wide-range Voltage Controlled Resistor--for use as variable load

[siberhusky]

I'm trying to find something that would work as a voltage-controllable resistive load. I thought something simple might be available--where a DC control voltage could be used to yield a range of resistance values. Here are the basics:
[ul]
[li]Resistance: span ~ 200-20,000 ohms.[/li]
[li]Sink capability: DC source voltage/current up to 7 Volts/50 mA.[/li]
[li]Control voltage: negotiable.[/li]
[/ul]

The digital potentiometers I've seen don't seem to be able to handle the current, and the FETs and Photo FETs I've viewed don't seem to cover that range. Any thoughts or ideas would be appreciated.

 

[catserveng]

How about a motor operated potentiometer with the appropriate size pot?

http://www.p3america.com/motorized_potentiometers_text.htm?gclid=CNeY8KGRzMICFcZbfgodGwoAbg

http://www.google.com/url?sa=t&rct=...52t9rUoI6CgJK6xu9fBD4ww&bvm=bv.82001339,d.cGU

MikeL

 

[IRstuff]

?? Huh ?? 50mA*20kohm = 1000V even 200ohm*50mA = 10V, which is higher than your 7V?

You do realize that the word transistor is the compounding of "transfer resistor?" You can use a couple of transistors in a current mirror configuration, and control the mirrored current.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

Of course I can. I can do anything. I can do absolutely anything. I'm an expert!

 

[MacGyverS2000]

You do realize that the word transistor is the compounding of "transfer resistor?"

TIL...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[siberhusky]

My apologies for not defining the boundaries more clearly. The numbers cited were meant to indicate the approximate upper limits, and they don’t occur simultaneously.

I’ll research the suggestons further. Thanks for the thoughts.

 

[jimkirk]

Look at the LM13700 dual transconductance amplifier. There are VCR applications for it.

http://www.ti.com/lit/ds/symlink/lm13700.pdf

Alone, it won't get you the current range you want, but the basic circuit topology could be made beefier with some additional circuitry.

A few questions: what frequency range do you need for the resistor? DC to a few kHz is one thing, MHz is another. How fast do you need to be able to modulate the resistance? Do you need this resistor to be floating, or can one side be grounded? If ground referenced, the job is much easier; an op amp and a multiplier chip can do it.

 

[siberhusky]

In essence I’m trying to get the functionality of a DC-DC converter or simplified MPP circuit, but on a very small scale.
[ul]
[li]The frequency is essentially DC.[/li]
[li]I’m looking to switch the load in milliseconds at worst.[/li]
[li]One-side grounded would work.[/li]
[/ul]

I've done a lot of reading/searching. A motor-operated pot does what I'm looking for in principle, but I don't know that it quite fits. I looked at the OTA’s, et al. I have the TI datasheet on the LM13700, and saw that VCR application referenced. I tried messing with a feedback/voltage-controlled constant current source that I put together, and looked at some buck-boost circuits. I think these things are close, but it seems tough to get the full range. I’m trying to match resistances at current levels down to ~1 mA or less… that’s what drives the higher resistance values noted above.

As a placeholder, I have precision resistors simply selected via analog switches, giving a brute force range of 150 ohms up to 6k ohms. The main issue is the lack of continuous coverage

Thanks for your thoughts….