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Electronics Experts please help(AC/DC voltage question)

U

ut1959

Member
I have a blood pressure monitor that has a port for an adaptor (says DC6V). Now it didn't come with an adaptor but I found an old sony one from a music device that says (4.5V AC 120V 60Hz 5W Output DC4.5V 500mA). Now is it safe to use this adaptor for the blood pressure monitor? The voltage is lower on the adaptor....
 
If the polarity of the Sony connector is the same as the correct adapter's, it shouldn't hurt anything. It may not work, but it shouldn't damage anything...

If you have a multimeter, it's easy enough to check the polarity of the adapter, but unless it's marked on the BP meter, it wouldn't be so easy to figure that out. I can't say for sure, but I think it's unlikely that connecting the power "backwards" would hurt the meter, so if you try it and it works, you'll know it works ("d'uh"), but if it doesn't work, you won't know if it's because the voltage or current is too low, or if the polarity is just wrong...
 
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Thanks for the info Mike! Yeah I forgot to mention that the monitor does work with the Sony 4.5V Adaptor. I was assuming that it would be okay since the voltage is lower.

Thanks again!
 
I'm far from an electronics "expert", but if it does in fact work, I doubt even the motor/pump in a BP meter could be damaged by even long-term use of too-low voltage and can't even imagine how it could damage the circuitry/electronic components themselves...
 
You can damage the BP monitor if the polarity is wrong. Make sure it's right. Tip positive or tip negative? I would double check both the polarity and the voltage before using it with a multimeter.
 
Both are tip positive so the polarity appears to be fine. The only problem I see now is that the device expects 6V but the power supply supplies only 4.5V.

Sometimes those power bricks are unregulated and may provide more than the rated voltage so you should check it with a multimeter just to see what it really is outputting.

In any event as others have correctly stated a low voltage condition would only cause the device to not function on that power supply but won't damage it. A high voltage might cause damage which is why I always double check non-stock power supplies before using them.

Go ahead and try it you might get lucky. If it doesn't work then a 6V @ 500ma or 1A wall wart power supply shouldn't be too expensive to buy if you need to.

Here's one with the correct polarity and rated voltage on Amazon for $6: https://www.amazon.com/DIKOO-Adapte...470775972&sr=8-1&keywords=6v+1a+power+adapter
 
disappoint said:
Both are tip positive so the polarity appears to be fine. The only problem I see now is that the device expects 6V but the power supply supplies only 4.5V.

Sometimes those power bricks are unregulated and may provide more than the rated voltage so you should check it with a multimeter just to see what it really is outputting.

In any event as others have correctly stated a low voltage condition would only cause the device to not function on that power supply but won't damage it. A high voltage might cause damage which is why I always double check non-stock power supplies before using them.

Go ahead and try it you might get lucky. If it doesn't work then a 6V @ 500ma or 1A wall wart power supply shouldn't be too expensive to buy if you need to.

Here's one with the correct polarity and rated voltage on Amazon for $6: https://www.amazon.com/DIKOO-Adapte...470775972&sr=8-1&keywords=6v+1a+power+adapter
Click to expand...

Hey thanks for the info! Yeah the device works with the 4.5V adapter fine, I'll continue to use it since it won't damage it. If worse comes to worse I'll get the adapter from amazon.

Appreciate it!
 
disappoint said:
You can damage the BP monitor if the polarity is wrong. Make sure it's right. Tip positive or tip negative? I would double check both the polarity and the voltage before using it with a multimeter.
Click to expand...
Maybe I was being hopelessly optimistic, but I kinda figured something as "sophisticated" and comparatively expensive as a BP meter that can run on batteries would have at least rudimentary protection against reversed polarity since there's a pretty good chance that a fair number of people will stick the batteries in backwards at some point during their ownership of them, and that the designers wouldn't go quite as far as building in that protection in a way that it only protected against reversal of the batteries, rather than farther along in the circuitry where it would work with any input power. But maybe that was a silly assumption...
 
Sometimes the wrong power supply will fry a device, even if the voltage matches what the port says. Some devices expect the power supply to do things like current limiting. Seems like cheaper the device the more likely it isn't fussy.
 
Running any device designed to run on a given voltage/ current rating with a different voltage/current will be much harder on the regulation circuits than the designer intended. This may result in premature failure and could void any warranties. Note that a voltage regulator attempting to "pull up" a voltage that is too low will obviously be heavily stressed in your particular situation, and will pull more current than normal on your power supply in it's attempts to compensate. YMMV....

And cost of device is less of a factor than the internal simplicity of it.
 
Mike64 said:
Maybe I was being hopelessly optimistic, but I kinda figured something as "sophisticated" and comparatively expensive as a BP meter that can run on batteries would have at least rudimentary protection against reversed polarity since there's a pretty good chance that a fair number of people will stick the batteries in backwards at some point during their ownership of them, and that the designers wouldn't go quite as far as building in that protection in a way that it only protected against reversal of the batteries, rather than farther along in the circuitry where it would work with any input power. But maybe that was a silly assumption...
Click to expand...

It might, but then you risk blowing a fuse or a diode. Those can be replaced, but less hassle just double checking your power supply is the right one.

Kartajan said:
Running any device designed to run on a given voltage/ current rating with a different voltage/current will be much harder on the regulation circuits than the designer intended. This may result in premature failure and could void any warranties. Note that a voltage regulator attempting to "pull up" a voltage that is too low will obviously be heavily stressed in your particular situation, and will pull more current than normal on your power supply in it's attempts to compensate. YMMV....

And cost of device is less of a factor than the internal simplicity of it.
Click to expand...

This is true and it's a good idea to check the voltage regulator's or buck/boost converter's temperature while the unit is powered on and running to make sure it can handle the power supply without burning out. Hopefully it has a heatsink. If not you can put one on it.
 
ut1959 said:
Thanks for your input guys, I'm thinking of just buying the 6V adaptor
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Don't forget that 4 x 1.2V rechargeable batteries, would be about 4.8V, and very slowly drop as they are used. Also a 4.5V adapter would tend to be a bit higher voltage in practice, depending on the load. 4.5V is very close to 4.8V, so surely it is designed to cope with that voltage ?
Or has the instruction manual BANNED rechargeables, because of the slightly lower voltage ?
tl;dr
If it seems to be working fine with the 4.5V adapter, and the battery low warning light is off (not showing), you should be fine.

Battery voltages usually drop, and drop while they get used up, so it should be designed to work at voltages below the 6V. Usually a battery warning symbol is designed to show, when the voltage is too low.

As regards some posters worries about it overheating (which I think is very unlikely to be an issue). That is usually when you give it TOO HIGH a voltage. 4.5V should be designed for, since the batteries, as they wear out, can easily reach that voltage and lower. So if overheating was going to be a problem, it would anyway, with battery use.
 
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SOFTengCOMPelec said:
Don't forget that 4 x 1.2V rechargeable batteries,....
Click to expand...
I did not take that factor into account, as most medical equipment I ran across (to include my own BP device) did in fact include a notice to only use alkaline batteries. (Some paranoia tends to be normal with medical devices, which to some extent is justified)
I do, however, stand by the idea that wrong voltage- while it may work fine indefinitely- is bad for the device's extended lifespan/ warranty.
 
Kartajan said:
I did not take that factor into account, as most medical equipment I ran across (to include my own BP device) did in fact include a notice to only use alkaline batteries. (Some paranoia tends to be normal with medical devices, which to some extent is justified)
I do, however, stand by the idea that wrong voltage- while it may work fine indefinitely- is bad for the device's extended lifespan/ warranty.
Click to expand...

It probably would be ok, as regards overheating. But I agree it is theoretically a possibility. Depending on how it has been designed, which I don't know.

If the blood pressure meter has an important and/or hard life. E.g. Doctors use and/or very ill patient, or is used many times a day. I guess a proper/quality 6V adapter would be best. Cheap/Chinese/Ebay ones could be problematic.

Maybe the full 100% accuracy is NOT achieved at voltages different to 6V, I'm NOT sure. But it could be an issue for the OP.

If the mains voltage dips (drops), the 4.5V may lower, and become too low, for accurate readings, or even NOT work at all.
 
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