Excuse me for butting in, @WBahn

But note that without combining multiple standard resistors, reducing the resistor by 500Ω isn’t easy. Standard Resistor Values are logarithmic.

I understand that, but I might be able to get close enough to make it work if that's what's needed.
Waiting for an answer from WBahn.
Thanks

 

Excuse me for butting in, @WBahn

But note that without combining multiple standard resistors, reducing the resistor by 500Ω isn’t easy. Standard Resistor Values are logarithmic.

It's worse than that. The 500 Ω figure is an upper limit on the on resistance of the CD4066. In most cases it will be lower. But it will vary as the voltages it sees varies. So it's not as simply as just reducing the resistances that are going to be switched in to compensate. That's why it's important to know what kind of tolerance range is acceptable.

If that resistance tolerance is too tight, then the 4066 is not going to be a suitable choice. There may be other integrated analog switches that are, but it's hard to get really low on resistances on a chip. So then you are looking at using discrete FETs, which is certainly doable, but not as clean and simple.

 

I understand that, but I might be able to get close enough to make it work if that's what's needed.
Waiting for an answer from WBahn.
Thanks

But what is "close enough"?

 

Please forgive me for repeating myself, and I'm striving to keep this friendly, but may I ask exactly what you are trying to do?

I've lost count of the number of threads on this forum where the Thread Starter proposes half a solution to a problem and asks how to implement it instead of presenting the problem and asking for possible solutions.

Looking at the circuit (where I previously mentioned it would be useful to know what the 20 pin IC actually is, so we could look at the datasheet) it's clear that resistors R1 and R2 are just being used to provide two reference voltages which are a fraction of the battery voltage. So I'm wondering if you are trying to keep those reference voltages fixed for two different battery voltages? 38V and 12V for example would be close. If this is the case, it may be easier to regulate those two reference voltages in a different way.

That said, two FETs as switches to switch in and out fixed resistors with 2 x 1K trimpots to do fine adjustment should give a quick an easy result.

 

Please forgive me for repeating myself, and I'm striving to keep this friendly, but may I ask exactly what you are trying to do?

I've lost count of the number of threads on this forum where the Thread Starter proposes half a solution to a problem and asks how to implement it instead of presenting the problem and asking for possible solutions.

Looking at the circuit (where I previously mentioned it would be useful to know what the 20 pin IC actually is, so we could look at the datasheet) it's clear that resistors R1 and R2 are just being used to provide two reference voltages which are a fraction of the battery voltage. So I'm wondering if you are trying to keep those reference voltages fixed for two different battery voltages? 38V and 12V for example would be close. If this is the case, it may be easier to regulate those two reference voltages in a different way.

That said, two FETs as switches to switch in and out fixed resistors with 2 x 1K trimpots to do fine adjustment should give a quick an easy result.

Thank you very much Jerry-Hat-Trick for your input......
Your FET & trim pot suggestion may very well be what's needed. The fixed values of
R1 & R2 determine the voltage output of the alternator in this application. I am looking for two different voltage outputs from the alternator unit. One "default" voltage and a second voltage when a ground is supplied to "switch out" the default resistor values with a second set of chosen resistor values for R1 & R2. The second set of resistor values will remain in circuit until the ground is removed, and at that point the "default" resistor values will be returned to circuit.
This is what I'm trying to do.
I plan to do simulations on the circuit and build prototype for testing different resistor values. I am seeking help with a circuit that can accomplish this and I would like to try what you have suggested with the FET/trim pot combination or any other suggested method someone might offer.
Thank you

 

I am seeking help with a circuit that can accomplish this

Thanks for your response. I'm still not sure I understand what the circuit does, but for R1 maybe something like:



An N-channel enhancement type MOSFET is normally turned on so the bottom of R1 = 4K2 is grounded , but when you switch the switch the transistor is turned off so the resistance to ground is switched to 20K5.

This assumes that that the transistor has infinite resistance when turned off and zero resistance when turned on, which isn't the case - hence the need for tweeking with pots.

See https://www.electronicshub.org/fet-as-a-switch/

How are you planning to switch in and out your ground to change the values of R1 and R2? I've just shown a physical switch. Grounding the gates of the MOSFETs for both R1 and R2 with one switch and their gates connected together should make them switch near enough simultaneously.

 

Thanks for your response. I'm still not sure I understand what the circuit does, but for R1 maybe something like:

View attachment 277057

An N-channel enhancement type MOSFET is normally turned on so the bottom of R1 = 4K2 is grounded , but when you switch the switch the transistor is turned off so the resistance to ground is switched to 20K5.

This assumes that that the transistor has infinite resistance when turned off and zero resistance when turned on, which isn't the case - hence the need for tweeking with pots.

See https://www.electronicshub.org/fet-as-a-switch/

How are you planning to switch in and out your ground to change the values of R1 and R2? I've just shown a physical switch. Grounding the gates of the MOSFETs for both R1 and R2 with one switch and their gates connected together should make them switch near enough simultaneously.

Thanks for your assistance on this project.
The ground signal to MOSFET gate will be provided by another device not part of this circuit.

Question please.......

1) When MOSFET is in the "Off State", is the total resistance amount 20.5 + 4.2 since the two resistors are in parallel?
2) Where in this diagram will the "Trim Pot" be located and what is his purpose?

Thank you

 

Thanks for your assistance on this project.
The ground signal to MOSFET gate will be provided by another device not part of this circuit.

Question please.......

1) When MOSFET is in the "Off State", is the total resistance amount 20.5 + 4.2 since the two resistors are in parallel?
2) Where in this diagram will the "Trim Pot" be located and what is his purpose?

Thank you

(1) instead of a 20K5 resistor I show a 16K3 resistor. When the mosfet is off that means that the two resistors are in series making 20K5
(2) maybe, instead of 16K3 and 4K2 make it 14K in series with a 5K trim pot and 3K9 in series with a 1K trim pot. When you adjust to get the results you want you know the value of fixed resistors you can use, or leave the trim pots in place