where to put oscilloscope probe of switch mode power supply  - Page 1
Forum section: electronics / beginners

author: khatus
author_rank: Regular Contributor
timestamp: August 19, 2019, 06:53:15 pm
content: Hello guys i want to measure switching frequency of a 220 to 12 volt dc switch mode power supply.But i am confuse where to put the oscilloscope probes.Can anybody tell me where i can put those probes..

author: MosherIV
author_rank: Super Contributor
timestamp: August 19, 2019, 07:13:12 pm
content: If you do not know - DO NOT DO IT !!!These types of smps are incredibly dangerous!They work by first rectifying the mains voltage to give you 240V DC , with all the energy of live main as well !Then this is switched/pulsed into the small transformer at high (50KHz or higher) frequency.The secondary side is rectified into a capacitor.This voltage is measured and the difference from the setpoint voltage is fed back to adjust the frequency of the pulses.The freq or pulse width is adjusted to keep the output voltage (or current for constant current psu) at the correct value.

author: tunk
author_rank: Super Contributor
timestamp: August 19, 2019, 07:25:41 pm
content: Connect them to the 12V output - the filtering isn't perfect so it should give you some idea of the frequency.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 19, 2019, 07:30:11 pm
content: Quote from: tunk on August 19, 2019, 07:25:41 pmOtherwise the anode of D7 should show you what you want, with the ground clip of the probe connected to the negative of the output. But beware, if you don't know what you are doing it can kill you, your scope and your cat!

author: Canis Dirus Leidy
author_rank: Regular Contributor
timestamp: August 19, 2019, 09:15:40 pm
content: Quote from: khatus on August 19, 2019, 06:53:15 pmFor some practical purpose or just out of curiosity? In the first case, just look at thedatasheet- the switching frequency of VIPer22A is fixed (60 kHz).P.S. But in any case, poking into the power supply with the oscilloscope (unless PSU is connected to mains through isolation transformer) is a really bad idea.

author: tggzzz
author_rank: Super Contributor
timestamp: August 19, 2019, 09:28:41 pm
content: Quote from: khatus on August 19, 2019, 06:53:15 pmTo measure the frequency you probably don't have to connect the probes to the SMPS. EMI should give you a regular "disturbance" at the switching frequency.If youreallyneed to connect a probe, an HV differential probe is the right type of probe to use.

author: Zero999
author_rank: Super Contributor
timestamp: August 19, 2019, 09:43:43 pm
content: Quote from: khatus on August 19, 2019, 06:53:15 pmYou can probably just connect the probes to the output and use the AC coupled setting to monitor the ripple which should correlate well enough to the switching frequency.Never mind that, those switched mode power supplies don't appear to be very safe or comply with the EMC standards. There's no Y capacitor and there appears to be insufficient creepage distances, but it's impossible to know, without seeing the other side of the board. Please post some pictures of the other side of the PCB.

author: khatus
author_rank: Regular Contributor
timestamp: August 20, 2019, 08:23:18 pm
content: Here is the picture of back side

author: perdrix
author_rank: Frequent Contributor
timestamp: August 24, 2019, 05:36:54 am
content: Arggh! Not a single anti-creepage slot and clearances look way too low for 230V AC. Where did that dangerous rubbish come from?David