Topic: Chumby Classic AC adapter

I just got a used Classic, which looks to be really sweet.  Since my previous Classic was killed by a bad AC adapter, I want to be very careful with this newer one.

When I check the adapter [no load] output with a DC voltmeter, the voltage seems to fluctuate.  It slowly drops down to about 11.5, then quickly moves up to about 12.2, then slowly drops down to about 11.5, etc. etc. endlessly.

Is this situation normal / typical?  Will it hurt the Classic if I plug in this adapter (I've read specs that say it will work from 6.5v to 14v, so in theory this fluctuation is still within acceptable range).

Thanks for any enlightenment!

Re: Chumby Classic AC adapter

You're not really going to get an accurate reading with no load on it.  I think many power supplies usually read way high with no load, for example, 12V adapters read 15V.

Linux Guy - Occasional Chumby Hacker

3 (edited by gminpa 2017-04-24 04:02:16)

Re: Chumby Classic AC adapter

What you describe is typical of an unregulated power supply. 

A regulated 12v supply should always put out a steady voltage very close to 12v, until the load becomes larger than the supply can handle.  At that point, voltage will start to drop, probably pretty quickly.

OTOH an unregulated supply will behave as you described.  With no load the output voltage will be higher than the nominal voltage.  With heavier and heavier loads, the voltage will go lower and lower.

An unregulated supply killed my previous CC.  The label said 14v.  And the CC should work with voltage up to about 14.5v (based on what I've read several places).  The previous owner assumed it was OK; I bought it on eBay and did not check the voltage when I received it.  The CC worked for a few days, then smoke came out and it was dead.  After that happened, I checked the supply and found that with no load it put out 20v!  (definitely unregulated)  That was much more than the CC could handle, and eventually something failed on the motherboard.

Having said all that, I have never seen a supply that behaves like the one I'm asking about.  Even with no load (actually the meter is a very small load, several thousand ohms), the supply output voltage "pulsates" if you can picture that.  Connect the meter, and it reads ~12.2v.  Then as you watch the meter, the pointer slowly drops down to ~11.5v.  At that point the pointer jumps quickly up to ~12.2v.  Then it starts slowly dropping again.  If I had to guess, I would say it's a regulated supply (after all, the voltage swing is relatively small) but there is something odd about the time constant of the regulator circuitry ... maybe an open output capacitor.

So the key question:  is this normal for an original CC power supply?  (It has the Chumby name stamped on it.)  Or is it malfunctioning, and thus liable to damage the CC if I use it?

I guess I need to get an appropriate sized load resistor, and look at it on a scope, to see if there's a lot of ripple.  I was just hoping Duane (or someone else) could tell me offhand whether this behavior is typical and safe.

Re: Chumby Classic AC adapter

It would be interesting to check out that power supply on a scope rather than a voltmeter to see what it's doing in realtime.  The voltmeter may not be able to show you any voltage spikes quickly enough.

I can't really say one way or another whether that particular power supply is good - all I can say is that the failure rate of chumby Classic power supplies has historically been very low, especially as compared to the C1 power supply.

Re: Chumby Classic AC adapter

Duane, thanks for the comments.

I don't have a scope here at home, but I have one at the shop.  I'll check it when I have a chance.

FWIW, I got those readings using a Simpson 260 VOM.  Input impedance is 25k ohms/volt, and I was on the 25v range, so the load placed on the power supply was ~ 625k ohms.  That's lower than a typical modern DMM.  I can imagine the 625k slowly discharging the output cap from 12.2 down to 11.5, at which time the supply would "turn on" and recharge the cap.  Basically it's acting as a sawtooth generator.

I'll let you know what I see, when I have a chance to do a scope test.  I will have to dig up some appropriate power resistors to put an appropriate load on it without smoking the resistors.