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Circuit to reverse polarity not working

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Zebo

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Attached is a circuit to switch the polarity to a 12V solenoid. The problem is that it only gives me 7V instead of 12V and overheating some components. Would anyone be able to shed some light on the problem? Or suggest a better solution other than using relays? I have a solenoid where I need to change the magnetic field depending on a condition of the main circuit which is not shown here. The current to the solenoid is normally off untill a voltage is put on the "Left" or "Right" in which case the solenoid is energized with the selected polarity. Your help will be so much appreciated.
 

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  • Polarity Switch.gif
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Hi,


It doesnt look like it was designed right in the first place. Perhaps the NPN types are swapped with the PNP types?
It looks like the two transistors on the bottom on each side should be on the top and the two on the top on the bottom, with associated resistors, or else the inputs can not be driven through resistors and must be driven to full ground (0v) or full +Vcc.
 
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Do you have a link to the solenoid you are using? Your H bridge is using 2N3904 and 2N3906 transistors so the current to the solenoid coil will be limited to the maximum current those transistors can handle which isn't much. The 2N3906 & 2N3904 are complimentary but the best they can handle is about 200 mA. That being with heat sinks on a good day. You want to remain well below that current limit. Additionally what is the current limit of your supply?

<EDIT> or what MrAl said. :) </EDIT>

Also, my bad as I looked again and the 3904 & 3906 aren't driving the solenoid.

Ron
 
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Hi Ron,


Those transistors are driving bigger transistors though right? Check out my previous post too see what you think.
 
Do you have a link to the solenoid you are using? Your H bridge is using 2N3904 and 2N3906 transistors so the current to the solenoid coil will be limited to the maximum current those transistors can handle which isn't much. The 2N3906 & 2N3904 are complimentary but the best they can handle is about 200 mA. That being with heat sinks on a good day. You want to remain well below that current limit. Additionally what is the current limit of your supply?

<EDIT> or what MrAl said. :) </EDIT>

Ron

Thanks Reloadron.
The circuit is connected to a 12v 1,3AH battery with no limitation to the circuit. The solenod is home made and draws 400mA, but I would like to provide for 1Amp in case I need to drive two solenoids at the same time in future. Do I simply use higher rated transistors and can you tell me which ones?
 
Hi,


It doesnt look like it was designed right in the first place. Perhaps the NPN types are swapped with the PNP types?
It looks like the two transistors on the bottom on each side should be on the top and the two on the top on the bottom, with associated resistors, or else the inputs can not be driven through resistors and must be driven to full ground (0v) or full +Vcc.

Okay, I have already fried one set, so I will swap NPN with PNP and see what happens. But, Reloadron suggested that the transistors can only drive 200mA anyway and you were asking if there are further transistors, so I will first need to get higher rated transistors for this test. Any idea which transistors will work? Can I simply add aother transistor for each side or must I change what I have?
 
Okay, I have already fried one set, so I will swap NPN with PNP and see what happens. But, Reloadron suggested that the transistors can only drive 200mA anyway and you were asking if there are further transistors, so I will first need to get higher rated transistors for this test. Any idea which transistors will work? Can I simply add aother transistor for each side or must I change what I have?

I had a My Bad in there I later confessed to. The transistors doing the work should handle an amp. Sorry about that. I looked and saw the 3904 and 3906 and looked not enough further. I do agree with Mr. Al in that something does not seem right about the circuit. Again, I was off base with the 200 mA. Sorry. :(

Ron
 
Hi Ron,


Those transistors are driving bigger transistors though right? Check out my previous post too see what you think.

Yeah, I screwed it up. I saw your post right after I posted and looked at the drawing and edited my post. I will be back after I scrape some egg off my face.

Ron
 
If your input signals are +12V and 0V then the solenoid gets 10.4V, not 7V.
If your input signals are only +8.4V and 0V then the solenoid gets 7V.
 
If your input signals are +12V and 0V then the solenoid gets 10.4V, not 7V.
If your input signals are only +8.4V and 0V then the solenoid gets 7V.

I have it connected to a 12V battery. The fact that the transisters are overheating means something is wrong as it is connected to a 400mA solenoid and I measure 7V, so the power is drained somewhere. I am at a loss as to what to try next.
 
You have all the output transistors turned on all the time, shorting the power supply and getting very hot.
You want one side to pull up while the other side pulls down like this:
 

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  • transistors H-bridge.PNG
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You have all the output transistors turned on all the time, shorting the power supply and getting very hot.
You want one side to pull up while the other side pulls down like this:

Thanks, I will try that. I have two photocells that will turn on the reverse and forward, so will there not be a dead short if stray light should switch on both legs of the bridge at the same time?
 
Thanks, I will try that. I have two photocells that will turn on the reverse and forward, so will there not be a dead short if stray light should switch on both legs of the bridge at the same time?

Photocells will not work as inputs unless pull-down resistors are added to quickly turn off the input transistors in the dark.
The inputs should be digital logic that abruptly goes from 0V to 5V and back to 0V.

The circuit must not have both inputs high at the same time.
 
Hello,

audioguru:
You can probably change that design to eliminate Q7, Q9, Q11, and Q13. That would reduce the number of transistors to 6 total and at the same time make the output voltages go even closer to the rails, possibly to within 0.2v of each rail.
 
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Photocells will not work as inputs unless pull-down resistors are added to quickly turn off the input transistors in the dark.
The inputs should be digital logic that abruptly goes from 0V to 5V and back to 0V.

The circuit must not have both inputs high at the same time.

I said "photo cell" for short not to go into long explanations. Actually, the pulse is from two infrared photo pairs (L-53F3BT & L-53P3C) which are triggerred by a swinging pendulum, one at a time, if another condition is met for one of the two photo transistors which only happens a few times per hour. The pulse from the photo transistor goes via a gate (CD4093) to a 555 which will latch for say 10 seconds IF a signal is received. My original H-Bridge was designed to prevent both legs going on at the same time, but with an obvious flaw. Perhaps your suggestion can be used with extra components to prevent both 555's sending a signal to the Bridge at the same time. Any ideas?
 
You have all the output transistors turned on all the time, shorting the power supply and getting very hot.
You want one side to pull up while the other side pulls down like this:

I have now also realized that perhaps the part of my circuit triggering the H-bridge should originally have been show. The 555 (pin 3) triggering the H-bridge can sink 200mA so perhaps the transistors are not all on at the same time, not so?
 
You could add a set-reset digital flip-flop circuit to ensure that both sides are not turned on together.

Your first circuit has the inputs of the transistors connected together so they are four diodes (the base-emitter diodes of the four input transistors on each side) in series across the 12V supply which causes an extremely high current and an awful lot of smoke.
 
audioguru:

This is similar to the circuit i was talking about. You'll have to find better values for the base resistors of course but we'll save four transistors this way.
 

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  • H2-TransBridge.GIF
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audioguru:

This is similar to the circuit i was talking about. You'll have to find better values for the base resistors of course but we'll save four transistors this way.

Thanks MrAl, I will certainly give that a try.
 
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