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DC wall transformers
d nixon
Why is it that wall transformers don't output the stated voltage? Seems to me this does nothing but cause confusion. How do I know what rating to buy?
For a project I'm working on I need a wall transformer that outputs between 14 and 16V DC. And silly me bought a 14VDC unit. But it outputs about 19V! Is there some rule of thumb? Argh! -Mike _________________________________________________________________ Get your FREE download of MSN Explorer at |
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Re: Capacitors in power supply circuits
d nixon
Yes, I'm interested.
-Mike There are other IC's available for voltage regulation other than the LM3xx & _________________________________________________________________ Get your FREE download of MSN Explorer at |
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Re: HP 772C Printer Cord
Jim Thibeault (Tebo)
Hi Jon:
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It looks like a ferrite choke to me as well, although I talked to two different tech's from HP and one told me that it was a diode, and the other said that it was a current limiting resistor, neither seemed to know for sure. I took it out of line and the printer works great, thanks for your help, this is a great list. Jim Jonathan Luthje wrote:
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Re: HP 772C Printer Cord
Jonathan Luthje
G'day Jim,
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The "thingie" that you describe sounds like an RF ferrite choke - found on some proprietry equipment ... beats me why it would melt though ... seemingly an impossibility. If it was heat-shunk (as is more often the case) rather than rubber encased, heat shrink is reasonably brittle anyway and would have cracked merely because you were trying to peel it. I'm only guessing of course, but I hardly think that anyone would place a diode inline with a power cord .... that's what PCB's are for. More than likely a requirement for FCC regulations or line noise reduction. It's really difficult to say without physically seeing it. Sorry I couldn't be of more help. Regards, J0n ----- Original Message -----
From: "Jim Thibeault (Tebo)" <JIMT.KF4NBG@...> To: <Electronics_101@...> Sent: Thursday, August 30, 2001 9:53 PM Subject: Re: [Electronics_101] HP 772C Printer Cord Hi Jon, |
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Re: HP 772C Printer Cord
Jim Thibeault (Tebo)
Hi Jon,
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I carefully pealed the rubber off the device and it fell apart as if the heat caused it to crack, I don't know what the material was but it looked like a choke of some kind, about 3/8 " in diameter, 1/4 " wide and the wire was passed through it. The reason that I refered to it as a diode is that the tech rep from Hewlett Packard called it a diode, but it didn't look like any diode that I have ever seen. I cut the wire back behind this device and put a new jack on it and it put out the required 18 volts, the printer works. Now you have me thinking that maybe I should insert a fuse, or fusible link inline. Hewlett Packard won't give me any more information, I'be called every HP phone number that I could dig up. Jim Jonathan Luthje wrote:
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Re: 12 volt motor speed control
Tavys Ashcroft
followed by or preceded by a variable pulse-width-modulated controllerYeah. Something like this: -Tavys |
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Re: 12 volt motor speed control
Jonathan Luthje
Darle,
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I would be tempted to have a look at the plan for the 6V controller first up, it should be easily modified to suit 12, or even 24V, by changing a few components. If you want to start from scratch, I would suggest a "bridge" circuit of some type for direction reversal (either relay or transistors - depending on whether you wanted to keep it solid-state or not), followed by or preceded by a variable pulse-width-modulated controller (probably something like an astable 555 driving a couple of hefty power transistors. This all depends of course on the type of interface you intend to feed it, but it is all very possible. But the first thing I would do is try and modify the existing circuit, if you would like to post it to the group then I'm sure there are a few people (incl. myself) who would be glad to have a look at it. Regards, J0n ----- Original Message -----
From: <dwb4926@...> To: <Electronics_101@...> Sent: Thursday, August 30, 2001 9:35 AM Subject: [Electronics_101] 12 volt motor speed control Hello Group , |
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Re: 12 volt motor speed control
Darren Reid
From: <dwb4926@...>
I have been looking for a plan to make a 12 volt speed controller butI bet that it would take only a very minor change to make that circuit work for the full 12V range. It may not require *any* change. Post a link to the circuit and folks will surely help out. -Darren --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (). Version: 6.0.265 / Virus Database: 137 - Release Date: 7/18/01 |
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Re: Capacitors in power supply circuits
Jonathan Luthje
Mike,
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Let's assume that from your 25VA transformer you are producing an unregulated output voltage of 28 volts (quite reasonable), and lets say that you need to draw the full current capacity from your 5V rail (2A), now you need to remove or regulate down 28-5 = 23V @ 2A, which is roughly 46W - say 50W for arguments sake - now 50W is a hell of a lot of heat to for a poor 'ol TO-220 flat-pack to dissapate (i.e. it can't do it, the maximum heat you can dissapate with a 10deg/W heat sink (for example) is approximately 8.5W. As Larry pointed out in his earlier post, if you really must use a 25VA transformer, then a current limiting resistor needs to be applied to the input/source pin of the LM7805. Using say a 5deg/W heatsink, which gives you a theoretical maximum of approximately 17W. Taking 23V off the supply, we need the Wattage W = VA so ... 17 = 23 * A A = 0.73 (lets say 700mA for arguments sake) You need to limit your 28V source to 700mA or less V=IR ; 28=0.8 * R A 35 ohm, or probably a 33R resistor would be the next closest value, needs to go in front of the 7805 transistor, a 3 or 5 watt wire wound would be preferable. You can regulate this amount of current with one or more bypass transistors and a large heatsink (@50W, say a 0.5 deg/W should do the job, however the cost of implementing this outweighs the cost of just buying a reasonably sized transformer (5-8V). The same goes for the LM317 - another TO220 Flat-Pack, not designed to dissapate large amounts of heat - very easy to overheat and shutdown, I had one on a 5deg/W heatsink regulating 12V to 3V (a 9V drop), and it the heatsink got so hot that the device shutdown (sink temp was approx 115degC - who knows what the case temp was ... 150deg?) ... and I wasn't exactly drawing a lot of current out of it either - 200mA or so ... But anyway - the point is that you may need to think about the range of your variable supply and the heatsinking capacity of your regulator, and whether or not you will need to use bypass transistors to help spread the load. Oh ... another point while I think of it - most of the LM317T & LM7805T series reg's have a max current drain of 1A, so putting a 2A supply at the other end may not be such a good idea in the first place, unless, of course you are implementing bypass trannies. There are other IC's available for voltage regulation other than the LM3xx & LM78xx series. I have the details somewhere around here .... (not in my head, obviously), which provide more effective regulation for dropping large amounts of voltage from the supply rails - with minimal heat dissapation. Let me know if you are interested in this alternative approach and I will endevour to find out what it is. I would suggest that perhaps the circuits that you have obtained are written by people who never actually bother to build the things to check them out - I come across a lot of circuits like that, not bad circuits fundamentally, but a few critical errors which would prevent them from operating effectively. See how you go, Regards, J0n ----- Original Message -----
From: <dnixon9@...> To: <Electronics_101@...> Sent: Thursday, August 30, 2001 12:27 PM Subject: [Electronics_101] Re: Capacitors in power supply circuits What if I created a voltage divider at the power rails and ran the 5V |
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Re: New to group and question
Darren Reid
From: <ethanzer0@...>
I'm new to the group and am currently working on building an analogEagle is very good. If you are using the demo version, there exists a library for it that adds full spice simulation. -Darren --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (). Version: 6.0.265 / Virus Database: 137 - Release Date: 7/18/01 |
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Re: HP 772C Printer Cord
Jonathan Luthje
Jim,
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I would double check the device in the cord - if it got so hot that it melted, it may be a fusible resistor or a thermal fuse, designed to prevent fires and things of the like, if it is one of the above, then it would point to a fault furthur up the line, so it may not just be a 50 cent component, or a power cord for that matter, it is potentially related to a serious fault in your printer / power supply. Regards, Jon ----- Original Message -----
From: "Jim Thibeault (Tebo)" <JIMT.KF4NBG@...> To: <Electronics_101@...> Sent: Thursday, August 30, 2001 1:23 AM Subject: Re: [Electronics_101] Re: Capacitors in power supply circuits I need help again, I have a Hewlett Packard 722 C printer and |
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Re: Capacitors in power supply circuits
What if I created a voltage divider at the power rails and ran the 5V
regulator off a 9V tap? And if so, would I use low or high ohm resistors to create the voltage divider? And why? I appreciate the help as I'm a bit confused...I have several power supply circuits that show a 5V regulator run right off a 25V or 36V rail. -Mike Larry Hendry <hendrysr@y...> wrote: Maybe (you decide) a decent approach might be to put a powerresistor of small ohms (you calculate based on expected load on the sourcehigh voltage drop is not a heat problem. When the current goes up, theat the regulator is reduced (transferred to the resistor actually). |
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Re: New to group and question
Jonathan Luthje
Ethan,
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Eagle is not a bad product, especially if you are not doing anything complex. I prefer Protel layout software, but it is quite expensive just for hobbyist work. It's expense is due to it's large component, track & layout library, along with multi-layer, multi-sheet capability and auto-layout features for PCB's ... quite nifty ... You can download a demo from Orcad is also another high-end bit of software to have a look at Seems to be a matter or preference Regards, J0n ----- Original Message -----
From: <ethanzer0@...> To: <Electronics_101@...> Sent: Thursday, August 30, 2001 7:22 AM Subject: [Electronics_101] New to group and question Hello, |
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Re: LEDs and relays in Autos
Jonathan Luthje
Jayson,
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A "real" brake light (at least in this country) leaves the parking / tail lights on simultaineously with the brake light, so the easiest way out of it would be to just hook them up without any switching circuitry whatsoever - load 1 to "switched", load 2 to "permanently on", if you really wanted to switch one off and the other one on, then I would suggest that you use a small-ish SPDT relay to switch from one to the other - hook up the common contact to +12V, the normally closed contact to Load 2, the normally open contact to Load 1, and the coil to "Switched +12V". Relays are a lot more robust than transistors, especially in automotive applications, where voltage spikes are common due to high loads on starting / charging etc. Also, make sure you include current limiting resistors in your design for the LEDs, as vehicle system voltage can be quite high during charging (up to 16V), which wouldn't do you LED's much good! Regards, Jon ----- Original Message -----
From: "Jayson" <woodrufj@...> To: <Electronics_101@...> Sent: Thursday, August 30, 2001 11:15 AM Subject: [Electronics_101] Re: LEDs and relays in Autos Well I'm doing something similar. I've planed out to have the "A" lit |
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Re: LEDs and relays in Autos
Well I'm doing something similar. I've planed out to have the "A" lit
with normal leds, while cluster "B" is super bright leds. Now i'm trying to think of a circuit that turns A off when B goes on, check out my previous post. This way it'll go 10% to 100% bright, more like a real brake light. Thanks Jay W --- In Electronics_101@y..., Larry Hendry <hendrysr@y...> wrote: I'll try this again with an actual answer this time. :)second 12 volt line would light the other 50%. So you r cluster has a "A" |
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Re: Capacitors in power supply circuits
Larry Hendry
--- ". ." <dnixon9@...> wrote:
And the other is a 7805TI would be somewhat concerned about using a 5V regulator from a source voltage so high. Jon makes a very good point (below) about the heat that will be generated by the regulator. You really need to know how much current you will be drawing from the regulator. Maybe (you decide) a decent approach might be to put a power resistor of small ohms (you calculate based on expected load on the source side of the 5 volt regulator. When the current draw is low, the high voltage drop is not a heat problem. When the current goes up, the voltage delivered to the regulator drops, and the power dissapated at the regulator is reduced (transferred to the resistor actually). While this is not as elegant as a supply with an unregulated voltage only a few volts above your regulator rating, it might be what you need for this application. From: "Jonathan Luthje" <jluthje@...> then you would need a 35V rated transformer, __________________________________________________ Do You Yahoo!? Get email alerts & NEW webcam video instant messaging with Yahoo! Messenger |
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Re: LEDs and relays in Autos
Larry Hendry
I'll try this again with an actual answer this time. :)
--- Jayson Woodruff <woodrufj@...> wrote: There are two problems I have. 1.) Both the front andSince you are dealing with brightness caused by LED clusters, what about making your cluster (your arrangement of series / parallel combinations) in two parts. Not two discrete parts, but two integrated parts. All LEDs would be the same. But, one 12 volt line would light 50% (or some other appropriate %) of the LEDs. the second 12 volt line would light the other 50%. So you r cluster has a "A" and "B" light with the LED arranged like this: ABABABABA BABABABAB ABABABABA Obviously, when the second line comes on, the total brightness will increase. Larry H __________________________________________________ Do You Yahoo!? Get email alerts & NEW webcam video instant messaging with Yahoo! Messenger |
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Re: LEDs and relays in Autos
Larry Hendry
--- Jayson Woodruff <woodrufj@...> wrote:
Hey Guys, I'm new here and have only very basic __________________________________________________ Do You Yahoo!? Get email alerts & NEW webcam video instant messaging with Yahoo! Messenger |
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