820 V digitally controlled rheostat

Thread Starter

George Osama

Joined Mar 17, 2018
8
Hello

I'm trying to turn on variable resistances from 2 MOhms to as small as 5 kOhms from a PIC 40X2 my initial design had the PIC controlling a relay driver ULN2003A which then turns on different relays each connected to a specific resistor value between a 820 V supply. However, I was looking for an improvement to this in which I can have like a variable pot in which I can tune in the resistance I want from the PIC. I could only find digitally controlled resistors but they have 30 V operating voltage max. Does anyone have any idea if that is even possible?

Thanks in advance
 

Thread Starter

George Osama

Joined Mar 17, 2018
8
Not sure for that high a voltage.
What type of adjustment resolution do you want?
Can the resistor go to ground or must it be floating?
It will be going to ground which is connected to the chassis. The resolution maybe like 10k or so but the idea is getting rid of the relays and relay drivers.
 

OBW0549

Joined Mar 2, 2015
2,984
I could only find digitally controlled resistors but they have 30 V operating voltage max. Does anyone have any idea if that is even possible?
You will not find any digital pots rated for 820 volt operation. 30V is about as high as they go.
 

crutschow

Joined Mar 14, 2008
23,347
What exactly is the application?

Do you realize that 820V into a 5kΩ resistance means the resistor will be dissipating over 134W? :eek:
 

TeeKay6

Joined Apr 20, 2019
382
Hello

I'm trying to turn on variable resistances from 2 MOhms to as small as 5 kOhms from a PIC 40X2 my initial design had the PIC controlling a relay driver ULN2003A which then turns on different relays each connected to a specific resistor value between a 820 V supply. However, I was looking for an improvement to this in which I can have like a variable pot in which I can tune in the resistance I want from the PIC. I could only find digitally controlled resistors but they have 30 V operating voltage max. Does anyone have any idea if that is even possible?

Thanks in advance
@George Osama
So you wish to place varying loads across the output of an 820V power source? Are you trying to assess how the voltage varies with loading? Or have you some other purpose? If the 820V is DC, you could consider replacing resistance with a current load; that is, you could let current flow through an electronic device that can control the current flow; such devices are available. However, as @crutschow has noted, you would be handling quite high power levels. Give more info if you wish to proceed.
 

Thread Starter

George Osama

Joined Mar 17, 2018
8
So about power dissipation in resistors I have 5 1206 resistors connected to each relay and there will be forbidden resistances as you mentioned so I won't be turning on 820V across a 5k resistor. I will be turning on relays depending on the resistances i want across the 820 V. I have this sorted out already.
All what I am asking if there are some way to have analogue rheostat controlled by the microcontroller, instead of having fixed resistor values being switched on and off by a relay.
 

TeeKay6

Joined Apr 20, 2019
382
So about power dissipation in resistors I have 5 1206 resistors connected to each relay and there will be forbidden resistances as you mentioned so I won't be turning on 820V across a 5k resistor. I will be turning on relays depending on the resistances i want across the 820 V. I have this sorted out already.
All what I am asking if there are some way to have analogue rheostat controlled by the microcontroller, instead of having fixed resistor values being switched on and off by a relay.
@George Osama
I will ignore your first paragraph as without more detail I really don't know what you intend to do.
As for your question in the second paragraph, yes there are ways to simulate an analog rheostat but they are complicated. If acceptable, there are devices called "digital potentiometers" (search for) that can perform a rheostat function but they are limited to very low voltage (most to 5V, but some a bit higher) and extremely low power (milliwatts max); these devices do not produce an analog output, they produce a selection of outputs that simulate many taps of a resistor/rheostat (e.g. 1%, 2%, 3%...99%, 100%). With (complex) supporting circuitry, their effective power level and voltage limits can be increased. "Digital pots" can be directly connected to the I/O pins of suitable microcontrollers to select which tap is connected.
There may be straightforward solutions to what you are trying to do, but you must supply much more information about your circuit (and its purpose) before those can be considered. You say "I have this sorted out already"...but we do not have this sorted out; we have no idea what you are trying to do.
 

Alec_t

Joined Sep 17, 2013
10,362
@Alec_t
It likely has nothing to do with automotive electronics. Why do you ask?
Because the post was in the Automotive Electronics forum and to make the TS aware of this. The mods have now moved it to a more appropriate forum.
 

AlbertHall

Joined Jun 4, 2014
8,364
@AlbertHall
Would you consider a motorized pot a "digital" pot?
UPDATE: Your suggestion was not out of order. However, I believe that subsequent discussion makes it clear that it would not be what the TS is after.
The motor driving the pot could certainly be controlled by some circuitry driven by the digital output of an mcu.
This could be a stepper motor or a more conventional motor controlled as in a servo.
It would require extra circuitry not needed for a digital pot but it would serve the purpose and it eliminates their voltage restriction.
 

crutschow

Joined Mar 14, 2008
23,347
I have 5 1206 resistors connected to each relay and there will be forbidden resistances as you mentioned so I won't be turning on 820V across a 5k resistor.
So is the 5k just the resolution you want and not the minimum resistance?
Then what is the minimum resistance you want across the 820V?

If you explain exactly what you are doing, we could likely make better suggestions instead of guessing. :rolleyes:
 

TeeKay6

Joined Apr 20, 2019
382
Because the post was in the Automotive Electronics forum and to make the TS aware of this. The mods have now moved it to a more appropriate forum.
@Alec_t
I never saw the post in the Automotive Electronics section. Thanks for explaining the confusion.
 
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