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I have a problem with a simple rectifier circuit that is designed to drive a DC motor. The motor is part of a ripple tank for a high school physics lab.

The input to the circuit is 12 VAC from a bench supply that appears to be good. This goes through a simple rectifier consisting of 2 diodes. There is a smoothing capacitor (1 µF, 450 V electrolytic) and what I assume is a voltage regulator, but I can't see any identifying markings.

This feeds into a potentiometer before going to the output. The diodes have been tested in-circuit using a DMM in diode mode and seem OK. The potentiometer also checked out OK when tested.

The voltage output varies from 0-9 V with no load on the output but crashes as soon as any load is applied. I had to use a light bulb as the load because the DC motor terminals are corroded beyond worthwhile repair.

Is it reasonable to assume that the capacitor has failed?

EDIT: I have added a photo of the "rectifier" and the motor terminals, I will need to take the unit apart again to draw a circuit diagram but the only thing missing from the picture is the diodes, a 1.5 kΩ input resistor and a 17 kΩ output resistor.

EDIT 2: I've identified the mystery device as a 2SD509 transistor and tried to draw a circuit diagram.

ripple mechanism

tank assembly

front panel

reverse of panel

schematic

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    \$\begingroup\$ Would you be able to draw up a schematic? Powering anything through a pot, unless it’s a very high power low resistance type, isn’t going to work well. Also 1uf is not sufficient input capacitance to power a motor or lightbulb. \$\endgroup\$
    – Bryan
    Commented Sep 14, 2022 at 4:54
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    \$\begingroup\$ Some photos of the circuit (including the 'what I assume is a voltage regulator') might help too. \$\endgroup\$
    – Bruce Abbott
    Commented Sep 14, 2022 at 4:59
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    \$\begingroup\$ It's really cheaply put together as well as being a terrible circuit. I was just asked to "get it working", I definitely need a new motor because the terminals are corroded beyond salvage so I can't even try that yet, \$\endgroup\$
    – JamieP
    Commented Sep 14, 2022 at 8:13
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    \$\begingroup\$ @JamieP Maybe it would just be better if you scrapped the whole thing and just told us what you'd like to achieve? Provide us a diagram of the ripple tank and the system that generates the ripples. Let us work out the details for the circuit to build. Start with the physical basics. If we know what that is, the rest can readily follow from there. This isn't rocket science. But I think you need to scrap what you have (circuit-wise) and figure on creating something new and better, for now. Just an opinion. Maybe we can help out. But we need details. \$\endgroup\$
    – jonk
    Commented Sep 15, 2022 at 8:58
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    \$\begingroup\$ This WAS done by someone who apparently didn't know the basics of electronics. The first clue are two diodes where the second one is pointless. You could short either one and you would get the same functionality with less voltage drop. The answer by Bryan contains a workable schematic - yours wouldn't work. \$\endgroup\$
    – Edin Fifić
    Commented Sep 16, 2022 at 3:26

3 Answers 3

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The following schematic may be closer to the requirement.

enter image description here

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Based on the new photos I have created a different schematic:

schematic

simulate this circuit – Schematic created using CircuitLab

(I guessed at the potentiometer resistance)

I would encourage you to verify that this is correct, and as to why it doesnt work, my first suspect would be the potentiometer: they dont like... existence, let alone water - the wiper may no longer be making contact with the track. Diagnosis will require some basic multimeter skills, and a multimeter.

Also your transistor model number is probably incorrect as I get different package transistors when I search that part.

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    \$\begingroup\$ The R2 is 18k, as there isn't a 17k value and the second band color is clearly gray (8). R1 is 1k8. \$\endgroup\$
    – Edin Fifić
    Commented Sep 16, 2022 at 3:20
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    \$\begingroup\$ I have managed to get confirmation that the motor is a Mabuchi Eg - 530ad - 2 F Dc motor. I have ordered a replacement online and will put things on hold for now until I get that. Thanks for everyone's help so far. \$\endgroup\$
    – JamieP
    Commented Sep 16, 2022 at 5:24
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    \$\begingroup\$ @EdinFifić noted and corrected, thanks. \$\endgroup\$
    – Bryan
    Commented Sep 17, 2022 at 1:26
  • \$\begingroup\$ @JamieP I agree with this circuit diagram - it's the same as what I arrived at "by inspection". Try connecting the transistor base (connects to pot wiper) to the R2/R1 junction. Motor should run at near full speed. If motor does not run at all try shorting out C and E on Q1 (connects top of load to top of R1. If motor does not run it is dead or a wire is broken - perhaps between R2 bottom and D2 OR D1 otr D2 is dead. || One at a time short out D1 and then D2 \$\endgroup\$
    – Russell McMahon ♦
    Commented Sep 17, 2022 at 8:02
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I agree with Bryan's circuit diagram - it's the same as what I arrived at "by inspection".

Try connecting the transistor base (connects to pot wiper) to the R2/R1 junction. Motor should run at near full speed.

If motor does not run at all try shorting out C and E on Q1 (connects top of load to top of R1. If motor does not run it is dead or a wire is broken - perhaps between R2 bottom and D2 OR D1 otr D2 is dead.

To test D1 and D2, One at a time short out D1 and then D2

Motor connected across C1 should run.

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