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General Category => Your projects => Topic started by: thampiverghese on April 12, 2014, 22:54:26 PM

Title: Repair Ecolite NF-L 200
Post by: thampiverghese on April 12, 2014, 22:54:26 PM
I am trying to find the value for emitter bias resistor for a MMBTA42 BJT oscillator which is a wireless charger for a flashlight. Please see the attached circuit. The transistor and R3 burned out. I replaced the transistor and used a 2K resistor for R3. It works. But I don't know whether it is the correct value.The markings on R3 is unreadable.Please help.Thanks.
Title: Re: Repair Ecolite NF-L 200
Post by: kam on April 14, 2014, 23:25:07 PM
to me it looks like 2-something-2, so its either 2.2mOhm, 2.2KOhm or 2.2 MOhm. I'd never bet on the 2.2mOhm. A 2.2 MOhm sounds more realistic
Title: Re: Repair Ecolite NF-L 200
Post by: thampiverghese on April 15, 2014, 00:33:40 AM
Thanks.I saw that too.I tried 2.5K and the collector voltage was 94.The emitter voltage was 54.It worked as it should.When I changed to 5K VCe was 112 and VEe was 57. At 10K VCe was 122 and VEe was 48. One guideline said to use a value to get a VEe, 10% of VCe. But this was for low voltage circuits. I couldn't find the guidelines for high voltage circuits. The flash light charged at 2.5K, 5K and 10K as it should. The resistor ran a little hotter at 2.5. How do I decide which resistor to use?
Title: Re: Repair Ecolite NF-L 200
Post by: kam on April 15, 2014, 23:22:12 PM
There is no definite guideline or rule of he thumb for choosing an emitter resistor unless you know what output you wish. As far as i can tell from a quick look at your circuit, C1 charges through the coil until the transistor turns on (or saturates) and C1 and coil discharges through R2-R3. You'd rather measure the frequency and see if the circuit oscillates correctly. I cannot tell much from this circuit.
Title: Re: Repair Ecolite NF-L 200
Post by: thampiverghese on April 16, 2014, 00:27:43 AM
Thanks.I have to buy a frequency meter to measure the frequency. At 2.5K it is oscillating alright because it is wireless charging the flash light.
Title: Re: Repair Ecolite NF-L 200
Post by: kam on April 16, 2014, 23:06:57 PM
actually you need an oscilloscope. this is a must tool if you are into electronics.
Title: Re: Repair Ecolite NF-L 200
Post by: thampiverghese on April 17, 2014, 00:31:38 AM
Thanks.I had a home made one several years ago. I should find a new one. Can you please recommend any inexpensive ones?
Title: Re: Repair Ecolite NF-L 200
Post by: kam on April 17, 2014, 14:01:58 PM
for inexpensive go for PC-based oscilloscopes. Remeber though, you pay peanuts, you get monkeys.
Title: Re: Repair Ecolite NF-L 200
Post by: cheerio on April 17, 2014, 21:19:40 PM
check the Rigol DS1052E   
i have it and it is good quality for the money.  it is a low end scope though
Title: Re: Repair Ecolite NF-L 200
Post by: thampiverghese on April 17, 2014, 23:55:09 PM
Thanks.I am 81 years old and retired. I will look for PC based one.
Title: Re: Repair Ecolite NF-L 200
Post by: kam on April 18, 2014, 00:00:07 AM
81yo and still quite active and creative as it seems. GJ !