.6
  2.5           5.4
  3             6.1
  3.5           6.9
  4             7.5
  4.5           8.2
  5             8.8
  5.5           9.5
  6            10
  6.5          10.6
  7            11.2
  7.5          11.7
  8            12.2
  8.5          12.7
  9            13.2
  9.5          13.7
 10            14.1
 10.5          14.6
 11            15
 11.5          15.4
 12            15.8
 12.5          16.2
 13            16.6
 13.5          16.9
 14            17.3
 14.5          17.6
 15            17.9
 15.5          18.2
 16            18.5
 16.5          18.8
 17            19
 17.5          19.3
 18            19.5
 18.5          19.7
 19            19.8
 19.5          19.9
 20            20

[steve24944]

Bob-

Thanks for the reply.  So what you are saying is that the CW-80 leaks voltage, but there is no amperage ? I don't know a lot about electricity but this sounds strange to me.  Is this by design, or was this a unwanted result ?   I take it this does not effect normal use of the transformer.

Steve

[lionelsoni]

When you connect your meter (with the CW-80's control all the way off), you are connecting the meter and the capacitor inside the CW-80's box in series and putting the combination across the transformer's secondary winding.  A very small current does flow through both capacitor and meter.  The meter has a much higher impedance than the capacitor, so it gets the lion's share of the transformer's 20 volt output.

It is an unintended result of putting that capacitor across the triac.  The purpose of the capacitor is to prevent sudden voltage changes across the triac, which can turn it on at times when it should be off.  The designers probably knew that it would mess up voltage readings but figured that that would be harmless, since the thing is usually connected to a load.  They could have fixed the problem by putting a small load inside the box, like a resistor or a lamp.

[Texas Pete]

Bob -

Would analog volt and amp panel meters, such as the ones from lightobject .com, be useful when used in conjunction with the CW-80?

Thanks.

Pete

 

[lionelsoni]

Not if you want measurements that are at all accurate.  It is possible to make a new scale for a voltmeter, following the table I posted above for example.  But using an ordinary ammeter is hopeless, since there is no fixed relationship between the waveform and the RMS current, as there is between the waveform and the RMS voltage.  You can buy "true RMS" meters that will give you an accurate reading; but they are not cheap.

[Texas Pete]

Thanks, Bob. I suspected not, but wanted to hear it from someone who actually knows what's up.

Pete