The dangers of using too many volts and amps from a power adapter

Hello,
I recently received a motorized telescope that runs off of 8 AA batteries. A power adapter is available but I have yet to find one
available and my local telescope stores. The power requirements, as stated from the manual, are: 12 VDC, 750 mA (tip positive). I found a power adapter lying around at home that has printed for output: 12 VDC, 1.2 A (tip positive). First of all, if this power adapter is outputting the listed amount of power, would it be safe to use on the telescope? I pulled out my voltmeter and measure the output power of the adapter, and it hovered around 16 VDC. I tried some other power adapters around my house and they all seemed to be reading more VDC than was printed on the adapter itself. Is this normal? Or is my voltmeter just screwed? I measured the VDC from the 8 AA battery pack and it came up with about 12.2 VDC, which makes sense seeing as how each battery is 1.5 VDC.
Any help would be greatly appreciated! I don't want to hook an adapter up to my telescope that might fry it!
Thanks! Derek
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snipped-for-privacy@shaw.ca wrote:

Power adapters come in two basic types - regulated and unregulated.
Unregulated ones give about the right voltage when on load - but the voltage is typically much higher than that off load. Which is not a problem if the load is constant and the right amount for the adapter.
If you have a load with electronics in it where the load can vary, say depending on whether motors run or not, or how high the volume is turned up, then an unregulated supply can be a very bad idea.
Regulated adapters have electronics in them which keep their output pretty constant over a wide range of load.
You should be able to buy a regulated supply with the right output. They are a pretty common item - often with an adjustable output voltage and a number of different tips and a way of changing tip polarity. I would use one of those. Places like Radio Shack and even Home Hardware should have them, I would have thought, plus any shop selling portable radios, etc.
It may be that the telescope could work from an unregulated supply - but it is not worth the risk.
--
Sue



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Thanks for the advice!!!
Are the number of amps an issue? I have read that a device will only take in as many amps as it requires, so it is ok to use an adapter that has a higher amperage output than the device requires. (in my case, the adapter putting out 1.2 A and the telescope only needing 750mA).
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Sorry, one more question!
Is there an easy way to tell if a power adapter is unregulated or regulated? Will it usually say on the adapter itself?
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snipped-for-privacy@shaw.ca wrote:

Unless it says "regulated", you may assume that it isn't. They can sell regulated ones for more than the others, so they tend to mention it as a selling point.
Anyhow, you have a tester. If the adapter says 12v and it is giving out 12v, with no load other than the meter, it is regulated.
I would generally always at least measure a supply before connecting it to an (expensive) load. A double check that the tip polarity is indeed OK and that the regulator is, well, regulating. Taking a 3GBP faulty adapter back for an exchange doesn't get your 500GBP electronics replaced as well..
--
Sue

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Sorry, one more question!
Is there an easy way to tell if a power adapter is unregulated or regulated? Will it usually say on the adapter itself?
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: Sorry, one more question! : : Is there an easy way to tell if a power adapter is unregulated or : regulated? Will it usually say on the adapter itself? : Yes, it will.
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snipped-for-privacy@shaw.ca wrote:

Yes it is OK to use an adapter with a higher amp rating - within reason. Some adapters (typically big ones) need a minimum load to regulate properly. So, perversely, a 1.2A supply will be fine, a 120A supply may not.
--
Sue

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wrote: : > Thanks for the advice!!! : > : > Are the number of amps an issue? I have read that a device will only : > take in as many amps as it requires, so it is ok to use an adapter that : > has a higher amperage output than the device requires. (in my case, the : > adapter putting out 1.2 A and the telescope only needing 750mA). : > : Yes it is OK to use an adapter with a higher amp rating - within reason. : Some adapters (typically big ones) need a minimum load to regulate : properly. So, perversely, a 1.2A supply will be fine, a 120A supply may not. : : -- : Sue
No, that's not necessarily true. It depends on what the supply is feeding. He'll be getting bout 19V into the scope, which could be plenty high enough to cause problems, especially if it drives any electronics at all. If it's only driving servos/motors, then maybe; but the heat might become an issue and seriously shorten the life of those. The lower the current draw, the higher the voltage with an unregulated power supply.
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Pop wrote:

I was writing in the context of regulated supplies. I would not use an unregulated supply, or recommend one, unless I was very sure that no damage would result and the best advice to anyone, where there is any doubt, is to stick to using a regulated supply.
I am sorry that I didn't state specifically that someone would have to be a total idiot to use an unregulated supply to power something that /may/ contain electronics that could be damaged by over-voltage.
The lower the current draw, the more unstable the output from some switchmode regulated supplies can be. This is particularly true with some very high output switchmode supplies (and a 12v 120A power supply is probably going to be a switch mode design). They are typically designed for a minimum load - which, in the case of a 120A unit, is likely to be greater than a few hundred milliamps.
--
Sue




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----------------------------

It appears that the power supply is needed only for the motor. There is a good chance that the motor doesn't have electronic speed control. In that case, your power supply, even unregulated, will be fine (and even if it has some control). If the voltage is a bit high, the speed will be a bit higher but it is likely that this will be small in your case. The current rating of your supply is adequate.
--

Don Kelly @shawcross.ca
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: remove the X to answer : : You're brave: If the motors overheat and the life becomes, say, a couple of months, will you pay to replace them? It'd only take one overly long series of uses to create enough heat at 19V vs 12 (a 1.6 ratio) to begin causing heat damage to the cores and windings.
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I see some silly, some almost reasonable, some wrong respones here.
First, unless it says it needs a regulated supply, it does not.
Next, if the supply you have is capable of 1.2A at 12V, then at 0.75 amps, the "12" V will be higher than that by the same ratio. It's a nearly linear relationship. So no, if your telescope electronics is senstive to voltage maximum, you shouldn't use the 1.2A supply. That supply will only be 12V as it's rated at 1.2A and you're only drawing 0.75A. Not a good idea to use it. At 0.75A, the voltage would be about 19 V, definitely high enough to burn our any 15V regulators/components (as are often used) in the telescope cktry.
Try radio shack; 12V 750mA supplies are VERY common. If you look at your paperwork, they probably even recommend a specific part number which could be crossed to get what you need.
HTH,
Pop
: Hello, : : I recently received a motorized telescope that runs off of 8 AA : batteries. A power adapter is available but I have yet to find one : available and my local telescope stores. The power requirements, as : stated from the manual, are: 12 VDC, 750 mA (tip positive). I found a : power adapter lying around at home that has printed for output: 12 VDC, : 1.2 A (tip positive). First of all, if this power adapter is outputting : the listed amount of power, would it be safe to use on the telescope? I : pulled out my voltmeter and measure the output power of the adapter, : and it hovered around 16 VDC. I tried some other power adapters around : my house and they all seemed to be reading more VDC than was printed on : the adapter itself. Is this normal? Or is my voltmeter just screwed? I : measured the VDC from the 8 AA battery pack and it came up with about : 12.2 VDC, which makes sense seeing as how each battery is 1.5 VDC. : : Any help would be greatly appreciated! I don't want to hook an adapter : up to my telescope that might fry it! : : Thanks! : Derek :
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Pop wrote:

Recommending an unregulated supply for something where you have little idea of what electronics it contains is the height of recklessness.
This telescope may have complex electronics to generate the appropriate motor speeds to achieve cancellation of precession. It may even have a computer interface and send such overvoltage back into that and cause damage there.
The power supply rating, given in the manual, may not be, and probably is not, indicative of its normal run current - which could be close to zero when no motors are running. Thus, even a 750mA rated unregulated supply could over-voltage the electronics.
Sue

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Well, if the telescope is very complex, it's a safe bet there is a power supply regulator included. It would be very bad design to do otherwise. The proper place for a regulator is near the load, not the source.

Not with a decent regulator at the load end. -Bill
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snipped-for-privacy@att.net wrote:

I only gamble with my own chips. ;)
--
Sue

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Palindr?me wrote:

My digital camera uses four AA cells. The manual says otherwise it needs a certain expensive DC supply sold as an accessory for the camera. If I knew more about what's in the camera, I might try a run-of-the-mill supply.
The telescope manual gives only the voltage and current required. If there were something especially delicate in the circuitry, wouldn't the manufacturer have been more specific about what sort of supply was required?
Thirty years ago we had a couple of computers. A service call cost at least $10,000 and we couldn't be sure service would be available when a million dollars was on the line. So they sent me to the factory for instruction. The day I graduated, I was shown a computer in the other room. It had a bug that factory engineers had been trying to catch for a month.
My first bug was like that. I caught it after working five days at 21 hours a day. The fanatical hunt was so much fun that finding the bug was a disappointment.
The logic of each computer required 200 circuit boards running on 5 VDC. A board had 64 contacts. When a computer was out of service for couple of weeks, I'd pull a board at random, use a cheater cord to apply 120 VAC methodically across lots of points on the board, and reinstall the board. Then I'd wait a week so I'd forget which board I'd zapped.
Was that the height of recklessness? I was trying to keep my skills honed. After a week I'd run extensive diagnostics, hoping to encounter a frighteningly elusive bug.
I attempted to sabotage boards on three occasions. It never resulted in any evidence of a problem. That frustration left me with the conviction that "electronic" is not synonymous with "delicate."
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Sawney Beane wrote:

And if you knew enough about electronics, you could build or buy a supply to eliminate the 4 AA cells.
It does *NOT* require knowing what's in the camera. You *do* need to know what connector to use.

Perhaps. Perhaps not. Assuming they would is reckless.
Ed
<snip>
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ehsjr wrote:

You're right. If I knew more about electronics, I could emulate the battery with a regulated, filtered 6 volts through 1 ohm. To me, that wouldn't be a run-of-the-mill supply.
The camera could well have a diode to keep cells from exploding if a power supply is used. It could also have a regulator in the battery circuit to provide a steady voltage as battery resistance increases. A voltage suitable for the battery could be too high at the power-supply jack. So, not knowing what's in the camera or the power-supply specs, it would be prudent to solder my jack to the battery contacts. The batteries would be permanently eliminated. I'd have to bring objects indoors to photograph.

I would hesitate to connect anything to the power-supply jack without knowing the specs. I have a computerized shortwave three-cell radio that came with a power supply. The jack says 4.5 V and gives the polarity. That's enough for the consumer to buy a run-of-the-mill replacement supply if necessary.
I decided to change to nicads. The voltage from three was inadequate, so I needed four. That would be about 5 V. I'd want to charge the nicads in the radio. That would be about 5.5 V. I'd want to be able to charge them as I listened. With an impedance suitable for charging the nicads, that would mean nearly 6 V with the radio turned off.
Before changing from the OEM setup, I opened to case to see what components would be affected, what wattage they handled stock, and what they would have to handle at 6 V. I made minor changes and it has run fine with a run-of-the-mill 6 V supply for 20 years.
As I don't know the power-supply specs for my camera, I would want to analyze what's inside before trying a run-of-the-mill supply.

Then call me reckless.
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He said 16 V open. If it's 12 V at 1.2 A, that's 3.3 ohms, inductive and resistive. 16-(.75*3.3) = 13.5 V at 750 ma.
I'd risk it, especially if his telescope is like my neighbor's. It's so flimsy that you can't touch it to adjust it. You hold a double-throw switch on the end of a cord and use that to adjust the telescope with a cheap electric motor. You'll have trouble even aiming at the moon that way. If you succeed, you'll find the optics so bad that you'll put the telescope away and get your binoculars.
Pop wrote:

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