What could I have Done Wrong? - Page 1
Forum section: electronics / beginners

author: ServiceXp
author_rank: Contributor
timestamp: October 07, 2014, 12:26:30 am
content: I'm very new to electronics, so please take it easy on me.I took apart an old Z-Wave range extender to take a look at the inners. I looked it over fairly carefully and saw the MB6S sitting there. So I thought to myself, lets see what kind of input and output voltage was there. Using my trusty Fluke 16, I measure the input and output legs with out any issue. If memory serves me ~7.5vac input and 5vdc out. So my next ideal would prove a little more exciting.I wanted to see what the 5vdc output voltage looked like on the scope, so I carefully soldered a wire on the negative output (the arrow) pad, connected the DSO probe ground wire to the wire I attached, and then proceeded to plug in the range extender. Well the end results.... I let the smoke out of the MB6S in a fairly violent manor. It wind up also taking out the zero ohm SMD resister and pad.Did I cause a short through the DSO negative lead (the attached wire, and DSO ground were not shorted to anything) to earth ground?

author: IconicPCB
author_rank: Super Contributor
timestamp: October 07, 2014, 01:32:56 am
content: Not much else

author: David_AVD
author_rank: Super Contributor
timestamp: October 07, 2014, 02:18:31 am
content: Maybe this will help?

author: ServiceXp
author_rank: Contributor
timestamp: October 07, 2014, 11:11:17 am
content: Quote from: David_AVD on October 07, 2014, 02:18:31 amThank you Dave, Yes very good video. That said this Zwave Extender does not have an earth ground post, but it also appears to utilize a non-transformer step down circuit. It appears to be a capacitors/resistor design, is this where I ran into problems?. Struggling to find my mistake, any insight?

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 07, 2014, 11:36:13 am
content: Quote from: ServiceXp on October 07, 2014, 11:11:17 amThere is your problem: nothing inside the Zwave is isolated from mains because there is no transformer, so while connecting the scope ground wire (which is connected to earth via the scopes mains cable) you shorted your mains to earth via the MB6S, probably killing 2 diodes in there and who knows what else.

author: mcinque
author_rank: Unknown
timestamp: October 07, 2014, 11:52:57 am
content: I'm not an expert, but when you're measuring things connected to mains on the same pcb, I ALWAYS use 2 probes as differential probe, setting them at 10x and putting scope to use MATH>A-B.Having a real differential probe is always a better solution but they are not cheap.

author: ServiceXp
author_rank: Contributor
timestamp: October 07, 2014, 12:50:25 pm
content: Quote from: PA0PBZ on October 07, 2014, 11:36:13 amOk so at least now I understand where I made my mistake. With this type of power supply, what is the best way to check the output of the rectifier? Isolated Scope (hand held)?

author: rs20
author_rank: Super Contributor
timestamp: October 07, 2014, 12:53:28 pm
content: Power your device using a mains isolation transformer.

author: ServiceXp
author_rank: Contributor
timestamp: October 07, 2014, 01:02:19 pm
content: Quote from: rs20 on October 07, 2014, 12:53:28 pmBut won't that just damage the transformer? Would I not still be shorting the MB6S negative output through the transformer? The ZWave extender is not earth grounded.

author: w2aew
author_rank: Super Contributor
timestamp: October 07, 2014, 04:30:06 pm
content: Quote from: ServiceXp on October 07, 2014, 01:02:19 pmNo, the transformer will isolate it. In the US, 120VAC receptacles are wired with one side (neutral) wired to ground at the service panel. So, even though your device doesn't have a ground lead, it still is connected to ground since there is no transformer in the supply.

author: ServiceXp
author_rank: Contributor
timestamp: October 07, 2014, 06:45:07 pm
content: Quote from: w2aew on October 07, 2014, 04:30:06 pmUnderstood.Thank You.

author: Precipice
author_rank: Frequent Contributor
timestamp: October 07, 2014, 07:00:29 pm
content: I hate to suggest this, but: Are you sure you want to be playing with this device yet, isolation transformer or no?It's still got the ability to hurt you and your test equipment. Sure, it'll be educational, but it could also be expensive, in many ways.Could you pick another widget, and come back to this one when you're a bit more experienced?(you'll also hit a brick wall quite quickly - Zigbee repeater chips are more or less a black box, you apply power and a few passives, and they do their thing. There are probably more fun things to investigate lying around).

author: ServiceXp
author_rank: Contributor
timestamp: October 07, 2014, 11:48:00 pm
content: Quote from: Precipice on October 07, 2014, 07:00:29 pmI appreciate your feedback and you're probably correct with your point on damaging test equipment, that would be a hard lesson learned. As an HVAC/R HA tech, working with 120- 480vac, the high voltage part is very familiar.