Probable circuit design error in zener voltage regulator? - Page 1
Forum section: electronics / beginners

author: pplaninsky
author_rank: Regular Contributor
timestamp: September 02, 2023, 08:55:17 pm
content: Hi,I’ve bought a ready kit from my local electronics shop.(Yes, I still have one physical shop in my city.)The kit is a MOSFET DC Motor speed driver.Naturally, the kit never worked. After attaching a 12V power supply and a 12 volt motor nothing worked. Then I found out:D3 was blownZD1 was blownthe mCU was blownThen I reversed engineer the PCB and I am pretty sure the schematics is as follows (see attachments).Questions:1. Should there be a current limiting resistor in series with D3 and ZD1. The kit claims up to 90V in. That would always blow the zener, wouldn’t it? Is this a design mistake? I triple checked my reverse engineering process. Also the only resistor on the PCB is the 10 ohm in series with ZD2. There are no other resistors on the PCB.2. D1A is reverse polarity protection. But why they attached D1B to ground? What is D1B doing? It seems like a waste.3. What is the ZD2 doing? Is it over-voltage protection for the input of the mCU?Thank you in advance for any hints.P.S.Info about the mCU can be found here:https://www.eevblog.com/forum/beginners/identify-chip-mosfet-low-side-driver/

author: WimWalther
author_rank: Regular Contributor
timestamp: September 02, 2023, 10:04:25 pm
content: Based on everything I've come to expect from a zener or gas-discharge shunt regulator, D3 should actually be a resistor.. or at least a resistor should be added in series with D3.Once the ZD zeners-on, it becomes a very low impedance, sure to burn up without current limiting.

author: Circlotron
author_rank: Super Contributor
timestamp: September 02, 2023, 10:25:09 pm
content: Yes, D3 should be a resistor.

author: pplaninsky
author_rank: Regular Contributor
timestamp: September 03, 2023, 08:41:52 am
content: I think D3 is part of temperature compensation for the zener.However, a resistor should be added.http://basicelectronic.blogspot.com/2006/08/thermally-compensated-zener-circuit.html

author: magic
author_rank: Super Contributor
timestamp: September 03, 2023, 09:53:43 am
content: 5.1V zeners have low thermal drift, it's the higher voltages which need diode compensation. Check any zener datasheet and see yourself.Besides, it's just a supply for a digital chip, so who cares?IIRC, AVR ADC can use VCC as the reference, which makes supply stability irrelevant if the pot is connected between VCC and GND.WTF is ZD2?

author: jpanhalt
author_rank: Super Contributor
timestamp: September 03, 2023, 10:37:13 am
content: 1) Agree on the need for a resistor in series with the first zener. Without a resistor, it will blow.2) I don't understand the need for the second zener that is connected to the pot apparently for adjusting the pwm. Could it be a axial lead capacitor?3) The MCU looks like a typical 8-bit unit. The RISC instruction set is very similar to similar 12F6xx and 12F1xxx PIC's from Microchip.4) Searched on ADC and "analog". No hits for ADC. Analog has only one mention. Uness the chip has a special hardware voltage input --> PWM, one would need an ADC. That pin would need to be set to analog input. Writing a new program shouldn't be too difficult, but knowing how to set up the ADC or ADC-like input would be critical. The chip looks like it is designed almost specifically for PWM, so there may be some hardware special function built-in.

author: pplaninsky
author_rank: Regular Contributor
timestamp: September 03, 2023, 11:16:48 am
content: ZD2 is marked as a diode on the silkscreen. Also, it measures a diode drop voltage in forward bias and doesn’t conduct in reverse on the multimeter. So, it doesn’t seem to be a capacitor.I thought the same about the lack of ADC and that chip probably doesn’t have an internal ADC. It has only digital ports.Totally, agree on the temp compensation for the ZD1.The whole PCB seems to be either wrongly copied or made by someone who didn’t know what they are doing.Finally, in simplest form - I think you can drive the MOSFET just with the POT, no need of mCU with PWM for this purpose.

author: pplaninsky
author_rank: Regular Contributor
timestamp: September 03, 2023, 11:18:54 am
content: Forgot to mention, the big heat sinks on the D1 and D2 Schotky diodes. I don’t think reverse polarity protection or flyback diodes need that much of a heat sink.

author: jpanhalt
author_rank: Super Contributor
timestamp: September 03, 2023, 11:40:51 am
content: Driving the gate of a mosfet just with a pot to control a DC motor means it is in the "linear" region (that steep part of the Vgs versus effective R curve, attachment). It will get very hot according to the W = I^2R equation.

author: macboy
author_rank: Super Contributor
timestamp: September 05, 2023, 04:31:08 pm
content: Quote from: jpanhalt on September 03, 2023, 11:40:51 amThe pot does not drive the gate, it goes into an analog-capable pin of the micro. The micro then outputs PWM for the MOSFET to drive the motor.There are too many issues with this circuit. R1 is 10 ohm? Imagine what happens with the 10k pot set all the way to the top.

author: jpanhalt
author_rank: Super Contributor
timestamp: September 05, 2023, 07:33:38 pm
content: Quote from: macboy on September 05, 2023, 04:31:08 pmMy response was directed at this comment by the TS:Quote from: pplaninsky on September 03, 2023, 11:16:48 amThere would be no MCU pin to which one could attach the pot. BTW, can you say with certainty that PA-1 of the original MCU has an ADC or its equivalent function? My guess is that it is a specialized MCU with an incomplete datasheet for those specialized functions.

author: macboy
author_rank: Super Contributor
timestamp: September 05, 2023, 08:40:48 pm
content: The PIC12F675 has ADC on that pin. It has timers to implement software-driven PWM. The power and INT pin connections match too. It is fairly common for a company to buy a batch of microcontrollers with custom marking, and implement a special function in firmware. It is certainly cheaper than developing an ASIC for low volume parts. A PWM motor controller with an analog input would be an excellent candidate for such a thing. I'm not saying that this is what this chip is/was, but it is possible. Regardless, the OP could potentially replace the blown chip with a 12F675 and write custom firmware.edit:The original microcontroller appears to be aChineseclone of a PIC micro.https://www.eevblog.com/forum/beginners/identify-chip-mosfet-low-side-driver/msg5028406/#msg5028406

author: jpanhalt
author_rank: Super Contributor
timestamp: September 05, 2023, 08:50:19 pm
content: @MacBoyThis is not about the PIC12F675 or any other PIC. Read the original post and look at the datasheet for that MCU.The TS later suggested removing the MCU and driving the mosfet gate directly with a potentiometer. What part of that is not clear? That is the suggestion to which I replied, basically, don't do it.As for being a clone, it is not a clone of the 12F6xx series. Similar instruction set, but not a clone. Again, look carefully at the datasheet.

author: macboy
author_rank: Super Contributor
timestamp: September 06, 2023, 02:34:04 pm
content: I'm done here. Good luck.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 06, 2023, 03:36:19 pm
content: Hmmm... A thought:What's the pinout of the through hole pads vs the SMD pads? (The Fremont FT60F21 was mounted on the SMD pads) If you wanted to use another MCU, you might be able to mount a DIP version on the bottom side. Or take a DIP MCU, bend some pins up, and wire them to the correct points. It'd be a bit ugly, but how ugly would be up to you.