Electronics > Beginners
Help me design a PSU
not1xor1:
--- Quote from: VEGETA on November 14, 2018, 09:18:07 am ---
--- Quote from: not1xor1 on November 14, 2018, 08:28:55 am ---
--- Quote from: Cliff Matthews on November 14, 2018, 01:38:08 am ---Here's one from Banggood (it's about $6). Someone posted a schematic.. Funny drawback of producing a negative rail with a bucket load of cheapo 1N4007's is the 1 amp maximum. Real efficient heater though..
--- End quote ---
It is easy to make a much better (more stable, more efficient) PSU with about the same amount of components (dual opamp and power transistors), but if current regulation is not important even a TL431 + darlington would work.
A negative rail and another TL431 would allow to start voltage regulation from 0V.
It would be wise anyway to add a foldback current limit via an additional BJT and resistor.
image link removed - see above for the schematic
--- End quote ---
This schematic doesn't seem to have constant current mode. Adding one will make it better since it is already easy schematic. I guess the purpose of Q1 is for short circuit protection, is it correct?
I am not a fan of having negative rails from diodes due to the heat and other issues. My initial suggestion was a TL431 + ICL7660 which is probably the best choice.
--- End quote ---
the original poster stated: no current limit needed
I anyway added a foldback current limit as I think it is the bare minimum for safe operation.
The diodes used to produce the negative rail dissipate few mWs because the circuit is just a charge pump.
There is absolutely no need for a further IC which might introduce just more noise besides the complexity and the cost.
not1xor1:
--- Quote from: Efe_114 on November 14, 2018, 10:34:39 am ---i think that Q1 is for short circuit protection too. But i didnt understand does this circuit have negative output? Can i build this thing with opamps instead of programmable zeners and 2n2222s as transistors
Or bd138 pnps?
--- End quote ---
The negative rail is not meant for outup.
It is there just to let the output range from 0V.
It does that by bringing the "anode" of the top TL431 to -2.5V.
Of course it is possible (and better) to use an opamp to do the same, or even a dual opamp to provide proper current regulation.
In my spare time I'm testing (just on LTspice at the moment) a very simple circuit inspired by that designed by NSC during the 70:
https://www.eevblog.com/forum/projects/very-low-noise-preregulator-for-benchtop-power-supply/msg1375039/#msg1375039
My circuit is based on modern ICs (LT1013) and is simpler than the original.
I just want to make some more tests before posting it here.
not1xor1:
--- Quote from: rstofer on November 14, 2018, 02:28:40 pm ---There is mention of a 48 VAC transformer above and the output, when rectified, is about 62V (plus or minus the filter capacitor and current draw/voltage drop. Nevertheless, a really big number.
When the PS is outputting, say, 1A at 1V, the pass transistor(s) drops 62V - 1V and dissipates 61 Watts (more or less). That's also a really big number.
That's why real power supplies use transformers with tapped secondaries and some number of relays that kick in at various voltage settings.
If you want to output 5V, there is no point in having more than 8v DC going into the pass transistor (more or less). Clearly, not 62V...
--- End quote ---
IMHO the power is still manageable by using more power BJTs and a big heatsink.
The main problem is rather the high voltage which would require a much more complicated PSU design, with a floating supply for the opamp and less common BJTs (i.e. the high voltage version of 2N3055 or something like the 2SC5200).
I think a beginner should start with a simpler design and a more manageable voltage and power, i.e. something around 15-20V 1A.
rstofer:
--- Quote from: not1xor1 on November 15, 2018, 08:16:02 am ---
--- Quote from: rstofer on November 14, 2018, 02:28:40 pm ---There is mention of a 48 VAC transformer above and the output, when rectified, is about 62V (plus or minus the filter capacitor and current draw/voltage drop. Nevertheless, a really big number.
When the PS is outputting, say, 1A at 1V, the pass transistor(s) drops 62V - 1V and dissipates 61 Watts (more or less). That's also a really big number.
That's why real power supplies use transformers with tapped secondaries and some number of relays that kick in at various voltage settings.
If you want to output 5V, there is no point in having more than 8v DC going into the pass transistor (more or less). Clearly, not 62V...
--- End quote ---
IMHO the power is still manageable by using more power BJTs and a big heatsink.
The main problem is rather the high voltage which would require a much more complicated PSU design, with a floating supply for the opamp and less common BJTs (i.e. the high voltage version of 2N3055 or something like the 2SC5200).
I think a beginner should start with a simpler design and a more manageable voltage and power, i.e. something around 15-20V 1A.
--- End quote ---
Yes, it can be managed but pretty soon you have a space heater - 60W of waste heat is a LOT. Parallel pass transistors with the bases driven by another power transistor is fairly common. It just needs to be considered.
Efe_114:
After changing the lm317 it wasnt going under 1.25 when i plugged the previous one back it was stabilized at the input voltage and didnt have a voltage drop i was feeding 12v and it gave 12 volt out i think something funny is going inside the chip also i ordered them from the most unreliable seller in my country
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version