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Intermittent Problem with Circuit Design. Asking for Assistance.

D

Derrick Tennant

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.
 
A

Andrew Holme

Derrick said:
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.
Click to expand...

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?
 
D

Derrick Tennant

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!
 
W

Walter Harley

Derrick Tennant said:
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.
Click to expand...

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.
 
D

Derrick Tennant

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.
 
R

Rich Grise

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.
Click to expand...

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:

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

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
 
D

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?
 
J

Jim Thompson

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?
Click to expand...

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.
 
D

Derrick Tennant

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.
 
N

Noway2

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.
 
J

Jim Thompson

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.
Click to expand...

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 |
| http://www.analog-innovations.com | 1962 |

Anyone can be rude, but it takes a Democrat to be a real dirtbag.
 
K

Keith Williams

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.
Click to expand...
It looks to me like you're getting "0V" out when the input is -24V
and +5V out when the input is greater than -(24-Vz) (i.e. non-
inverting, as Jim indicates).
 
D

Derrick Tennant

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.
 
D

Derrick Tennant

Yes, that is right.

Good lord, I think I've confused myself.
 
J

Jim Thompson

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.
Click to expand...

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 |
| http://www.analog-innovations.com | 1962 |

Anyone can be rude, but it takes a Democrat to be a real dirtbag.
 
J

Jim Thompson

Didn't you want the opposite? Your circuit is non-inverting.
Jim's is inverting (and simpler; no zener needed).


It happens. ;-)
Click to expand...

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 |
| http://www.analog-innovations.com | 1962 |

Anyone can be rude, but it takes a Democrat to be a real dirtbag.
 
J

Jim Thompson

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

...Jim Thompson
Click to expand...

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.
 
D

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
From the testing I've been doing today, it seems to work alright. Of
Click to expand...
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?
 
F

Fred Bloggs

Derrick said:
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?
Click to expand...

That doesn't look like any circuit anyone suggested to you. Where is the
12V pulse coming from exactly?
 
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