I have a heated plant propagator for raising seedlings, rooting
cuttings etc. One of these http://tinyurl.com/qayxzke The temperature
sensor is connected to a little control box (on the LHS in the
picture), and a potentiometer sets the control temperature. Inside the
control box is a little printed circuit board with various components,
the three largest of which are an encapsulated relay rated at 10A
277VAC, a 50v 100µF capacitor and I think a rectifier (the yellow
thing). There's no wall-wart or external low voltage transformer, and
the heater is mains powered. I can see no sign of dry joints or
overheating on the circuit board. Image of the components here
Sensor input bottom RHS, power
in/out top centre. Image of the underside here
Sensor input now bottom left.
It's recently started developing a problem, in that when the
controller calls for heat and the relay clicks in, it often doesn't do
so completely and buzzes loudly and the 'heating on' indicator light
doesn't glow at full brightness. While this isn't a big problem at the
moment as it still manages to control the temperature and the buzzing
is out of earshot, it offends me in that it can't be doing any good
and it may deteriorate to a point of failure or even worse,
Any suggestions as to what I need to do to get the relay to close
properly? I have modest soldering capabilities, but certainly not for
the tiniest components on the board!
Yes this needs to be fixed and not just put out of earshot!!
The yellow item is a capacitor, in front of it is the reservoir
capacitor which I think is probably defective. The rectifier, is to the
right of the black capacitor with 4 legs one on each corner.
The 8 legged device is the heart of the controller.
Acquire a similar replacement capacitor of a similar value and at least
the same voltage rating and solder it temporarily in parallel with the
existing one observing the correct polarity (grey line with minus signs
on it) If the chattering stops, then whip out the old one and fit the
new again making sure it is the right polarity.
On Tuesday, 10 November 2015 13:07:03 UTC, Bob Minchin wrote:
You can skip the parallelling, just replace the part. Polarity needs to be
right. If your junkbox doesn't contain one of the same capacity/voltage, in
creasing either by 50% is no problem - if it still fits.
On Tue, 10 Nov 2015 10:53:28 +0000, Chris Hogg wrote:
I might be wrong but the 024-1Zs suggests the relay coil is designed for
24v operation. The yellow block is probably a 1 or 2 microfarad 250v AC
(400vdc) dropper capacitor feeding a bridge rectifier (probably the small
black four legged i/c next to the 100 microfarad electrolytic).
The symptoms rather suggest that one of the bridge rectifier elements
has failed short circuit, degrading it from full wave to half wave
rectification which, in this marginal PSU cct will produce very high 50Hz
ripple and a significant reduction in the average voltage feeding the
larger 8 pin dc/dc converter chip.
There's an example of this type of compact mains to low voltage DC
supply circuit shown here:
Watch out for line wrap.
On Tue, 10 Nov 2015 22:20:55 +0000, Chris Hogg wrote:
If you've got a suitable spare, handy to drop in, that's worth a shot.
However, removing that smoothing cap and the big yellow dropper cap will
give you much better access to replace the rectifier chip if it tests
If you've got a multimeter to test with, you should be able to probe the
pins to test for short circuits between the pins if googling the chip
number confirms my suspicion that it's a rectifier chip. The nice thing
about testing for bridge rectifier diode short circuits in this circuit
is the lack of a low resistance dc shunt path from a transformer
secondary winding which normally requires that one of these connections
be opened to properly test the rectifier.
The resistance of the load usually looks high when testing with a
multimeter with the test voltages typically applied to measure
resistance. If you're testing with a classic moving coil multimeter set
to its x1 ohms range (driven from a single uncladded AA zinc carbon
cell), silicon diodes will give a reading close to mid scale when testing
the forward 'resistance' of a single diode. You should be able to
identify a shorted diode element in the bridge rectifier chip without
having to disconnect it from the circuit so it's a test worth applying if
you have a multimeter of one sort or another to test with.
A DMM uses a constant current source and measures the volt drop to
produce an "Ohms" reading, essentially translating the voltage directly
to ohms, eg 10mA test current will produce a 500mV reading when testing a
50 ohm resistor on the 200 ohm max scale (or about 670mV - 67 ohm reading
for a silicon diode - dropping to a slightly lower value when using a 1mA
test current on the 2K max scale, perhaps showing 0.63K ohms or so).
I would expect that DMMs that offer 'autoranging' in the ohms
measurement mode could become confused by this non-linear behaviour. They
may either use this behaviour to automatically identify or warn you that
you're measuring a diode or else ignore this and leave it entirely to you
to manually select the 'diode test' function (often associated with a
continuity test buzzer feature).
Thanks for all that. I have a little DMM with a diode test function,
as you describe. I'll test the rectifier before I set about replacing
the cap. I should be able to get the test prods in there without too
I've tested the bridge rectifier using the 'diode' range on my little
DMM, and AFAICT it's OK with no shorts. I'm glad it's OK, because it's
_very_ small, an MB 6S type, and I wouldn't even attempt to replace it
I've checked the DC output of the rectifier: 29 VDC when the relay
isn't activated; 18VDC when it is, i.e. it drops significantly when
the relay is activated. No significant AC on the output of the
rectifier. The relay coil is 24 volt.
The voltage applied to the relay coil when activated is 16.6 VDC
The big yellow capacitor is 0.27 µF, 275VAC.
I'm awaiting delivery of a suitable reservoir capacitor to replace the
possibly faulty one, as I don't have one suitable.
Many thanks for all the help and suggestions so far.
Not unreasonable sounding measurements. 16.6 volts is a bit low/marginal
for a nominal 24v relay. I'd expect to see significant AC on the
rectifier output when the relay is energised.
I would still have reasonable expectations of the replacement
electrolytic capacitor sorting it out.
Bit of a uncommon value for the yellow capacitor 0.22, 0.33 or 0.47 more
commonly found values - maybe they had a job lot of 0.27uF in stock.
Yes but no .... often tends to be the quoted rating of those X rated
types for continuous across the ac mains use.
I've just looked through my stock and find lots of 275v and also 305v
both AC voltage and some other non branded ones marked 630 v which I
presume to be the DC peak voltage - I think they came out of some 3
On Thu, 12 Nov 2015 12:44:59 +0000, Bob Minchin wrote:
In most cases that would be true. Using a larger value than the minimum
mandated by most circuit design requirements generally improves the
performance of the circuit. However, you have to remember that in this
case picking a slightly larger or smaller value would be like replacing a
270 ohm dropper resistor with either a 220 or a 330 ohm one.
In this case, a 220nF cap wouldn't be able to supply enough voltage to
the relay and the use of a 330nF may require the use of 400mW zenner
diodes in place of 300mW ones. The next available rating up from 300mW
may jump to a more expensive 500mW type.
Whilst it may be possible to use higher wattage zenner diodes *and* be
able to safely dissipate the extra heat, this design choice (330nF and
400mW diodes) would raise the energy consumption to an unnecessary level.
As the circuit stands, it only draws some 600mW or so from the mains
supply when it's not calling for heat from its 50W heating element.
In any case, the choice of 270nF is taken from the E12 range which, for
this capacitor type is standard and not uncommon. Electrolytics, otoh,
*are* commonly supplied from the wider E6 value range (1, 1.5, 2.2, 3.3,
4.7, 6.8 and then into the next decade sequence - 10, 15, 22 and so on).
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.