# Help understanding a circuit driving a high power solinoid.

Hello,

I need help understanding the attached schematic. I am a noob and while I do understand the basics, I can’t figure out how this works. Hoping to get some help, more detailed the better.

What I do know:

I numbered the components because they are hard to read in the picture. Here they are

1. 1K resistor
2. 470 resistor
3. 2N4403 transistor
4. 1N4004 diode
5. 56 resistor
6. 2.7k resistor
7. tip102 transistor
8. 220 resistor
9. tip36c transistor
10. 1N4004 diode.

I am pretty certain that the VCC is 5VDC regulated. The line coming in from the far left is the control line that I also believe is high or low 5VDC. I will do some more testing when I get a chance, but lets assume that is the case.

The line coming in from the right is +50VDC.

This circuits sole purpose is to supply a path to ground for a high power coil/solenoid. The coil has a diode on it for protection against spike voltage.

I would like to understand this much better, and maybe it can help some other folks too. Lets just say there is very little here I understand and go for the most complete explanation possible. My ultimate goal, aside from learning and understanding this, is to mimic the functionality with the Arduino as the control. I may mimic it exactly to see it all work, but if there is a better way please share.

Rather complicated. Where does this circuit come from? Simpler designs are usually adequate.

I figured most of it out, I just wished I knew more about the resistor values and there placement.

The output for the control is closed in its normal state and there is no positive voltage, so the vcc going through the 1k resistor makes the base of the pnp 2N4403 HIGH, and in turn closes off the vcc to the base of the tip102. The tip36c is also a pnp and the 50VDC passing through the 220 resistor keeps the path to ground closed.

From what I can tell, the control goes to ground when it is switched making the base of the 2N4403 low which causes the vcc current to pass through the 2N4403 and turn the base of the tip102 HIGH. This opens the flow of the +50VDC through the 220 resistor and the tip102 to ground. That instant, the base on the tip36c goes low and dumps the full current to ground as it is direct path, instead of going through the 220 resistor to the ground created through the tip102.

Seems to me, you could do this much easier. But there has to be a reason they did it like this. I am sure a lot of this circuit is built in safety, but I don't know whats what.

What I am thinking is that I will just drive the tip102 with the arduino. I know what the diode after the 2N4403 is for, but I am not sure why there is a 56 ohm resistor in line with the base(limit the current to the tip102 to 90mA?), and I certainly don't understand why there is a 2.7k to ground in front of the tip102(pulldown? why 2.7k?). The rest makes sense to me.

Thanks for checking.

would this help with some ideas

http://ruggedcircuits.com/html/circuit_gallery.html

Want arduino to control a solenoid ?

This URL may be of help. http://wiki.bildr.org/index.php/Controlling_a_solenoid_(TIP120_Arduino)

But there has to be a reason they did it like this

There could be many reasons. Maybe it was a government contract, and the more components the higher the payments. Who knows? When was this designed? Who designed it? Why did the chicken cross the road? I do believe that whoever designed this, did not know, or follow the KISS principal.

that circuit looks like its designed to take 20 plus amps, does your relay need that much drive current ?

Maybe it was a government contract, and the more components the higher the payments.

That was exactly my first reaction to the circuit. Great minds think alike :)

Given that the government prefers 5 bureaucrats when 1 could do the job why not extend that thinking to transistors?

the 2k7 is to pull the base down as you say.

with out it, you can drive the base high, but there is no way to take the base low, due to the diode.

The value, is probably a compromise.

when the tip is 'on'', i.e. the base high, then the 2k7 is dropping most of the power , any lower and the dissipation of the resistor is going to go up. any higher, and the base leakage is probably going to be high enough to bring the base high,

4k7 would probably work, 10 k might not,

My advice is to forget that circuit. It's way over-complicated. The TIP36C will get hot and need a lot of heatsinking if the load current is high. An n-channel power mosfet + 2 resistors + flyback diode is a much better (and much simpler) solution.

I suspect some of it is there to protect the driving circuit should one of the big transistors fail and feed 30V back along the chain - the arrangement with the diode for instance. Or maybe there are large transients induced in the wiring from the switching leading to the need for protection (not too hard to imagine near a 20A solenoid).

Perhaps the circuit has to assume lots of ground-bounce, or ground-mismatch?

But its way more complex than is typically needed (a MOSFET, a flyback diode and two resistors).

my bet is its an old circuit,

from before MOSFET chips were common, cheap , large enough or reliable.

having said that, in some circles Darlingtons are considered more robust than MOSFETS,esspecialy in military products,