Shop OBEX P1 Docs P2 Docs Learn Events
Problems trying to power a BoE with a 9volt 300mA wall wart — Parallax Forums

Problems trying to power a BoE with a 9volt 300mA wall wart

Brann FenixBrann Fenix Posts: 57
edited 2006-09-04 15:41 in BASIC Stamp
Ok here is an odd one...

I wanted to grabbed a 9v wall wart from Parallax, but since they were on back order and I am not really that good at waiting, so I went on the hunt elsewhere. Now... I found an old Radio shack 9v@300mA power supply in my house right after I ordered a new 9v300mA one from another website that had them in stock*is'nt that always the way*. So, I just tested both wall warts with my multimeter and to my horror I got these readings:

Radio shack 9v@300mA: Reads 14.46V

New I just got 9v@300mA: Reads 14.98V


I found it kind of odd that both read close to 15v when they should of been 9v... I was looking for the max voltage the BoE accepted and I saw that the BS2 accepts 5-15v, but I was not about to plug in either of them and risk blowing my BoE. Could I use the 14.46V one with my serial BoE and BS2? Or am I stuck waiting for Parallax to get more 9v wall warts that don't suck and output 9v like the should? Lastly... why would they both being outputting near 15v? Just cheap Smile parts?

Thanks in advance,
Brann

Comments

  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-09-04 05:20
    Brann,

    ·· Typically Wall Adapters are unregulated.· This is the case even with the units we carry.· Unregulated supplies tend to have a higher output voltage than they are rated at when not at their rated current.· Basically once a load is on the output it will draw down near the 9V it is rated at.· As you can see the BS2 is rated at 15V yet the board is rated for 6-9V.· This is done intentionally for this reason.· If you have a 15V unregulated supply you may see smoke.· =)· You can use the ones you have if the polarity is correct.· Take care.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • allanlane5allanlane5 Posts: 3,815
    edited 2006-09-04 12:40
    Yes, you can use the adapters you've got.

    An "unregulated, DC adapter" typically has a transformer inside it, along with a diode to give 'half-wave rectification', and a capacitor or two to hold the resulting DC and reduce the 'ripple' a little. And then they 'rate' the voltage and current -- in your case, 9-volts at 300 mA. This means, if you DO load the supply with 30 ohms, you should measure 9 volts (with 300 mA going through the resistor -- V == I * R, so V / I == R, so 9 / 0.3 == 30 ohms)

    Now, typically people read the supply "open circuit" -- just plug in the supply and put a volt meter across the leads. But 'open circuit', there's no load on the supply to 'drag down' the voltage. So yes, you read quite a bit higher voltage in this case. However, this is NOT a problem for the BS2.

    The BS2 has on the BOE a "linear regulator". This device acts to drop the voltage on its input side down to the desired output on its output side. These devices tend to be quite flexible -- they have a "drop-out" voltage, below which they won't regulate any more, and they have a "max voltage", above which they'll get too hot and shut themselves down for a while.

    The "Max voltage" tends to be around 25 volts, so your 15 volts or so won't hurt it at all.

    Now, what Chris is saying above about 15 volt adapters -- his point is, you're using a 9-volt 'rated' adapter, which has 15 volts "open circuit". This is not a problem. If you were using a 15-volt 'rated' adapter, ITS open circuit voltage could be 22 or 25 volts. This could be a problem -- the linear regulators would be running quite warm, and might shut down every so often, depending on what other devices you had running in the circuit.

    It's very unlikely you'd "see smoke" with a 15-volt adapter, those linear regulators are pretty robust. Now, if you hook the voltage up backwards, then yes, you'll definitely see smoke and do damage. "Forwards" here is the plus voltage to Vin, and the ground voltage to Vss.· (Only a regulated +5 volts can be applied directly to Vdd, by the way).

    Post Edited (allanlane5) : 9/4/2006 3:43:35 PM GMT
  • Brann FenixBrann Fenix Posts: 57
    edited 2006-09-04 15:01
    allanlane5 thanks for breaking it down for me [noparse]:)[/noparse]

    I have it running fine this adapter now. I just figured it was better to ask a simple question than risk toasting my BoE... and I even managed to learn something new, so it is a double win.


    Thanks again!
  • allanlane5allanlane5 Posts: 3,815
    edited 2006-09-04 15:41
    Oh, darn, I meant the plus voltage to "Vin", ground to "Vss". The only Vdd voltage you can use without damage is a regulated·+5 volts.

    Hope nobody broke anything!
Sign In or Register to comment.