need help with a variable power resistor

Discussion in 'General Electronics Chat' started by sdowney717, Jul 18, 2012.

  1. sdowney717

    Thread Starter Member

    Jul 18, 2012
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    I emailed Vic at Raritan on this old convertor-charger to see if they can tell me what the specs are on the component. Input 120 volts AC, output about 13.5 volts DC depending on the variable resistor setting.

    It is the older “R” series which predated both the “B” and the “F” series.

    It is the lowest part on the circuit board. Riveted on the ends and soldered into the circuit.
    [​IMG]

    I measured 14 million ohms across the terminals which must be too high dont you think?
    Perhaps corrosion has affected the winding under the adjuster.
    I cant see on the other side for any markings.

    Also any ideas on where to go for a replacement?
    even a rheostat could work here.
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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  3. #12

    Expert

    Nov 30, 2010
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    The danger of reading a resistance (in circuit) are about the readings being too low because something is in parallel with the resistor. At 14 million ohms, I'd say the resistor isn't IN the circuit! (14 million is incredible for a wirewound resistor.) I'm thinking corrosion has eaten through a turn of the winding and/or a connection to the board. You can try to measure from one end to a bare place in the adjustment area, then from the bare place to the other end. If you get numbers like 12 ohms and 32 ohms, you can guess the resistor is about 50 ohms. You can remove the resistor and read the label on the backside. The labels are usually bulletproof.

    I'll bet you are going to have to take that board out and work on the corrosion because other parts are no longer connected.
     
  4. sdowney717

    Thread Starter Member

    Jul 18, 2012
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    yes, extremely high resistance.
    I moved the adjuster out of the way, held one probe on the right soldered connection and dragged the probe across the winding.
    From extreme right to about 1/3 left I was getting a reading that varied as the probe moved. Perhaps up to a couple million then after it just reads about 14 million ohms all the way to the left side.

    I will take the charger board out and see if it is marked. It is on a boat. I noticed lately it was not charging like it should, too low.

    I really hope it is marked. Where will be a good place to get something similar to fix this?

    I dont mind a round control. What do you think would be the power rating for this resistor?
     
  5. #12

    Expert

    Nov 30, 2010
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    I buy parts at www.mouser.com but I can't tell what country you are in. There is also Digikey, Jameco, and Farnell.
    The power rating is related to the size of the resistor. Measure it. It might also be marked on the back. I don't think there is a round control to replace this part.

    Your resistance readings are still absurd. A million ohms at 120 volts would use .01 watt. Your resistor is in the 10 to 20 watt range.
     
  6. sdowney717

    Thread Starter Member

    Jul 18, 2012
    385
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    I am in the USA. Plan to get this off boat today and will discover some more on the part. I will put up some more pictures.
     
  7. PackratKing

    Well-Known Member

    Jul 13, 2008
    850
    215
    That looks like a rather hefty unit you have there! Hope you can find a datasheet for it, an adequate rheostat - Ohmite ceramic - will do nicely
    The bench power supply I built, somewhat similar, is based on the LM317, configured as adjustable. Mains powered to a hefty transformer, A 50v. PIV 25 amp monolithic bridge rectifier, on a large heatsink, and a cap-input filter. Output is 8 amps @ 0 - 12 volts.
    I know that sounds high for the LM317 output... tho' I ganged 6 of them in parallel on a "rather large" heatsink, and let Ohms' Law deal with the fact that a parallel circuit splits current evenly between devices........it never broke a sweat.

    * Clarostat * 10 turn potentiometer for the varying element. I may eventually replace that w/ a large Ohmite rheostat.
    It is a " battery replacement " supply used for various camera equipment, to keep me out of K-mart or the $ store forever buying batteries...
     
    Last edited: Jul 19, 2012
  8. sdowney717

    Thread Starter Member

    Jul 18, 2012
    385
    13
    Took off the outer cover
    picture of the side SCR diodes, 5 stud diodes.
    2 are 368C 7147 which take voltage off transformer
    3 are 368 SCR 7120 BT which output to batteries
    one large stud zener GE C38U on plate heat sinks.

    [​IMG]

    shows the circuit board. I took off the nuts and examined the underside and it is coated and fine. The variable power resistor I resoldered and amazingly it is working. It reads .331K ohms. Strange dont know why it was not working before! Looked at the windings very closely and they are ok.
    [​IMG]

    I managed to read the numbers off the parts with a glass. the gold colored diode I think is a zener number N3829 or N3828
    The two studded diodes are IN1343 which detect if the ignition turns on, it does something to the charger output.
    The 2 transistors, one is 2N525, other says thermalloy 22118

    I turned it on without a battery and get 10 volts. Attach a 12 volt blower and get 11 volts, so I think it must be attached to a battery to produce the 13.5 volts it should have. I put the adjustment back in the middle, so will test it and see.
    Someone used white silicone to reaatach the overheat breaker to the transformer. I peeled all that off and tied it to the armature.

    So do you think this somehow detects a battery and adjusts the voltage, I never ran it without a battery hooked up so was wondering why the voltage without a battery was so low.
     
  9. sdowney717

    Thread Starter Member

    Jul 18, 2012
    385
    13
    I tested on the car this am and it is working. Puts out 13.6 volts.
    It must sense the battery voltage using a feedback and adjust the voltage higher.
     
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