To set up my question: I am needing to power a 6v 6666.67 mA solenoid using two 1200mAh LiPos in series.
I know that transistors are capable of amplifying a current signal. Specifically the common collector amplifier configuration. While this seems to be a basic concept, I am having some difficulties understanding how to apply this to my project. Specifically how to determine which transistor to use.
To further complicate the issue, I need to control the solenoid using the solenoid driver example. I have successfully used this driver circuit with smaller current solenoids, but am having trouble visualizing how to combine it with a second transistor for signal amplification.
Looking at examples of other circuits online [such as this one] I have attempted to first simulate the amplifier circuit, and then to combine it with the solenoid driver circuit.
Can I please ask for some wisdom from the community on whether or not I am on the right track?
A darlington transistor has two transistors inside, but it has a large voltage drop.
A mosfet has to be a "logic level" mosfet, or it won't turn on with only 5V.
For example this one : N-Channel MOSFET 60V 30A - COM-10213 - SparkFun Electronics
Is that a 5V Arduino board ?
The Arduino output pin can be connected to the gate of the mosfet, but a protection resistor and a resistor to keep the mosfet off is better. For example 10k from gate to GND and 220 ohm from Arduino output to gate. http://www.pighixxx.com/test/portfolio-items/connect-a-dc-motor/?portfolioID=610
Is that a 7A motor ? Why do you have a resistor of 220 ohm to the motor/solenoid ?
We like schematics, not those pictures, sorry. With such a picture, we have to build the schematic in our head to know what is going on.
You don't use emitter-follower for switching. "Common collector" isn't generally used to name this configuration,
its used for buffering analog signals, not power electronics.
You use the standard common-emitter to switch as you can saturate the transistor and reduce voltage losses
to well below a diode voltage drop.
But these days noone uses BJTs as MOSFETs win on nearly every parameter.
As for the solenoid, two small LiPo's in series are 7.4V or maybe a little less on high load, so that's
fine for a 6V solenoid if your duty-cycle is low.
You absolutely must have a free-wheel diode or other snubbing circuit across the solenoid or it will
destroy everything on the first turn-off. A diode rated at 7A peak would be enough. A 7A solenoid
stores enough magnetic energy to push 1000's of volts back without suitable precautions. Google
free-wheel diode, inductive spike, etc.
For 7A a good low-resistance logic-level MOSFET is the correct choice, something like 0.005 ohms
on-resistance would be ideal.
BTW its a common confusion about amplifying and switching - amplifying is a linear,
analog thing using the active area of the transistor. Current gain parameter (beta or
hfe) is appropriate.
Switching uses the off-state (cutoff) and the saturated state only (passing as quickly as possible
through the active area to reduce losses). The current gain parameter doesn't apply to
either region.
Koepel, aarg, septillion, MarkT. Thank you for adding your thoughts. To answer a few specifics:
Arduino is indeed the 5v model.
Solenoid current should be accurate: It's rated at 6v with 0.9 internal resistance.
The resistors I show in the breadboard are placeholders. Which indicates a mistake on my part for not mentioning the fact.
I will put together a schematic to replace the breadboard layouts, make some adjustments for what you've taught me here, and then ask that you kindly take another look.
My apologies for not producing a schematic in the first place. As you can probably gather, I am just scratching the surface here.
For schematics, Eagle is often used. It takes time to get to know it, but it is worth the effort.
You can write the schematic on a piece of paper and make a photo of it.
A photo can be attached when you use the "REPLY" button.
You presumably mean a 6volt 40watt solenoid
It is plain nonsense to specify 6 volts with only 1 significant figure and zero decimal points against an amperage of 1 significant figure and 5 decimal points
Thanks to the advice given I have made some adjustments to my thinking; and, more importantly have learned a few things.
For this design:
MOSFET instead of BJT
Replace the small signal diode with a 10A diode
Here is a schematic of the new solenoid switching driver. I'm not sure how to proceed regarding the amplifying the current signal though. As it stands, my 1200mAh power source does not satisfy this solenoid. Should I be thinking about a second MOSFET or a will an NPN suffice?
BSS138 RDS on 6 ohms , Id continuous 0.22 A
totally unsuitable to drive your solenoid , its more suited to light a led
you need Id > solenoid current and RDS more like 0.06 ohm
fdd5670 or something like it
BlackLab:
As it stands, my 1200mAh power source does not satisfy this solenoid. Should I be thinking about a second MOSFET or a will an NPN suffice?
If your power supply isn't up to driving the load, no amount of extra semiconductors are going to make any difference. An amplifier does not increase the amount of current available, it amplifies the small Arduino signal so the load is controlled. The power to drive the load is wholly derived from the supply.
Although a 1.2Ah LiPo pack would normally be happy driving 7A load. Replace the BSS138 with a
suitable logic-level power MOSFET and it looks very plausible.
Grumpy_Mike:
If your power supply isn't up to driving the load, no amount of extra semiconductors are going to make any difference. An amplifier does not increase the amount of current available, it amplifies the small Arduino signal so the load is controlled. The power to drive the load is wholly derived from the supply.
That makes sense, thank you. I got caught up in trying to solve my power problems, and have misunderstood the concept of amplification. I was under the impression that it applied to current, rather than signal. I've gone back and re-read the article that got me started on this (here) and see that they are also referring to signal.
MarkT:
Although a 1.2Ah LiPo pack would normally be happy driving 7A load. Replace the BSS138 with a
suitable logic-level power MOSFET and it looks very plausible.
Thank you! Glad I didn't buy any MOSFETs before getting feedback from here. I greatly appreciate the advice I have received from these forums.
All-in-all I believe that I need to source a lower power solenoid. This is a 10% duty solenoid, and the 25% duty version of this model requires only 6v 12w. That should be easier for me to provide with a small LiPo.