PCB pin 18 IC1 Voltage not right

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PCB pin 18 IC1 Voltage not right

Postby rsluiman » Fri May 19, 2017 6:32 pm

Hello,

I've recently created the complete PCB with components and everything is good except for the voltage on pin 18 of IC1.
The voltage when pin 3 is floating is 0V.
The voltage when pin 3 is connected to ground it's 1.125 V so its quite a bit shy of the supposed 3V.

I've measured and tested all the things i can think of. Has anyone got an idea?

thanks.
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Re: PCB pin 18 IC1 Voltage not right

Postby hvxl » Sun May 21, 2017 8:12 pm

With pin 3 connected to ground, which voltage do you measure across R3?
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Re: PCB pin 18 IC1 Voltage not right

Postby rsluiman » Mon May 22, 2017 4:05 pm

The voltage over r3 when pin 3 of ic1 is connected to ground and terminals x1 and x2 are connected is 2.8V
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Re: PCB pin 18 IC1 Voltage not right

Postby hvxl » Mon May 22, 2017 4:52 pm

That's too low. But at least the good News is that the problem doesn't appear to be a bad opto-coupler.

You mentioned "everything is good except for the voltage on pin 18 of IC1". So, the check "connect pin 3 of the IC1 socket to ground (e.g. pin 5), the current between the two terminals of X1 should be between 17mA and 23mA" produced the expected result? Assuming the low end and losing 2mA through R1, you should still have 15mA left. That should give you 3.3V across R3. With only 2.8V, OK1B doesn't get enough current to fully activate. That's why the voltage on pin 18 doesn't go high enough.

  1. Check that R1, R2, R3 and D9 have the correct value.
  2. Please recheck the current from X1 when pin 3 on the IC1 socket is connected to ground.
  3. With pin 3 of IC1 grounded and terminals X1 and X2 connected together, measure the voltages across R1, R2, D9, and D10.
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Re: PCB pin 18 IC1 Voltage not right

Postby rsluiman » Mon May 22, 2017 5:24 pm

The resistors got the right value. I'm not sure how to check if the zener diode is correct.
There is 21 mA running through X1.
The voltage over the components are: (with X1 and X2 connected and pin 3 of IC1 connected to GND(pin5))
- D9 : 3.94V
- D10 : 15.13V
- D11 : 0.342V
- R1 : 0.62V
- R2 : 1.101V
- R3 : 2.855V
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Re: PCB pin 18 IC1 Voltage not right

Postby hvxl » Mon May 22, 2017 8:49 pm

Things just don't add up: There's 21mA coming into X2. That's all going through either D7 or D8. 0.62V / 330 Ohm = 1.9mA takes a detour through R1. So, 19.1mA is left to go into the emitter of Q1. Q1 is operating in linear mode (it's not saturated), and the Hfe of 120 applies. This means that the current out of the base is 1/120th of the collector current and only 0.02mA can escape out of the base of Q1. That leaves more than 19mA coming out of the collector of Q1. The voltage across D9 is below the zener voltage, causing D9 not to allow any current through. So all 19mA should be going through R3. But you only find 2.855V / 220 Ohm = 13mA. Where did the missing 6mA go?

Is your multimeter capable of measuring current? If so, please put it between X1 and X2 as one of the two wires to check that you actually still get 21mA in that configuration. If it's only about 15mA, we've been looking at the wrong part of the circuit for the problem. If the 21mA is confirmed, the only possibility I can come up with is that D9 is a 3V9 zener instead of 4V7. Carefully try to desolder one side of D9 and pull the lead out of the hole in the circuit board. Then rerun the check for the voltage on pin 18.
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Re: PCB pin 18 IC1 Voltage not right

Postby rsluiman » Tue May 23, 2017 1:10 pm

The current is still 21 mA. I've desoldered D9 and now the voltage on pin 18 goes to 4.25V so it's the diode it seems. However the writing on the diode indicates a 4V7 (http://www.farnell.com/datasheets/688897.pdf) This diode has a dissapation of 1.3W instead of the 0.5W of the recommended part. It seems like the store may have sent me the wrong diode.

I've installed a new diode (http://assets.nexperia.com/documents/da ... /BZX79.pdf) and the voltage is around 2.8V so everthing works now.

Thanks for helping me out.
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Re: PCB pin 18 IC1 Voltage not right

Postby hvxl » Thu May 25, 2017 1:19 pm

I'm glad to hear that it's all working for you now.
The different power rating should not be the reason for the issue. The zener simply kicked in way too early.
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