Voltage across Zener Diode is less then Vz? - Page 1
Forum section: electronics / beginners

author: jwhitmore
author_rank: Regular Contributor
timestamp: January 03, 2018, 08:42:25 pm
content: I'm going to have to attach a picture of the circuit given in the datasheet  for the Profet I'm experimenting with as I'm struggling with the current sense circuit.I should say that I don't have a microcontroller connected to the Profet at the moment as I'm just testing it on a breadboard so have a multimeter sitting in place of an A/D Channel of a uC. Earlier sections of the datasheet  (Figure 25 Pg32) just illustrate a single resistor Ris ("A typical value is 1.2 k?"). Across which you'd measure the current sense output current.Now instead of Figure 25's single Ris, Figure 52 (pg49 of 53) uses a much more complicated circuit between the Profet and the A/D Channel of a uC. (Like I say it's a multimeter at the moment instead of an A/D Channel?). I've attached the current sense part of Figure 52 to this message. I'm struggling with the output from this sense circuit. The problem is that I'm putting a 3v3 Zener diode into the circuit as I'm considering using a 3v3 based uC.My problem is that given this current sense circuit I have an output load current of 4.9Amps and my multimeter is reading a sense voltage of 1.927 Volts when the zener diode is in the circuit. When I remove the Zener I measure a sense voltage of 4Volts. If I reversed the calculation 4V would mean a sense current of 4/1k2 = 0.00333333 Which when multiplied by the current sense ratio Approx 1500 suggests that the output current is 5A. The Load current is measured as 4.9A so I'm happy with a calculated sense estimate of 5A.That all makes sense with the Zener excluded from the circuit, but when it's in the circuit I measure a sense voltage of 1.927V. Without the Zener I'd expect 4Volts so given it's a 3v3 Zener I'd expect to be measuring that Zener break down voltage not less then the Vz?The datasheet  recommends an Ris of 1k2 and since I'm looking at using a 3v3 uC I'll change the sense circuit to split the 1k2 into two 600 Ohm resistors in series and measure across a single 600 ohm rather then the 1k2. That'll mean that I will have less chance of going over the 3v3 of a uC. Having said that however there's something missing in my understanding of zener diodes. Been looking at loads of resources but below Vz the zener is always off as far as I can see, and no current should flow through it. Unless my On Semi MMBZ5226B isn't correct?Maybe I should replace the Zener diode with a new one. I can't think why I'd be measuring 1.9V across it when it should be over Vz (3v3).Thanks in advance for taking the time to read this message. If you can see the school boy error I'm making I'd love to hear it.https://www.infineon.com/dgdl/Infineon-BTS5045-1EJA-DS-v02_02-EN.pdf?fileId=5546d4625a888733015aa411125b1069

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 04, 2018, 01:25:10 am
content: What's the definition -- how is Vz measured, anyway?I think you will find the answer informative.For further reading, there's an excellent old Motorola zener diode appnote out there (I don't have a link handy unfortunately) that discusses the V(I) characteristics of zeners, how that varies with rated Vz, and many applications of them.Tim

author: mikerj
author_rank: Super Contributor
timestamp: January 04, 2018, 01:00:13 pm
content: Low voltage zeners are a waste of space for clamping since they have such soft 'knee' characteristics.

author: perieanuo
author_rank: Frequent Contributor
timestamp: January 04, 2018, 04:58:51 pm
content: You cant put 3v3 zener on 3v3 proc. Very bad practice. If you want to clamp I don't recommend zener on adc input. The capacitance also is critical in Adc. Maybe you fried the adc port. Anyway first observation is mandatoryMaybe for a prototype you just put old-timer solutions like diodes in series and think to have 3 V max as top scale.

author: jwhitmore
author_rank: Regular Contributor
timestamp: January 04, 2018, 05:14:30 pm
content: Well at least I didn't fry the Adc port as I'm not using one at present. I was trying this out with a multimeter before it goes near a uC. I am however intending using a 3v3 uC. For the moment I'm sticking to a multimeter.It appears that I've made a big oops in putting a Zener into the circuit but I was blindly copying the recommendations of the data sheet for the Profet. I'll redesign and simply use a current limiting resistor into the ADC and split the 1k2 so I'm detecting a smaller voltage out of the current sense pin. I've got other questions about that profet but for the moment I'll nail down the current sense part and simplify the circuit.