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Electro Fan
Beginner questions about determining DC power supply requirements
«
on: May 31, , 07:08:41 pm »
Some beginner questions about determining DC power supply requirements.
1. If a device doesn't clearly specify the DC input power connector as being center pin positive or center pin negative, how would you determine the polarity? Is it necessary to open the device and trace the circuit or is there a way to probe/test the center pin and/or outer barrel to determine the polarity? (Remember, in this situation, no power is applied to the device through the connector because we don't yet know with any confidence the connector's polarity ' so we are trying to figure out the center pin polarity with the device being in an off state.)
2. (Separate question from above, in this case we know the center pin polarity.) If a device doesn't specify it's current requirement (no label or documentation) but it does specify voltage (along with the correct center pin polarity) on a chassis label ' but the voltage is specified as a range (such as 13V to 24V), how would you go about a) determining the current requirement? And b) once you know the current requirement either because you figured it out through testing or because you found the documentation, how would you go about deciding what voltage to apply within the range? For example, let's say through testing or documentation we learn that the device draws a maximum of 3 Amps. How would you select the voltage within the 13V ' 24V range? Would go toward the low end (13V), the high end (24V), or somewhere in the middle? What would drive your decision on the particular voltage within the specified range?
Thanks
« Last Edit: May 31, , 07:29:59 pm by Electro Fan »
ArthurDent
Re: Beginner questions about determining DC power supply requirements
«
Reply #1 on: May 31, , 07:37:47 pm »
On #2, if you know it takes 13-24VDC and you know max current and the polarity, I'd say-it depends. Using a bench supply with V and A meters I would apply an increasing voltage from 13-24 VDC and see if the current remains pretty constant. If it does that probably means it has a linear regulator inside so any voltage over the minimum required will be dissipated as heat. In this case I'd go with 13-15 VDC depending on what you have available for a supply in that range and this will allow the device to run cooler and waste less energy as heat.
If when you increase the voltage from 13-24 VDC you see the current drop to almost 1/2 value then the device probably has a switching DC-DC converter inside and you could use any voltage in that range but I'd favor close to 24 VDC unless you want to power it in your car. I have some telco GPS units that will operate from 18-72 VDC and the current draw varies greatly depending on the supply voltage I use.
On #1 I'd check inside to see where the power goes to a big capacitor and see if I could figure it out from that. Sometimes the input may go through a diode for reverse protection and you can either see that or you can try to see if you can figure it out with a DMM in the input pins without taking the device apart.
rstofer
Re: Beginner questions about determining DC power supply requirements
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Reply #2 on: May 31, , 07:47:45 pm »
Plug polarity: I would use a DMM. With a positive voltage in the display (as opposed to a negative value) and given that the red probe is positive, it's pretty easy to figure out.
I would choose a supply voltage that is convenient. If the application is automotive, I would be on the low voltage end of things.
If I didn't know the required current, maybe I could find a wattage and work backwards. Either that or I would just use a car battery knowing that it can provide up to A for a brief period. I would measure the current with my DMM. I would probably put a 5A or 10A in series. The meter probably has a 10A fuse and I wouldn't really want to blow it.
If you do use a car battery, make sure you get the polarity right or you'll see just what A of 12V can do!
An alarm battery or motorcycle battery would probably be safer.
All above assuming you don't have a bench PS capable of delivering, say, 3A at 30V.
Somehow, you should be able to intuit whether the thing is going to take milliamps or megaamps.
Why not tell us what kind of device you're working on?
« Last Edit: May 31, , 07:54:40 pm by rstofer »
Electro Fan
Re: Beginner questions about determining DC power supply requirements
«
Reply #3 on: May 31, , 09:10:52 pm »
Plug polarity: I would use a DMM. With a positive voltage in the display (as opposed to a negative value) and given that the red probe is positive, it's pretty easy to figure out.
I would choose a supply voltage that is convenient. If the application is automotive, I would be on the low voltage end of things.
If I didn't know the required current, maybe I could find a wattage and work backwards. Either that or I would just use a car battery knowing that it can provide up to A for a brief period. I would measure the current with my DMM. I would probably put a 5A or 10A in series. The meter probably has a 10A fuse and I wouldn't really want to blow it.
If you do use a car battery, make sure you get the polarity right or you'll see just what A of 12V can do!
An alarm battery or motorcycle battery would probably be safer.
All above assuming you don't have a bench PS capable of delivering, say, 3A at 30V.
Somehow, you should be able to intuit whether the thing is going to take milliamps or megaamps.
Why not tell us what kind of device you're working on?
Hi rstofer,
Thanks. On the polarity, I don't see how you can get a DMM to tell you the polarity if there is no power present; and to apply power without damaging the powered device you would have to know the polarity. See the catch 22 here? In this "use case" there are no red or black wires visible or any markings on the chassis cover - just an opening for a power plug. By convention, it's probably center pin positive, but it could be center pin negative. It's easy to test under power, but if you power up in the wrong polarity you could damage the device. I get that inside there might be some markings or color coded wiring. So maybe the answer is there is no good way to know without removing the covers.
For the second set of questions, we are assuming we don't know the current requirement or the wattage specs, we just know a range of acceptable voltages. (For whatever reason the device has relatively poor markings and no documentation.)
As for the device I have in mind we can be pretty sure it will require a few Amps (maybe anywhere from 1-5 and probably 2-4), so I don't think we need or want a car battery. (Yes, a bench linear supply is available). Here, the question is what is the best way to choose the specific voltage to be applied given the range of voltage that has been specified.
In summary, the questions are really aimed at learning a) how to determine what the polarity is before powering up a device (so as not to damage it by guessing wrong), and b) then how to determine what voltage within a range of voltages would be best.
Electro Fan
Re: Beginner questions about determining DC power supply requirements
«
Reply #4 on: May 31, , 09:32:59 pm »
On #2, if you know it takes 13-24VDC and you know max current and the polarity, I'd say-it depends. Using a bench supply with V and A meters I would apply an increasing voltage from 13-24 VDC and see if the current remains pretty constant. If it does that probably means it has a linear regulator inside so any voltage over the minimum required will be dissipated as heat. In this case I'd go with 13-15 VDC depending on what you have available for a supply in that range and this will allow the device to run cooler and waste less energy as heat.
If when you increase the voltage from 13-24 VDC you see the current drop to almost 1/2 value then the device probably has a switching DC-DC converter inside and you could use any voltage in that range but I'd favor close to 24 VDC unless you want to power it in your car. I have some telco GPS units that will operate from 18-72 VDC and the current draw varies greatly depending on the supply voltage I use.
On #1 I'd check inside to see where the power goes to a big capacitor and see if I could figure it out from that. Sometimes the input may go through a diode for reverse protection and you can either see that or you can try to see if you can figure it out with a DMM in the input pins without taking the device apart.
Hi ArthurDent, Thanks! Sounds like for #1 it requires removing the covers or at least being able to peak inside to examine the circuit. But if I'm missing something and there is a handy dandy way to determine polarity by just probing the plug and measuring something at the unpowered receptacle on the chassis that would be good to know. (I think the answer is that end users shouldn't have to measure and test everything and that's why there are standards and in lieu of standards, there should be markings. In this case there just didn't happen to be sufficient markings, labeling, documentation, etc.)
On #2 your answer helped me learn that I should use a bench supply and start on the low end while observing the current draw. Your answered helped me learn that if the current stays reasonably constant that the device probably has a linear regulator, and that if the current decreases (nearly in half in this case) that the device probably has a DC-DC switching converter. I get why with the linear regulator it would make sense to operate near the low end of the voltage range, but please say more about why with the DC-DC converter the high end of the voltage range makes more sense. Is it because we have to assume that the designer specified the range (while using a DC-DC converter) so that that peak current requirements can be met? And if so, why didn't the designer just specify the higher end of the voltage range? Also, why is telco equipment spec'd with voltage ranges? Just to make it easy to find a power supply to power the equipment? Or is there some other reason? Thanks!!
magic
Re: Beginner questions about determining DC power supply requirements
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Reply #5 on: May 31, , 09:39:08 pm »
Usually center is positive, but there are exception. Usually if something is shorted to the chassis, it's the negative, but there are exceptions. Usually probing in diode mode in both directions would be safe (and perhaps give some clue), but there may be exceptions.
You didn't even say what kind of device it is.
Take it apart if possible and don't take chances.
Electro Fan
Re: Beginner questions about determining DC power supply requirements
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Reply #6 on: May 31, , 09:56:33 pm »
Usually center is positive, but there are exception. Usually if something is shorted to the chassis, it's the negative, but there are exceptions. Usually probing in diode mode in both directions would be safe (and perhaps give some clue), but there may be exceptions.
You didn't even say what kind of device it is.
Take it apart if possible and don't take chances.
Thanks, and Roger..... don't turn it on, take it apart - sounds like something that's been heard before, wonder where ?
The first device that drove question number one is an old SWR/power meter that had some markings and instructions but they were a tad ambiguous with respect to center pin polarity.
The second device that drove question number two on selecting a specific voltage within a voltage range is a GPSDO. Sometimes equipment made overseas comes with less than great markings, no or limited documentation, and/or language translation/interpretation issues.
In both cases I could search for info on the specific models but I'm more interested in learning how to solve the problems. That's the challenge/fun part almost as much as using the equipment.
lordvader88
Re: Beginner questions about determining DC power supply requirements
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Reply #7 on: May 31, , 09:57:21 pm »
IDK but LED's sure use a lot of current compared to plenty of simple transisotr circuictuits, so adding lots of pretty LEDs really is expensive.
As for question 1, the circuit probably has tank capacitors whose polarity would be set by the desired voltage rails in a circuit
If the x # of rails are just a stack of - to + voltage sources, then the caps are easy to follow from looking at
I guess common sized +1uF electrolytic caps will always be with the - on the lower voltage point. So follow the big Cap polarities, and u should deduce the rail polarities and likely current if u compare the cap size to other devices current draw after rectification, etc
« Last Edit: May 31, , 10:13:47 pm by lordvader88 »
Electro Fan
Re: Beginner questions about determining DC power supply requirements
«
Reply #8 on: June 01, , 08:46:17 pm »
Another idea....
Maybe if either the center pin on the DC plug connector on the device or the outer barrel connection point of the DC plug connector on the device is connected to the chassis of the device then it might be possible to determine which is positive (the center pin or the outer barrel connection point) by testing for ohms with one DMM probe on the chassis and the other probe checking the center pin vs the outer barrel connection point?
For example, if the resistance from the center pin to the chassis is relatively low, say 50 ohms, and the resistance from the outer barrel connection point to the chassis is relatively high, say 300k ohms, then the center pin is likely positive? And to confirm, using a continuity check from the center pin to the chassis should/might give a beep? And, assuming center pin positive, from the outer barrel connection point to the chassis would give no beep?
Bonus lesson: I think I discovered that the continuity beep will work up to about 50 ohms or maybe a few ohms less depending on the DMM.
soldar
Re: Beginner questions about determining DC power supply requirements
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Reply #9 on: June 01, , 09:30:11 pm »
Plug polarity: I would use a DMM. With a positive voltage in the display (as opposed to a negative value) and given that the red probe is positive, it's pretty easy to figure out.
I would choose a supply voltage that is convenient. If the application is automotive, I would be on the low voltage end of things.
If I didn't know the required current, maybe I could find a wattage and work backwards. Either that or I would just use a car battery knowing that it can provide up to A for a brief period. I would measure the current with my DMM. I would probably put a 5A or 10A in series. The meter probably has a 10A fuse and I wouldn't really want to blow it.
If you do use a car battery, make sure you get the polarity right or you'll see just what A of 12V can do!
An alarm battery or motorcycle battery would probably be safer.
All above assuming you don't have a bench PS capable of delivering, say, 3A at 30V.
Somehow, you should be able to intuit whether the thing is going to take milliamps or megaamps.
Why not tell us what kind of device you're working on?
I have no idea how any of this relates to the question being asked. A I missing something?
All my posts are made with 100% recycled electrons and bare traces of grey matter.
Johnny10
Re: Beginner questions about determining DC power supply requirements
«
Reply #10 on: June 02, , 06:47:17 pm »
DMM
Resistance to the chassis is pretty good start for negative or ground.
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- How to Choose the right Power Adapter for your Device
Author: Aus Electronics Direct
Date Posted:1 August
How to choose the right AC to DC Power Adapter for your device.
Do you have a working electronic device with a missing, failed or damaged power adapter or power supply?
In this article, we will tackle how you are able to identify a suitable replacement AC to DC Power Adapter or Power Supply for your electronic device.
What is an AC to DC Power Adapter?
Basically, its a power supply which converts 240V AC Mains Power into a lower usually DC voltage that your electronic device needs to operate.
What to look for?
The easiest way to replace a power adapter is by looking at the original power adapter supplied with your device. The power adapter will have a rating label usually on the underside or top of the power adapter. Below is a sample image of the rating label.
However, this may not be case where the original power adapter is at hand to be able to view the rating label. In this guide, we will run you through what you need to look for to successfully replace your devices power adapter.
Using the above rating label as an example, lets tackle the markings and explain what they mean and highlight what you need to note and look for.
- Model # - This model number refers to the original manufacturers part number. Unless you are communicating with the Original Equipment Manufacturer, this would have no relevance to power adapter suppliers and sellers.
- INPUT Specifications ' This shows the voltage range, frequency and current consumption of the power adapter. Australian Appliances including Power Adapters requires 240V AC. If the Power Adapter is sourced in Australia, this will meet the requirements. However, if they are being sources
- OUTPUT Specifications ' This is the most important information we require. Using the same rating label, the output states the output voltage, whether the output is AC or DC and the Maximum Current output. This particular Power Adapter indicates it has an output of 5V DC with 1A supply.
In essence, what we need to identify either through the power adapter rating label; looking for markings on the actual electronic device or instruction manual booklets are the following:
- Input Voltage
- Type of Voltage output = AC or DC
- Current requirements = measured in Amps (A)
- Size of DC Connector
- Polarity of Connections
The general rule of thumb is to match the Voltage (V) and equal or beat the current (A) requirements.
Checking the Voltage (V) Output
AC or DC ' We need to identify whether the voltage output require is AC od DC. Some power adapters indicate this with AC or DC immediately following the voltage output, however in most instance, they are signified by the below symbols.
AC is signified as a sinewave whist DC is signified with a straight line with 3 broken lines underneath.
Checking the Current (A) Output
Once you have confirmed the voltage of the device, we need to find the amount of current that it uses. Current tells us how much power the device needs to operate.
Amps: The number of Amps is usually stated right after the Voltage and is shown in A or mA. Please note that 1A is equivalent to 1,000mA. To select the appropriate power adapter, simply match or exceed the current (A) rating. Do not worry if the current rating is higher as all our power adapters are switch mode power supplies which provide regulated output. This means that your device will simply draw what it requires.
Amps and volts what about Watts?
Voltage carries the power and the Amps are the measurement of how much power it consumes, think of it like a river, the V is how wide the river is and the A is how fast the water is flowing, so a low volt high amp circuit is a small river flowing fast.
If we look at the river, we could calculate how much water is flowing into the river per hour. This would be the Watts. There is a direct relationship between these three variables.
A 12V 2A adapter (12m river flowing at 2m per hour) would have the watts of 12 X 2 = 24W.
The basic equations are here
Are you interested in learning more about a Dc Power Supply? Contact us today to secure an expert consultation!
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How to Choose Condensing Units SizeWhat Should You Consider When Choosing a Scroll Condensing Unit?Top 7 Water Cooling Advantages for Performance BoostTop Benefits of Water-Cooled Condensing Units ExplainedEnergy-Saving Cold Room Panels: Enhance Efficiency and SavingsW = V X A
A = W/V
V = W/A
Reading the polarity of the connector
Once you have the V and A down then comes the tricky part, most appliances will have a circular connector with the inside hole carrying one pole and the outside carrying the other.
The most common is centre positive (+) connectors.
There is also the less popular centre negative (-) connector.
The adapter and the device must match so always double check the symbol on the AC adapter and the device show the same polarity.
A centre negative connector.
Choosing the right DC plug
Hopefully, you have the size of the devices plug and can just check the adapters measurements, there are two measurements for these types of connectors.
- OD ' Outer diameter of the plug.
- ID ' Inner diameter of the plug.
The most common type of DC connectors used is the 2.1mm DC connector. This has a 2.1mm centre pin and has an outside diameter of 5.5mm.
Most of our single voltage power adapters adapt this 2.1mm DC plug. However, we do offer universal power adapter with interchangeable DC Plugs.
Below are different types of DC plugs with inside pin and outside diameter measurements for your reference.
If you are measuring, you need the diameter of the centre pin for the (ID) and the inner diameter of the port (OD).
ID an OD measurements for both the female and male connector must match to connect properly.
The universal power adapter
If you are not sure on the pin type or are looking to use it for a few devices, we recommend looking at a universal adapter. A universal power adapter is supplied with multiple interchangeable DC plugs. These allows you to be able to output at different voltages, with different plugs, configured in different polarities.
As USB slowly takes over, the charging system is becoming universal, USB is now becoming available in lower power devices and as chargers become more powerful there will likely be bigger and powerful devices running natively on USB. Until then remember to be safe with all electronics and double check everything, as incorrect voltage or polarity can damage your device.
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