What is required to make simple board design more robust? - Page 1
Forum section: electronics / projects

author: dugherd
author_rank: Contributor
timestamp: January 26, 2017, 07:38:39 pm
content: Hello, all. I am designing a simple board to provide control to a small servo. Input is from a 10K pot to modulate the output PWM pulse width from 1 to 2 ms with frequency of about 50 Hz. The controller is powered by 5V supply and the servo is powered by an independent 12V supply. This only provides the control signal.My question is that for a basic circuit design like shown below what should be done to make the board more robust (this will be delivered to a customer)?Should there be on board power supply/regulator?The pot is panel mounted and wired to the board, is it ok to leave the third terminal floating? Should this input signal be filtered?What kind, if any, output protection should there be? I added a zener on the output to protect for overvoltage. Is this appropriate? Input to the servo should be 3.5 to 5.5V.Is there anything else that I am missing? Something that is considered mandatory on every PCB design?Thanks in advance for your suggestions.Jeff

author: james_s
author_rank: Super Contributor
timestamp: January 27, 2017, 04:31:38 am
content: What is the sort of end user this is expected to deal with? Are they likely to connect it to overvoltage or reverse polarity? Sometimes I include a diode in the power input, but normally only if it uses a barrel connector for power since there's no polarity standard for those. Otherwise I'd say just make sure you use keyed connectors and label the pins on the silkscreen so it's difficult to hook anything up wrong.

author: dugherd
author_rank: Contributor
timestamp: January 27, 2017, 05:48:30 am
content: Quote from: james_s on January 27, 2017, 04:31:38 amThe connection is wired to a terminal block so reverse polarity is possible. What is best protection for this?

author: lilshawn
author_rank: Regular Contributor
timestamp: January 27, 2017, 07:52:03 pm
content: Quote from: dugherd on January 27, 2017, 05:48:30 amjust a simple diode is fine ... something like a 1n4002 or 1n4004 right in line after the + input.depending on the voltage, if reverse connection, the diode will block it or sacrifice itself.

author: Fungus
author_rank: Super Contributor
timestamp: January 27, 2017, 08:43:41 pm
content: It's always worth putting in a fuse.Onanyproject.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: January 27, 2017, 09:23:45 pm
content: I don't understand how the circuit is supposed to work? where do you connect the load? where does the 12V go?it seems like you have NPN as output transistor, but you have it connected with 1.2k pull up, this is not the way you connect it to a motor, a voltage source with 1.2k of resistance will provide large voltage drop in low currents. it also connected to the 5V rail and not 12V.I would use N-MOS as an ouput mosfet, with 100 base resistor, 100k pull down on the gate.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: January 27, 2017, 09:58:49 pm
content: Quote from: lilshawn on January 27, 2017, 07:52:03 pmThat will only work if the system can handle a full diode drop on Vin. If the circuit expects 5V that's a ~15% error, that'll piss a lot of circuits off. I prefer the reverse biased diode approach (reverse polarity will short itself out through the diode), but only if the power supply can handle a short circuit on its output without blowing up. Otherwise a Pch MOSFET wired for reverse polarity protection is super easy to add to the circuit.I'd also add a TVS diode on the power line to protect against transients and overvoltage (such as the spike you get when plugging an inductive source (long wires) into your capacitive 5V input).

author: james_s
author_rank: Super Contributor
timestamp: January 27, 2017, 10:57:53 pm
content: A series diode works fine on low power stuff depending on the supply voltage, and a Shottkey diode has a very low forward drop. The crowbar diode is valid approach though, but should use additional protection, I like PTC thermistors personally. They have a low impedance until overload causes them to heat up and thus self limit. A fuse blows and then the device is dead. Most people just throw something away these days rather than open it up to see if it contains a fuse.

author: dugherd
author_rank: Contributor
timestamp: January 28, 2017, 12:03:32 am
content: Quote from: ZeTeX on January 27, 2017, 09:23:45 pmThis does not drive the load. Only provides a control signal (PWM) such that a variable pulse width from 1 - 2 ms varies the position of the servo output shaft from 0 - 90 deg. Motor power is provided by a separate 12V supply. The transistor and pullup resistor invert the output signal of the LM555.Quote from: suicidaleggroll on January 27, 2017, 09:58:49 pmI added a fuse for short circuit protection, a P-ch MOSFET for reverse polarity protection, and a TVS diode for transient/overvoltage protection. My concern with this is in the case that polarity is correct so the FET is on if there is an overvoltage that is longer than a fast transient such that the diode is in reverse conduction this could nuisance blow the fuse. Should I be concerned? Maybe this is a good thing.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: January 28, 2017, 12:14:03 am
content: Quote from: dugherd on January 28, 2017, 12:03:32 amI would switch to a bidirectional TVS and put it before the fet so the fet doesn't have to pass the current pulse the TVS is shunting away.If you're worried about the fuse you could use a slow-blow, as long as it blows before the TVS.