DC Power Supply TM5020E - Page 1
Forum section: electronics / repair

author: Sello
author_rank: Newbie
timestamp: December 23, 2024, 12:17:37 am
content: Hi Kindly help I have a problem with my power supply doesn't show on the screen no output voltage also the fan does not run the board does not show anything burnt the components seems to be fine no shows to blown but i do not receive any voltage on the output can i get some advice on how to go about finding the fault or what may be the fault causing the power supply not to have an output.

author: _rushin_or_draggin_
author_rank: Regular Contributor
timestamp: December 23, 2024, 04:40:05 am
content: The simplest thing to check first would be the fuse. There might be one where the cable plugs in at the back. If not, it should be somewhere inside (but I am betting on the plug). Since so many things seems to not be working, that typically points to a single failure point. Also, check to see if you're getting voltage to and from the regulators. The fan is most likely connected directly to one of the regulators. It looks like maybe 4 are mounted on the heatsink.Update with your findings and we'll see where that leads us!

author: Sello
author_rank: Newbie
timestamp: December 23, 2024, 05:09:32 am
content: I have checked the fuse is working fine also i get voltage from the regulators but on the transformer the secondary side i get a very small voltage

author: Swake
author_rank: Frequent Contributor
timestamp: December 23, 2024, 01:27:42 pm
content: The displays are coming on and read 0 volt, do I understand that correctly?There is what I think is a klixon on the heatsink (see pic). It is a thermal fuse that resets itself. It should only switch when the temperature is higher than specified, but these sometimes stop working entirely. Could you check the type (read what is written on it) and measure if it is open of closed?Power supplies in the same form factor and case are very common and have been made in many versions for many brands including Farnell. If someone could share a schematic of a similar unit that would be very helpful.Warning: This is a switched mode power supply. Be very careful poking around in it as there are lethal voltages in that thing a bit everywhere.

author: Andy Watson
author_rank: Super Contributor
timestamp: December 23, 2024, 03:15:53 pm
content: Quote from: Swake on December 23, 2024, 01:27:42 pmAppears to be a re-badged Mastech. See first few replies of this post:https://www.eevblog.com/forum/repair/mastech-hy5020e-50v-20a-bench-supply-economical-to-repair/Or perhaps the OP could post the circuit that he has on his laptop!

author: Sello
author_rank: Newbie
timestamp: December 24, 2024, 06:14:36 am
content: There is no display the power supply just stop working no component shows to be burnt and there are no output on the power supply. Here is a schematic to fault find I have been using but it not for the TM5020E but it similar to it.

author: Swake
author_rank: Frequent Contributor
timestamp: December 24, 2024, 07:35:55 am
content: Did you check power is coming in? I mean the fuse and the power switch, you measured them and you had 230V?Did you check that klixon?If all that is confirmed good remove the power plug and with a multimeter confirm there is no more juice in the big caps. Then do an easy check with the multimeter in diode mode and verify all diode junctions in transistors, fets, diodes for any suspicious open or shorted ones.

author: Sello
author_rank: Newbie
timestamp: December 24, 2024, 10:56:45 pm
content: The power switch, the fuse and the klixon are fine and i get the 230V. I did the diode testing no short or open circuit

author: Swake
author_rank: Frequent Contributor
timestamp: December 25, 2024, 07:54:17 pm
content: Check if the TL494 is alive: Verify if Vcc is present, it comes in via H1 and C107. If that is ok then tell us what the signal looks like on pin 5 of the TL494.

author: MathWizard
author_rank: Super Contributor
timestamp: December 26, 2024, 06:03:49 am
content: Those are some funny looking gate drives on those primary side MOSFETs. I'm making some of it in LTSpice to see if the current in/out of the gates, can actually build up a voltage on those windings, or what current some induced voltage makes. But then I have no idea what size inductors to use yet. I'll have to try and make a full sim, to really see what's going on. I don't get yet the way some of the windings are tied to other transformer windings.

author: Swake
author_rank: Frequent Contributor
timestamp: December 26, 2024, 07:11:23 am
content: Herewith the schematic of a very similar power supply. Difference is lower voltage / amps capability but concept is the same.

author: MathWizard
author_rank: Super Contributor
timestamp: December 26, 2024, 07:57:34 am
content: Yeah I don't think I've ever seen MOSFET's driven that way, and with windings, and then throw the SCR into the mix, and control loops, I bet it's a lot of work to calculate whats going on.Ok so the secondary side windings are tied into the controller Vcc, and are being switched, and generating a little voltage and current to drive the gates I guess.

author: Swake
author_rank: Frequent Contributor
timestamp: December 26, 2024, 08:33:23 am
content: U301 (TL494) needs some power to work (Vcc).This Vcc power comes in on S4 in pins 3/4(GND) and 5/6(Vcc +) from H1And now I'm lost because H1 is the output of a transformer and that can only be AC voltage.... There is no rectification unless Vcc is connected in the wrong position.I think the schematic is also wrong or at least inaccurate in the naming of the pins GND and Vcc on S4 as it is still AC voltage.My new question is: what voltage is coming out of H1 ?- If there is nothing or if it is very low then the issue is 'before' T101.

author: MathWizard
author_rank: Super Contributor
timestamp: December 26, 2024, 09:05:52 am
content: After looking at it again, I guess it's some type of LC resonant tank, with 2 1/2 bridges, and the gate drives being driven from the other side, and not some other source on the primary side.But hey I got a 36V out model working, with values like I might find in a computer PSU gate drives.I would think the H1 winding should be control Vcc for the TL494, but they do have it drawn incomplete, because pin12 is only connected to the supply cap. I would think D105 rectifies H1 for the IC, and the top of the 47uF C107 is local Vcc.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: December 26, 2024, 08:18:43 pm
content: Quote from: Sello on December 23, 2024, 12:17:37 amDo they not use punctuation in South Africa???

author: Sello
author_rank: Newbie
timestamp: December 27, 2024, 06:03:34 am
content: We use them.

author: Swake
author_rank: Frequent Contributor
timestamp: December 28, 2024, 10:41:44 am
content: QuoteBeen able to gather this information?