Transformer Problem

BillPall · Aug 6, 2020

I have a 50VA 240/24V transformer driving, at different times as needed, one, two or three Central heating zones' 24V DC relays, 680 ohms each. DC is achieved using Bridge rectifiers type KBPC1-005 and 2200uF capacitors. The transformer replaced a failed smaller VA transformer. I designed and modified the system over the past fifty years. This is what happens:
with only the DC relays as loads, the DC Voltages at the relays'
coils are 35. As a Mechanical Engineer with a DIY knowledge of
electronics, I think that level of voltage could shorten the life of the relays. I tried to reduce the voltage by inserting a suitable resistor, after a fuse that was located immediately after the transformer's 24V output.
This worked fine and got my desired 24VDC at one relay coil. However, there was insufficient voltage to operate even one more relay.
I now think the capacitors and the transformer may be too large.
One potential solution, I thought, was to reduce the resistance as a second and third relay was activated, using a voltage value trigger. This is stretching my DIY a bit. Or is 35VDC OK on a 24-volt DC relay?
Please, is there a solution that does not include scrapping the new transformer?
Your help would be very much appreciated.
Best regards,
BillPall

markz528 · Aug 6, 2020

BillPall said:
I have a 50VA 240/24V transformer driving, at different times as needed, one, two or three Central heating zones' 24V DC relays, 680 ohms each. DC is achieved using Bridge rectifiers type KBPC1-005 and 2200uF capacitors. The transformer replaced a failed smaller VA transformer. I designed and modified the system over the past fifty years. This is what happens:
with only the DC relays as loads, the DC Voltages at the relays'
coils are 35. As a Mechanical Engineer with a DIY knowledge of
electronics, I think that level of voltage could shorten the life of the relays. I tried to reduce the voltage by inserting a suitable resistor, after a fuse that was located immediately after the transformer's 24V output.
This worked fine and got my desired 24VDC at one relay coil. However, there was insufficient voltage to operate even one more relay.
I now think the capacitors and the transformer may be too large.
One potential solution, I thought, was to reduce the resistance as a second and third relay was activated, using a voltage value trigger. This is stretching my DIY a bit. Or is 35VDC OK on a 24-volt DC relay?
Please, is there a solution that does not include scrapping the new transformer?
Your help would be very much appreciated.
Best regards,
BillPall

24AC full wave rectified single phase will give you around 26.5 volts DC. That is a big cap. For relays you don't need caps at all. I would get rid of it.

motion guru · Aug 6, 2020

DC

C Converter that holds 24VDC output with variable DC Input

BillPall · Aug 6, 2020

markz528 said:
24AC full wave rectified single phase will give you around 26.5 volts DC. That is a big cap. For relays you don't need caps at all. I would get rid of it.

Thanks so much for your quick reply. Tomorrow the Caps will be out. (I have noticed it is just after midnight here in the UK, so it will be later today.) I should have said that the No-Load transformer output is 27V AC.

BillPall · Aug 6, 2020

Thanks for you quick reply. Learning all the time. I did not know such a device existed since I have not had that kind of problem before. They look a bit pricey.

motion guru · Aug 6, 2020

Lots of different kinds . . . range of prices, this was just the first I came across that was close to what you said you needed

SomeoneSomewhere · Aug 6, 2020

No need for a transformer; there are power supplies that get you 24VDC straight off a 230V mains supply.

E.g. one of these. 60W for GBP20.

Edit: If you're only driving those three relays, a 15W version is a bit cheaper.

Something like this is in nearly any industrial control cabinet from the last 20 years if not more.

Or buy some 24VAC relays.

rabler · Aug 7, 2020

markz528 said:
24AC full wave rectified single phase will give you around 26.5 volts DC. That is a big cap. For relays you don't need caps at all. I would get rid of it.

Not sure how you computed 26.5 VDC, but that's not correct. AC voltage is actually measured RMS (root mean square), which is roughly a way of saying "average" voltage. (It is actually computed to give the same effective power as DC voltage). The peak voltage is higher by the square root of two, so 24 * sqrt(2) is 33.9 volts. *Unloaded* full wave bridge rectifier going into capacitor, the capacitor charges to and holds the peak voltage.

markz528 · Aug 7, 2020

rabler said:
Not sure how you computed 26.5 VDC, but that's not correct. AC voltage is actually measured RMS (root mean square), which is roughly a way of saying "average" voltage. (It is actually computed to give the same effective power as DC voltage). The peak voltage is higher by the square root of two, so 24 * sqrt(2) is 33.9 volts. *Unloaded* full wave bridge rectifier going into capacitor, the capacitor charges to and holds the peak voltage.

Crap. I divided instead of multiplying. Good catch. 24 VAC single phase full wave rectified is around 21.5 volts and 27 VAC is around 24 volts DC. Not used to working with single phase rectifiers. Normally work with 3 phase.

That is what you get without the cap. And yes - a cap can hold close to peak voltage which at 27 VAC is 38 volts and the bigger the capacitor and/or the smaller the load the closer to the 38 volts it will hold.

Dump the capacitor and you get 24 volts. This just powers relays - correct? If so, don't overthink it.......... If it powers electronics its another story.

BillPall · Aug 7, 2020

I got rid of the capacitors and everything is fine within 1V of the 24V. Thanks Markz528 for your valuable suggestion.

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