Perplexed about volts. - Page 1
Forum section: electronics / general

author: J.Xerneas
author_rank: Contributor
timestamp: June 13, 2016, 01:25:13 am
content: I have a Radioshack weather radio, this one to be exact:http://support.radioshack.com/support_electronics/doc69/69290.pdfIt is supposed to run on 12v 300mA but the power supply I got with it from a relative is 4.5v 800mA. I didn't realize it when I got it, only reading the rating on the supply, I initially presumed it was 4.5v. After reading the pdf on it and seeing that it was 12v it got me wondering, and after testing the voltage while plugged into the 4.5v supply I got confused. It is reading 6.5v... Something doesn't add up. A 12v radio running on a 4.5v supply I can accept, but a 12v radio on a 4.5v supply running at 6.5v*Deep BreathI can not.What the Hell am I missing.

author: Rerouter
author_rank: Super Contributor
timestamp: June 13, 2016, 01:35:18 am
content: Ok, so there is a few things going on here,First up cheap power bricks that plug into the wall are rated for a voltage at a load, (800mA in your case), they regulate rather poorly so open circuit they will be higher, and over the rated load they will be lower, (Its not uncommon for a 5V brick to output about 9V open circuit) but stick on the rated 800mA load and it should be within half a volt or so,Next up the radio being rated for 12V input, This could well be a leftover from the first batch of power supplies they found, and later found the 4.5V for less and switched to them, (these devices are built to a price, and a power brick is a fair chunk of it) It also means people wont get hung up on plugging it into a car (12V is 12V right?)Now the radio working on as low as 4.5V is just down to what device they are using for the power supply, at a guess a linear regulator for 3.3V, or it could even be a 5V if they are running it really on the edge of the ratings,So yeah a few things going on, mostly down to "built to a price" reasonings

author: StillTrying
author_rank: Super Contributor
timestamp: June 13, 2016, 01:52:43 am
content: The 'weather radio' in the pdf will need 12V DC at 300mA, I doubt it will do anything at 6.5VDC.I think they've given you the wrong DC adapter.

author: Brumby
author_rank: Unknown
timestamp: June 13, 2016, 02:19:08 am
content: This:Quote from: Rerouter on June 13, 2016, 01:35:18 amThe two points are:1. Transformer based power adapters are notoriously variable in their output voltage depending on load (SMPS less so). The figures you have presented are completely normal. (I have a box of old adapters and once I went looking for one to power the charging circuit of a beard trimmer. The charging circuit worked best with a 6V supply, which I was able to get out of a 4.5V plugpack that was rated for five times the current I needed.)2. Equipment, especially low power stuff, is often able to operate over a wide variety of supply voltages (unless you're tied into something inflexible, such as TTL). This can be due to internal supply regulation or just a very tolerant design. So the question is - Does it work with the supplied adapter?

author: Brumby
author_rank: Unknown
timestamp: June 13, 2016, 02:27:15 am
content: Quote from: J.Xerneas on June 13, 2016, 01:25:13 amQuote from: J.Xerneas on June 13, 2016, 02:20:44 amWhich is it - 6.5v or 4.6v ?

author: Skimask
author_rank: Super Contributor
timestamp: June 13, 2016, 02:32:52 am
content: What's the AC component of your 4.5 or 6.5 or 12v power supply? Or whatever the hell it is...Your posts/numbers/facts are about confusing...

author: Rerouter
author_rank: Super Contributor
timestamp: June 13, 2016, 02:41:30 am
content: Well are you measuring AC or Dc voltage, and on which nodes.4.5V AC becomes ~6V DC after being rectifier through a diode,Or if your measuring DC on both nodes, they must be using a switching regulator, 4.5V DC in, 6.5V DC out.

author: MarkF
author_rank: Super Contributor
timestamp: June 13, 2016, 02:42:10 am
content: 1) Does the radio work with the supplied adapter?2) Re-read the specifications....It says the radio power requirements are 120 VACpower source (supplied AC adapter) or 12 VDCvehicle power source (optional adapter).The specifications never say what the actual operating voltage is.

author: Delta
author_rank: Super Contributor
timestamp: June 13, 2016, 02:42:22 am
content: Where and how are you measuring "the voltage of the radio"?

author: MK14
author_rank: Super Contributor
timestamp: June 13, 2016, 02:45:22 am
content: Quote from: J.Xerneas on June 13, 2016, 02:20:44 amOne possibility that comes to mind, is that the adapter (4.5V), may be just a transformer and rectifiers. Since what it was designed to power did NOT need capacitors (removal of ripple), and/or the capacitor(s) were expected to be designed into what the adapter plugs into.So when you plug it into the radio, any capacitors in it, may make the voltage go up by approx 50%, as they will tend to remove the ripple.I.e. charge to the peaks of the rectified DC.Hence approx 6.5V.

author: StillTrying
author_rank: Super Contributor
timestamp: June 13, 2016, 03:09:12 am
content: Quote from: Delta on June 13, 2016, 02:42:22 amI'm stuck on that one as well.

author: MarkF
author_rank: Super Contributor
timestamp: June 13, 2016, 03:20:09 am
content: What does the adapter say? Most are labeled with their output voltage and AC or DC.Your adapter may output either AC or DC.

author: Brumby
author_rank: Unknown
timestamp: June 13, 2016, 03:47:48 am
content: Quote from: J.Xerneas on June 13, 2016, 02:30:56 amBatteriser?[ Just to be clear, that wasnota serious suggestion, BTW ]

author: Delta
author_rank: Super Contributor
timestamp: June 13, 2016, 11:20:55 am
content: Quote from: J.Xerneas on June 13, 2016, 02:54:31 amYou are failing at that I'm afraid...Please draw a sketch to enable us to know what on earth you mean by "the voltage of the radio".What meter are you using for these measurements?

author: MK14
author_rank: Super Contributor
timestamp: June 13, 2016, 11:25:33 am
content: Quote from: Delta on June 13, 2016, 11:20:55 amAs I understand him to mean.When he measures the power supply output, un-connected to anything (open-circuit), (using his multimeter, scope NOT available), he gets about 4.5 volts.But when he then connects it up to the radio, it AMAZINGLY/PUZZLINGLY jumps up to about 6.5 Volts. Measured by somehow connecting the multimeter up, maybe by opening the radio, or unscrewing the plastic of the connector plug (hence exposing the contacts for measurement), or something ?

author: Tandy
author_rank: Frequent Contributor
timestamp: June 13, 2016, 11:38:31 am
content: The radio scanner will have a voltage regulator internally,I don't have access the the scanner schematic but it could be buck/boost regulator that will enable the radio to work on a wide range of voltages.Edit: It looks like the radio has an AN77L05 voltage regulator on the input, the internal circuitry runs at 5V so theoretically any input voltage between 5.03 to 15.7V will run the radio.

author: StillTrying
author_rank: Super Contributor
timestamp: June 13, 2016, 12:13:17 pm
content: Quote from: MK14 on June 13, 2016, 11:25:33 amYeah, I did think of that but most of these connectors are molded-on to the wire.4.6 X 1.414 = 6.504According to what Tandy has just posted this radio should be 'working', and believe it or not the OP doesn't actually say that the radio is not working!This thread is a bit like trying to decorate a room through the door's keyhole.

author: cowana
author_rank: Frequent Contributor
timestamp: June 13, 2016, 12:21:09 pm
content: Quote from: Tandy on June 13, 2016, 11:38:31 amThe AN77L05 datasheet seems to say 5.3v to 15.7v?

author: MK14
author_rank: Super Contributor
timestamp: June 13, 2016, 12:22:52 pm
content: Quote from: StillTrying on June 13, 2016, 12:13:17 pmI think he has VERY old equipment, from a long long time ago, by the sound of it. So they tended to be made by hand then (I think ?). So the plastic tended to be un-screw-able, from the plugs. But it is a VERY long time ago, and I may be remembering it wrong.I think the commonly molded ones, came later on.Good point about the 4.6V/6.5V ! (root 2).Anyway I hope/bet that the OP can explain HOW ? (the voltages were measured).Mr Bean would find a way of decorating a room via the keyhole and a suitably small sized firework.Edit:Or maybe the adapter has exposed/exchangeable connector pins (which leave enough gap to put probes into it), as in this picture:

author: CJay
author_rank: Super Contributor
timestamp: June 13, 2016, 12:43:58 pm
content: Quote from: J.Xerneas on June 13, 2016, 01:25:13 amThey lost the PSU and found one that 'fits', there's enough headroom in the crappy 'regulation' of the transformer to allow the regulators inside the radio to produce close enough to the correct voltages for it to work.

author: StillTrying
author_rank: Super Contributor
timestamp: June 13, 2016, 01:24:37 pm
content: Quote from: MK14 on June 13, 2016, 12:22:52 pmI'm looking forward to the explanation of how the radio boosts the adapter, and then seeing the real time NOAA images!

author: Delta
author_rank: Super Contributor
timestamp: June 13, 2016, 09:27:22 pm
content: What model Fluke do you have?Set the meter to AC volts, and measure the open circuit voltage of the PSU.

author: Skimask
author_rank: Super Contributor
timestamp: June 13, 2016, 10:43:46 pm
content: Quote from: StillTrying on June 13, 2016, 01:24:37 pmNot boosting, rather the radio itself is filtering a full wave rectified DC voltage from the wall wart thereby giving the appearance of a higher voltage.4.5 volts of 120Hz full wave DC / .7071 = about 6.36v filtered-ish DC.And do we know what kind of meter?DISREGARD-Fluke, doesn't matter what model, it's probably going to do a good job of approximating the RMS of any wavy lines...

author: MarkF
author_rank: Super Contributor
timestamp: June 14, 2016, 12:56:55 am
content: NO WHERE IN THE DATASHEET DOES IT SAY THE RADIO NEEDS 12V TO WORK!Where is this coming from? The datasheet says it will accept a 12V adapter for use in a vehicle. That's 12 volt into the adapter. Not the adapter output!

author: Monkeh
author_rank: Super Contributor
timestamp: June 14, 2016, 01:51:08 am
content: Quote from: MarkF on June 14, 2016, 12:56:55 am... but it does say to use a 12V power supply connected to the DC 12V input...

author: MarkF
author_rank: Super Contributor
timestamp: June 14, 2016, 03:23:38 am
content: Quote from: J.Xerneas on June 14, 2016, 03:02:08 amQuote from the specifications on page 34 of the pdf:"Power Requirements...................................... 120V AC, 60 Hz, 8W(with supplied AC adapter),vehicle’s 12V power source(with optional DC adapter)"Show me in the pdf where it says the radio operates at 12V.

author: MK14
author_rank: Super Contributor
timestamp: June 14, 2016, 03:30:52 am
content: Quote from: MarkF on June 14, 2016, 03:23:38 amHave your reading glasses fallen off ?It mentions it in a number of places.Quote is from the pdf in the opening OP's post.Quote

author: Tandy
author_rank: Frequent Contributor
timestamp: June 14, 2016, 12:07:30 pm
content: To clarify the radio was originally supplied with a 12V AC adaptor but 12V is not required internally there is a voltage regulator that will take an input of anywhere between 5.3 & 15.7V. In fact the original manufacturer supplied a 9V power supply with the unit, I don't know why RadioShack chose to include a 12V power supply with the RadioShack branded version, presumably they were able to source 12V supplies cheeper due to large order for other equipment.

author: StillTrying
author_rank: Super Contributor
timestamp: June 14, 2016, 04:43:42 pm
content: So...The instruction manual is wrong in insisting that this version needs 12V DC, maybe the manual writer just copied the power supply bits from an earlier manual.As for the rest of this thread, I think I'll have to go for lie down!

author: MK14
author_rank: Super Contributor
timestamp: June 14, 2016, 05:44:59 pm
content: Quote from: StillTrying on June 14, 2016, 04:43:42 pmThe instruction manual was NOT necessarily wrong at all.A cheap/basic 12 volt DC rectified transformer adapter, from many years ago. Would have had fairly bad specifications, by todays standards. (E.g. typically no switch mode power supplies etc).As you have to allow for the voltage to drop by as much as 20% (maybe more), under high load. You (ideally) want to allow the mains supply voltage to drop by 10% or 15% or maybe more, as it can vary, between homes/locations and what time it is, etc.Then the ripple (since finite valued filter capacitors) will allow the voltage to droop (since it is unregulated BEFORE the radio DC in connector), during the 50/60Hz mains cycles (100/120 Hz half cycles).The rectifier diodes will have an increasing voltage drop, which gets even higher, at max currents.You then may want a safety factor.The regulator may need a fair bit more than the 5V, to work well/properly.The 5.3V figue some are quoting earlier in the thread, does NOT seem to take into account, more complicated factors within the voltage regulator. Such as its junction temperature, which inevitably rises, under load.The 5.3V was at exactly 25 deg C junction temperature. There are other factors I don't want to go into (makes the post too long), which may also increase the value. Really the 5.3V is the BARE MINIMUM, not the usable/reliable/safe value.In practice it wants at least 6V, but probably 7V or even better 8V. Then add the various other stuff above. Hence 12 Volts is probably NOT too unreasonable.9V sounds a bit tight to me.On the other hand, too high a voltage, may excessively heat the (possibly) un-heatsinked, voltage regulator.tl;dr5.3V is the bare min, and really you need considerably more, for it to work well, if we are talking about linear regulators, with ripple voltages and possible mains voltage variations etc etc.EDIT:Obviously I have NOT got access to TANDY's/Radio-shacks datasheets and specifications. So I could be partly wrong, on my estimated values. The transformer could be specified such that the 20% drop, is included, so that it makes 12 Volts or more. Depends on the specifications etc.Also things tend to be set to "STANDARDIZED" voltages. So 12 Volts is much preferable to whatever the bare minimum (inc losses/drops), actually was/is. E.g. 9.67835253V.

author: Red Squirrel
author_rank: Super Contributor
timestamp: June 16, 2016, 03:01:53 pm
content: If the radio has a built in regulator, it probably runs on 5v or 3.3v internally (ex: the ICs and such) but can accept 12v. Wall warts tend to be off a bit as well for various reasons such as open circuit being higher than with load.Now question is, will the radio take 120vac directly.

author: rrinker
author_rank: Super Contributor
timestamp: June 16, 2016, 04:35:05 pm
content: Don't turn it on, blow it up!