Hello All,

I have a really simple question for all of you; not so simple for me or I would not be here!

I think I need a diode but don‘t know what kind. I am hoping you can help. It will be quite obvious this is not my primary job.

Here is the situation:

I have a prototype device that I am trialing for a company. The device is very clever and use is a 24 volt DC motor that is controlled by a simple SPST switch that has an LED dependent light that turns on when the switch is closed. Simple enough. However I noticed when the SPST switch is opened and the motors is spinning down the LED continues to faintly glow! What appears to be happening is that the motor as it is spinning down is acting as a generator. I confirmed this by measuring a potential of 4VDC as it spinss down. I would like to eliminate this potential since other sensitive electronics attached to this circuit might not like this. It seems to me that a diode is what I need to prevent “backflow” from occurring as the motor spins down.

Can anyone recommend a diode? Again, the voltage is 24VDC, although I did not measure the current in the “back flow“ direction, in the forward direction it is never greater than two amps and so is almost certainly much less than this.

I apologize for the simplistic language but would greatly appreciate anyome’s help!!

Thanks,
Mike

 

Welcome to AAC!

I apologize for the simplistic language but would greatly appreciate anyome’s help!!

Circuit, block, and/or wiring diagrams are always helpful.

 

A simple diagram of how you have things connected would be extremely useful. In particular, how is it that the switch controlling the motor is leaving these sensitive electronics attached to it?

How is the LED connected in the circuit? How is the current in it being limited?

 

I don’t have a diagram but I will send a few photos that I think will help. This is a system for a boat which as you may know is usually a 12VDC system but the device I am trialing uses 24VDC so there is a 12-to-24VDC converter involved but I do not believe it comes into play in my question. I will also attach a diagram of the simple SPST switch that has a single dependent LED which is illuminated when the SPST switch is open and the motor is spinning down.

In the picture below the SPST switch is in the position where the white arrow is pointing that says “MS3 Motor”. The card with the circle on it represents the motor.


Here is a diagram of the SPST switch whose LED glows when the switch is open:



The contacts on the motor measure 4VDC(no AC) as the motor spins down. Potential decreases to zero as the motor approaches 0 RPM.

Appreciate your help. Please let me know if you need more information.

 

You know, I just thought of something. I am using a DPDT ON-OFF-ON switch that I wired as a SPST switch while I wait 8 weeks for my SPST switches to arrive from Mouser. I imagine the current generated by the spinning down motor is making it’s way to ground through the switch and allowing the current to flow through the switch and illuminating the LED.

Does this sound plausible?

Here is the switch I am using in place of the SPST ON-OFF switch:

 

Here is how it is wired:


I think as the motor is winding down it is acting as a generator. The current is making it’s way to ground through one of the LEDs and causing it to illuminate.

I think this will not be a problem when I use the SPST switch that does not have an alternate pathway to ground so a diode is likely not needed.

I would appreciate some validation on this from some who actually knows what they are talking about since obviously I do not

Thank you for your help!
Mike

 

Well, I just went back and looked at the SPST switch and since the current from the motor can still make it to ground via the LED even when the switch is open I believe the LED will stI’ll illuminate as the motor is spinning down.

So, I am back to my original question:

Can someone suggest a diode that would prevent this ”back flow” current from occurring? Remember it is at max 4 volts and likely much less than an amp.

 

I am going to ask the question in a slightly different way since I think I understand the situation a little bit better now. This drawing illustrates what I believe is the the situation:


This picture shows the SPST switch with a Dependent LED. The LED is to illuminate only when the switch is closed. However when the switch is open and the motor is winding down it is acting as a generator and back current is entering at point 3, passing through the diode and exiting at point 7 to ground.

I would like to eliminate this reverse flow by placing a diode where I have indicated by the white arrow.

The measured voltage at the motor when the switch is open and the motor is spinning down is 4VDC initially and goes to zero when the motor reaches 0 RPM(about five minutes). I do not know the current but I doubt it is much since even at start-up of the motor it is less than 2A and is slightly less than 1A when the motor reaches its maximum speed.

So, could someone make a recommendation for a diode in this situation?

Thank you!
Mike

 

You're going to have to provide the current rating of the heater and motor before you can determine a suitable diode. (if the switch is also in series with the heater)

Also be aware that you will then have a voltage drop across the diode.

It may be a better solution to snub the back EMF of the motor instead.

 

Thank you for your response!

-The switch is NOT in series with the heater.

-The current of the motor is <2.5 A at start-up, 1.0 A at speed At 24 VDC.

-Voltage drop is OK; motor is designed to work between 16-18 VDC.

Can you recommend a part no. and where to purchase?

Thanks again,
Mike

 

I apologize for the simplistic language but would greatly appreciate anyome’s help!!

Does the LED have a discrete series resistor or is it in an assembly? Another solution is to install a resistor across the LED.
Never mind, misread post.

 

Does the LED have a discrete series resistor or is it in an assembly? Another solution is to install a resistor across the LED.

I believe the OP also wants to protect the converter.

I'm going to leave the diode recommendation to someone else.

 

I am using a DPDT ON-OFF-ON switch that I wired as a SPST

Use a standard DPDT switch wired as a brake or short across the motor when OFF.

 

Thanks All, I appreciate your help. I just ordered some 50V, 6A diodes. I figure a diode rated at twice the voltage and twice the current that I expect would be a pretty safe way to go.

Thanks again!

 

Sghioto, your solution is pretty cool, I will keep it in mind.

Thank you!

 

Sghioto, your solution is pretty cool, I will keep it in mind.
Thank you!

Or use the DPDT center off switch on hand wired to isolate the motor from the other circuits.

 

sghioto, thanks again, but I am now starting to get a headache

I really appreciate your input and have saved both diagrams you sent!

 

Or use the DPDT center off switch on hand wired to isolate the motor from the other circuits.
View attachment 311378

Sghioto, 50V, 6A diode worked but .75V voltage drop. This is acceptable.

Thanks again for your help,
Mike