Digi pot for pwm board

[mathphobe]

I've recently bought one of these pwm boards for testing , they've got a much wider frequency and duty cycle than I've tried on previous projects.
I'd like to actually mount the board to try out its practicality, I'd buy more as they're cheaper than I can make myself bizzarIey. Problem is I need to replace the pots with smd tactile switched digi pots, freq up/down and duty up/down.
I believe they're 2M ohm and 10k ohm. I've found numerous 10k digi pots and a few related schematics, can't find a 2M digi pot nor any schematic.

Anyone know how to swap out the pots on this, maybe have a schematic you can share?

Cheers Mick.

 

[dorke]

The largest value they make is 1M.
If that alone isn't enough you can use a dual device(2 in series) to get 2M.
Look here

 

[kellys_eye]

You could switch a 1M resistor in/out of series with the 1M version that's available or even use a load of 200k resistors and give yourself 10 ranges!

Given the variable resistors are all 15-turn presets you'd need to have switched ranges to get any form of accuracy as the 'steps' available on a single fixed (variable) resistor may not offer the resolution you require.

 

[mathphobe]

What happens if I use a 1M pot? Do I lose some of the frequency range or just zip through the range quicker?

 

[kellys_eye]

What happens if I use a 1M pot?

Funny you should ask. If you Google that board it shows a number of different versions, same board layout, same component layout BUT with a 1M pot in place of the 2M pot your image shows....????

It also shows the schematic:


The 1MΩ pot shown can easily be changed for a 2MΩ given the range it covers.

Interestingly, the datasheet for the SG3535 shows the timing components for the RT pin in relation to a maximum of a 200kΩ resistance (for values of C1 ranging from 1nF to 100nF - the value of 10nF (as fitted) gives a charge time of between 20μS and 1000μS (5kHz to 1kHz) for a resistance span of 200kΩ.

Of course, the values for RT and CT will depend on the inductor you're using on the output of the controller but I suggest you download the data sheet and have a look at the parameters yourself - seems like you could chose the appropriate capacitor to suit a 10KΩ digi pot quite easily.

 

[mathphobe]

Very nice post there Kellys_eye thank you. I'd not seen any of that info or I'd have put it in my op.

The inductor I'm using? I was under the impression I could apply the circuits output directly, or with maybe some passive, to the gate of a logic level mosfet, did I misunderstand??

Tell me more. All I need the board to do is rapidly switch a mosfet/s carrying a good load, I could swap or add anything to the board if need be.
If you had a 'just put x and y in..' type idea while you were writing your post I'd be very grateful to hear it.

If I'm honest I don't understand data sheets, or more specifically the math they pertain to. I've enough IQ and manual dexterity to follow direction and most schematics though.

 

[dorke]

Can you define "Rapidly"?
i.e what is the range of frequencies you need?
larger values of the pot equate to "very slow" frequencies.

 

[mathphobe]

I bought the board to test a wider range of frequency and duty cycle than I've previously been able to with 555's. For my loads I know 10-30Khz 50% duty is good, under 50hz is poor. I've never been able to test a full range of duty cycle and never tested over 30Khz. I guess the answer to your question is I don't know exactly but it won't be low frequencies.

Higher resistance = lower frequency? Cool, that's the kind of info I never find, the info I need is always presented in mathematical terms that mean absolutely nothing to me like Vr(t)=Vro+Vr1cos(2πf1t+∅1) or my favorite when I looked into smps
x˙=[A01δ(t)+A02(1−(δ(t))]x+[B01δ(t)+B02(1−δ(t))]uxA01δA02(δ+[B01δB02(δ
There's just no way I can make a circuit from that, I can barely look at it!

 

[kellys_eye]

The device itself is designed specifically to generate the gate drive voltages for SMPS circuits in as much as it delivers the respective pulse width outputs that don't overlap i.e. dead time (essential to prevent short circuits) but can also be modified by a feedback signal for stabilisation.

The outputs would normally drive a transformer for the step-up/down function required.

Data sheets aren't that difficult to understand if you take the time to absorb what they are telling you. The symbology they use is pretty standard, the technical spec is straightforward and they all start by describing the basic function of the device that should give you a good enough idea of whether or not the device is going to meet your needs.

The better datasheets also include some basic design circuits that can be built (and work!) just to get you started.

If there is any aspect of a datasheet that you don't understand or can't Google to your satisfaction then just ask here and we'll try to explain, in a simplified manner, what the developers are trying to tell you.

 

[dorke]

About the SG3525A datasheet ,the product ,and your needs:

First thing is to look at how is the frequency set with the SG3525A.
It is given by the manufacture:

That is a fairly simple formula,in which:
CT= timing capacitor.
RT=timing Resistor.
RD=Dead_time resistor.

second thing is to look at the "limitations":

As can be seen(table above):
1. RT should be 2Kohm to 150Kohm.
2. CT should be 1nF to 100nF.

From that it is clear that the Chinese manufacture allowed himself to "stretch" the value of RT to 2Mohm which is way above the recommended value.
He did that so that he can cover the full frequency range with a single capacitor value,
but that is bad engineering

That means two things:
1. It will not work over the recommended IC limits.
2. You shouldn't use such large a resistor!

The left graph above shows the relationship between the frequency(actually time period) and different values of CT and RT under the assumption that RD=0 (zero dead_time).

Now lets see how you can configure this device to your needs.
I'm assuming you would like to operate in the 20Khz to 100Khz for testing,
(you can change this range to be any frequency in the operating range of the IC i.e 100Hz to 400KHz).


In blue the lines corresponding to 20-100Khz
In green the 10K digipot.(note that it should be in series with a 2Kohm resistor to assure the minimum, RT value!)
Looks like a single value of about 6.8nF(in pink) would do nicely to cover this range.
If you need a higher range say 40Khz to 200Khz,simply change the capacitor to 3.3nF(use a switch?)

I hope this helps.

 

[mathphobe]

The device itself is designed specifically to generate the gate drive voltages for SMPS circuits in as much as it delivers the respective pulse width outputs that don't overlap i.e. dead time (essential to prevent short circuits) but can also be modified by a feedback signal for stabilisation.

The outputs would normally drive a transformer for the step-up/down function required.

I looked for info about it but found very little, one thing I did read several times was "gate driver", guess my imagination did the rest. The boards output could work in place of s1 in this schematic though right??

You said outputs plural, hadnt realised that, should I assume they're a hi and low? I think I could further develop my project with that function.

Dorke, I must confess I had to read your post a few times to get a grasp of it, pretty sure I'm there now.. sort of. The visual on the resistance, capacitance, frequency grid helped me no end, thank you!
I can use a 50k pot and a 1nf cap to get 0-100Khz? If so right there would be the ideal!

Bad engineering from chinese retailers on ebay sadly doesn't surprise me, I bought a 2a micro usb charger board for a lipo once, it worked as described but the dam thing got so hot I was scared the solder would melt!

 

[dorke]

I can use a 50k pot and a 1nf cap to get 0-100Khz? If so right there would be the ideal!

Well not quit, from the graph a 50k pot would give you about 20KHz-500khz,blue on the graph.
(notice the discrepancy as the max. should be 400kHz) .

Using a 6.8nF and 50K pot will give you about 4KHz-100Khz,pink on the graph.
Is this o.k for your needs?

You should also note that digipots have actually discrete(not continuous) levels of resistance (depending on their resolution),so you can not get all the range covered only discrete points!

 

[mathphobe]

Well not quit, from the graph a 50k pot would give you about 20KHz-500khz,blue on the graph.

Schoolboy error, my bad.. I was reading the highest number as the highest frequency, of course 2ms is faster than 500ms. Lesson learned!



There's a lot of digi pots I could get, the 10k is an easy swap as there's plenty of ready made schematics around.
I've chosen to use a 100k in place of the 1M but I've no idea which one to get. Understanding various interfaces and volatile or not is a little over my pay grade.

Here's how I envision the my circuit using a generic pot for illustration (swapping r2 for a 2k and swapping r3 for the 10k digi pot)
Here's the pin out of the 100k non volatile I2C pot I intend to buy

I'm assuming the 'interface' and 'volatility' are just for communication and that the pot doesn't need programming, or if it does it at least has a workable base map? If my assumption is correct, can anyone tell me where I connect the pins please? I've read the data sheet (attached) but it's been no help to me.

 

[kellys_eye]

The device is controlled over a serial comms bus (I2C) using the SDA and SCL lines (plus the AD0 and AD1 lines) and it would be best to use the likes of an Arduino as there are 'libraries' already written for the device that can control it very simply.

 

[mathphobe]

I was quite hoping tactile switching or resistive switching would work but its not a problem, for my circuit I'll get a 'contact/closure' interface ic like the ds1869s-100 as there's little chance of fitting an audrino in my enclosure.
I looked at the ad5259 because it had many more taps, 256 vs the 64 of the ds1869, I will likley endeavour to find a another c/c ic with more taps now I know I2c won't work. Thank you kellys_eye.

Pretty sure I've got all the info I need to carry on with my project now so I'd like to thank everyone for their invaluable help.

Mick.

 

[dorke]

You can operate the 100k/256 steps MAX5454EUB device with switches(and some simple logic).
It is a dual device which can cover the 2 pots you replace in a single IC.

 

[mathphobe]

... or am I

Sorry, thought I was done but then got thrown a curve ball.

The 'choosing correct digipot' pdf says "tap up/down" digi pots can be used with buttons.
I've found a 256 tap up/down pot but the data sheet doesn't mention or give a schematic for buttons..

Is the first pdf incorrect or is the button interface just not listed on the data sheet?

If it is indeed correct the pin out is labelled differntly to other schematics I have, I've no idea how to wire it with buttons, could anyone help me with that please?

 

[mathphobe]

Started writing my post earlier but got destracted, hadn't refreshed the page before I finished my question, didn't see your post at all there dorke.. great minds n all that

Yeah, I saw the dual version but again, no idea how to add buttons or limit the other side to 10k.

Anywhere you know where I could find a schematic?

 

[kellys_eye]

Page 7 of the datasheet explains how the U/D pin works in conjunction with the chip-select and INC pins.

 

[dorke]

Here is a simple circuit that may work for manual switch operation.
First you select the pot you want to operate and the direction,
then you push the INC switch for a "single step".