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Post by Deleted on Apr 5, 2013 17:23:28 GMT
Is it possible to take a normal DC supply (say +24vdc) and split it into two supplies of +12vdc?
I've only been able to find voltage dividers, which only give a single reduced voltage.
If it is possible, are there any major drawbacks in its implementation?
TIA
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jonclancy
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Post by jonclancy on Apr 5, 2013 17:51:20 GMT
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Post by Deleted on Apr 5, 2013 21:24:32 GMT
Correct me if I'm wrong but I thought a railsplitter leads to a positive and a negative supply rather than the dual positives I'm after?
Cheers
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Post by Deleted on Apr 5, 2013 22:50:32 GMT
Correct me if I'm wrong but I thought a railsplitter leads to a positive and a negative supply rather than the dual positives I'm after? Cheers Chris Perhaps Marc is the best one to answer this, but I was under the impression if say you took say a +24V supply, and used a power amplifier I.C. such as the LM380 that you would then end up with a centre tapped 24V supply provided that the original earth side remained 0 volts . In other words, +24 and +12V referenced to the original 0 volts/earth line. Perhaps we both need a refresher here via Google and a Wiki ? Kind Regards Alex
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Post by Deleted on Apr 6, 2013 0:07:36 GMT
Correct me if I'm wrong but I thought a railsplitter leads to a positive and a negative supply rather than the dual positives I'm after? Cheers Chris Perhaps Marc is the best one to answer this, but I was under the impression if say you took say a +24V supply, and used a power amplifier I.C. such as the LM380 that you would then end up with a centre tapped 24V supply provided that the original earth side remained 0 volts . In other words, +24 and +12V referenced to the original 0 volts/earth line. Perhaps we both need a refresher here via Google and a Wiki ? Kind Regards Alex Lost me there Alex! I had got as far as a railsplitter with a negative to positive inverter to create to equal positives. Unfortunately that chip based inverter seriously limits the power available, drawing board beckons....
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Post by Deleted on Apr 6, 2013 0:20:50 GMT
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pagan
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Post by pagan on Apr 6, 2013 5:09:05 GMT
If your only after 2 positive rails, then why not just use a pair of LM7812's?
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Post by Deleted on Apr 6, 2013 5:46:11 GMT
If your only after 2 positive rails, then why not just use a pair of LM7812's? Allan Depending on the current draw from the + 24V supply, the 7812s could be getting pretty hot with 12V drop across each of them. Alex
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pagan
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Post by pagan on Apr 6, 2013 11:01:40 GMT
If your only after 2 positive rails, then why not just use a pair of LM7812's? Allan Depending on the current draw from the + 24V supply, the 7812s could be getting pretty hot with 12V drop across each of them. Alex Alex Yes very true. It does depend on current draw, but then could also use the high current LM3** series devices instead. Either way, they would be better than a voltage divider. Allan
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Post by Deleted on Apr 6, 2013 11:10:18 GMT
Allan You are still going to be dissipating the same amount of heat. That still means suitably large heat sinking. Regards Alex
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pagan
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Post by pagan on Apr 6, 2013 12:08:11 GMT
Allan You are still going to be dissipating the same amount of heat. That still means suitably large heat sinking. Regards Alex Alex If your dropping from 24v to 12v, it's still 12v, then it just depends on the current draw that gives the power dissipation. no way out of it, well maybe stepped voltage drops,,, but.. It depends on the power needed for cj's circuit, and what it's providing voltage/power for. Also heatsinks aren't a major deal. unless it's a Class A amp,,,, then you just make it look pretty.
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Post by Deleted on Apr 6, 2013 12:13:21 GMT
If your only after 2 positive rails, then why not just use a pair of LM7812's? Hi Chris The above would have been my best guess but as Alex has pointed out heat may be an issue. Even so just use large heat sinks. What are you planning to use this for? What’s the current draw? With the above info we may be able to come up with something a little more imaginative. Take care
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Post by Deleted on Apr 6, 2013 12:33:58 GMT
Thanks for the answers so far; I was trying to put together a twin 12v charger/psu. The draw is minimum of 1a for each dc supply. As I had another 1x 24vac transformer hanging around I thought I'd try and put this together from the "parts-box". I was originally thinking along the lines of splitting the 24vac to two bridge rectifiers but then, as Alex pointed out, was concerned about dropping 12vdc across a regulator due to heat dissipation. I do have a couple of medium sized sinks in the box if they are man enough? (diode there purely for size reference)
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Post by Deleted on Apr 6, 2013 12:52:45 GMT
Thanks for the answers so far; I was trying to put together a twin 12v charger/psu. The draw is minimum of 1a for each dc supply. As I had another 1x 24vac transformer hanging around I thought I'd try and put this together from the "parts-box". I was originally thinking along the lines of splitting the 24vac to two bridge rectifiers but then, as Alex pointed out, was concerned about dropping 12vdc across a regulator due to heat dissipation. I do have a couple of medium sized sinks in the box if they are man enough? (diode there purely for size reference) Hi Chris They would do the job provided that the transformer has a high enough VA rating. As you would be drawing 2A using bridge rectifiers, the transformer would need to be rated at 3A, so an 80VA transformer. (Bridge rectifiers will only give you a little over 60% of the transformers current.) However you should use 3A regulators when dissipating that much heat, as the 7812 is right on it's max rating of 1A. Something like MC78T12 would do the trick if you can get them. Regards Alex
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pagan
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Post by pagan on Apr 6, 2013 13:11:44 GMT
Thanks for the answers so far; I was trying to put together a twin 12v charger/psu. The draw is minimum of 1a for each dc supply. As I had another 1x 24vac transformer hanging around I thought I'd try and put this together from the "parts-box". I was originally thinking along the lines of splitting the 24vac to two bridge rectifiers but then, as Alex pointed out, was concerned about dropping 12vdc across a regulator due to heat dissipation. I do have a couple of medium sized sinks in the box if they are man enough? (diode there purely for size reference) Hi Chris They would do the job provided that the transformer has a high enough VA rating. As you would be drawing 2A using bridge rectifiers, the transformer would need to be rated at 3A, so an 80VA transformer. (Bridge rectifiers will only give you a little over 60% of the transformers current.) However you should use 3A regulators when dissipating that much heat, as the 7812 is right on it's max rating of 1A. Something like MC78T12 would do the trick if you can get them. Regards Alex Alex What I was thinking about was the "K" series LM3**'s. TO-3 packages. Or/and use a current pass transistor... or 2 Allan 24volts ac * 1.414 = Volts dc 24vac = 33.936vdc so even with diode drop, etc could be 32vdc Another thought The psu for the 15 Watt Class A amp
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Post by Deleted on Apr 6, 2013 13:39:06 GMT
OK Alex, thanks for the info (the tranny is 90va).
Chris
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Post by Deleted on Apr 6, 2013 23:01:15 GMT
Hi Allan, I'm not quite sure how your your current pass transistor differs to what's been said thus far, lack of knowledge I'm afraid. I found this which I think is what you meant. Could you explain for a know-not, cheers.
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pagan
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Post by pagan on Apr 7, 2013 0:28:30 GMT
Chris
That's the same circuit principle used in the psu for SC/Alex's class A amp.
SC used 317/337's with pass transistors, because they needed plus and minus voltage and pass trannies for extra current capabilities.
The LM318/338 are variable voltage, capable of 5amps and to3 packet for better heat. The LM340 are fixed voltage output, ie +5v, +12v or +15v also 5 amp.
Still I would not use these for 5 amp, being there max and heat would be a problem. The heat can be shared between devices when used with a pass transistor.
The pass transistor doesn't need to be something fantastic, the venerable 3055 is suitable. Or if you have old SC amp kits, as I do, with unused output transistors ie 21193/94's, well there is a use for them.
So, depending on whats available in the parts bin,, you can use the LM7812 or LM317 with a pass transistor. If you have an old psu board you can always hack the pass tranny in there, of mount the reg and pass tranny on a heatsink and run wires back to the circuit.
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Post by Deleted on Apr 7, 2013 0:49:49 GMT
Allan You need to be careful there, as the LM340T5 for example is a 1A voltage regulator. Regards Alex
P.S. I have emailed Chris an explanation from S.C. about how their 15W Class A PSU works.
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pagan
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Post by pagan on Apr 7, 2013 1:42:55 GMT
Allan You need to be careful there, as the LM340T5 for example is a 1A voltage regulator. Regards Alex P.S. I have emailed Chris an explanation from S.C. about how their 15W Class A PSU works. Alex ooops you're right mixed up datasheets between lm338(K) and lm34**(K) Allan
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Deleted
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Post by Deleted on Apr 7, 2013 12:19:28 GMT
Thanks again folks
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Post by Deleted on Apr 7, 2013 12:32:33 GMT
Chris Does this transformer only have a single 24V AC winding ? If not, the idea seems to be bordering on unworkable, without heaps of heat sinking and the use of 2 x 3A voltage regulators. We are talking about a possible total heat dissipation of around 40 Watts ! Kind Regards Alex
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Post by Deleted on Apr 7, 2013 13:14:12 GMT
Hi Alex,
Yes, this transformer is a single 24vac.
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pagan
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Post by pagan on Apr 7, 2013 14:07:07 GMT
Chris Does this transformer only have a single 24V AC winding ? If not, the idea seems to be bordering on unworkable, without heaps of heat sinking and the use of 2 x 3A voltage regulators. We are talking about a possible total heat dissipation of around 40 Watts ! Kind Regards Alex Alex that why I was thinking pass tranny or two to spread the heat around. Chris Will till be dissipating a bit of heat. Allan
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Post by Deleted on Apr 7, 2013 21:50:38 GMT
Allan We are talking here about 2 pissy little 12V 1A supplies that could have been put in a fairly small ABS case with internal heatsinks for each regulator about the size used in the class A HA main PCB by Jon,Will and others, provided that a transformer from 12.6V to 15V was used. The need to work with a 24V transformer has caused it to need something like a large rear mounted heatsink that both voltage regulators could be bolted to. Regards Alex
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