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Relay trouble with ATX power supply

dukeofmarshall

Full disclosure: I'm a beginner electronics enthusiast. Here is my setup:

 

(1) Arduino Nano

(1) Arduino 24v 3 amp Relay Shield

(1) MR5A001A1 12v 30 amp Relay from an Isuzu Rodeo

(1) ATX Power Supply Model ATX-1956D

(1) 12v 50w Halogen Light Bulb

 

So the end goal is to drive the halogen bulb with the Arduino. Be able to turn it on and off as needed. For my first setup, I connected the Arduino to the relay shield, ran a 12v line from the power supply unit to the common on the relay, and the NO from the relay to the halogen bulb. I did not take the amps of the bulb into account at first but soon realized my mistake when the relay fried. I replaced the Arduino relay and the PSU just to be sure and added the car relay. My current setup is Arduino Nano connected to the Arduino relay, one 12v line from the PSU is connected to the common on the Arduino relay, the NO on the Arduino relay is connected to the trigger(?) of the car relay, one 12v line from the PSU is connected to the common on the car relay, and the bulb is then connected to the NO of the car relay. The Arduino and the Arduino relay shield are both powered from a 5v line from the PSU.

 

The issue I'm having is when the Arduino relay switches on and turns on the car relay, the PSU just shuts down. Now, I can power the bulb directly with the PSU, no problem. But when connected up in this setup something's causing the PSU to shut off. Strangely enough, If I connect the bulb to the NC of the car relay, the bulb powers up with no problem. Then, as expected, when the Arduino relay switches on the car relay, the bulb goes off, but when the Arduino relay switches off and turns off the car relay, activating the NC of the car relay again, the PSU immediately shuts down like before.

 

Anyone have any ideas or suggestions about this? All the parts seem to be in order. There's not too much voltage going into any part and, now, all the parts should be able to handle the amperage with the PSU being more than enough to power the whole system. What am I missing here?

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  • Hi Todd,

    Another possibility is that you are getting enough of a voltage spike from the relay coil to trigger the ATX into protect mode. Make certain that you have a diode, reverse biased across the relay coil. Another possibility is the way the load of the bulb is being applied. A 56 watt halogen bulb could easily draw 10 amps for a few milliseconds until the rising temperature of the filament brings the resistance up to operational levels. Make sure that you are using the 12 Plus supply of the ATX as the negative 12 volts may only be rated for a couple amps.

    John

  • in reply to jw0752

    Just did two more tests. I powered up the PSU and then connected a 12v computer fan, then disconnected and reconnected several times. No problems. I repeated the same test with the bright LED pads and no problems there either. So your suggestion about the bulb drawing lots of amps on connection may be correct, but how do we address that or can we? Thanks.

  • in reply to dukeofmarshall

    Hi Todd,

    Sorry it took so long to get back to you. One thing that you might try is to use an In-Rush Thermistor in series with the bulb. I discussed these Thermistors a while back in a previous blog.

     

    https://www.element14.com/community/people/jw0752/blog/2016/03/15/identifying-the-triplets-in-a-power-supply-front-end

     

    The In-Rush Thermistor presents a current limiting resistance initially and then drops to a very low resistance as it warms up. These are used frequently in circuits and power supplies where there is an initial very high current demand.

     

    Here is one offered by Newark:

     

    http://www.newark.com/amphenol-advanced-sensors/cl-30/ntc-inrush-current-limiter-2-5/dp/81F3387

     

    The specifications on this Thermistor say that it begins at room temperature at 2.5 Ohms. This would limit the initial current to about 5 Amps. When it warms up its resistance drops to 0.06 Ohms. It is rated to handle 8 amps which is well above the 5A draw of the bulb.

     

    If you have access to scrap circuit boards you may find an inrush thermistor that you can salvage. As I mentioned in my blog these will look like a large disk capacitor and will be in series with the power feed to the circuit.

     

    You could also experiment with a capacitor across the ATX 12 volt as has mentioned. If you use a capacitor that is too large it will also shut down the ATX but if you get just the right value, not to large and not too small it may solve the problem.

     

    John

  • in reply to jw0752

    Mr. Wiltrout,

    Thank you for the information and reading/educational material. I was able to find an SCK 037 NTC Power Thermistor from a non-working ATX supply I had lying around. That seems to do the trick. My initial test was just powering up the PSU and then connecting the live wire with the thermistor in series to the spare bulb. I was able to do this multiple times with no issues. So then I went ahead and attached the thermistor in series to the common on my car relay. Powered the whole system up and currently don't see any problems as far as the PSU shutting down. The Arduino can turn the bulb on and off at will.

     

    Thanks again for helping out. I do have two more questions if you don't mind. 1) The thermistor seems to get REALLY hot. Is that normal for a thermistor or should I try to do something about it? 2) When the diode suggestion didn't solve the PSU issue, I removed the diode, but in some of my research it was suggested that always having a diode across a coil like you suggested is good practice. Would you agree? Should I reattach the diode to the coil?

     

    I really appreciate all the time, information, and education you've given me. Take care.

  • in reply to dukeofmarshall

    Hi Todd,

    It is normal for the Thermistor to get hot. As the temperature increases the resistance goes down. As the resistance goes down the amount of heat produced also goes down. For a given current flow the device should heat up to an equilibrium position and then stabilize. I do not know the specifications on the device that you salvaged but if it can handle the 5 amps of your bulb it should be OK. Make certain that you have given it adequate space away from other components so the heat doesn't damage anything. You can also check around for other Thermistors to see if a different one will run cooler. The diode across the coil is usually a good idea. The only difference it might make in the circuit that you have constructed is that the contacts in the relay on your relay shield will last a little longer. I am pleased that you got things working. As frustrating as it is to have problems I always find that it provides a good learning experience.

    John

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  • in reply to dukeofmarshall

    Hi Todd,

    It is normal for the Thermistor to get hot. As the temperature increases the resistance goes down. As the resistance goes down the amount of heat produced also goes down. For a given current flow the device should heat up to an equilibrium position and then stabilize. I do not know the specifications on the device that you salvaged but if it can handle the 5 amps of your bulb it should be OK. Make certain that you have given it adequate space away from other components so the heat doesn't damage anything. You can also check around for other Thermistors to see if a different one will run cooler. The diode across the coil is usually a good idea. The only difference it might make in the circuit that you have constructed is that the contacts in the relay on your relay shield will last a little longer. I am pleased that you got things working. As frustrating as it is to have problems I always find that it provides a good learning experience.

    John

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  • in reply to jw0752

    Mr. Wiltrout,

    I completely agree. It was, and still is, a learning experience for a novice like myself. And as irritating as it was at the time, it's rewarding now. I kept playing around with different configurations and when that didn't work and internet searches didn't turn up anything, I decided it was time to seek professional help. Thanks again for your help.