Any electronics gurus?

airspirit · 08-17-2003, 03:10 PM

I am looking to make a DC power converter that takes input from a 5V line and outputs -18V to -20V (I'd like to control that with a pot or something ... for contrast). I am SO rusty in my electronics design it isn't even funny. Do any of you have an idea about how to make one of these bastages?

murray13 · 08-17-2003, 05:52 PM

For a CCFL or the like??? Buy one. 1. Its easier. 2. It will most likely be cheaper in the end. 3. It will perform better.

Creating -18V from a 5V line isn't easy. It would be a little easier from a 12V line.

airspirit · 08-17-2003, 06:15 PM

Okay, then how do I do it from 12V? I'll be running this from molex (it is the contrast for an LCD) and I can't find a part at any of the supply houses that'll do it. If you could point me in the right direction, I'd be most appreciative.

TerraMex · 08-17-2003, 06:34 PM

There's a power converter from Maxim:

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1705

I think this is it. Also see the related IC's.

http://www.dz98.com/wjj-pdf/pt5021-31.pdf

Or a little brother.

airspirit · 08-17-2003, 07:11 PM

That first link looks really REALLY nice. The only problem is that the circuit diagram at the bottom is completely unlabelled ... doesn't do me too much good. Do you know where I can find a labelled one? This is making my head hurt ... hehe.

TerraMex · 08-17-2003, 07:54 PM

Reference: http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1729
Direct Link :http://pdfserv.maxim-ic.com/arpdf/MAX629EVKIT.pdf

Check the second schematic on the PDF. It's labeled and with a -20v output.

It's all i could find. The rest just redirects me to thermistor related and similar pages.

I'd be more worried about the output amperage than anything else. Those circuits are rated very low (20mA for what i read).

airspirit · 08-17-2003, 08:06 PM

Wait ... I'm starting to think here: -20V, that's like just reversing the leads, right? So I could use a 20V thingy, and just attach the ground side to the -20V input and attach the ground from that to the 20V lead, right? Does that make sense? Reversing flow would cause the negative voltage, right?

The reason I ask is that I found a 12V->20V converter for car to notebook power, and I'm thinking that if I cut the lighter plug and attach a molex and then cut the power plug and solder that to the pins, I should be good ... right?

Or alternately, here is a picture of exactly what I need:

http://www.optrex.com/SiteImages/Par...C/50773age.pdf

In section 4.2 it gives an example of a power supply for the contrast to provide the -18 to -20V. I can find the 2SA1202, I think, but I don't understand the rest of the diagram. The resistor symbol with "VR" next to it is unfamiliar, as is the symbology used around "Tr". If someone could explain this I would be ever in their debt.

edit: this loop uses a maximum of 4.5 mA for the contrast adjustment. 20 mA would be more than enough. I found the premade eval kit you directly linked to (already built ... very nice!), but I can't seem to order it as it is unpriced. I'm going to try to contact them on Monday if I can't find another solution.

TerraMex · 08-17-2003, 08:58 PM

> Does that make sense? Reversing flow would cause the
> negative voltage, right?

I see your point, but power supply units were not built to be run on reverse. IMO, it wouldnt work... and if it did, not for long.

That's why there's inverting circuits. If it was that simple, we just connect things on reverse and save alot of electronics.

Anyway , i just realized (damnit, it took alot of time), you have a -12v rail on the PSU. You can actually use that, if the current draw isnt very high ; mine is rated for 0.8 A, which is pretty good.

And as far as i know (no certains) the -12v is just a reference voltage rail.

And you can also use a small DC-DC converter and pump the -12v to -20v.

>The resistor symbol with "VR" next to it is unfamiliar, as is the
>symbology used around "Tr". If someone could explain this I
>would be ever in their debt.

As far as i can relate VR equals Variable Resistance.

And it makes sense. The middle resistance has the "->" which means it doesnt have a fixed value, thus can be ajusted.

Edit : misread, the Tr refers to something else.

http://www.chipdocs.com/pndecoder/da...S/2SA1202.html

PS: The "Module" there supplies positive and negative voltages, the circuit attached serves as power regulator, imo.

airspirit · 08-18-2003, 11:58 AM

Yar, screw this. I'm ordering a LCD with a built in controller ... I don't have the patience to deal with this as well as the standard wiring headaches.

Thanks for the help though. I didn't realize how much I lost until I actually had to use it ... I haven't done any of this kind of stuff in any amounts in years.

TerraMex · 08-18-2003, 12:10 PM

Taking the easy way out eh?

Anyway, be careful with the one you buy. Some LCD's even with controller dont come with the inverter for the light. If your LCD uses some ccf or el-wire type layer it will need an inverter.

airspirit · 08-18-2003, 05:57 PM

It's a LED backlit unit, so no worries there ... I just need to step the Vled down to around 4.2V or so.

punish3r · 08-18-2003, 06:17 PM

I know I got into this one kind of late, but for anyone else reading this, I just wanted to clarify about the "switch the leads = negative PS"
The easiest way to understand why this wouldn't work is to imagine what a +12VDC PS looks like when piped through an oscilloscope. Properly connected, you'll have a horizontal line 12V "above" the 0V centerline. By switching the leads, all you're doing in reversing the hole and electron flow, thereby creating a "ground" that equals +12VDC, with a VCC of 0V, thereby pumping your energy "backwards" through whatever circuit you're powering, in most cases destroying it. (A side note: manufacturers of battery powered components have known for years that people are notrious for putting batteries in backwards, and therefore install a diode to prevent reverse flow)
Anyway, back to our oscilloscope model. A real -12VDC supply (properly connected) will show a horizontal line 12V "below" the 0V centerline.

airspirit · 08-19-2003, 11:15 AM

Very nice. Thank you for the clarification. I'm properly kicking my own ass for my stupidity, because I should have known better, hehe.

08-17-2003, 03:10 PM	#1
airspirit Been /.'d... have you? Join Date: Jul 2002 Location: Moscow, ID Posts: 1,986	Any electronics gurus? I am looking to make a DC power converter that takes input from a 5V line and outputs -18V to -20V (I'd like to control that with a pot or something ... for contrast). I am SO rusty in my electronics design it isn't even funny. Do any of you have an idea about how to make one of these bastages? __________________ #!/bin/sh {who;} {last;} {pause;} {grep;} {touch;} {unzip;} mount /dev/girl -t {wet;} {fsck;} {fsck;} {fsck;} {fsck;} echo yes yes yes {yes;} umount {/dev/girl;zip;} rm -rf {wet.spot;} {sleep;} finger: permission denied

08-17-2003, 06:15 PM	#3
airspirit Been /.'d... have you? Join Date: Jul 2002 Location: Moscow, ID Posts: 1,986	Okay, then how do I do it from 12V? I'll be running this from molex (it is the contrast for an LCD) and I can't find a part at any of the supply houses that'll do it. If you could point me in the right direction, I'd be most appreciative. __________________ #!/bin/sh {who;} {last;} {pause;} {grep;} {touch;} {unzip;} mount /dev/girl -t {wet;} {fsck;} {fsck;} {fsck;} {fsck;} echo yes yes yes {yes;} umount {/dev/girl;zip;} rm -rf {wet.spot;} {sleep;} finger: permission denied

08-17-2003, 06:34 PM	#4
TerraMex Cooling Savant Join Date: Nov 2002 Location: Portugal, Europe Posts: 870	There's a power converter from Maxim: http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1705 I think this is it. Also see the related IC's. http://www.dz98.com/wjj-pdf/pt5021-31.pdf Or a little brother. __________________ "we need more cowbell."

08-17-2003, 07:11 PM	#5
airspirit Been /.'d... have you? Join Date: Jul 2002 Location: Moscow, ID Posts: 1,986	That first link looks really REALLY nice. The only problem is that the circuit diagram at the bottom is completely unlabelled ... doesn't do me too much good. Do you know where I can find a labelled one? This is making my head hurt ... hehe. __________________ #!/bin/sh {who;} {last;} {pause;} {grep;} {touch;} {unzip;} mount /dev/girl -t {wet;} {fsck;} {fsck;} {fsck;} {fsck;} echo yes yes yes {yes;} umount {/dev/girl;zip;} rm -rf {wet.spot;} {sleep;} finger: permission denied

08-17-2003, 07:54 PM	#6
TerraMex Cooling Savant Join Date: Nov 2002 Location: Portugal, Europe Posts: 870	Reference: http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1729 Direct Link :http://pdfserv.maxim-ic.com/arpdf/MAX629EVKIT.pdf Check the second schematic on the PDF. It's labeled and with a -20v output. It's all i could find. The rest just redirects me to thermistor related and similar pages. I'd be more worried about the output amperage than anything else. Those circuits are rated very low (20mA for what i read). __________________ "we need more cowbell."

08-17-2003, 05:52 PM	#2
murray13 Cooling Savant Join Date: Sep 2002 Location: Omaha, NE USA Posts: 216	For a CCFL or the like??? Buy one. 1. Its easier. 2. It will most likely be cheaper in the end. 3. It will perform better. Creating -18V from a 5V line isn't easy. It would be a little easier from a 12V line.

08-17-2003, 08:06 PM	#7
airspirit Been /.'d... have you? Join Date: Jul 2002 Location: Moscow, ID Posts: 1,986	Wait ... I'm starting to think here: -20V, that's like just reversing the leads, right? So I could use a 20V thingy, and just attach the ground side to the -20V input and attach the ground from that to the 20V lead, right? Does that make sense? Reversing flow would cause the negative voltage, right? The reason I ask is that I found a 12V->20V converter for car to notebook power, and I'm thinking that if I cut the lighter plug and attach a molex and then cut the power plug and solder that to the pins, I should be good ... right? Or alternately, here is a picture of exactly what I need: http://www.optrex.com/SiteImages/Par...C/50773age.pdf In section 4.2 it gives an example of a power supply for the contrast to provide the -18 to -20V. I can find the 2SA1202, I think, but I don't understand the rest of the diagram. The resistor symbol with "VR" next to it is unfamiliar, as is the symbology used around "Tr". If someone could explain this I would be ever in their debt. edit: this loop uses a maximum of 4.5 mA for the contrast adjustment. 20 mA would be more than enough. I found the premade eval kit you directly linked to (already built ... very nice!), but I can't seem to order it as it is unpriced. I'm going to try to contact them on Monday if I can't find another solution. __________________ #!/bin/sh {who;} {last;} {pause;} {grep;} {touch;} {unzip;} mount /dev/girl -t {wet;} {fsck;} {fsck;} {fsck;} {fsck;} echo yes yes yes {yes;} umount {/dev/girl;zip;} rm -rf {wet.spot;} {sleep;} finger: permission denied

08-17-2003, 08:58 PM	#8
TerraMex Cooling Savant Join Date: Nov 2002 Location: Portugal, Europe Posts: 870	> Does that make sense? Reversing flow would cause the > negative voltage, right? I see your point, but power supply units were not built to be run on reverse. IMO, it wouldnt work... and if it did, not for long. That's why there's inverting circuits. If it was that simple, we just connect things on reverse and save alot of electronics. Anyway , i just realized (damnit, it took alot of time), you have a -12v rail on the PSU. You can actually use that, if the current draw isnt very high ; mine is rated for 0.8 A, which is pretty good. And as far as i know (no certains) the -12v is just a reference voltage rail. And you can also use a small DC-DC converter and pump the -12v to -20v. >The resistor symbol with "VR" next to it is unfamiliar, as is the >symbology used around "Tr". If someone could explain this I >would be ever in their debt. As far as i can relate VR equals Variable Resistance. And it makes sense. The middle resistance has the "->" which means it doesnt have a fixed value, thus can be ajusted. Edit : misread, the Tr refers to something else. http://www.chipdocs.com/pndecoder/da...S/2SA1202.html PS: The "Module" there supplies positive and negative voltages, the circuit attached serves as power regulator, imo. __________________ "we need more cowbell." Last edited by TerraMex; 08-17-2003 at 09:05 PM.

08-18-2003, 11:58 AM	#9
airspirit Been /.'d... have you? Join Date: Jul 2002 Location: Moscow, ID Posts: 1,986	Yar, screw this. I'm ordering a LCD with a built in controller ... I don't have the patience to deal with this as well as the standard wiring headaches. Thanks for the help though. I didn't realize how much I lost until I actually had to use it ... I haven't done any of this kind of stuff in any amounts in years. __________________ #!/bin/sh {who;} {last;} {pause;} {grep;} {touch;} {unzip;} mount /dev/girl -t {wet;} {fsck;} {fsck;} {fsck;} {fsck;} echo yes yes yes {yes;} umount {/dev/girl;zip;} rm -rf {wet.spot;} {sleep;} finger: permission denied

08-18-2003, 12:10 PM	#10
TerraMex Cooling Savant Join Date: Nov 2002 Location: Portugal, Europe Posts: 870	Taking the easy way out eh? Anyway, be careful with the one you buy. Some LCD's even with controller dont come with the inverter for the light. If your LCD uses some ccf or el-wire type layer it will need an inverter. __________________ "we need more cowbell."

08-18-2003, 05:57 PM	#11
airspirit Been /.'d... have you? Join Date: Jul 2002 Location: Moscow, ID Posts: 1,986	It's a LED backlit unit, so no worries there ... I just need to step the Vled down to around 4.2V or so. __________________ #!/bin/sh {who;} {last;} {pause;} {grep;} {touch;} {unzip;} mount /dev/girl -t {wet;} {fsck;} {fsck;} {fsck;} {fsck;} echo yes yes yes {yes;} umount {/dev/girl;zip;} rm -rf {wet.spot;} {sleep;} finger: permission denied

08-18-2003, 06:17 PM	#12
punish3r Cooling Neophyte Join Date: Nov 2002 Location: Colorado Posts: 34	Reversing Leads I know I got into this one kind of late, but for anyone else reading this, I just wanted to clarify about the "switch the leads = negative PS" The easiest way to understand why this wouldn't work is to imagine what a +12VDC PS looks like when piped through an oscilloscope. Properly connected, you'll have a horizontal line 12V "above" the 0V centerline. By switching the leads, all you're doing in reversing the hole and electron flow, thereby creating a "ground" that equals +12VDC, with a VCC of 0V, thereby pumping your energy "backwards" through whatever circuit you're powering, in most cases destroying it. (A side note: manufacturers of battery powered components have known for years that people are notrious for putting batteries in backwards, and therefore install a diode to prevent reverse flow) Anyway, back to our oscilloscope model. A real -12VDC supply (properly connected) will show a horizontal line 12V "below" the 0V centerline. __________________ Fix reason firmly in her seat, and call on her tribunal for every fact, every opinion. Question with boldness even the existence of a God; because, if there be one, he must more approve of the homage of reason than that of blindfolded fear. -Thomas Jefferson

08-19-2003, 11:15 AM	#13
airspirit Been /.'d... have you? Join Date: Jul 2002 Location: Moscow, ID Posts: 1,986	Very nice. Thank you for the clarification. I'm properly kicking my own ass for my stupidity, because I should have known better, hehe. __________________ #!/bin/sh {who;} {last;} {pause;} {grep;} {touch;} {unzip;} mount /dev/girl -t {wet;} {fsck;} {fsck;} {fsck;} {fsck;} echo yes yes yes {yes;} umount {/dev/girl;zip;} rm -rf {wet.spot;} {sleep;} finger: permission denied

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