I have just burnt a DC/DC converter ... - Page 1
Forum section: electronics / beginners

author: Reminx123
author_rank: Contributor
timestamp: October 11, 2015, 08:27:24 am
content: Hi All,I have just burnt a new Murata MTU1D1215MC, a 12V to +-15V DC/DC converter, but I don not understand why.I start my experiment by connecting the two input, pin 1,3 to 12V power supply and then I measured the output, my multimeters read +-17V but this is ok since in the specs there is a note that explain that without load this can happen.I then add 4 x 4.7k resistors in parallel, equivalent to 1.175k, between pins 4 and 6. This was done in order to draw 25mA, close to the maximum output of 33mA. At this point I measured again the output voltage and found that the value was ok, close to +-15V, but there was an offset of 15V between my input GND and output 0V.The situation was like this:- input: +12V, GND- output: GND (that was supposed to be -15V), +15V (that was supposed to be 0V), +30V (that was supposed to be +15V)At this point I thought that as the DC/DC converter is insulated it could be a good idea to "offset" the output buy connecting the input GND to output 0V. When I do that I just burnt my DC/DC converter.Only after seeing the "magic smoke" I thought that it could be a good idea to add some resistance (instead of just shorting pins 3 and 5) but still I can not understand why I burnt the converter.Some detail about the usage of this converter: I need to generate +-15V in order to power an instrumentation amplifier for ADC conversion, the 0V of +-15V needs to be at same level as circuit GND.Looking forward your commentsCiao!Remix

author: Rerouter
author_rank: Super Contributor
timestamp: October 11, 2015, 08:55:29 am
content: It is indeed isolated, the fact that the output was a perfect 15V while measuring (an isolated converter should drift very quickly to 0V) means you had a short between -15V output, and your input ground somewhere,Meaning when you tied the output ground to input ground, you shorted out the negative converter, and poof

author: alho
author_rank: Contributor
timestamp: October 11, 2015, 11:32:49 am
content: Polarity of your meter was wrong when measuring -15V?Voltage from A to B is +15V, B to C is +15V and A to C is +30V. So B is GNDQuote from: Reminx123 on October 11, 2015, 08:27:24 amGND is +15V above -15V. (And +15V is +30V above -15V)

author: Reminx123
author_rank: Contributor
timestamp: October 11, 2015, 06:24:44 pm
content: Many thanks for your reply, I have prepared a sketch to illustrate the setup.In black pencil color there is the "supposed" situation, in orange what I measured, a guess of A,B, and C from alho and the connection I added.I still does not understand why it has burned, I will wait one week to receive another part and repeat the experiment. But this time I will add a 100k resistor!CiaoRemix

author: Zero999
author_rank: Super Contributor
timestamp: October 11, 2015, 06:48:22 pm
content: Quote from: Reminx123 on October 11, 2015, 06:24:44 pmThat's exactly what you should expect to measure. If you'd kept the meter's negative lead on point B, then you'd see +15V on C and -15V on A.QuoteAdd the 100k resistor where? If it's in series with the 12V supply, then nothing will happen. Try a 100R resistor in series with the 12V and 100µF capacitor in parallel with pins 1 & 3.

author: Deathwish
author_rank: Unknown
timestamp: October 11, 2015, 06:55:31 pm
content: i dont understand why you have added a link to the primary side input ground and the secondary output 0V, the 0v is a ground with reference to the isolated +15 / -15V secondary output.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 11, 2015, 07:57:34 pm
content: Using an isolated dc/dc brick is an easy way of creating a bipolar supply. If you do not need the isolation, you can then simply wire through -Vin to 0v (or any) and use the created voltages to go bipolar, higher or lower than Vin.You could (theoretically) use three of these bricks to go to 120V if you're having a crazy day.It should be noted that some of these power bricks do not like assymetric load. That is, more current from 0V to -Vout than from +Vout to 0V or vice versa.That you've smoked it is probably your own mistake, shorted by the measurement probe perhaps?

author: Reminx123
author_rank: Contributor
timestamp: October 11, 2015, 08:03:09 pm
content: Quote from: Hero999 on October 11, 2015, 06:48:22 pm

author: Reminx123
author_rank: Contributor
timestamp: October 11, 2015, 08:05:26 pm
content: Quote from: Deathwish on October 11, 2015, 06:55:31 pmYes, but I would like to have a common GND in my circuit. What I have shown is a part of the complete system, I need to have GND and +15V and -15V referenced to GNDCiaoRemix

author: Reminx123
author_rank: Contributor
timestamp: October 11, 2015, 08:08:49 pm
content: Quote from: Jeroen3 on October 11, 2015, 07:57:34 pmJeroen, many thanks for your explanation.I understand, that was the same idea I had when I decided to short -Vin with 0V. Now I need to wait to receive the new part and test it again, this time I will have much more look into details and I will report my experience.CiaoReminx