powering homework board with wallwart?
iamdenteddisk
Posts: 66
I picked up a [url=mailto:9vDC@700ma]9vDC@700ma[/url] wallwart-transformer at the fleamarket for a nickel im wondering if its suitable for powering my BS2 homework board during bench test &programing? I have a solar charger&nimh setup on my project but it dosent get enough light indoors for continuous opperation and these alkalines eat $5 in an hour ,ifso I will just disconect my charger and use·the transformer·during indor opperation.
can someone confirm or reject this idea?
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can someone confirm or reject this idea?
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Comments
1) The Homework board is designed to use a 9V battery. It's got a 9V battery clip and a space for the battery to sit and the instructions talk about using a 9V battery, so that must be what it's designed for.
2) Looking at the schematic for the Homework board in the documentation on it, it uses either an LM2936 or an LM2940 voltage regulator depending on the revision (A/B vs. C/D) of the Homework board. You can download the documentation from National Semiconductor and it states that the maximum input voltage for the LM2936 is 40V and the maximum input voltage for the LM2940 is at least 26V with short transients to at least 45V. The LM2936 can handle up to 50mA while the LM2940 can handle up to 1A. The LM2940 is also limited practically by the amount of heat it can dissipate. The heat depends on the current drawn and the amount of excess voltage (above 5V) that it has to get rid of as heat. Given that the Homework board is designed for 9V use, the LM2940 can probably handle 1A with an input voltage of 9V (dissipating (9V-5V)x1A = 4W).
3) Some AC adapters are well regulated and others are not regulated. Unless they're marked as regulated or advertised as such, you need to assume that they're not. An adapter marked as 9V may put out 12V or more if lightly loaded. It may also put out only 8V or 7V when loaded near its maximum rating. The current rating is a maximum. A quality AC adapter can put out its rated current while its output voltage is still what's marked on the package.
4) For your specific case, it looks like the adapter should work as long as your project doesn't draw more than 700mA, realistically no more than 600mA. The one thing to be careful of is the polarity of the adapter. Use a multimeter ($10 from RadioShack) to verify that the positive output is connected the way the markings on the power cube indicate. Since the Homework board is designed for a 9V battery, make sure the battery clip is wired up properly with the right polarity as marked on the Homework board and in the documentation. The fastest way to destroy electronic equipment is to connect it backwards to a power supply.