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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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8th May 2022, 12:05 pm | #1 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
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Stacked power supply puzzler.
I'm knocking up a power supply to power 24v relays and 12v fans, so went down the stacked power regulated supply route using LM317's. The fans both on at the same time draw 120mA, one relay is on all the time and requires 60mA, but the 24v rail also will need to supply another identical relay in a fault condition.
So, simple yes? One circuit board with two identical LM317 12v regulated supplies, one stacked on top of the other. After an initial screw up where I soldered in the caps the wrong way round and tried to power to bridge rectifiers from a 15v 0v 15v wired transformer, it's working....ish. Without a load I'm getting, 0v 12v 24v, with two 100r resistors in series as load the bottom supply goes up in value to 13.8v. After mucking about changing the adjust R the penny finally dropped, the load is acting as a potential divider and either feeding back to the bottom supply or doing what it's supposed to do, IE acting as an equal voltage divider. After all that waffle, what is the best way to do this? Why not use a split rail supply using a LM317 & 337 to get +15v 0v -15v? The fans won't care as long as they see 12v and the relay will see 24v. But will I run into a similar issue? Idea's on a postcard, message inna bottle or post a reply please, Andy.
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8th May 2022, 12:21 pm | #2 |
Dekatron
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
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Re: Stacked power supply puzzler.
The LM317 will not sink current as you have found.
Neither of those parts need a regulated power supply and are unlikely to even need smoothing. A capacitor will only be needed to stop any mechanical buzzing. |
8th May 2022, 1:46 pm | #3 |
Dekatron
Join Date: Jun 2013
Location: East Sussex, UK.
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Re: Stacked power supply puzzler.
Put the fans in series then you only need a 24 volt supply.
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8th May 2022, 2:40 pm | #4 |
Rest in Peace
Join Date: Apr 2008
Location: London, UK.
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Re: Stacked power supply puzzler.
To understand why your existing setup isn't working it would be helpful to see the circuit. Certainly there will be trouble if any 12V load is powered from +24 to +12 unless there is consistently greater load from +12 to 0, due to the aforementioned point of the +12V rail not being able to sink current via a positive regulator.
But a split rail using positive and negative regulators giving +/-12V will accept load 12V loads from +12 to 0 and 0 to -12, and also 24V loads between +12 and -12 and should work fine. I am not sure about series-connection of DC fans with electronically commutated motors. I experimented with this once, got some odd unpredictable voltage-sharing, and have not investigated further as to whether that was just an exception. |
8th May 2022, 4:14 pm | #5 |
Dekatron
Join Date: Jun 2013
Location: East Sussex, UK.
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Re: Stacked power supply puzzler.
I only put 2 in series once about 25 or more years ago. It was in an Amstrad PC which had a little motor running very fast in a sort of tunnel and making a lot of noise. I found a second one and put one at each end and running more slowly, at half the voltage, they still shifted the air but we’re much quieter.
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8th May 2022, 4:20 pm | #6 |
Heptode
Join Date: Jan 2011
Location: Croydon, London, UK.
Posts: 773
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Re: Stacked power supply puzzler.
You should be able to fix this problem very easily by changing and rewiring one resistor.
The voltage setting relay for the LM317 that is used for the 24V output should be recalculated to give 24V rather than 12V. Connect the lower end of the resistor to 0V (= negative side of the supply) rather than the 12V that it is connected to now. Paula |
8th May 2022, 11:55 pm | #7 |
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Re: Stacked power supply puzzler.
I like what Lucien said, and in fact its quite an elegant solution.
But a split rail using positive and negative regulators giving +/-12V will accept load 12V loads from +12 to 0 and 0 to -12, and also 24V loads between +12 and -12 and should work fine. Joe |
9th May 2022, 6:07 am | #8 | |||
Dekatron
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Location: Sleaford, Lincs. UK.
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Re: Stacked power supply puzzler.
Quote:
Quote:
It seems to work fine with actual relay's and fan's Lucien, just not with a potential divider as load. Probably because both loads are reactive and not purely resistive. That's a good point, don't think I remembered to fit a reverse diode across the relay's; is one needed for reg IC rather than a tranny own as SW? Quote:
So, as Lucien says if two separate 12v supply's are needed a split rail supply is better. thinking about it valve scopes use a combination of the two, stacked & split. Stacked to get the various rails for the anodes, focus etc and split for the cathode. Which gets me thinking, Tek used stacked regulated supply's in the 500 series scopes, but they must have done it in such a way that various regulated supply's could sink current, unlike as per the 317. Might be wrong about that though. Thanks all,learning a lot about PSU's this week, Andy.
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9th May 2022, 8:01 am | #9 |
Dekatron
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Re: Stacked power supply puzzler.
Circuit diagram would help massively, hand-drawn on a piece of paper and photographed?
Paula's suggestion, make an LM317-regulated 24V supply, and then power a further LM317 from that to get your 12V supply, on the face of it sounds good advice. There's lots to learn... you'll get an idea why people can make an entire career out of power supply design! |
9th May 2022, 12:11 pm | #10 | |
Nonode
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Re: Stacked power supply puzzler.
Quote:
Chris
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9th May 2022, 1:42 pm | #11 |
Dekatron
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Re: Stacked power supply puzzler.
Or to help reduce overall dissipation and improve efficiency, feed the regulator for the 12V rail from the transformer secondary centre tap, this makes use of 2 of the diodes in the existing bridge as a bi-phase full-wave set-up, this raw DC needs its own reservoir.
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9th May 2022, 5:32 pm | #12 | |
Heptode
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Re: Stacked power supply puzzler.
Quote:
Paula |
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11th May 2022, 5:48 am | #13 |
Dekatron
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Re: Stacked power supply puzzler.
Some good suggestions which I'll implement, thanks all, Andy.
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