I think a by-pass transistor messes things up a bit unnecessarily. As mentioned on the drawing I linked to before you can use LM350 instead of the LM317 if you need higher current. It’s also possible to use the LM338 part with can put out 5A and handle 40V at its input.
Obviously if using LM350 (3A) or LM338 (5A) you must change both IC1 and IC2!
If you need more current (over 5A) then you can go for a by-pass transistor.
There’s also the LT1083 adj for 7,5A but it’s more expensive and cannot handle more than 25V on the input.
The power dissipation is well explained in the datasheets which everybody reads (I hope) when constructing things. To spread the “heat” you can very nicely parallel some LM317 or any of the other named regulators for higher power if you like.
About your comment on the LT1083, I’m not sure exactly what you mean “below ground”.
One thing to keep in mind when using potentiometers for high-current applications in that the maximum power is usually with the pot on its highest resistance setting. If you set it halfway, only half of the surface is used, so the maximum power is approximately halved. Since the current will increase linearly with the inverse of the resistance in this circuit, and the power is increased with the square of the current, it's really easy to burn a pot at high current settings.
I'd change the circuit so the pot is not in the high-current path and use a separate shunt resistor in series with the current. I believe there's an example in the LM317 datasheet (either the National or the Onsemi one).
Does anybody have a schematic for a DIY switching power supply with voltage and current limiting? Ideally, I'd like to get about 7A out of it. Too ambitious maybe?
The ELV supply is nice but you have to buy the thing (very expensive!) since they probably won’t hand out the source code and a few other things.
And it’s not really a full SMPS it’s a secondary switched (as mentioned) which means you still need a large and heavy transformer.
Why not make yourself a primary switch mode 0 – 30V 0 – 12A almost for free? Find an old ATX computer supply and make a few modifications almost without spending anything. Sounds good? well this guy knows all about it: http://www.chirio.com/switching_power_supply_atx.htm
The ELV supply is nice but you have to buy the thing (very expensive!) since they probably won’t hand out the source code and a few other things.I didn't look at it in depth, but I assumed that the micro controller firmware would be easy to replace since the regulation seemed all analog, so the only thing it would do is control a few DAC/PWM signals, and measure some voltages.
This might seem simple enough for a professional programmer but there sure are many traps and pitfalls that will jeopardize the reliability regarding stability and voltage control unless you know exactly what you are doing. This also applies to the electronics of a supply of curse.
I’m not saying it cant be done just that it’s not for everyone to manage, the interaction between electronics and software are crucial.And it’s not really a full SMPS it’s a secondary switched (as mentioned) which means you still need a large and heavy transformer.True. The fact that none of the electronics magazines or websites (that I'm aware of) have published a variable-voltage/current primary switched power supply says something about the complexity.
I agree the complexity is high compared to linear supply’s however the principle is not that far from the secondary switchers except maybe for the very dangerous high voltages involved.Why not make yourself a primary switch mode 0 – 30V 0 – 12A almost for free? Find an old ATX computer supply and make a few modifications almost without spending anything. Sounds good? well this guy knows all about it: http://www.chirio.com/switching_power_supply_atx.htmNot sure how much easier a good modification is compared to building from scratch. You need to confirm that the loop will remain stable under all conditions, since the duty cycle will be outside of the normal specifications, and I wouldn't trust them to have enough margin for a 200%+ increase in output voltage either. He also disabled the over voltage protection. Many ATX power supplies need a minimum output current at several voltages to remain stable, a load resistor would screw up your current regulation (you'd need a constant current sink). I'm not a big fan of reusing ATX power supplies, except when you need a high-current 12V output without much protection, and don't demand a clean signal. If you want to try it, I'd use a high-quality name-brand one, since they tend to be the most stable and are better protection.
The less it’s modified the better, you will benefit form the original well tested supply and the proposed mod is not very complex at all. As the author points out there are only a few controllers which are suitable for this kind of mod and this tells me that he has been there, tried other controllers and more. I’m sure you can appreciate the way he extended the voltage range with minimum effort and still kept the original parts in the high current section. And of curse it was necessary to disable the over voltage, haven’t seen many of these implemented in variable PSU’s.
I don’t believe you need a minimum load for this to work but if so it could be done without interfering if it’s put before the current shunt.
I've found the service manual for an Agilent primary switched power supply (3MB PDF). Reading the theory of operation section may give you a rough idea, although the implementation of a high-power switcher is quite hard. Note that circuit layout and magnetics are likely to be very critical.
That’s why I think it’s a great idea to use an already made and well tested base like the ATX supply to start on.
The Agilent supply is very advanced and that’s just not a project for homebrew. If you need a supply like this I believe you have to buy it.
I'd say that the extra complexity does not outweigh the lower costs for a DIY project. 30V, 7A is well within the range of linear supplies, you just need good cooling. Of course there's nothing wrong with doing it for fun, but I would suggest you to become very familiar with simple buck/boost converters and fixed-voltage primary switchers before you try to make something like this.
BTW, I have a old HP 6255A linear power supply that I purchased used on Ebay. I've owned it for about 7 years, but i have no clue how old it really is. Do you think I can still trust this power supply or is it likely that the electrolytic capacitors have degraded over time?
The less it’s modified the better, you will benefit form the original well tested supply and the proposed mod is not very complex at all. As the author points out there are only a few controllers which are suitable for this kind of mod and this tells me that he has been there, tried other controllers and more. I’m sure you can appreciate the way he extended the voltage range with minimum effort and still kept the original parts in the high current section. And of curse it was necessary to disable the over voltage, haven’t seen many of these implemented in variable PSU’s.
I don’t believe you need a minimum load for this to work but if so it could be done without interfering if it’s put before the current shunt.
That’s why I think it’s a great idea to use an already made and well tested base like the ATX supply to start on.
The Agilent supply is very advanced and that’s just not a project for homebrew. If you need a supply like this I believe you have to buy it.
Yes, it’s possible to build analog supply in any range but even here you can make mistakes. One issue in analog technique is self-oscillation which suddenly under some condition can blow things up for you. Also depending of what you are using it for there are hazards involved like ham radio equipment for instance.
Who here is capable of analyzing circuits like this? I'm reluctant to build something that I don't understand so I've been staring at this schematic off and on for a few hours. I'm not necessarily asking for someone to explain this to me -- I'm just wondering if this type of circuit analysis should be trivial for a seasoned professional EE.
http://www3.telus.net/chemelec/Projects/MC1466/MC1466-1.png