# PH Hobbies ?



## Trains (Jan 2, 2008)

I have a old PH hobbies power pack. (21 Volt, 10 Amps) If I use a meter and measure across the ac side of
the bridge rectifier I get 21 volts. When I measure across the dc side I get 30 volts? I use to get 21 volt on 
the output side. 

Thanks 
Don


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## Curmudgeon (Jan 11, 2008)

Got a load on it? 
Stick a light bulb or old motor across the output and try again, just for grins.


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## George Schreyer (Jan 16, 2009)

21 VAC on the input side of a bridge SHOULD produce 28 VDC on the output side IF there is a filter capacitor installed. Sounds like it is working like it should.


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## Trains (Jan 2, 2008)

OK guy's thanks, I guess my train engineer bit the dust. I been running the 5471 for about 15 years on the PH haven't used it in about a year.
No output from the T/E. The green light blink but no volts coming out.


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## Greg Elmassian (Jan 3, 2008)

Yep, conversion from sinusoidal AC to DC (and filtered with a big enough cap) is (George check me here) 1.414 if I remember correctly. 

21 * 1.414 is 29.694 in my book, pretty close to 30. 

Was the TE rated for that high a voltage? 

Regards, Greg


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## Trains (Jan 2, 2008)

Greg,

The T/E was 24 I think, but I ran it with the PH for about 18 years with no problems. I hooked it to the N scale 
and ran about two years. Turned it off, haven't used it in about 6-8 months. Turned it on and nothing. 
I think the output of the PH was about 24 volts when I purchased it. I would not have hooked it up if it was 30 volts.


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## George Schreyer (Jan 16, 2009)

Greg, you forgot to subtract the voltage drop of the bridge....


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## Greg Elmassian (Jan 3, 2008)

oops!!! 

so 29.694 - 1.4 (assuming standard silicon diode voltage drop) = darn close to 28.3 volts... still seems to be over rated voltage... yes, might have run many years... maybe something got tired, or you got a voltage spike. Solid state electronics can age... 

Anyway, sounds like your TE finally gave up. Your first post says you measured 30 volts, might double check it. What if a diode (or a few) went in the bridge? Would that explain 30 volts? 

Wild idea, but maybe you are getting AC or some poorly rectified power out. 

Might be worth checking the bridge. 

What do you think George? He did say he measured 30 volts, if it is REALLY 30 volts, then I think this is possible. 

Regards, Greg


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## Trains (Jan 2, 2008)

Greg,

Is there away to check the bridge? There has to be something wrong. I would thing you would have 21 volt on the AC side and 30 on the dc side.
How would voltage go up? Not very good at electronics I know just enough to get zapped!


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## George Schreyer (Jan 16, 2009)

The output of the bridge rectifier will charge the filter capacitor up to the PEAK value of the AC voltage coming in. Not counting the rectifier drop, than is the (square root of 2) x (the RMS input voltage). This is what is expected. If there is some noise on the input voltage and the power supply is unloaded, you might actually find a somewhat higher voltage. As the load increases, the output voltage will take on a sawtooth look to it as the capacitor discharges between peaks of the AC input.


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## Dan Pierce (Jan 2, 2008)

Note that home line voltages can vary from the nominal 117VAC by 10 percent which means the output of a transformer at 21 volts could go as high as 23 volts. 
Now multiply by 1.414 and subtract the diode drop of 1.4 volts for a full wave bridge, or worse yet, a center tapped transformer feed that gives only one diode drop of .7 volts. 

This is the reason I like regulated supplies. 

Aristo spec on the 55474 and 55475 is 5 to 18 volts. 

You can get away with higher DC voltages, but I personally would never go over 24 volts with this unit.


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## Trains (Jan 2, 2008)

Was going to send the TE in, I took it apart and found a 10 amp fuse blown. Got that replaced works good.
Then I opened up the PH hobbies 10 amp power pack. Found the bridge rectifier bad replace it with a Radio Shack
one I had, work good. Hope the two are interchangeable? The old one put out 30 volts the new one puts out 20.5 volts.


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## Curmudgeon (Jan 11, 2008)

You got a 10V drop by putting in a RatShack bridge rectumfryer?


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## Greg Elmassian (Jan 3, 2008)

I suspect a short in the old rectifier, now corrected. 

Regards, Greg


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## Curmudgeon (Jan 11, 2008)

Shorted, bypassing the rectumfrication. 
I suppose a 'scope would have showed a lot.....or at least an AC Voltmeter stuck on the output. 

Not real good for permag motors..........


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## Trains (Jan 2, 2008)

I think the rectifier went bad then took out the Train Engineer. The out put of the 
PH should have been about 21 volts.


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## Curmudgeon (Jan 11, 2008)

And gave you 30V with AC component


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## Greg Elmassian (Jan 3, 2008)

That would be my guess too... a scope is a wonderful thing, don't have to use it often, but for some things it really helps. 

Greg


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## Trains (Jan 2, 2008)

Thanks for all the help guys!


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