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Topic: Using BC327 instead of BC337 but not working as expected? (Read 8 times) previous topic - next topic

dtokez

I just tried that circuit from DC on the MT7201, could see some dimming, but i'd only say 75%-100%. I tried a few different resistors on the base but still the same. I tried it connected to the arduino and running the 'Fade' sketch from the basic examples.

I keep thinking the ADJ pins are connected to something, I could measure about 0.5M ohm between it and some of the others so I tried lifting it, but managed to snap this one too. No ADJ pins to play with anymore, but I have some more bulbs on order :(

dc42

The datasheet for the MT7201 says of the ADJ pin:

Multi-function On/Off and brightness control pin:
? Leave floating for normal operation.(VADJ=VREF=1.2V giving nominal average output current IOUTnom=0.1/RS)
? Drive to voltage below 0.2V to turn off output current
? Drive with DC voltage (0.3V<VADJ<2.5V) to adjust output current from 25% to 200% of IOUTnom
? Drive with PWM signal from open-collector or open-drain transistor, to adjust output current. Adjustment range 25% to 100% of IOUTnom for f>10kHz and 1% to 100% of IOUTnom for f<500Hz
? Connect a capacitor from this pin to ground to increase soft-start time. (Default soft-start time=0.8ms. Additional soft-start time is approx. 0.8ms/nF)

So you should be able to get 1% to 100% of Iout using the Arduino default PWM frequency of 490Hz. Note that the PWM will work in reverse, i.e. analogWrite(pin, 255) should turn the LED off, while analogWrite(pin, 0) should turn the LED fully on. I'm assuming that you haven't bypassed the bridge rectifier in the current regulator, that's why I included the diode in the circuit (to prevent Vadj going more than 0.3V below the ground connection of the MT7201).

To drive it from a TLC5940 I suggest this circuit. I've moved the diode to the emitter so that you can share 1 diode between all the channels.

To avoid breaking off the ADJ pin, I suggest you use a fine enamelled copper wire to make the connection to it.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

dtokez

Thanks DC! I notice that the same info is in the ZXLD1360 datasheet too. I hope the that the other bulbs use the same regulators. Shame I will have to wait for them to arrive, hopefully won't be too long.

I'm looking forward to trying your circuit when they do get here, hopefully I won't break the pins off! I used some kynar wire to connect to them but I tried to lift the pins in case the PCB connected them to anything else but I don't think it did :(

I have not removed the diode bridge from them, it would be nice not to have to modify them too much. So am I right in thinking that I should connect all the emitters to the the same diode?

Many thanks

dc42

#28
Jan 02, 2013, 10:21 am Last Edit: Jan 02, 2013, 10:36 am by dc42 Reason: 1

I'm looking forward to trying your circuit when they do get here, hopefully I won't break the pins off! I used some kynar wire to connect to them but I tried to lift the pins in case the PCB connected them to anything else but I don't think it did :(


The only thing it might be connected to is a capacitor, to increase the soft-start time. You could use a multimeter to see whether the pin is connected to anything else, in particular to any capacitors.


I have not removed the diode bridge from them, it would be nice not to have to modify them too much. So am I right in thinking that I should connect all the emitters to the the same diode?


Yes. Make sure that each lamp you are controlling has a good connection between one of the pins and ground. To avoid damage in the event of a broken ground connection, you might want to connect a resistor of about 470 ohms or 1K between the transistor collector and the ADJ pin.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

dtokez

Thanks DC! I don't think they were connected to anything, I lost the pad on the MT7201 but the pad on the ZXLD1360 seems to be O/C

Looking forward to the other bulbs arriving now for more testing! Hopefully we can get it to work :)

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