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Intermittent Problem with Circuit Design. Asking for Assistance.

Hello,

I'm currently working on a project for work where we need to take in a

-24VDC (+/-10%) voltage, and a -12/-24VDC (+/-10%) Oscilatting voltage and convert it to +5VDC for use in standard TTL chips. The kicker is that at -24VDC it needs to be converted to +5VCD, and at -12VCD, it needs to be at ground. The circuit I have works sometimes, but it seems to have an odd problem where at times, the transistor I'm using to flip the voltage from negative to positive doesn't alway work. If I put a piece of metal, or my finger onto the input, it will after a few seconds (depending on the capacity/inductance?) allow current to flow. This is my current design (please use fixed width font):

-24VDC D1 / ____|\\|____ |/| | / | / \\R /1 \\ | | \\E Q1|B-----Gnd /C | | / \\R /2 \\ | |-------------------U1 | | --- +5VDC

D1 - 1N4746A R1 - 5KOhm R2 - 10KOhm Q1 - 2N2222 U1 - 7414

-24VDC is going into the -18V Zener Diode. This should seperate the

-12/-24VDC pulses so that -12V is ground. On the output of the doide, I get about -5VCD. It then goes into the transistor so that the -5VDC is flipped to +5VCD (This is where I'm getting my problem at). From there, the +5VCD is going into a 7414 chip to make sure it is at a stable level (buffered) and to help protect the rest of the circuit after that (in case of some weird catastrophic failure).

Does anyone have any ideas or design tips that might be able to help me?

Thanks for any help you can give.

Sincerely,

Derrick Tennant.

Reply to
Derrick Tennant
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I presume the output is taken from Q1 collector not the 5V rail.

How about a resistor between Q1 base and emitter to help discharge the base-emitter junction, and switch off faster?

Reply to
Andrew Holme

Yes, it is. It's my mistake. I put it wrong.

\\E Q1|B-----Gnd /C | |-------------------U1 | / \\R /2 \\ | | | --- +5VDC

I have just tried putting a capacitor between the cathode of D1 and Gnd, and it seems to have possibly helped. I just hate these intermittent problems. I will also try a resistor between the base and ground to see if it might help.

Thanks!

Reply to
Derrick Tennant

I don't understand where the fixed -24V is in your scheme.

But if it's what I think you mean, I'd just use a photocoupler.

+5V | .-. | | | | 2k2 '-' -12v to -24v o------. | | o------- OUT V -> |/ Photocoupler | |>

___ | | -24v o---|___|------' | 2k2 === GND

If the fixed -24v line I showed doesn't exist, then you need to be more clear about what you're working with.

Reply to
Walter Harley

I'm sorry. I guess I wasn't very clear. My mistake.

The input voltage is -24VDC. It will at times go to -12VDC during a state change in the device. So, depending, it might be -24VCD constantly for a long time, or it could fluctuate between -12V and -24V every few seconds.

Reply to
Derrick Tennant

Thanks for all the help on this.

The -24V does not stay as a reference. The input fluctuates at some point when an external device is triggered.

What I currently have right now is:

-24VDC D1 / ____|\\|____ |/| | / | / \\R /1 \\ | C1 +--||-- GND | \\E Q1|B-----Gnd /C | +-------------------U1 | / \\R /2 \\ | | | --- +5VDC < D1 - 1N4746A R1 - 5KOhm R2 - 10KOhm Q1 - 2N2222 U1 - 7414 C1 - 330pF

It seems to work, but would it be more stable/more correct with a 10K resistor instead of C1?

Reply to
Derrick Tennant

But when the "input" is fluctuating from -12 and -24, is there a constant

-24 available as a reference?

I haven't analyzed your circuit, but it looks like you forgot that common-base doesn't invert - more negative to the emitter makes it conduct more, pulling the collector to ground. (or, it's supposed to.) Less negative at the emitter, and the transistor conducts less, so the collector can be pulled up. Lessee:

-24VDC D1 / ____|\\|____ |/| | / | / \\R /1 \\ | | \\E Q1|B-----Gnd /C | | / \\R /2 \\ | |-------------------U1 | | --- +5VDC

I'd change this to:

-24VDC D1 / ____|\\|____ |/| | / | / \\R /1 \\ | R3 +--/\\/\\/-- GND | \\E Q1|B-----Gnd /C | +-------------------U1 | / \\R /2 \\ | | | --- +5VDC <

D1 - 1N4746A R1 - 5KOhm R2 - 10KOhm Q1 - 2N2222 U1 - 7414

R3: 10K

Cheers! Rich

Reply to
Rich Grise

So, why then does it work the way I have it now? I'm taking the ~-5V from the zener diode, putting it into my transistor circuit, and coming out with a nice clean +5V.

Reply to
Derrick Tennant

This may sound a bit naive, but for the input source (-24V that can go to -12v) there has to be a ground and a (-)input. My understanding is that he wants to convert this input to a +5 (opposite polarity relative to the common).

What if he just used a linear regulator and treated the ground as the positive supply and used the -input as the "ground" reference? I am not certain if this would work - or if a 7805 (positive regulator) or a 7905 (negative regulator) would be better.

Another, albeit safer, implementation would be a dc-dc converter.

Reply to
Noway2

I've simulated it, and now I've built it.

What I have is: -5V going into the Emitter (Through a 5K Resistor). Base is Ground. Collector is at +5V (Through a 10K Resistor). Output is +5V at the Collector when the input is -5V. Output is Gnd when input is Gnd.

Reply to
Derrick Tennant

Yes, that is right.

Good lord, I think I've confused myself.

Reply to
Derrick Tennant

Perhaps I didn't draw my circuit right? I'm not the best ASCII artist.

This is what I have made (thanks to Jim Thompson for the circuit I stole and reconfigured):

+5V o------------------o | | 10K | o-----> OUTPUT | |/ Gnd---o-----| | | | |\\ | E _|_ | 330pF ___ | | | | | | 5K | | | |

-5V/0V INPUT >-o-------o

course, my original problem was that it was intermittent. All I did was add the capacitor, and it so far has been working 100% of the time. Any thoughts? Or should I give up and never touch any components ever again?

Reply to
Derrick Tennant

I thought your original spec was...

-24V Input --> +5V Output

-12V Input --> 0V Output

Common base is non-inverting.

You need something like this...

+5V o-------o-------o | | | | 10K 10K | | | o-----> OUTPUT | | | |/ o-----| | | | |\\ --- E 12V / \\ | | | | o----GND | 10K | | INPUT >----o

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
Anyone can be rude, but it takes a Democrat to be a real dirtbag.
Reply to
Jim Thompson

Are you building (i.e. breadboard), or simulating.

As you've _drawn_ it, it's NON-inverting...

Are you sure you have an NPN, connected properly?

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

formatting link
| 1962 | Anyone can be rude, but it takes a Democrat to be a real dirtbag.

Reply to
Jim Thompson

Well, I left out the zener diode, just for laziness. :)

I guess this would be the exact circuit I have: +5V o---------o | | 10K | o-----> +5V/0V OUTPUT | |/ Gnd---o-----| | | | |\\ | E _|_ | 330pF ___ | | | | | | 5K | | | | o-------o __|__/ / /_\\ (18V) |

-12V/-24V INPUT >--o

...and the way it is now, during a -12V pulse, the output is +5V, and during a -24V pulse the output is 0V.

This works for the application, because I then run the +5 through an inverting buffer, which would give me my required correct output.

Reply to
Derrick Tennant

Data does not compute with drawing. Think about it.

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

formatting link
| 1962 | Anyone can be rude, but it takes a Democrat to be a real dirtbag.

Reply to
Jim Thompson

You need either a zener or another resistor, to keep from breaking down the base in reverse bias.

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

formatting link
| 1962 | Anyone can be rude, but it takes a Democrat to be a real dirtbag.

Reply to
Jim Thompson

Sorry. Should have said the above plus... or you'll have very weenie base drive ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
 Anyone can be rude, but it takes a Democrat to be a real dirtbag.
Reply to
Jim Thompson

Maybe you had a bad solder joint? But your above circuit does not do what you specified originally.

As drawn above, and working properly...

-24V Input --> ~0V Output

-12V Input --> ~0V Output

;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
 Anyone can be rude, but it takes a Democrat to be a real dirtbag.
Reply to
Jim Thompson

I'd put the threshold at mid-signal... zener = ~15V.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
 Anyone can be rude, but it takes a Democrat to be a real dirtbag.
Reply to
Jim Thompson

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