Do I need a lossless resistor?

cmartinez

Joined Jan 17, 2007
6,593
The point is, use a 15V zener for reference (you can connect at 47k resistor in series with it. Since you now have a Vcc - 15V reference, then you can always use a switcher to set the 65VDC supply to 50 or less.
I'm under the impression that zeners are not very good as references. Please correct me if I'm wrong, but don't they normally have a 10% tolerance and have a large thermal coefficient? Why not use an Op-amp configured as a voltage subtractor instead?
Or perhaps there are no op-amps capable of running at up to 65 V?
 

GopherT

Joined Nov 23, 2012
8,012
I'm under the impression that zeners are not very good as references. Please correct me if I'm wrong, but don't they normally have a 10% tolerance and have a large thermal coefficient? Why not use an Op-amp configured as a voltage subtractor instead?
Or perhaps there are no op-amps capable of running at up to 65 V?
This guy just wants to drop his 65V DC to 50V. He didn't say anything about accuracy. Even if the Zener is off by 10%, then buy an 18V zener and you will always be at least 15 volts from the 65V source.

Also, Zeners get a bad rap on the stability question. Below 8 volts there is a pretty good knee on Zener voltage vs. current. On zeners above 8 v, there is almost no knee. A constant current source improves things a lot.
 

Thread Starter

Xerox

Joined Aug 6, 2015
13
The point is, use a 15V zener for reference (you can connect at 47k resistor in series with it. Since you now have a Vcc - 15V reference, then you can always use a switcher to set the 65VDC supply to 50 or less.
One big problem, where do you find a switcher that will take a 65V input?
 

AnalogKid

Joined Aug 1, 2013
8,135
Buck regulators can be very efficient, but in an application with varying input voltage and output current, expect around 75-80%. So for every ten watts of load, you have to remove 2.5-3.3 watts of heat. By limiting the input voltage range and the output current range, and building a circuit with oversized switching components and synchronous rectification, you can get the efficiency well over 90%.

ak
 

cmartinez

Joined Jan 17, 2007
6,593
Buck regulators can be very efficient, but in an application with varying input voltage and output current, expect around 75-80%. So for every ten watts of load, you have to remove 2.5-3.3 watts of heat. By limiting the input voltage range and the output current range, and building a circuit with oversized switching components and synchronous rectification, you can get the efficiency well over 90%.

ak
That's a lot of heat... if heat were an issue, why not go from DC to AC, and then a servo-controlled variac with a rectifier circuit at it's output, then stabilize it with capacitors ... mmmhhhh... my idea might very well qualify for a Rube Goldberg cartoon! :)
 

GopherT

Joined Nov 23, 2012
8,012
That's a lot of heat... if heat were an issue, why not go from DC to AC, and then a servo-controlled variac with a rectifier circuit at it's output, then stabilize it with capacitors ... mmmhhhh... my idea might very well qualify for a Rube Goldberg cartoon! :)
Could you imagine the issues if you used the variable output DC to power the servo motor! Positive feedback, oscillations, variac getting slapped from limit pin to limit pin. Oh NO!!!!
 

cmartinez

Joined Jan 17, 2007
6,593
Could you imagine the issues if you used the variable output DC to power the servo motor! Positive feedback, oscillations, variac getting slapped from limit pin to limit pin. Oh NO!!!!
As we say down here... The remedy would be worse than the illness... :p
 

mcgyvr

Joined Oct 15, 2009
5,394
Well that's the trouble if I did explain everyone would go off on a tangent and give a zillion reasons why it should be done another way.
Maybe the zillion reasons are right and what you are attempting to do shouldn't be done for valid reasons..

It is outside of the box.. Which makes me think you wandered that way due to lack of knowledge/details..

The devil is always in the details..
Luckily you have many people here who don't seem to care and will just through out any answer without really understanding the details of the problem..
Good luck..
 

Thread Starter

Xerox

Joined Aug 6, 2015
13
Good luck..
Thanks. When I was very young the town where I lived had a power station not connected to any national grid that supplied dc 250V to all and sundry. I remember us not being able to have a fridge or a vacuum cleaner because they all worked off ac. Its always been a philosophical point with me, what would have happened if ac had not come along. The concept of a dc-dc transformer has me stumped not because I lack the skills which I do to some extent its just that I cannot get past dc-dc converters that have fixed voltage outputs. The feedback arrangements are so straight forward but not so for a tracking dc-dc converter. We have lossless diodes but is a lossless resistor a contradiction in terms. It cannot it seems reduce voltage without generating heat. The nearest you can get to this is to convert it to ac and then back to dc using a switcher.

Thanks to everybody who has given or continues to give this some of their valuable time.
 

cmartinez

Joined Jan 17, 2007
6,593
a lossless resistor a contradiction in terms. It cannot it seems reduce voltage without generating heat
Keep in mind that a resistor reduces current, converting it to heat... not voltage. If you wanted to reduce voltage only, then a resistor voltage divider would do the trick... but yes, if you wish to convert dc to dc efficiently, a switchmode circuit is what it takes.
 

AnalogKid

Joined Aug 1, 2013
8,135
1. By definition, a lossless resistor is a contradiction in terms. A resistor does not store energy in either an electric or magnetic field, so there is no reactive component to its impedance. Because of that, there is no phase shift between voltage and current, so there is no "imaginary power" dissipation. It's all real power, going up as real heat.

2. There is nothing magical about a fixed-output DC/DC converter. They are well developed, plentiful, and cheap because they are what the market has demanded for a few decades. If in a parallel universe a constant delta-V converter was the norm, they would be on ebay for $3. Still, it is not a difficult circuit modification to a standard buck circuit.

3. By definition, there is not and can not be such a thing as a true DC transformer. A transformer primary is governed by Ampere's Law, and that law allows for a steady-state magnetic field (such as an electromagnet that can pick up a car) created by a direct current. But the transformer secondary is governed by Faraday's Law of Induction, which accepts only a *changing* magnetic field (sine wave, square wave, audio, whatever). So you can build a black box that looks like a DC transformer at its terminals, but there can not be such a thing as a basic electronic component.

ak
 
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