Solving Circuit Board Problem: 18vdc Motor, 8 Ohm Resistor

In summary, the person is trying to power a Dremel tool with an 18 V DC motor that is rated at 1.96 amps. They have replaced the capacitor with a piece of wire and still cannot get 18 VDC out of the end to power the charge mat. Can anyone help them?
  • #1
I'm having issues with this simple circuit. If you look in the top left corner of the image attached, I have a dremel tool powering an 18vdc motor which puts out current at 1.96 amps. Then it goes to a switch. The wires then go to an 8 ohm resistor(white rectangle) then to a 4700μF capacitor(blue cylinder). Now where the pencil is in the picture, I still have 18 volts being regulated by the capacitor. Where I have soldered the ground wire from the switch and the hotwire to the capacitor I still get 18 volts. There is a connector at the end of the black wire at the bottom right that plugs into a universal powermat, and I cannot get 18 v out of the end to power the mat. Can anyone help me?


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  • #2
You seem to have a DC motor with a capacitor and a resistor in series with it, fed from an 18 V DC supply.

Capacitors do not pass DC, so the motor will not work like that.

So, you can remove the capacitor (replace it with a piece of wire) and try again.
  • #3
It did not work. I know the specs require 18 v at 1 amp to power the charge mat, and the motor is 18v at 1.96 amps. I have used an 8 ohm resistor and a 10 ohm resistor as well, any other suggestions?
  • #4
If I'm reading this right, vk6kro seemed to be hinting that you need the capacitor in parallel with the load, not in series. I'd agree with him.

The capacitor is a filter. Put it *between* the V+ and return line, and it blocks DC from shorting to ground, but allows the ac component to bleed to ground (=stabler DC). If you put it *in* the return line, it blocks all DC current from going through the circuit (but you'd still read a V+ *potential* as you noted)

The most important function of the resistor is to prevent an initial short-circuit through the capacitor as it charges. That would burn out your generator motor.
  • #5
alright, I'll go try this. I'll post again in about an hour or so
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  • #6
Leave the capacitor out anyway.

Re-reading your post, are you using the motor as a generator to provide power to charge up other devices?
This would be a way of converting 12 volts DC (which powers a Dremel tool ) to 18 volts DC

If you have 18 volts at one end of the output wire and not at the other, then maybe you have a faulty wire?

Can you measure the voltages relative to the shield on the output cable? With the capacitor removed, what voltage do you get on each side of the resistor, again, relative to the shield on the output wire?
  • #7
The dremel tool is spinning a small dc motor at max rpm to produce 18volts at 1.96 amps. The motor is meant to power the universal charge mat
  • #8
The voltage was fluctuating on both sides of the resistor
  • #9
Here's a closeup of it redone


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  • #10
Here's the closeup in parallel


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  • #11
The capacitor is still wrong.

It should go across the output, ie from the centre of the output wire to the shield of the output wire.

Note that the + sign on the capacitor has to go to the + side of the supply.
  • #12
I can't contain my curiosity -- why are you building this device?
  • #13
vk6kro I am relatively new to circuits, could you please explain what you mean by the shield of the wire?
  • #14
jhendren said:
vk6kro I am relatively new to circuits, could you please explain what you mean by the shield of the wire?

The wire in your photo looks like shielded wire. This is a central wire surrounded by a woven shield of copper wire.
This is called a shield as it stops interference reaching the central wire if it is grounded.
It may have more than one insulated wire inside the shield which makes it a little different to coaxial cable, which normally only has one conductor inside the shield.

It is usually not needed for power wiring, as in this case, but it can be used if you have it on hand.
  • #15
jhendren said:
vk6kro I am relatively new to circuits, could you please explain what you mean by the shield of the wire?

Why are you not taking the point about the Capacitor? It will never work with a Capacitor in series. Deal with the most important issue first then move on if it still doesn't work.
  • #16
Centaur,I thought by soldering the resistor and capacitor at the same point makes it in parallel?
  • #17
Sorry, the last picture has them in parallel. But are you sure you have the capacitor connected the right way round? I can't tell, from the picture, which is positive and which is negative but the capacitor mustn't be connected the wrong way round. What's it there for anyway?

But, without a more detailed 'schematic' diagram, it's hard to see what you are actually doing. It would be normal to supply a diagram such as you see in most other posts, making clear what is what. How are you testing whether there is any current coming from the motor generator? What is the purpose of the Capacitor and the power resistor? What voltage does the motor actually provide when the power mat is connected across it? Where did you get the info about the current that the motor can supply?

I wonder whether your motor can, in fact, provide enough power to warm up the mat. You could get a clue by observing how the pitch of the motor changes when the mat is connected. If any power is being delivered, the motor should slow down audibly.

Why not just connect the motor straight to the mat and see if it warms up?
  • #18
The motor does slow down audibly, I noticed that a few days ago, I also need to place the switch after the capacitor and resistor. I believe that the current will fluctuate, so having the capacitor drain and power the mat steadily is my goal. However, I am not quite sure how long(if it even can) the capacitor can support the mat. But the capacitor is 4700μF and the resistor is 8 ohms. I have tried series and parallel and it seems the paralle works best.


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  • #19
The diagram doesn't include where you have connected the heat mat (fairly important). Also, the two way switch would appear to apply a short circuit to the motor output if you put both contacts the other way. Have another look and try to improve the circuit diagram. We could be almost there!

Also, it's nice that the motor slows down. That implies some power is being generated and dissipated (somewhere). Does the resistor get hot?

That 100k resistor is much too high to achieve anything, btw.
  • #20
You should have something like this:

A 5000 uF capacitor is not really very big and it won't power anything for long.
At best, it will remove some minor variations in the output of the generator.

I think the power mat is a charging board that looks a bit like a skate board, so it won't take a huge current.

However, powering it like this is not the way to go. You could look for an old charger from a laptop. Many of these put out 18 volts DC.
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  • #21
I got it to work
  • #22
How warm does the heat mat get?
It should do better with no series resistor. But you really need a meter for this.
  • #23
i have a control technique drive type UNI 4401 , 45 KW
can i obtain the circuit diagram of the two internal boards ( UD45 ISS.1 , UDISS.2 ), and also the wiring diagram of this drive ?
  • #24
This looks like a new topic. Please start a new thread.

Related to Solving Circuit Board Problem: 18vdc Motor, 8 Ohm Resistor

1. How do I determine the voltage required for a circuit board with an 18vdc motor and an 8 Ohm resistor?

The voltage required for a circuit board is determined by the Ohm's law equation V = I x R, where V is voltage, I is current, and R is resistance. In this case, the voltage would be 18vdc, the resistance is 8 Ohms, and the current would be calculated by dividing 18vdc by 8 Ohms, which equals 2.25 amps.

2. What is the purpose of a resistor in a circuit board?

A resistor is used to regulate the flow of current in a circuit. It is used to limit the amount of current that can pass through a specific part of the circuit, protecting components from being overloaded and potentially damaged.

3. How do I troubleshoot a circuit board problem with an 18vdc motor and an 8 Ohm resistor?

The first step in troubleshooting a circuit board problem is to check for any loose connections or damaged components. Then, use a multimeter to test the voltage and current at different points in the circuit to identify any potential issues. If the problem persists, it may be necessary to replace the motor or resistor.

4. Can I use a different resistor with a different value in place of the 8 Ohm resistor?

It is not recommended to use a different resistor with a different value in place of the 8 Ohm resistor, as it could affect the overall performance of the circuit. Resistors have specific values that are chosen based on the needs of the circuit, so it is best to use the designated resistor for optimal function.

5. How do I ensure the circuit board with an 18vdc motor and an 8 Ohm resistor is properly grounded?

To ensure proper grounding, make sure that the circuit board is connected to a grounded power source and that all components are securely connected to the ground plane on the circuit board. You can also use a multimeter to test for continuity between the ground points on the circuit board.

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