3296W series multiturn trimpot

[baldguyfromblackpool]

3296W series multiturn trimpot...

e.g.
https://docs.rs-online.com/d700/0900766b8069ccf4.pdf

https://www.bourns.com/docs/technical-documents/technical-library/trimmers/technical-articles/ap_proc.pdf

Does anyone know if 3296W trimpots have a linear or logarithmic response?

I can't see anything in the datasheet

I was planning on using one in the base lead of a 2n3904 transistor, with 12V fed through a relay coil into the collector, and a 0V / 3V on-off signal feeding into the trimmer, so I can try and get the transistor to switch the relay reliably. I was hoping for a component with a nice linear characteristic rather than all the adjustment at one end.

 

[bertus]

Hello,

You do not need a trimmerpot.
For switching applications there is a rule of thumb.
The base current should be 1/10 of the collector current.

Bertus

 

[baldguyfromblackpool]

From its datasheet, relay is G6Y-2P, 12V coil, 16.7mA, 720Ω

So I want 1.67mA into the base when the control line goes to 3V?

The indicator LED wants 1mA to light up. I'll have to get maths head on.

So R1 must supply 2.67mA, and in fact delete R2 altogether?

I'm getting R1 = 112.36Ω, seems low?

 

[bertus]

Hello,

Yes, the base resistor would be (3 - 0.7) Volt / 0.00167 A = 1377 Ohm.
( the 0.7 Volts is the base-emmitor voltage)
You can try 1200 or 1500 Ohms as base resistor.

Bertus

 

[baldguyfromblackpool]

Ahh ok, I've got an LED hanging off the end of the base resistor as an indicator. So if I said (3 - 0.7) / 0.00267 A (the LED takes 1mA) that'd be.. 861.42Ω

Would I be better with the LED in parallel with the relay coil?

 

[bertus]

Hello,

Can you post a schematic?
The voltage drop of a led depends on the color.

Bertus

 

[baldguyfromblackpool]

HLMP-D150 Avago Technologies Led Red T-1 3/4 (5mm) 637 Nm 1.6 V 1 Ma 3 Mcd
Bulb Size:    T-1 3/4 (5mm)
Forward Current If:    1mA
Forward Voltage:    1.6V
LED Colour:    Red
LED Mounting:    Through Hole
Lens Shape:    Round
Luminous Intensity:    3mcd
Packaging:    Each
Viewing Angle:    65°
Wavelength Typ:    637nm

HLMP-D150 PDF

Part #: HLMP-D150. Description: T-1쩐 (5 mm), T-1 (3 mm), Low Current, Double Heterojunction AlGaAs Red LED Lamps. File Size: 574.46 Kbytes. Manufacturer: Broadcom Corporation..

www.alldatasheet.com

Sorry, best I could do for schematic. Top, as I have it now. Bottom.. would that be better?


This whole thing is to be attached to a 3-pin JST socket on a solar inverter, which signals a generator when to start and stop. the 0V and 12V lines are constant but the 3V line cycles between close to 0V and 3V, 3V means "run the generator", 0V means "stop the generator". The generator controller expects a "volt free" style relay contact. The manufacturer tells me the JST port on the inverter can stand up to 0.1A being drawn from it, although they didn't tell me if that was each line, the total, or what. The box they used to make to plug into this JST port, they don't make anymore.

 

[bertus]

Hello,

I have shown your schematic full size.

In the top schematic, the led will not light as the voltage is limited by the base-emittor of the transistor.
In the bottem schematic, the led will burn as there is no current limiting.

Bertux

 

[baldguyfromblackpool]

Thanks. I'll try and attend to that..

 

[baldguyfromblackpool]

 

[bertus]

Hello,

Yes, that is correct.
The led will go on when the input is 3 Volts, together with the relays.

Bertus

 

[baldguyfromblackpool]

Thanks very much indeed for the help you've given me! Some soldering to do now

I don't know why the G6Y-2P relay has a polarised coil, they draw specific attention to making sure it's correctly connected in the datasheet. I'm wondering if it has a spike protector diode already built into it.

 

[bertus]

Hello,

Find the datasheet of the relays and look if they mention the diode.
It will not hurt to have an external diode.

Bertus

 

[baldguyfromblackpool]

I managed to get hold of one of the interface boxes they used to make. It's a circuit board immersed in a block of potting compound.

I need two of these units, so I decided to carry on with trying to make my own, but use the one original unit I managed to source to reverse engineer & be sure my 'aftermarket' design matches up as closely as possible with their original.

Luckily the potting compound was really pliable and easy to peel apart without damaging the components inside the block.

The original design is remarkably similar to what has been arrived at so far!

Uses an Omron G5Q-14 relay - datasheet - coil uses more current than the relay I chose so hopefully I'm safe there?

Can anyone help identify the transistor they've used? marked N3M. Is it a bipolar transistor or some sort of FET? See pic of Q1

In addition to my circuit, they have a tiny beige SMT capacitor across the incoming constant 12V supply.

They also have a resistor going from the base of the transistor to 0V which is marked 01C, I think that's 10k?

The resistor they have in series with the base is marked 01B, that's 1k?

 

[olivia_49]

The 3296W multiturn trimpots are linear, so they’ll give smooth, predictable adjustment for your transistor-relay circuit.

 

[baldguyfromblackpool]

Thanks Olivia. Years and years ago as a boy in an electronics shop on Dale st. in Liverpool, the guy behind the counter told me that all trimpots are linear, but that was in the late 70's!

Hopefully with a bit more knowledge about the circuit the trimmer is no longer needed but, good to know for the future.

I looked up a few references for the N3M device the original manufacturer used and got several hits, unfortunately split 50/50 between bipolar NPN transistors and N-channel FETs. Out of curiousity, if I measured between red and black, if it were an NPN transistor would I see a lowish resistance, whilst if it were a FET I would see 11k ohms or so?

I'm seeing 1.44k, both ways round, on both the 20k resistance range, and diode test mode. It's a FET and I've not deciphered the resistor values correctly? If it were a BJT I would have seen different readings with polarity and mode?

I guess it's academic as long as the circuit operates the relay reliably and doesn't unduly stress the inverter's output.