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Topic: Relay control to start and run my car (Read 6072 times) previous topic - next topic


I am using a arduino duemilanove to read a rfid tag and control 5 realys to start and keep the car running till i push the kill button.
every thing is working properly but the relays. i am running off the 12v constant behind the stereo in the car. it will run all five relays for the 1000 ms. after the starter relay turns off and the other four to stay on, after about 5 min the transtors start burning up.

Can anyone help??


May 07, 2010, 07:33 pm Last Edit: May 07, 2010, 07:36 pm by saabie22 Reason: 1
Hi Milo,

What is the part number of the transistor that you're using?

What is the DC resistance of the relay coils?

Your schematic shows direct connections from the Arduino to the base of the transistors.  Do you have resistors in series with the base?


May 07, 2010, 09:02 pm Last Edit: May 07, 2010, 09:11 pm by milo1112 Reason: 1
the part number for the transistors are 2N2222.

i have 4K7 resistors for output pins connected to base of the transistor.

and need new relays.

but the ones i had were nominal voltage = 12  DC resistance 10%(ohms) = 144   nominal current(mA) = 83
just forgot the resistors in the digram


May 07, 2010, 09:38 pm Last Edit: May 07, 2010, 09:43 pm by saabie22 Reason: 1

If you assume about a 0.7V drop from base to emitter then your base current should be (5-0.7)/4K7 = 0.9 mA.  If the gain of the transistor is 100 then you should be getting 90mA through the coil which is enough.  In my opinion you don't have a lot of margin to account for variation in hfe and coil resistance.

I would check the voltage from C to E when the transistor is on, just to be sure it's really saturated and acting like a switch.  If I saw something more than say 0.5 volts I'd probably lower the base resistance and try to drive it harder.  Otherwise the transistor is behaving resistively, heating up, and possibly leading to failure.

For reference, I use 2N2222A's as capacitor discharge switches in analog timers and use 1K base resistance driven by 5V with absolutely no problem.  So in my case I have about 4mA base current.

good luck!



thanks ill try that and let you know my results.


The 1N4001 suppression diodes have a peak inverse voltage of only 50 volts, which is a little low for your application.  You might well expect 100+ volts being generated by a 12 volt relay coil when it releases.   If you don't believe me try putting your fingers across the coil and then de-energising it !

It may be that the diodes have broken down and are passing current to the transistor when the relay is energised.  In other words the transistor is having to pass a lot more current than just the coil current.

I'd suggest you go for higher rated diodes, such as 1N4003 or 1N4004, they don't cost any more and certainly won't break down at the voltages expected

One thing you should do is check the transistor current when the relays are energised to determine if they are passing more than expected.



May 08, 2010, 01:29 pm Last Edit: May 08, 2010, 01:31 pm by milo1112 Reason: 1
i tested the diodes as i took it apart to see what had gone wrong.
the diodes were still operational, but when i tested the transistors they were burned up. only one that still worked was the transistor that is on for 1000 ms to start the car, then off.
i believe the transistors are what failed.


Did you measure the resistance of the relay coil to see what it actually is? You could also measure the current being pulled while the relay is energized. Radio Shack has some power transistors that have higher current capabilitys, so you may want to give one of them a try.

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The comment about using diodes with a higher PIV rating won't hurt but when the transistor turns off, the high side of the coil is the side connected to the collector of the transistor and the anode of the diode.  

The diode will be in forward conduction back to the battery so PIV is not the important characteristic.  

At the instant of turn-off the diode will allow the voltage at the collector to rise to battery voltage + the diode drop due to the coil back EMF and it will conduct at 83mA, falling to zero once you dissipate the coil energy.  The max continuous forward current rating is 1A for any of the 1n4001-1n4007 so you are safe.

Jackrae's suggestion to check the collector current is a great idea though.  If you have the diodes accidentally reversed then you would have essentially a dead short through the transistor when they were on.  That would smoke them fast!


May 10, 2010, 03:20 pm Last Edit: May 10, 2010, 03:22 pm by milo1112 Reason: 1
they didn't smoke fast, it took like a hour. i had it working and installed in the car i was test driving it when they burned up.
here are the new relays i am using http://relays.tycoelectronics.com/datasheets/T9A_DS.pdf
using and 1N4003 diodes, 1k resistors, 2N2222A transistors.
can't do any tests till relays get here. :(

on the diode the white stripe is the direction of flow right?


on the diode the white stripe is the direction of flow right?

The white stripe is normally on the cathode. This should go to the + supply end of the coil, so that the diode is not normally conducting.

If the relays cooked that is because they had too much current down them. This is probably caused by running them off too high a voltage.



In your circuit, you want the white stripe on the diodes to connect to the battery side of your circuit.

The white stripe shows the cathode end of the diode.  When the diode is conducting (forward) this will be the low voltage connection.

I ran a little simulation of your circuit and if you have the diode accidentally backwards, your transistor has to dissipate about 1.5W.  The absolute max rating, if you have the plastic package, for the 2N2222 is .625W.  If this is the problem you should notice the transistors getting really hot (like over 100C!)


Giving this problem a little more thought , I think the earlier replies have nailed down your problem.

Your original circuit shows the transistor bases connected directly to the Digout pins of the Arduino.  In other words there was NO control over the level of base current.  Yes your transistors would be fully turned on but the base-emitter current was being limited by the capability of the Arduino output circuit.

This would not only burn up the base-emitter junction but also potentially damage the Digout circuits since you were applying an effective short circuit to them.

Fitting resistors between the Digouts and the base connections will resolve the base current problems.  Assuming a transistor gain of say 50 (it could be 100+) and a coil demand of 80ma then you need around 1.5ma of base current so a resistor of 3k3 (3300 ohms) should be satisfactory.  If there isn't enough drive (transistor not turned fully on) you could reduce the resistor to 1k without damaging either transistor or Arduino



I ran a little simulation of your circuit and if you have the diode accidentally backwards,

No they are the right way round, they are needed to conduct away the back EMF from the coils. The cathode should be on the +ve supply and they are on that diagram (unless it has been changed) it wasn't showing up earlier today when I made my first reply.

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