Monday, February 27, 2017

Communicating with AVRs via serial (USART). Poorman's isolated RS232 to TTL interface

AVR and serial (RS232) communication

I've been trying to build a new project based on the AVR for about a week and I needed to find a way to communicate with the MCU on the breadboard for debugging purposes. Up until now the projects I worked on were quite small and simple so no communication with the PC was needed, but this has changed. One way of achieving my goal is by using the USART capability of the AVR which has some nice features:

  • Full duplex connection
  • Serial frames with 5 to 9 data bits and 1 - 2 stop bits
  • Sync or Async operation modes
  • High resolution baud rate generator
So far so good, the only thing left is to find a way to interface it with the PC.


First try: disaster

Since my desktop computer has a serial port (RS232) header, I decided to use it instead of an USB RS232 to TTL converter. The main reason for this choice is because I had a few ST232 chips in my parts box that I wanted to put to use. The ST232 is a 5V powered multi-channel RS-232 driver and receiver compatible with MAX232.  By following the datasheet, I built this circuit:
First try at a RS232-to-TTL interface

As you can see, it's made up of the ST232 level converter, a few polarized capacitors used by its charge pumps and the DB9 connector. Note that both the ST232 and the ATMEGA share the same power supply and ground. The +5V power comes from my lab PSU.

I have used the above circuit for a few days without any trouble until one time when the microcontroller simply refused to program. Oddly enough, it was correctly detected by avrdude, it accepted all the commands and received the data sent to it, but upon verification the flash memory was completely filled with nulls. Thinking it was just a bad microcontroller, I replaced it with another one (bought in a different store to eliminate the probability of a bad batch) and then with third one (ATMega 8). Again, everything went well until the MCU simply died between 2 power cycles. This is when I understood that the ST232 circuit was somehow responsible for killing the ATMEGA chips. There were no other components on the breadboard, just the MCU and ST232 circuit. ESD is also excluded since I had ESD protection gloves on at all times.

Here all the voltage levels for the circuit while it's powered on and after the lab power supply is turned off and the ST232 circuit remains plugged in the computer RS232 port:


Measuring some points in the above circuit

As you can see from the picture, the circuit is working normally while it's powered, but when the lab power supply is disconnected (and the RS232 connection is still maintained with the PC), the power rail (VCC) dips to -513 mV (-0,513 volts). If you are unlucky to have other ICs on this rail, they could be destroyed.

According to the Absolute Maximum Ratings table in the datasheet of the ATMega8, voltage on any pin except RESET (with respect to Ground) should be between -0,5V and VCC + 0,5V and voltage on the VCC pin 1.8V to 5.5V. Clearly, the -0,513 volts that is leaked by the ST232 IC is out of spec for the MCU, thing that lead to it being destroyed after a few power cycles. While AVRs are pretty tolerant, there are some other ICs out there that are much more sensitive so if you are using such a RS232 to TTL converter chip (be it ST232 or MAX232), make sure to first disconnect the RS232 plug from the PC and only after that turn off power to the circuit.


A solution

In order to communicate with the AVR without risking to destroy it, I made an inexpensive RS232 to TTL interface using only common parts. The interface is galvanically isolated so the AVR will have great protection against power surges or current leaks. One drawback of using cheap components is that transfer speeds will not be too big, but it should be enough for debugging some simple projects.

Specifications:
  • Principle of operation: voltage level conversion with PC817C (or CT817C, EL817C, etc.) optocoupler / optoisolator
  • Operating voltage (TTL side): 3V to 5V (one resistor value needs to be changed for switching the level)
  • Operating voltage (RS232 / PC side): same as the serial port DTR / RTS lines (it harvests power from these lines)
  • Maximum data rate: 4800 Baud half-duplex

Requirements:
  • It needs the software to set the RTS line to logic 0 (which provides +10V) and the DTR line to logic 1 (which provides -10V). The circuit will not work if these lines are not configured correctly. Not all software can toggle these lines. For Linux one can use GTKTerm and for Windows RealTerm.
Schematic: 
Poorman's isolated RS232 to TTL interface

The circuit is pretty simple. The positive part of the Tx RS232 signal from the PC passes through D1 then fed into the led of U1 via the resistor R1. The led sees about 5 mA of current. On the output side, optocoupler U1 is wired as common collector so the signal will be the same polarity as the one in the RS232 port, but at TTL voltage. R2 is chosen with a low value in order to improve the rise / fall time of the waveform and decrease the phototransistor switching time. The signal is then fed into an inverting, common emitter buffer comprised of Q1 and load resistor R4.

On the transmitter side, the signal is fed into the led of U2 via the resistor R5 (which can be changed to work with 2 different voltage levels, see the schematic). On the output of U2, the voltages used for the signals are harvested from the RTS (+10V), DTR (-10V) lines, so the software must toggle the lines accordingly. The phototransistor and the PNP transistor Q2 are arranged in a cascode configuration to get an improved rise and fall time. The cascode configuration is great for a circuit like this because the phototransistor does not see the load resistor R7, it only sees the internal resistance of Q2, which is quite small.

Here's an example transmission on the screen of an old scope:

Sending a letter via GTKTerminal

The waveforms above were created by continually sending the "V" character at a rate of 4800 bps, 8 data bits, 1 stop bit, no parity. The power supply on the TTL side is 5 Volts and the scope is set to 5V / div & 0,5 mS / div for the timebase. The first picture shows the TTL output (that goes to the MCU's Rx pin) and the second one the RS232 output (that goes to the PC's Rx pin).

The voltage levels on the RS232 side are not perfect (e.g. +-12V) because the control lines can only supply 10V a few mA of current and there is also a voltage drop across the diodes. But this shouldn't matter since for RS232 the space (0 or ON) condition is indicated by +3 to +12V and a mark (1 or OFF) condition anything from -3 to -12V.  Judging from the oscillogram above, the circuit should work just fine (not to mention that modern RS232 transceivers are forgiving, with some sensitive enough to work with +-2V signals).


So what speed can this be used at?

First, it should be noted that PC817C (and equivalents) are quite slow (with maximum Tr / Tf of around 18 μs with RL of 100R depending on the manufacturer and sample). To be sure that the circuit works reliably I'll use the worst case scenario for the rise and fall times (e.g. 18 μs) for both optocouplers, but just as an exercise, let's calculate the speed for each of them:

For U1, speed is dictated by the load resistor R2, so the slowest time will be 18 μs (as per the datasheet).

For U2, the phototransistor only sees the input resistance of Q2. Considering that the led of U2 is powered with around 5,5 mA of current and with a failsafe value of 135% CTR for CT817C (taking into account temperature and aging of the device and substracting them from the 200% CTR that the manufacturer claims), Ic will be at

Ic = 1,35 * 5,5 = 7,42 mA. 

To calculate the internal resistance Re of Q2, we use the following formula:

Re = 25 mV / Ic (in mA) = 25 / 7,42 = 3,36 Ohms

This low resistance will ensure good switching time, but I want to be on the safe side so I'll consider it equal with the time above (18 μs), not to mention that the datasheet doesn't show the timing values for load resistors less than 100R. The minimum bit time should be kept to 1/10 of the baud rate so we can calculate it like this:

Mbt = 18 μs * 10 = 180 μs

To work reliably, the circuit must work at a baud rate that has a bigger bit time than the one calculated above. Knowing that the bit time for an arbitrary baud rate can be calculated by dividing 1 second to the baud rate, the following table can be devised:

Baud rate Bit width (μs) Max rise/fall time (μs)
1200 833 83,3
2400 416 41,6
4800 208 20,8
9600 104 10,4
19200 52 5,2
38400 26 2,6
57600 17 1,7
115200 8 0,8


Since the fastest speed with a bigger bit width than 180 uS is 4800 baud, this will be the max rate at which this circuit circuit is considered reliable. Higher speeds would be attainable by using fast optoisolators like 6N137, but much of the schematic would need modification.

Saturday, January 7, 2017

Modding a cheap 60W SMPS (CR6853T + TL431) from 12V to 24V


Before reading on
 
Warning! This project uses dangerous / lethal high voltage! If you're not experienced with power electronics, do not attempt anything you read in this article.

Capacitors can remain charged to lethal voltage even after the circuits are disconnected from mains! Everything you do on your own risk and responsibility!


The original power supply

A few years ago I picked up a small SMPS power supply (Melarre M-13.8V 5A) for use as a charger for 12V lead-acid batteries. It was very cheap at the time and it worked quite well. The output is 13.8V at a maximum of 5A, although I am a bit skeptical about it being able to continuously pump 60W of power. The construction looks somewhat like a laptop power brick.

Original case

Since I didn't need it anymore as a charger, I decided to modify it to output 24V and use it as a base for a lab power supply. The build quality is not that good, especially the soldering which had lots of splattering, risking short circuits (but nothing that can't be fixed).


The internals

The unit is based on the Chip-Rail CR6853T PWM controller, Toshiba 2SK3067 primary switching transistor, FMB26L Schottky diode for secondary rectification and a transformer with a size of 20 x 20 x 30 mm and 10 mm core diameter.

Transformer core, 1 cm diameter

Unfortunately I couldn't find a schematic for this SMPS, so I had no info on the number of turns inside the transformer. The only way to find out was to remove it from the board then count each winding.


Unwinding the transformer

The transformer had the typical SMPS layout (primary split in 2 parts to minimize leakage):
  • The first half of the primary was wound closest to the core, with bifilar wire, total thickness 0.7 mm, 30 turns
  • The secondary, made from hexafilar wire, total thickness 1.2 mm, 7 turns
  • The second half of the primary, with bifilar wire, total thickness 0.7 mm, 15 turns
  • The auxiliary / feedback winding, thickness 0.4 mm, with 8 turns
So, in total there are 45 turns for the primary, 7 for the secondary and 8 for the feedback.


The modifications

To make it output 24V, I rewound the transformer completely, keeping the same number of turns for both the primary and auxiliary and doubling the number of turns for the secondary (to a total of 15 turns). Also, instead of the hexafilar wire, I used normal, 0.8 mm magnet wire. It's thinner than the original one, but it should hold just fine since it will be used at maximum 1 amp. For the insulation between the windings I used Kapton tape in addition to the original yellow polyester tape.

Transformer after rewinding

Next up, I have drawn the schematic for the secondary part of the SMPS in order to see how the voltage stabilization is done.

Original secondary of the SMPS


The isolated feedback loop is provided with the help of a TL431 adjustable shunt regulator, an optocoupler (PC817) and a few resistors. The filtering is made with the help of 2 electrolytic capacitors (in parallel) of 1000 uF each, which insufficient for a flyback SMPS (the ripple is quite high in this kind of design).

The output voltage is given by the voltage divider created by R1 and R2 and is given by the formula:

VOut = VRef * (1 + R1 / R2)
, where VRef is the TL431 reference voltage (2.5V)

In the original circuit, R2 is made from 3 other resistors in parallel (probably hand picked by the manufacturer in order to get close to the desired voltage). The resistance of R2 is thus somewhere around 3.5 KOhm. Solving the above equation for VOut, we get 2.5V * (1 + 14.7 KOhm / 3.5 KOhm) which is 13V, pretty close to the intended value.

Now, to get 24V, I decided to make R1 10 KOhm. R2 needs then to be 1.16 KOhm so I put in 1K + 100R in series. The modified secondary part of the SMPS looks like this:

Modified secondary

I will also add an inductor and another filter capacitor (on the wires, there's no space on the PCB) to form a PI filter and get rid of some of the ripple.

To test the SMPS, I first ran it at mains (220v) in series with a 100W light bulb, just to be on the safe side. It worked from the first try, so I call this a successful experiment.

Reassembled power supply

Conclusion

To summarize, here are the general steps for modifying a flyback-style SMPS to another voltage:

  • Take out the unsuitable components from the secondary (the capacitors which might be too low voltage, zener diodes) and make sure the secondary rectifiers are able to cope with the current you're willing to draw
  • If a higher voltage is needed rewind the transformer accordingly. Rewinding is usually not necessary if the desired voltage is lower than the one the supply originally provided unless you want to draw bigger current than what was originally provided.
  • Find the 2 resistors that set the voltage of TL431 and change them. Keep the top resistor (R1 in the above schematic) bigger than 5 KOhm so that the TL431 doesn't see excess current.
  • You can choose an arbitrary value for R2, something close to 1 KOhm then solve the above equation for R1 like this: R1 = R2 * (Vo / 2.5 - 1). Or solve for R2 = R1 / (Vo / 2.5 - 1)
  • If you want to have a potentiometer for R1, make sure to put a resistor (at least 470R) in series with it otherwise the TL431 will get damaged when the pot is turned all the way down
  • Put back filtering capacitors with suitable voltage ratings. Derate them as much as possible (I derate them by 50% usually) because the ripple  and noise is usually high in SMPS designs.
  • Make sure the switching transistor and the rectifiers in the secondary have adequate heatsinks

That's about it, thanks for reading!