|Cornish Jack||19/11/2011 11:12:50|
|1190 forum posts|
Possibly one for Les Jones or one of the other wiggly amp gurus.
I've just got some DROs which should be powered by a 9 volt supply. The makers stipulate an UNregulated supply. My only previous experience with such things resulted in nadgering an FM radio (6 volt selected on a variable output 'wall wart' ) Checking the output (later) with 6 volt still selected showed 17 volts on the meter. The vendor of the supply confirmed that this was what one should expect. I am not the sharpest tool in the box re 'lectrics and I cannot see the point of having - a) variable selections on an UNregulated supply (if the output is always maximum) or
b) why would such a supply be stipulated.
Would much appreciate an explanation ... in simpleton terms , please?
|Les Jones 1||19/11/2011 14:28:52|
|2192 forum posts|
The unregulated plug in power supplies I've seen select the voltage by selecting taps on the transformer secondary. I would expect the "6 volt" setting to give up to about 10 volts with no load but not as high as 17 volts. Without examining the unit that gave this problem I could not say if it was faulty or just a very bad design. (For example just switching in resistors to reduce the voltage. Are these DRO's DRO350's by any chance ? The only reason I can see for it to say it must be unregulated would be if it needed a little more than 9 volts. If that was the case it would need to specify the current rating of the supply. If this was the case they would specify a supply with a higher current rating than the current drawn by the DRO so the output voltage was on the high side.
|John Shepherd||19/11/2011 15:20:11|
|218 forum posts|
I can think of no reason for using an unregulated supply. I am certain that if you tried a regulated 9v supply the DRO will come to no harm. I would not be so sure if that was the case if you tried an un regulated supply however.
If the DROs work with a 9v battery, then a 9v regulated supply will probably be a good investment..
Hope that helps.
|Clive Hartland||19/11/2011 16:24:56|
2631 forum posts
The term, 'Unregulated', is that only half wave DC, or are they saying that it does not need voltage regulation?
Two different things here I think.
Logically the DRO would need a useful DC at 9V but the voltage coming from a PSU rectifier is usually above 9V to allow for a regulator to work, so a voltage of 13 to 17 V would be OK.
You can give me a kicking if Im wrong.
|Steve Garnett||19/11/2011 16:36:12|
|837 forum posts|
All I think it means is that whatever regulation the device needs, it's provided internally. This means that it will be absolutely fine on a regulated or non-regulated supply. The only thing you don't want to do is to use a regulated supply with a significantly higher voltage, because then you'd stand a chance of exceeding the regulator's dissipation limit. This generally isn't such an issue with unregulated supplies, because they tend to drop their non-connected voltage when they're actually connected to something, and there's nothing like enough power available to threaten the regulator anyway.
Most modern DROs don't need anything like 9v to make them run, and if it's the Shumatech one it should be happy with anything above 7.5v - the regulator will drop out below 7v.
|Speedy Builder5||19/11/2011 17:13:47|
|2187 forum posts|
I am not an electro whiz, but you can make a regulated supply from a phone charger, a LM317 regulator, a fixed and variable resistor (Thats pretty basic) - yes, you can add capacitors etc. Just google LM317 and you will find plenty of circuits. The LM 317 (Or variants) will clamp the voltage irespective of load or input voltage within reason.
|Steve Withnell||19/11/2011 18:37:57|
825 forum posts
Do the makers say a regulated supply will not work? The note might be simply implying don't waste your money on a regulated supply. Most devices today seem to have a fairly wide tolerance on supply voltage.
Dredging the depths of my brain, I once built a regulated supply to produce a 15volt supply and then tapped off that to drive a regulator to deliver 12 volts off that - regulators in series.
My lecturer told me that was bad practice as there could be a tendency for the regulators to oscillate when configured like that, never figured that one out. If the kit you are driving has a regulator built in as Steve G's note implies, you will have two regulators in series and the voltage diferential between them will be similar to my "bad practice" design.
|Steve Garnett||19/11/2011 20:06:22|
|837 forum posts|
That's the trouble with some lecturers - they get it into their theoretical heads (quite incorrectly in this case) that two regulators in series could oscillate and then say that something that's been quite normal in industrial practice for donkey's years is 'bad practice'.
It's not regulators in series (which is actually what the Shumatech design has) that cause the potential problem at all - it's what they might end up supplying. Since the first regulator has to be working before the second one powers up, you could, with some circuit configurations, end up with an unstable system when it first powers up. For instance, if the higher voltage powers a generating system of some sort, and the lower voltage runs the feedback stabilising system for it, then when it starts, there's a potential for an uncontrolled over-run in the generator part.
The real issue here is that you have to be aware of what might happen - and then plan around it. The three-terminal regulators in general use these days are amazingly stable devices - I don't ever recall any stability issues around them other than those caused by poor construction techniques. As long as you've planned out exactly how the power dissipation is going to go, and there are no race-hazard conditions (as described above), then having them connected in series is fine.
And I can't think of a single system that will run off an unregulated supply that won't run off a regulated one, FWIW...
|Andrew Johnston||20/11/2011 10:16:08|
5837 forum posts
Generally there is no problem using linear regulators in series. However, there are some caveats, particularly at startup. This is especially true if using low dropout regulators. For most linear regulators the output capacitance and ESR are an important part of the feedback loop. Get the values wrong and the regulator may oscillate. I have seen this even in the venerable 78xx series.
To save me a load of typing, see this link for more details:
Running switch-mode regulators in series is a whole different ballgame. Been there, done that, had my posterior bitten! If you think about a simple buck regulator the input impedance is negative, which can really 'test' the incoming supply. The best thing to do is sequence the supplies properly using external circuitry and 'enable' pins on the regulator ICs.
|Cornish Jack||20/11/2011 11:00:21|
|1190 forum posts|
Thank you gentlemen, one and all.
What I think I understand is that I COULD use regulated or unregulated and the units will sort out what's needed??
Still can't get my limited brain cells around why THREE Unregulated supplies metered out at 17 volts with each one selected to the 6 volt selection
I have some voltage selectable regulated units lying around so will try those.
The DROs are the BW MPS versions - nicely basic, no confusing 'bells and whistles' and they MIGHT just improve my machining tolerances out of the present 'Oh well, nobody will notice that!' level
|Steve Garnett||20/11/2011 12:22:41|
|837 forum posts|
Andrew's quite correct - anything can go wrong if you screw up the design... and yes there are load conditions you could apply to a 78xx output that can screw with its stability if you don't design it right. All I'm saying is that if you follow the rather simple basic rules, a second 78xx isn't one of them!
Before anybody tries to jump down my throat, please bear in mind that I'm trying to keep this conceptually understandable, but not necessarily technically complete:
You have to consider what a 'lack of regulation' actually is. Put simply, in this case it's the degree of ability of a power supply to go on supplying more and more current at any particular stated voltage. So for practical purposes, it's a measure of voltage 'droop' under load. Invariably how good the performance of a transformer is depends entirely upon its physical characteristics. So, if you have a heftily built transformer with a dirty great thick secondary winding, and you power it up, you'll get a voltage reading across this secondary that will droop very little under load - the winding is quite capable of supplying current without the voltage drooping very much, partly because it's good at magnetically coupling the energy from the primary winding into it, and also because the wire has a good current-carrying capacity.
With your smaller transformer though, it has a very small-scale winding of thinner wire, and although you can measure 17v across the secondary when it's unloaded, virtually any load at all will make this drop quite significantly. Even though the magnetic coupling inside is still quite good, something else has happened...
The performance in each case is pretty much determined by what's known as the 'internal resistance' of the winding, and although that's by no means the only factor, it's probably the most significant one. A low internal resistance usually implies (all other things being equal) that the winding can supply rather more current without drooping than a higher internal resistance would (as per first example). With the smaller winding on the smaller transformer, the energy transfer available is essentially limited by the physical size of the winding wire, which limits its current-carrying ability.
One thing that's worth noting about devices that have poorer regulation is that they have, to a degree, an in-built safety mechanism - if the load becomes too great, the voltage drops lower. What that actually means is that the current the device can supply is limited to quite a low maximum value, and is less likely to cause collateral damage if anything goes wrong. With a higher current available in a larger winding, an increased load will cause a higher current to flow because the regulation is better - so more energy leaves. Obviously the mechanism is the same in both cases, it's just the amount that varies.
What an external regulator does is to take your 17v and make sure that what comes out is 6v, regardless of the current drawn. So it's regulating the voltage, and not the current. In other words, if the current varies within the limits of the transformer winding supplying it, the voltage will remain the same. But, if you try to draw too much current, then the winding still won't be able to supply it, and the device will shut down.
I hope that makes sense. If it doesn't, then ask for more explanation!
Edited By Steve Garnett on 20/11/2011 12:23:35
|Cornish Jack||20/11/2011 14:09:47|
|1190 forum posts|
Thank you very much, Steve.
That explanation managed to make sense to me as regards the no load/high voltage situation. In practical terms, I shall stick with regulated supplies, however it does seem to me that making an UNregulated supply with SELECTABLE output voltages which are meaningless, is a pathway to disaster for electrical thickheads like me!!
|Stub Mandrel||20/11/2011 16:39:15|
4311 forum posts
Dont forget when a simple (transformer, 4 diodes and a capacitor) supply is unloaded there are several effects that bump up the voltage:
6V nominal on load add two diode drops, so 6+(2*0.8)=7.6V RMS into the rectifier.
off load AC rises to 12.16 volts.
take away two reduced diode drops 12.6-(2*0.7)=11.2 volts
Take peak value 1.414 = 15.8 volts.
60% is not unusual for really small transformers, so 17V off load is quite believable.
|461 forum posts|
Bill (Cornish Jack), why not measuring the voltage of your unregulated supply under some load? Say a resistor between 300 - 1000 ohms, will absorb a handful of milliamps. Maybe the picture changes then...
I wondered why none of the electronic gurus advised that.
Doesn't BW show the power consumption in the user's manual? This would give a better clue to the value of the test resistor (Ohm said: r = U/I).
|Cornish Jack||20/11/2011 21:36:35|
|1190 forum posts|
Thank you Hansrudolph. Unfortunately the user's manual is pretty basic and just states that the units 'require a 9 volt unregulated supply' (sic). The load will be vartable, since the power feed is via 4 way splitter taking jack plug connections hence anything between 1 and 4 units loading. I'm greatly leaning towards Steve Withnall's viewpoint that BW are offering the least expensive suggestion ... I hope!!
|Les Jones 1||20/11/2011 22:47:13|
|2192 forum posts|
I agree that Steve's reasoning is probably right. If they say it must be an unregulated supply they imply that it would work with the worst example of an unregulated 9 volt supply which may be anything from about 7 volts and about 25 volts. (Scaling the values from your 6 volt supply.) If this is the case then 9 volts from a regulated supply is within this range. It would be better if they said something like an unregulated supply will be OK to use as the unit will work from X volts to Y volts.
|Steve Garnett||20/11/2011 23:21:02|
|837 forum posts|
|That's the thing - you can connect as many of these devices to an unregulated supply as you like as long as the voltage when they're all connected doesn't drop below 9v. If that happens, you either need two supplies to run them, or one beefier one.|
Edited By Steve Garnett on 20/11/2011 23:21:27
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