Hello to all. First time posting a Q here in Breaktime, although I have been reading the forum for months and posting a reply to Q’s here and there if I had something to offer. What a great source of information, expertise and charachters! 🙂 This isn’t a home building Q, but this seems the most appropriate place to post.
I need to reduce the voltage on a 3V DC adapter (actual metered output around 3.75V) down to the output of 2 C-cell batteries (actual output around 2.75V). I rigged the adapter into a piece of electronic equipment (DW’s stationary bike – so I don’t have to keep recharging batteries), but it seems obvious the voltage must be too high as the LCD diplay goes full on – like a calculator showing nothing but 8’s. (BTW, I wired in the adapter so that it is either adapter or batteries, not both, so I’m not putting 6.5V thru it).
The keypad functions still work and changes are visible on the screen, but the voltage increase must also be messing with the ability of the bike to also change resistance levels, as they do not change. The circuit board must be stepping up the voltage to 12V DC for the resistance adjuster, as it is labeled 12V as the wires leave the board heading down from the diplay to the adjuster.
Will a simple diode from Radio Shack inserted into the adapter power line do the trick? Is there more to it than that?
Thanks in advance,
Cheers, Paul
Replies
The nominal voltage of the dry cells is 1.56 v, or 3.12 total. So you are not that far off.
In fact I am really surprised that it does not work. I suspect that you have other problems, such as an unfiltered supply.
But it would not hurt to try place 1 or 2 diodes in series. Get ones that are rated at 1 amp or more.
Although I should have learned by now how poor my EE memory has become, I'll open my mouth and agree with Bill, I think it's very unlikely that the voltage difference is the issue, many of those AC adapters are poorly or un-filtered and give you pulsating DC output. I'd say to look for a very good quality one (do you have access to an oscilloscope to look at the output?).
Thanks Paul. I think you answered my Q to Bill while I was typing it. No oscilloscope available to me and I presume that my $20 digital multi-meter may read a steady voltage when it is really fluctuating?
Re: the oscilloscope - I had to google that word as I'd forgotten what it was. At the price to buy one, I could buy DW a new bike! And outfit the rest of a gym as well. Yikes.
I bought the adapter years ago from Radio Shack for a walkman (using 2 AA). Had a car radio amplifier handy (on a 12v 3A power supply also from RS) and speakers and that was my university stereo system. Even the EE in my dorm was impressed (although he rigged up a table lamp to pulsate with the output going to the speakers on his system - I have no idea how that worked, but it sure was cool!)
Re: the bike - I'm still flummuxed re: what the problem could be, presuming a variable output from the adapter is not the problem and if a .75 to 1V difference doesn't really matter.
Cheers, Paul.
Thanks, Bill. I think that the C-cell chargeables are stamped 1.25V and output was as you say, above that.
If I recall the numbers correct from the other nite at the workbench, the total output of the 2 C-cells was about 1V less than that of the adapter. Maybe .75V. I have to check again.
Can you explain what you mean by unfiltered supply? (FYI- the adpater is rated 3V, 800 mA). Do you mean that the adapter voltage fluctuates (it was steady on the meter reading)?
I wasn't sure if diodes were the ticket to reducing the adapter voltage. I might have some around and will tinker to see what happens.
Thanks for the info.
Cheers, Paul
If it uses rechargable batteries (NiCads) then there nominal voltage is about 1.2 per cell.
There are several class of wall warts. Some are just transformser and they output low voltage AC, but that will be clear on the markings.
Then there are AC/DC converters. They are a transformer with recifiers. The output will be 120 cycles of rectifeid AC. Instead of a sinewave that goes positive and negative it just goes to zero and to peak 120 times a second. That going to zero can confuse the electronics.
Then there are "power supplies". The best ones have filter and regulated outputs.
That have a capacitor on the output which acts as "storage reserve" so that the output will not go to zero, but it will still have some ripple depending on size of the cap.
Then you have regulation which can not only smooth out the last of the ripple, but also maintain a constant output voltage reguardless of load or line voltage (with in design limits).
Here are a couple of RS units that would appear to work from the online specs.
http://tinyurl.com/3bx89
http://tinyurl.com/3cre7
http://tinyurl.com/3962b
Or you could try adding a capacitor.
I am guessing at this, but I think that any of these rated at 1000 microF or higher would work.
http://tinyurl.com/rcbn
Bill - great info. I think you just compressed a month of high school physics into one post. Fantastic.
And now it makes sense that a multi-metre will not show flucuations in the voltage, whereas the oscillator would.
I'll see if some diodes do the trick and if not, then try a capacitor as well, as you suggest.
I'll check the adapter specs as well - as I said to PaulB, I bought it at RS about 15 years ago and do recall dropping at least $10 (CAD) at the time. I'd be surprised though if it didn't already have filtered/regulated output you describe.
Thanks again.
Cheers, Paul.
Make sure the adapter is putting out DC. Some don't include a rectifier bridge, may use a half-wave setup or may lack a large enough capacitor to smooth the waveform enough. Of course I don't think any of these are the problem even though it would be wise to check.
My bet is that you have the polarity reversed as this could cause the display to lock on in some designs. 3.75v should work just fine for a two cell replacement. Most designs are based on theoretical batteries that put out 1.5v each. Most good designers include enough leeway that anything close will work.
I was wondering about polarity (no offense to the poster ;) too but he mentioned the function indicators and etc worked. Think that would be the case if it were inverted?
4LORN1 (and PaulB),
It's a DC adpater, no question on that. As for a polarity reversal, oh man, would I have egg on my face if that is my problem. I had checked it when soldering, but maybe I swithed the wires - it was 10:30 pm, past my bedtime. Guess what I'm checking tonite!! And if that is the problem, I will humbly post same on Monday. :) That's like returning the nails because half of them were pointing the wrong way as they came out of the box.
Thanks for the great input.
Cheers, Paul
If it's a DC adapter, and it's not wired backwards, my next guess would be that it has too much hum. A relatively large 10-25v capacitor (say, 500-1000uf) should provide enough filtering. Note that the capacitor may be a polarized electrolytic, so pay attention to polarity.
If you really think it's the voltage, then a simple voltage divider is probably the best "fix". Since this thing probably draws very little current (if your meter can measure mA, it would be good to check), you can use, say, a 100ohm resistor in series with 37ohm resistor. Put the output of the wart across both resistors, then tap power off beween the middle andn the 100ohm end. Quarter-watt resistors would probably work for this, but half-watt would be better.
Or you can use an old trick: A forward-biased silicon diode has a voltage drop of around 0.7V. One or two diodes in series with the supply would drop the voltage enough for you.
The trouble with resistive dividers is the high impedence.
I suspect that this nomrally only draws a few ma. But they often have alarm that go off that will draw 10 maybe 100 times the nominal current.
That will cause the voltage to drop and it if has any setup then dump it.
Yeah, you are correct.
Of course one could do a whole electronic regulator thing, but that's probably overkill in this situation. The diode approach is probably simplest.
Checked the polarity on the weekend - it is correct, so no egg on my face on that.
Let me add some more info, and my apologies in advance if it raises issues I should have first brought up. The keypad/display unit takes 2 sets of C-cells, but they are not wired together in the battery compartment. The +'s run separate to the circuit board, but the -'s are wired together in the battery compartment (or am I wrong to think that the two sets of batteries are running independant of each other?).
The problem with the display and lack of function arises (when using the adapter to run both curcuits) when I hook up the display to the rest of the bike by attaching the wiring leading down to the resistance motor. As I mentioned before, the circuit board is stamped indictating the motor is 12V DC. There are two wire sets leading down to the motor - one is two wires that plug in up next to the 12V DC marker on the circuit board. The other is a 5 (or 6) pin connection that perhaps relays the changes in resistance settings to the motor (total guess on my part).
If I plug in the adapter when the display unit is not hooked up to the bike, it functions normally - the LCD panel displays all functions correctly, including resistance and time.
If I run the adapter on only one of the two C-Cell circuits and pop in 2 C-cells for the other circuit and hook it all up to the bike, it runs fine. If I switch that around (run the adpater on the other C-cell curcuit instead along with batteries in the other) I get the same problems as when I use the adapter for both circuits.
This is why I thought maybe the one circuit was more prone to increased voltage and needed to be reduced.
The adapter voltage was fluctuating on the multimeter, randomly going up and down .01V, but generally reading 3.94V. The two rechargeable C-cells were putting out voltage at their stamped rating of 1.25V each.
Perhaps the voltage fluctuations reflect Bill's suggestion for a better adapter. And hopefully then some diodes reducing the extra 1.44V would fix the problem if the quality adapter did not do the trick.
Difficult for you to comment without it in front of you, but thanks to all for your thoughts (and any further ones).
hook up a bike headlight generater to the exercise bike wheel..problem solved.
Spheramid Enterprises Architectural Woodworks
Repairs, Remodeling, Restorations.
LOL! Hey, I can't even hook up an adapter properly. Let's not make this even more complicated for me! ;)
Was the thing shipped with rechargeable batteries in it? Could be that it's intending to recharge them from the generator.
If the unit is designed for either nicad rechargeable or regular carbon batteries, it should be designed to handle at least 3.2V, maybe a little more. The carbon batteries put out a hair over 1.5V when fresh.
No, it takes either of the carbon or rechargable C cell batteries.
Always put rechargables in the bike, but they tend to drain when not used (as it their wont), which was the reason to try to hook the bike up to an adapter.
I really don't understand why this isn't working. I wouldn't think putting in another .5V or .75V would have made a big difference (if that is the problem). Maybe the diode (or 2) in the one power line to the board will do the trick. If I get an adapter that is putting out a steady 3.x V of power, then it would seem the only difference in the setup is the voltage of the adapter versus that of the batteries.
Well, ripple in the supply would be another difference.
Also, they may have two different sets of batteries for a reason. Maybe one side creates noise on the supply that would cause problems for the other side.
Dan (and Bill, 4LORN1, and all)
Thanks for the input. In the process of moving houses, so all tools are now boxed. Will post later when I get chance to test out further your theories on the problems and fixes.
Cheers, Paul.