User jason s - Chiphacker

moderately priced 6-layer PCB fab?

2010-07-28T21:04:16Z

I have a board I'm working on with a coworker; we are debating whether to go 4-layer or 6-layer board, and would like to use 6-layer but haven't found a reasonably priced fab house.

Is there any reputable PCB fab house out there that has good pricing on small-quantity (3-10 pcs) 6-layer PCBs? (approx 5"x6", nothing fancy) The best we've been able to do is about US$700, whereas there are specials on 2- or 4-layer PCBs that are in the $200-$300 range.

clarify background info: the board has 3 widely-used nets = AGND, DGND, +5V, which basically spells 5 layers, and there doesn't seem to be much advantage to 5 vs. 6. We've looked at sharing the AGND and +5V layers, that's an option; we've looked at routing the +5V by hand, that's an option; we've looked at combining AGND and DGND nets and living with the noise. The right thing to do from an engineering standpoint is a 6-layer board, but if it's going to be 2x the price it may be cost prohibitive given our particular situation (further details are beyond the scope of this forum).

Answer by Jason S for What cable assemblies do you recommend?

2010-07-15T14:44:11Z

We've used ITW PANCON "MAS-CON" series connectors. (single row, available in 0.1" or 0.156" spacing) They work pretty nicely, the crimper tool isn't very expensive.

PCB-mount connectors for thermocouples

2010-07-09T15:46:11Z

Has anyone had good/bad experiences with particular PCB-mount thermocouples?

I need to design a circuit board with a connector I can plug a standard thermocouple connector into.

(and yes, I am aware that when you connect thermocouple wires to copper traces, you create junctions of dissimilar metals that can cause errors if the junctions are at different temperatures)

Answer by Jason S for Communication between microcontroller and separately powered PCB

2010-07-05T02:24:28Z

If on separate boards, use a differential signaling approach such as LVDS, with a driver on one board, and a receiver on the other. (This assumes the two boards have ground voltages that are reasonably close together -- if not then you need an isolation barrier.) Reset signals are an example of something that is sensitive to glitches -- a momentary glitch on a reset line has an effect on the future state of the chip going to it.

You can get away with less-careful approaches if the signals in question are stateless inputs.

Answer by Jason S for 1-wire parasitically powered microcontroller?

2010-06-25T02:00:23Z

Use a capacitor to store the energy, and connect the negative end of the capacitor to ground, and connect a Schottky diode between the data line and the capacitor. Schottky diodes have a low forward drop.

Answer by Jason S for Is C++ suitable for embedded systems?

2010-06-20T19:53:52Z

C++ is absolutely suitable for embedded systems. I now use the presence/absence of good development tools (or lack thereof) as my primary criterion for whether or not to use a particular microprocessor.

Areas of C++ that are good to use on embedded systems because they have low resource costs:

modularity brought by good use of classes/structures
templates if the compiler does a good job of compiling them efficiently. Templates are a good tool for bringing reuse of algorithms to different data types.

OK areas:

virtual functions -- I used to be against this, but the resource cost is very small (one vtable per class, not per object; one pointer to the vtable per object; one dereferencing operation per virtual function call) and the big advantage of this is that it allows you to have an array containing several different types of objects w/o having to know what type they are. I used this recently to have an array of objects each representing an I2C device, each with separate methods.

Areas not to use:

dynamic memory allocation -- others have mentioned this, but another important reason not to use dynamic memory allocation is that it represents uncertainty in timing; many reasons to use embedded systems are for real-time applications.
RTTI (real time type information)
exceptions -- a definite no.

Answer by Jason S for How do I build a cheap/efficient mains adapter for some LED lighting?

2010-06-12T16:31:47Z

buy one -- for example Digikey sells 5V regulated power supplies for about US$7 and unregulated ones for less than this.

You would probably be hard pressed to make something less expensive that is safe.

Answer by Jason S for Can you run a BLDC motor backwards without damage?

2010-05-27T22:37:06Z

What you have to worry about most when you reverse direction of a motor, is that you do not put too much current into either the motor or the electronics/switches that control it.

When you connect a voltage source across a motor that is at rest and either has a large inertia or a locked rotor, you get a large current flowing through it = V / R where R is the stator winding resistance of the motor. This is called the stall current.

If you are running at full speed with a voltage source across a motor, and you immediately reverse the polarity of the voltage source, you can get up to 2x the stall current, because the voltage source is then at the opposite polarity of the motor's back-emf. This can be too much current, and if that's the case then you have to control the rate at which you reverse voltage across the motor, by using PWM or some other way besides a hard voltage reversal.

Answer by Jason S for I don't quite understand this FET-BJT preamp circuit

2010-05-18T12:50:04Z

Here's the deal. The capacitor provides constant voltage at high frequencies across the BJT base-emitter + resistor combination. This causes fairly constant current through the BJT and resistor, with some high impedance Z, probably determined mostly by the BJT base resistor Rb. The FET has a high transconductance (gm = Iout / Vin), and the net gain is gm * Z. This is the voltage across the FET drain-source. The BJE emitter resistor has constant voltage across it, so there's a bias voltage added to that. The constant current allows the BJT to act as a low-impedance output buffer (=Rb/beta).

Answer by Jason S for Choosing a switching device

2010-05-14T00:31:40Z

Almost definitely a MOSFET. The only issue really is what voltage is available to drive it.

Let's say you have a 15V battery with a 5V on/off signal referenced to battery negative. If your load can float and can attach directly to the battery positive, then you can use an N-channel MOSFET: tie MOSFET source to battery negative, drain to the side of the load not connected to the battery, and the gate to the on/off signal through a small (50-200 ohm) resistor.

If the load needs to be grounded, you need to use a P-channel MOSFET with drain tied to the ungrounded end of the load, the source tied to battery positive, and the gate drive circuit may be a little more complicated (* = I might post details later).

A 15V battery is small enough that you can get away with a MOSFET rated for 25-40V range and there are super-low on-resistances that can handle 60A loads.

edit: you don't say what kind of packaging you are looking for. If you are planning on using a circuit board for other things, go with a MOSFET in an SO8, DPAK, or D2PAK package. SO8 sounds iffy for 60A loads unless it's for a really brief time, but it's probably the most cost-effective, though the worst for thermal dissipation. DPAK and D2PAK are beefier.

If you're not planning on using a circuit board, an automotive relay would probably be much easier to use.

Answer by Jason S for High voltage/current open-drain output driver

2010-05-11T12:23:15Z

I would have recommended Allegro's relay/solenoid drivers, but they have been deemed "pre-end-of-life" and shouldn't be used for new designs.

Infineon SPIDER ic's look interesting.

MAX4896 might work but I avoid Maxim ic's like the plague as they tend to be made out of unobtainium.

best quick&cheap power supply inverter

2010-03-29T20:02:13Z

I have a +5V power supply and need to invert to a -5V supply sourcing about 25mA. I've got an LMC7660 running now. It's ok for the short term, it outputs -4.4V with 100mV p-p switching noise, but that's unacceptible for my long-term needs.

Anyone have any experience with LMC7660 / ICL7660 - like switched capacitor inverters? There must be some better parts on the market nowadays, we've had those LMC7660's in a drawer for at least 12 years. I can probably hunt something down, but figured I'd ask here.

Answer by Jason S for Prototype Options For Medium Current Application

2010-05-05T13:41:37Z

If there's significant power conversion / switching electronics, you may have to use a "real" (custom etched) PCB to get good ground planes and keep the parasitic inductances down. This can be the case even in the 0.5A-1A range.

There are other applications where you may need to keep voltage drops low, so plan carefully. We had a "smart" battery charger that used charging currents of 1.25A. The voltage drop along our charger cable was only about 0.1V, but we had SMBus (=I2C more or less) signals referenced to the ground return, and with a 0.1V difference between the ground return potential at either end of the cable, it ate into the SMBus voltage level margin, which isn't that much. Voltage drop along the positive power input wasn't a significant issue, but drops along the ground return had a big effect.

Bottom line: it depends. You need to study your system and decide what kind of voltage drops along the supply lines can be problematic. Then design your system around it. (e.g. star grounds rather than daisy chains in some circuits) Measure the drops in the final system to make sure you are doing OK.

Otherwise, what the other answers said.

Answer by Jason S for Whats the max current for this component?

2010-04-25T01:02:34Z

Technically, it is impossible to tell: switch manufacturers have to deal with the subtleties of arcing when designing circuits, and AC behavior is different than DC behavior. Arcs will be extinguished much more easily with AC (since voltage passes through zero) than with DC.

It's odd that the rating drops so much at 240VAC from the 6A rating at 125VAC. If the limiting factor were thermal behavior, I would expect the current rating would be the same. So I'm guessing they're probably using a particular test condition, like opening the switch with maximum rated current, assuming there's bouncing in the switch which may cause an arc.

Ask the manufacturer....

Answer by Jason S for Shielding Techniques for Digital Sensor

2010-04-22T12:36:10Z

I suggest you purchase / check out a copy of Henry Ott's "Noise Reduction Techniques in Electronic Systems". It talks about these things.

Don't ground the shield at both ends, as this creates a ground loop. The PCB end is probably the best place to ground the shield.

Answer by Jason S for How do I connect components when using a circuit board with pads but no traces?

2010-04-13T11:28:45Z

Take a length of wire-wrap wire (30ga or so).
Strip end of wire long enough to bridge the two pads. This leaves a short length of stripped wire and the remaining insulation, which now ends at point X.
Solder end of wire to one pad.
Cut wire at point X.
Solder point X to the other pad.

Answer by Jason S for H-Bridges and Motor Stall Current

2010-04-06T11:43:32Z

How do I determine how powerful an H-Bridge I need to control a DC-motor?

If you want to provide full stall current, measure the DC resistance of the motor leads. (e.g. a 0.3 ohm motor connected to a 12V battery will draw 40A at stall) If you want to be able to drive the motor to full speed, then reverse it, it will draw twice that current.

If you have a current sensor, you can (with suitable control circuitry) limit the current to any value you choose, for a "soft-start". This is usually a good idea. A less complicated route is to have a current trip sensor which then turns off the H-bridge (do NOT short the motor out, it will overcurrent if it is spinning) if there is an overcurrent detected, then retry some time period later. This has inferior performance to current limiting but will generally be adequate.

Answer by Jason S for When to avoid using a breadboard

2010-03-31T03:31:39Z

what Adam said, but also high current. The protoboards have high parasitic capacitance and inductance.

I use them to test pieces of circuits I know are OK, but not for larger systems.

Answer by Jason S for best quick&cheap power supply inverter

2010-03-30T12:39:14Z

After looking around I think I'm going to use a TPS60403, it's got a higher switching frequency (250kHz) than the LMC7660.

Answer by Jason S for Beginning On This Magic World

2010-03-23T21:50:47Z

this is perhaps a duplicate question to an earlier question:

Basic Electronics Book

where you will find several introductory books on electronics. (I vote for Horowitz & Hill.)

Question etiquette

2009-11-28T04:18:02Z

Various notes on question etiquette could go here....

(moderators: I have no idea how this should be structured... I was going to edit my response to the Electronics Exchange question but it got closed. Also I don't have enough points yet to add the tag "faq" -- could someone add that tag?)

Answer by Jason S for wireless temperature sensor

2010-03-16T18:52:11Z

Why not use a standard remote sensor for a consumer-grade weather station? Most of the ones you find in stores are made by Oregon Scientific. I'm not sure of the wireless band and the protocol, but it ought to be fairly simple and probably documented somewhere out there on the internet.

Answer by Jason S for Waterproof Temperature Probe

2010-03-15T12:39:47Z

I use one of these kitchen thermometers for my home-scale maple sugaring equipment:

http://www.target.com/gp/detail.html/181-2465896-2946763?asin=B0001BFJ54

The probe is foodsafe and works fine in boiling sap, although after one or two seasons it starts to pick up scale from the calcium impurities in the maple sap. (a tough problem to get around)

I'm not sure if the probe contains a thermistor or a thermocouple, but in any case it's a cheap off-the shelf solution that I'm sure you could use with your own circuitry if you experimented a bit.

Answer by Jason S for Theory: With an AC circuit adding capacitors will cause the current trend towards leading...

2010-03-15T12:32:33Z

Because with a resistive / capacitive parallel load, the total current is the vector sum of pure resistive current (current in phase with voltage) and pure capacitive current (current 90 degree lead from voltage). If the two components are Ir and Ic then the lead angle is atan2(Ic,Ir) which is 0 for Ic = 0, Ir > 0, and pi/2 = 90 degrees for Ir = 0, Ic > 0. So you are always between 0 and 90 degrees lead for RC parallel loads. It's not like the lead angle is proportional to the capacitance.

Answer by Jason S for MD5 implementation for microcontroller

2010-03-11T22:04:56Z

I would stick with a known reputable implementation of MD5, and stay away from libraries you find from 3rd-party vendors. The original RFC 1321 which described MD5 has a sample C implementation.

Reminder: the known weaknesses for MD5 are collision attacks, and not preimage attacks, so it is suitable for some cryptographic applications but not others. If you don't know the difference you shouldn't be using it, but don't discard it altogether. See http://www.vpnc.org/hash.html.

Answer by Jason S for Reading RTD Temperature Sensor

2010-03-11T21:25:33Z

You might also be running into EMI susceptibility. Active components (such as your PGA) are prone to a phenomenon called RF rectification where an AC disturbance at high frequencies at the input amplifier causes a DC disturbance at the output of an amplifier. This is very common in high-gain circuits (you'll see it a lot in thermocouple amplifiers) in an electrically noisy environment.

If this is the problem, shunt out high-frequency noise by putting one or more good high-frequency bypass capacitors (1000pf-10000pf ceramic probably best) across the closest points that are inputs of your circuit. (for instance, if you have a 4-resistor one-op-amp differential amplifier:

then put 2 capacitors at the input of the resistors -- from V1 to GND and V2 to GND, you may need a 3rd one from V1 to V2 if there is a lot of differential noise -- and NOT across the op-amp inputs)

Answer by Jason S for Reading RTD Temperature Sensor

2010-03-09T17:26:00Z

One common culprit is charge coupling between channels of the ADC (disregard if you are only using 1 channel).

Most microcontrollers with multichannel ADCs have a multiplexer and a sampling capacitor. The sampling capacitor might be in the 1-10pf range. When you switch from one channel to the next, that sampling capacitor initially retains charge from the previous channel's voltage. The sampling capacitor then has to charge up/down to the voltage on the next channel, and has a time constant that depends on the external impedance on the ADC channel input.

It is good practice to use an RC circuit right on the ADC channel inputs. (edit: if you have a voltage divider, you don't need the R; the Thevenin equivalent resistance acts as a resistor, so a 10K and a 1K divider will yield an equivalent resistance of 909 ohms.) I tend to use something in the neighborhood of 499 ohms, 100-300pf. What happens is the external capacitor in the RC network acts as a storage reservoir, so when the ADC multiplexer switches, the external capacitor very quickly charges the sampling capacitor. There's a tradeoff between using a small capacitance (fast time constant, but initial transient when ADC mux switches is very large) vs. a large capacitance (very little initial transient on external capacitor when ADC mux switches, but a long time constant) and you can solve this yourself to optimize.

You generally need to do this even if you are using an op-amp to buffer the voltage leading into the ADC. This is because op-amps aren't great at dealing with high-frequency nonlinear loads like a multiplexer + sampling capacitor.

If you are not buffering the voltage leading into the ADC with an op-amp, note that high source resistance can be a problem. This charge coupling causes a current that flows between one channel and the next, with current equal to f * C * deltaV, where f = sampling frequency, C = internal sampling capacitance, and deltaV = voltage between successive channels sampled by the ADC. Example: deltaV <= +/-3V, C = 5pf, f = 1000Hz yields a charge-coupling current of up to +/- 15nA. If your source impedance is 10K, you'll get an offset voltage of up to +/-150uV depending on the voltage difference between channels. (This really only becomes a problem with high sampling rates or high source impedances)

Answer by Jason S for It's been more than 10 years (since I designed hardware with RS-232), and we're still using RS-232?

2010-03-03T13:33:45Z

Distinguish serial UART communication from the various voltage levels used to transmit it: RS232, RS485, LVDS.

Serial UART communication is reliable and simple.

Answer by Jason S for Inductor for LM2575

2010-02-20T13:48:09Z

the datasheet suggests a Pulse PE-52627 for most designs. If you use National's Webench, and put in your exact design requirements (input/output voltage, current) then I'm pretty sure it will suggest a part number.

Answer by Jason S for MSP430 programmer

2010-01-30T14:44:36Z

Hmm. I don't quite see how you could beat US$99 for the MSP-FET430UIF USB debugging/programming interface from TI, or US$49 for the MSP-FET430PIF parallel-port debugging/programming interface, by trying to design/build/test something yourself.

The MSP-FET430UIF also is part of TI's university discount program and is 50% off ($49.50 instead of the $99), not sure what the details to qualify, but contact your TI rep.

I like MSP430's; for a basic low-end microcontroller they have been the easiest series for me to use/debug because of the development tool set.

Comment by Jason S

2010-07-29T13:16:42Z

"You'll still be paying $700 but at least you'll have a lower per-board cost." The board houses already do that to some degree; the difference in total cost between 5 boards and 10 boards might be only $50-$100.

Comment by Jason S

2010-07-15T14:41:54Z

Please edit to change "these pins" to mention a specific series. Otherwise readers have to follow the links in question to figure out what they are.

Comment by Jason S

2010-06-26T00:25:36Z

please make this question community wiki as it does not have a single answer

Comment by Jason S

2010-06-23T23:47:31Z

If I had to pick one, I would buy the #6342 pogo tips in a heartbeat. They weren't that expensive, and the springyness is invaluable for making good contact. You just can't do that with non-compliant wires or probes, without having to sit there and press hard against the component in question.

Comment by Jason S

2010-06-20T19:56:44Z

We use Advanced Circuits heavily in our company and have had very good results with them.

Comment by Jason S

2010-06-20T19:55:18Z

Please make this a "community wiki" question: it is likely to have multiple answers.

Comment by Jason S

2010-06-20T19:41:13Z

+1 -- this is the proper way to do it if you don't have an ADC taking in negative inputs.

Comment by Jason S

2010-06-20T19:39:58Z

-1: you'd better use precision resistors and a precision reference for 5V.

Comment by Jason S

2010-05-31T23:47:35Z

Not necessary -- it's generally much easier to do in-circuit programming on the board after the part is in place.

Comment by Jason S

2010-05-27T22:32:25Z

good answer! (says a motor guru)

Comment by Jason S

2010-05-27T00:55:55Z

please make this a community wiki question

Comment by Jason S

2010-05-19T01:13:27Z

"what's the benefit over a circuit like this": Good question. Gain looks about the same (dominated by BJT base resistor Rb... in your posted circuit it's the two bias resistors in parallel) in both cases. Output impedance looks about the same... when I first saw the circuit on this page I thought the capacitor was a battery, and I thought: "oh, of course, they're making the BJT into a constant current source, why wouldn't you just use a zener..." in which case you truly could use a constant current source w/r/t the BJT -- the advantage of this deals with parasitics in the BJT...

Comment by Jason S

2010-05-18T12:44:11Z

A 50-200 ohm resistor usually suffices, you don't want one that is significantly larger.

Comment by Jason S

2010-05-18T12:43:21Z

@Henrik, @jluciani: gate resistor is not for limiting current to the gate, per se (which you don't want to do). It's for several other reasons: controlling the turnon/turnoff time (resistor in parallel w/ diode allows turnoff to be faster), preventing ultrahigh-frequency oscillations due to device gain and device lead inductance, and isolating faults from propagating to the circuit that drives it (esp. if directly from a microcontroller pin).

Comment by Jason S

2010-05-14T13:39:29Z

60A is serious current, but if it's limited to a particular duration (e.g. 1msec) then it's not quite as much of an issue. But you have a point.