using TIP42 PNP paralleled

[TheEquineFencer]

hi Floyd,
Looking thru your drawing, one or two queries.

What current are you expecting thru opto TX diode when LED11 is lit?.

What current are you expecting thru the opto RX transistor when LED11 is lit?
Check the opto current transfer function.

You do realise that the TIP/Solenoid will be OFF when only LED11 [last LED] is lit.

If you plan to switch 8 solenoids, ie about 10Amp, have you checked that the two switches in series
with solenoids can handle 10Amps?. [ 'full-opon' and 'on-rise' contacts]

I believe that pin #3 of the LM3914 is going to the wrong end of R1 [10K] ,also whats the purpose of R1?
The LED current sinks of the LM3914 are constant current, fixed by R2 [680R]

Where is the 0V/Gnd connection for pin #8/#2 of the LM3914?

Who drew the circuit?

Eric
EDITED: viewed the pcb layout you posted on another thread, from which I was able to clear some of the questions in my post.
re-font as italic

I]What current are you expecting thru opto TX diode when LED11 is lit?.
Ans.1
Question #1 On the drawing it says LED#11, it's really #10, I have not a clue as far as Tx, not even what you mean, I'm really new at this and just picked an OPTO I thought would work. The only load going through the LED side is 10-20ma to turn the LEDs on as far as I know.

What current are you expecting thru the opto RX transistor when LED11 is lit?
Ans.2
Same as #1

Check the opto current transfer function
Ans.3
I have not a clue what you mean by that

You do realise that the TIP/Solenoid will be OFF when only LED11 [last LED] is lit.

Ans.#4
Yes, that is my purpose. I plan to have an OPTO attached to EACH LED and then have a dip switch so I can select which level of O2 I want to start cutting the solenoids off at.

If you plan to switch 8 solenoids, ie about 10Amp, have you checked that the two switches in series
with solenoids can handle 10Amps?. [ 'full-opon' and 'on-rise' contacts]

Ans#5
Now that you mention it, I really did not even think of the load on the supply going to the injector circuit through the two NO contacts on the limit switch and the pressure switch, thanks, I'll use a relay for that load.

I]Where is the 0V/Gnd connection for pin #8/#2 of the LM3914? [/I
Ans#6
I'm not really sure, the directions I was given for this ciruit had me make the connections as shown, I might have misplaced something on the drawing when I tried to look at the actual board and put it into the drawing, but the board does work as designed. O2 level goes up/down, the LEDs follow.

Who drew the circuit?
Ans #7
The drawing on the post I tried my best to do with a program disk I was given for a class they do at school. The actual circuit is from a
O2 Monitor/Voltage monitor design that can be purchased on the Internet.

Now I have another question, in the drawing you posted, what is the purpose of the 10000uF,16V Zener and the 10K R connected on the +12V/0V Grd side?
When I run the S1 with the Dc blocking/clamping diode Will I need to do the same to each of the njectors I plan to run off each FDS4935As. I'll go back and reread your post again, I was up at 3am EST to go on a fishing trip and just got home. I now have to replace the turbocharger on my GF's truck to top it all off, the impeller is coming apart.

 

[ericgibbs]

hi,

>> Now I have another question, in the drawing you posted, what is the purpose of the 10000uF,16V Zener and the 10K R connected on the +12V/0V Grd side?

Its a 10hm: resistor not 10K.
The 1N4745A is a 16V zener diode, if the incoming supply voltage exceeds 16Vdc the zener will clamp it to 16V, the 10hm: resistor is to limit the current thru the zener when it conducts, when an over volts or spike occurs.

>> Check the opto current transfer function
Ans.3
I have not a clue what you mean by that

The opto 4N25 has a current transfer ratio of 20%min to 50% typical.
This means for a typical 4N25 if 20mA flows thru the TX led, only 50% will flow in the RX transistor ie: 10mA.
BUT it could be as low a 20mA * 0.2 = 4mA!!

Did you look over my drawing?

Eric

Look at this link for a 4N25 simple application.
**broken link removed**

 

[TheEquineFencer]

https://www.electro-tech-online.com/custompdfs/2007/08/4N25262728.pdf is the spec sheet for the Digi-Key Part Number 160-1300-5-ND I used. I have no clue when you start talking about the math end of the stuff. I get the jest of the idea that the cuurent level might be wrong. Yes I did look at your drawing but not with a clear mind, Linda just woke me up to go borrow a truck for her and I've had 6 -9 hours total sleep in the past three days. I'm going to go get her a truck and go back to bed. I'll check back later, Floyd.

 

[ericgibbs]

https://www.electro-tech-online.com/custompdfs/2007/08/4N25262728-1.pdf is the spec sheet for the Digi-Key Part Number 160-1300-5-ND I used. I have no clue when you start talking about the math end of the stuff. I get the jest of the idea that the cuurent level might be wrong. Yes I did look at your drawing but not with a clear mind, Linda just woke me up to go borrow a truck for her and I've had 6 -9 hours total sleep in the past three days. I'm going to go get her a truck and go back to bed. I'll check back later, Floyd.

Hi Floyd,
The transistor current is not 'wrong', I am just pointing out that it could be as low as 4milliAmps when the opto LED current is 20milliAmps. the opto is only 20% efficient in current transfer.
So when you calculate the opto collector resistor value, use 4mA as the expected current.

Look at this GIF, from your weblink.

Does this make it any clearer

Eric

 

[audioguru]

The 680 ohm resistor to ground at pin 7 of the LM3914 sets the LED currents to only 9mA.
Then the output current of the opto's transistor could be as low as only 1.8mA.
There is no way the opto can turn off a TIP42 transistor that has a 1.6k base resistor to ground.

 

[ericgibbs]

The 680 ohm resistor to ground at pin 7 of the LM3914 sets the LED currents to only 9mA.
Then the output current of the opto's transistor could be as low as only 1.8mA.
There is no way the opto can turn off a TIP42 transistor that has a 1.6k base resistor to ground.

hi agu,
I believe the OP has now decided to go the P MOSFET route.???

I have indicated to him that the opto LED current should be in the order of 20mA, if is to be sure of say 4mA opto collector current. Using a 10K0 collector resistor should enable it to drive a P MOSFET.

Eric

 

[audioguru]

I was wrong because I have two broken calculators.

The third calculator shows the LED currents are 18mA when the resistor from pin 7 to ground is 680 ohms and the currents are 38mA (way too high) if the resistor is 330 ohms.

When the supply voltage is 13.8V, the LED currents are 18mA and they are all on, then the LM3914 dissipates about 1.8W and it will smoke and melt.

I can't remember if the 10 ohms resistor is in series with the LEDs' power line. Its value could be increased to 33 ohms then the resistor will share half of the heat with the LM3914 when all the LEDs are on.

 

[ericgibbs]

I was wrong because I have two broken calculators.

The third calculator shows the LED currents are 18mA when the resistor from pin 7 to ground is 680 ohms and the currents are 38mA (way too high) if the resistor is 330 ohms.

When the supply voltage is 13.8V, the LED currents are 18mA and they are all on, then the LM3914 dissipates about 1.8W and it will smoke and melt.

I can't remember if the 10 ohms resistor is in series with the LEDs' power line. Its value could be increased to 33 ohms then the resistor will share half of the heat with the LM3914 when all the LEDs are on.

hi,
As you can see I did edit my 330R back to 680R, like you I did my sums again!.
LED current approx = (1.25/680) * 10 =18mA

 

[TheEquineFencer]

hi,
As you can see I did edit my 330R back to 680R, like you I did my sums again!.
LED current approx = (1.25/680) * 10 =18mA

I just ran a test as theunit is presently configured. The Supply voltage from the 12V 2A charger I'm using was 14.69VDC
I connected the Fluke 87 set for Milli-amps record, between the LED and the Opto in series and energized the board.

Results: Ampreage spiked to 11.99ma and drop to a low of .5 after I left it on a while. Input voltage droped to 10.60VDC. I cycled the 0-1V siganal to drive the LED#10 (not LED#11 as in the misprint on the drawing) OPTO on/off several times with the same results. I also checked the LED current when I cut if off and got a sharp rise in current to around 11.33ma. The injector did energise and cut on/off. I hope this helps with what we need to know as far as what the current draw is on the LED side of the OPTO. I'll get a 12VDC battery to connect and retest later today, I do not think using the 12V 2A is working too well. I hope this does not smoke something on the board side of the O2 panel.

 

[audioguru]

I think your circuit design has many parts on the edge of working or not working.
I wouldn't use it to protect the expensive pistons of my car from blowing up.

 

[ericgibbs]

hi Floyd,
Noted the test result on the opto driver.
As the PMOSFET and Solenoid are switching when you expect them to, it sounds as though you have enough opto drive output.

While you are getting a 12V battery [car type??] I would recommend that you get a vehicle type fuse holder and say a 15A or 20A fuse, connect the fuse in series with wire going to the positive terminal of the 12V battery.

I'm sorry if I appear to be telling you how to suck eggs, BUT lead acid batteries will not tolerate shorts, be careful.

Lets know how it goes,
[ I'm going to put my fingers in my ears, close my eyes and hum very loudly]

Eric

 

[TheEquineFencer]

I realize at the present time it's "on the edge", my plan is when I get it working to try it w/o the turbocharger and such, natrually asparated until I'm comfortable it works.
As far as telling me how to suck eggs, there's nothing wrong with telling me or anyone else what you think if it's ment to be productive or related to safety. It shows me you care! Thanks! I'm working on a new drawing with the info provided to see if it looks like were on the same page, I'll post it just as soon as I finish it. Thanks.

 

[audioguru]

Your circuit will be a lot more reliable if you replace the 4N25 opto-coupler that has a minimum current transfer ratio of only 0.2 with a 4N35 that has a minimum current transfer ratio of 1.0.

Then replace the TIP42 transistors with TIP125 darlington transistors or P-channel Mosfets.

 

[TheEquineFencer]

I agree with you guys on using 4N35s over the 4N25s, I've got 47 more of them right now, later I will replace them because of the reliability issue.
Alright, here's what I've come up with using Mosfets, check it out and see if it looks about right. Is there supposed to be a blocking diode across the Mosfet source and drain as it looks in the drawing posted by ericgibbs?

I'll post the other one with the 8 Mosfets in a little while.

 

[ericgibbs]

I agree with you guys on using 4N35s over the 4N25s, I've got 47 more of them right now, later I will replace them because of the reliability issue.
Alright, here's what I've come up with using Mosfets, check it out and see if it looks about right. Is there supposed to be a blocking diode across the Mosfet source and drain as it looks in the drawing posted by ericgibbs?

I'll post the other one with the 8 Mosfets in a little while.

hi,
The diode across the Source/Drain is part of the internal structure of that MOSFET.
Looking forward to the final drawing.
Signing off for today.
Eric

 

[boxer4]

Since MOSFETs are voltage controlled and not current controlled, the 4N25s can be just biased for voltage swing, and a really high pulldown resistor can be used. You'll still get the voltage swing needed to turn on/shut off the MOSFET (only issue being turn on/turn off time to charge/discharge the gate cap, but that's usually in the few pF range and virtually negligible (calculate the RC constant with the pulldown and the gate cap to doublecheck, I doubt it would be anywhere in the "care" region.)

 

[audioguru]

Since MOSFETs are voltage controlled and not current controlled, the 4N25s can be just biased for voltage swing, and a really high pulldown resistor can be used. You'll still get the voltage swing needed to turn on/shut off the MOSFET (only issue being turn on/turn off time to charge/discharge the gate cap, but that's usually in the few pF range and virtually negligible (calculate the RC constant with the pulldown and the gate cap to doublecheck, I doubt it would be anywhere in the "care" region.)

No.
A power Mosfet has a total gate capacitance (including the Miller effect) of many thousands of pf. The 10k gate resistor will cause it to turn on slowly.

 

[TheEquineFencer]

OK, here's what I've come up with. My only other question is where the 10K R3 is located, do I use just that one or do I need to have one across each M1-M8 Mosfets? ALSO NOTE that in this drawing I did not add the "S" select switches but I do plan to add them as in the drawing with the one Mosfet design. The program I'm using is a student version and will not let me use more than 50 componets doing the drawings.

 

[TheEquineFencer]

No.
A power Mosfet has a total gate capacitance (including the Miller effect) of many thousands of pf. The 10k gate resistor will cause it to turn on slowly.

How slow is slowly? I want things to happen really fast. The faster the better I would think.

 

[TheEquineFencer]

Here's a picture of the PCB w/o the transistors or Mosfets being in the picture, as it is presently assembled. I realise the 10R looks a "little used", that's what happens when you land a wire wrong! But it still works and is within specs, I'll replace it later on.