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Darlinton Flyback Diode

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Wp100

Well-Known Member
Hi,

Am using a Darlinton transistor TIP122 or similar to drive a 2A 24Vdc solenoid.

As the Darlingtons have the anti parallel diode across output transistor do I need to fit a diode across the solinoid coil as well ?

Have searched around and cannot find a clear answer.

Similarly if I do have to use a separate diode on the coil what should is continuous current rating be, some say the same as the load, other say a much smaller values as it only takes the full current for such a short time its covered by the diodes peak current handling.

Thanks
 
Hi wp,
I looked at the datasheet for the TIP122, and I have to say I can understand your confusion.
The internal diode is to prevent reverse polarity voltages from destroying the darlington driver as far as I can tell.
You should use your diode across the solenoid coil. The advice about peak current rating and power dissipation is correct. The flyback diode is only carrying current at switch off.
 
i agree. it's always good practice to add a diode. not all darlingtons have an internal diode, and should the transistor need to be replaced in the future, the replacement might not have one internally.

generally a 1N4004 will work for most relays.
 
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That internal diode in the darlinton is no help to suppress the large positive voltage spike that will appear at the transistor collector when the solenoid is switched off. The diode would need to go from the collector to the positive supply voltage to work as a suppressor. Thus you need a diode across the solenoid as the others have suggested (cathode to the plus side).
 
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Hi,

I have to agree also that a diode across the solenoid is necessary. That's because when the transistor turns off the voltage across the solenoid reverses, so the diode in the darlington package can not shunt the current because it is connected the wrong way (and it needs to be that way for other reasons).

And yes, a diode of the same rating as the max current is usually used. That's because at the instant of turn off the current through the diode is the same as through the solenoid. Thus with 2 amps through the solenoid at turn off that puts 2 amps through the diode at turn off at least until the energy in the coil dissipates. How fast the energy dissipates depends on the inductance of the coil and the drop across the diode. With a large inductance it takes longer, and with a larger diode voltage drop it actually takes less time. Using two diodes in series means the energy dissipates in about half the time of one diode. Usually though you dont need two diodes unless you need to dissipate this energy quickly for faster solenoid de-energizing.

Most diodes have an average current rating and a peak rating. You really need the peak rating here not the average rating, but it would be good to check the data sheet. Some data sheets show data for one half cycle or something like that where it doesnt repeat for something like 100 cycles. You can probably use that as a guide. Most likely a 1N4007 diode or something like that would work, with a 1N54xx type definitely covering all the bases as long as it doesnt have to switch on and off faster than about 100Hz. These diodes are pretty cheap even in one piece quantities. The 1N40xx type are fairly rugged too, but the 1N54xx type are even more rugged physically and the leads are heavier so there is less possibility of it breaking off. This is often a concern also for relays and solenoids where there is more than average vibration or other physical shock.
 
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It is always safe practice to add a separate reverse back EMF Diode to protect any Transistor/Mosfet driving a coil. Be it a relay or a solenoid. That's the way I was taught.

No matter how slowly the coil is actuated. Simply good practice.

What say you guys :D

Cheers,
tvtech
 
Even IF the internal diode could protect the transistor, the transient voltage and current that appear at the internal diode also appear along the conductors between the diode and the relay. That can be an emi/rfi problem if those conductors are of significant length.
 
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Forgot to add...the protection Diode is directly across the Relay/Solenoid coils obviously..Duh..:eek:

Forgetful me

tvtech
 
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Forgot to add...the protection Diode is directly across the Relay/Solenoid coils obviously..Duh..:eek:
There is an exception to that. If you have a long line between the solenoid and the driver, then for high currents, the inductance of that line can cause a significant voltage spike at the transistor. In that case it may be better to connect the diode from the transistor collector to the power supply. Noise-wise it may actually be better with the diode at the transistor since it minimizes the line spike that could otherwise generate some EMI.
 
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