Voltage drop across Solid state relay?

tortuga

Senior Member
Location
(44.057116, -123.103394)
Occupation
field supervisor
Greetings all, I am getting some strange readings off a solid state relay;
I recently replaced a small 8A relay that controls a restive heating element and am getting about 20 volts drop across the relay with my meter, 118V to 98V.
I get the same amp reading on both sides of the relay 5.5A.
I did not get a chance to go thru and check all the other connections as my first instinct is a bad connection somewhere.
The control wires are 20 VDC.
The solid state relay is rated for 40 amps, the load is 5.5 amp.
Is there something about solid state relays that deforms the waveform/phase and could be fooling my meter?
 

Russs57

Senior Member
All I can tell you is I found the same thing on one used for a sump pump. Replaced it and all is good so far. Just like a triac I would expect to see a 1-2 volt drop but anything more than that gets replaced. These days all my meters are true RMS so can't comment on the rest.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
When you measured the voltage did you put the meter directly across the SSR or from each side to the neutral? It might be best to measure all three ways to make sure that the measurements come up with the same voltage drop. Are you using a true RMS meter as Russs57 mentioned?
Also, what is the make and model number of the SSR? Is the SSR bolted down to conduct heat away? 5.5A through a 40A unit that's not attached to anything could be marginal as far as heat dissipation.
A lot of questions I know, but it would help in figuring out what you are observing.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
If it's a thyristor ( SCR or Triac) type of SSR, the VD would be about 1V when gated fully on. What you might have is an SSR that is not gating fully on, i.e. a "soft starting" feature or a PWM that your meter is reacting to. Again, post the part number of the SSR.
 

tortuga

Senior Member
Location
(44.057116, -123.103394)
Occupation
field supervisor
When you measured the voltage did you put the meter directly across the SSR or from each side to the neutral?
I was going from each side to a ground screw.

It might be best to measure all three ways to make sure that the measurements come up with the same voltage drop. Are you using a true RMS meter as Russs57 mentioned?
Yes its a true RMS, Ideal 61-775

Also, what is the make and model number of the SSR?
I don't have that info, it was late and I should have noted it.
Its a 'cheap' one.
I grabbed it after hours, it matched the coil voltage and it was in a random parts bin and looked way better than the one that was there.

Is the SSR bolted down to conduct heat away? 5.5A through a 40A unit that's not attached to anything could be marginal as far as heat dissipation.
A lot of questions I know, but it would help in figuring out what you are observing.
No its not bolted to a heat sink, I believe the box its in is plastic.

Thanks for the replies, when I get back to it ill get it in a metal box and post the make/model of it.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Greetings all, I am getting some strange readings off a solid state relay;
I recently replaced a small 8A relay that controls a restive heating element and am getting about 20 volts drop across the relay with my meter, 118V to 98V.
I get the same amp reading on both sides of the relay 5.5A.
I did not get a chance to go thru and check all the other connections as my first instinct is a bad connection somewhere.
The control wires are 20 VDC.
The solid state relay is rated for 40 amps, the load is 5.5 amp.
Is there something about solid state relays that deforms the waveform/phase and could be fooling my meter?
Normally these relays are controlled by a PWM signal so you get pulses where it is on for just part of the cycle. The important thing is that current is flowing through it and not what the voltage drop across the relay is reading.
 

myspark

Senior Member
Location
SCV Ca, USA
Occupation
Retired EE
I agree with the concept.

This may (my comment) sound pedantic but the post is confusing.

When you say RESTIVE—are you saying idle, restful, non- functioning?

RESTIVE= uncontrollable. Beyond restraints, belligerent. A restive horse.
RESTFUL= Tending to oppose any movement. No apparent motion.
IDLE= At rest, not meant to be put in motion.

Being RESTIVE doesn’t mean something is at rest. It is being seen as agitated, and ready to spring into action.
A restive crowd of demonstrators. . . the actions are still going on.

Do you mean the solid state relay is not conducting, no trigger signal applied to turn on-- then the load that is connected to-- is not actively receiving power. (de-energized)

As you know solid state components (as mentioned by another poster), do not really turn off. They simply go from high state (conducting) to low state (non-conducting)
The reason you are getting those readings on your meter.

There is always inherent high resistance at the “contacts” when the semiconductor is not conducting.
It doesn’t mean it is totally open like you would expect from generic contacts-- that are de-energized when the electro-mechanical relay is not getting power.

So, the issue that needs to be clarified would be:

Are those readings you mentioned--taken--when the relay is functioning--doing its job . . . in earnest?
 

moonshineJ

Member
Location
USA
I believe tortuga meant RESisTIVE heating element; just a typo. He mentioned current thru relay, so the heating element seems to be On.
Synchro might be right, 5.5 A thru 40 A relay could be an issue perhaps. What the specification sheet says?
 

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
Synchro might be right, 5.5 A thru 40 A relay could be an issue perhaps. What the specification sheet says?
I haven't played with these things, I much prefer electromechanical relays. But my take is that 5.5A through a 40A rated solid state relay could be a problem because the load is so small. I too would like to see the spec sheet to see if there is a minimum current requirement.

-Hal
 

kwired

Electron manager
Location
NE Nebraska
I was going from each side to a ground screw.


Yes its a true RMS, Ideal 61-775


I don't have that info, it was late and I should have noted it.
Its a 'cheap' one.
I grabbed it after hours, it matched the coil voltage and it was in a random parts bin and looked way better than the one that was there.



No its not bolted to a heat sink, I believe the box its in is plastic.

Thanks for the replies, when I get back to it ill get it in a metal box and post the make/model of it.
What voltage is read if measuring from input terminal to output terminal?

20 volts dropped @ 5.5 amps through a true resistance should mean you are giving up 110 watts of heat.

I doubt you are losing that much heat or else you would likely have burned up the SS relay.

The voltage output is probably not true sinusoidal through this device though
 

gar

Senior Member
200707-2027 EDT

The majority if the responders to this thread are ignorant on the subject. and should never have responded, and muddied the waters.

Many solid-state AC SSRs are made with a power switch using either a TRIAC or reverse paralleled SCRs. Both types of devices remain conductive even if the trigger current is removed so long as the anode-cathode current remains above some low holding level. When the voltage across the device drops below some value around 1 V, then the current drops below the holding value, and the device turns off unless gate trigger current is maintained.

In the simplest AC SSR optical coupling is used to produce either an on or off signal to the gate. Off is no signal.

What might you expect to read? It makes no real great difference whether it is an average or RMS reading meter.

Mt experiment was with an OAC5 SSR, and a 25 W incandescent load. For ON 5 V DC was applied to the OAC5 input.

Had nominal 120 V sine wave input. Readings were not simultaneous and my line voltage fluctuates a little.
Fluke 27 average reading meter.
Voltage across the OAC5 0.99 V. Input 121.3 V, and load 120,2 V.
Beckman 4410 true RMS reading,
Voltage across the OAC5 1.02 V. Input 120.9 V. and load 120.4 V.

The OPs device may be a more complex device that has failed or something else. Need scope pictures.

. .
 

gar

Senior Member
200708-1508 EDT

Some limited information from the very first GE SCR Manual. No edition number, copyright 1960. The second edition is dated 1961

Type C10 page 204. Highest PIV 400 V. Nominal current rating is 4 A. Proper heat sinking would be required. On voltage drop in the range of 0.6 V (high junction temperature and 20 mA), to about 1.2 to 1.3 V at 4 A current. Typical holding current 2 mA.

In 60 years the basic characteristics have not changed, they are defined by physical law.

.

.
 

kwired

Electron manager
Location
NE Nebraska
200707-2027 EDT

The majority if the responders to this thread are ignorant on the subject. and should never have responded, and muddied the waters.

Many solid-state AC SSRs are made with a power switch using either a TRIAC or reverse paralleled SCRs. Both types of devices remain conductive even if the trigger current is removed so long as the anode-cathode current remains above some low holding level. When the voltage across the device drops below some value around 1 V, then the current drops below the holding value, and the device turns off unless gate trigger current is maintained.

In the simplest AC SSR optical coupling is used to produce either an on or off signal to the gate. Off is no signal.

What might you expect to read? It makes no real great difference whether it is an average or RMS reading meter.

Mt experiment was with an OAC5 SSR, and a 25 W incandescent load. For ON 5 V DC was applied to the OAC5 input.

Had nominal 120 V sine wave input. Readings were not simultaneous and my line voltage fluctuates a little.
Fluke 27 average reading meter.
Voltage across the OAC5 0.99 V. Input 121.3 V, and load 120,2 V.
Beckman 4410 true RMS reading,
Voltage across the OAC5 1.02 V. Input 120.9 V. and load 120.4 V.

The OPs device may be a more complex device that has failed or something else. Need scope pictures.

. .
Might be in over my head a little bit, but should he see about 20 volts drop on other than one with variable output ability?

Even if it is intentionally varying voltage then current should drop as well when supplying a resistance load.

Variable output ability ones I have seen typically use either 4-20 mA or 0-10VDC signal to drive them, presuming OP has basic SSR that passes full volts, less any inherent drop through the device, whenever there is sufficient input signal.
 

gar

Senior Member
200708-2100 EDT

If you go back to the original post it is stated that an electromagnetic relay was replaced with an SSR. At that point it would be assumed that an SSR with a comparable functionality would be used.

The so called SSR needs to be accurately defined as to what it really is. It can be tested in various ways that may provide that information.

.
 

tortuga

Senior Member
Location
(44.057116, -123.103394)
Occupation
field supervisor
Thanks all for the replies.
I believe tortuga meant RESisTIVE heating element; just a typo. He mentioned current thru relay, so the heating element seems to be On.
Synchro might be right, 5.5 A thru 40 A relay could be an issue perhaps. What the specification sheet says?
Yes thank you, I meant a resistive heating element.

200708-2100 EDT

If you go back to the original post it is stated that an electromagnetic relay was replaced with an SSR. At that point it would be assumed that an SSR with a comparable functionality would be used.

The so called SSR needs to be accurately defined as to what it really is. It can be tested in various ways that may provide that information.
.

Its a Fotek SSR-40 DA relay:
SSR-40-test-s.jpg


I finally got back to remove it and brought it home,
Just set it up on the test bench (on a proper heat sink).
With a known good 24 VDC power supply its working fine.
117 Volts on the line side 116 Volts on the load, 1 V drop across the relay. Rigged it up to a space heater set a 600 Watts and its right at 5.6 Amp with my meter.
So the issue is elsewhere in the machine.
 

Besoeker3

Senior Member
Location
UK
I finally got back to remove it and brought it home,
Just set it up on the test bench (on a proper heat sink).
With a known good 24 VDC power supply its working fine.
117 Volts on the line side 116 Volts on the load, 1 V drop across the relay. Rigged it up to a space heater set a 600 Watts and its right at 5.6 Amp with my meter.
So the issue is elsewhere in the machine.
Well done that man!
I don't greatly have a lot of support for those relays but that's for another date.
 

myspark

Senior Member
Location
SCV Ca, USA
Occupation
Retired EE
Thanks all for the replies.

Its a Fotek SSR-40 DA relay:
View attachment 2552885


I finally got back to remove it and brought it home,
Just set it up on the test bench (on a proper heat sink).
With a known good 24 VDC power supply its working fine.
117 Volts on the line side 116 Volts on the load, 1 V drop across the relay. Rigged it up to a space heater set a 600 Watts and its right at 5.6 Amp with my meter.

So the issue is elsewhere in the machine.
Here is an excerpt from the link below:

“The voltage drop in an SSR is usually 1 V to 1.6 V, depending on the load current. For small loads (less than 1 A), the heat produced is safely dissipated through the relay’s case. High-current loads require a heat sink to dissipate the extra heat. See Figure 6.
For example, if the load current in a circuit is 1 A and the SSR switching device has a 2 V drop, the power generated is 2 W. The 2 W of power generates heat that can be dissipated through the relay’s case.
However, if the load current in a circuit is 20 A and the SSR switching device has a 2 V drop, the power generated in the device is 40 W. The 40 W of power generates heat that requires a heat sink to safely dissipate the heat.


The link explains the in-depth view of solid state relays and brief explanation why you have a voltage drop at the relay output.


In summary:
There is always voltage drop at the output of the relay--as a consequence of heat generated by the semiconductor.
The load is functioning OK. . . there’s nothing wrong with it.
It is important to keep the temperature tolerance of the semiconductor to prevent further loss (voltage drop). . . brought by heat build-up.. . and that is-- deploying the correct size heat sink.

This is explained on that link.

Just something to keep in mind:
Solid State Relays (as mentioned earlier) do not relay turn off when not conducting. They conduct on the principles of semiconductor “DOPING”.. . which is done by introducing positive electrons to “EXCITE” the doped semiconductor to allow electrons to flow.

Unlike copper media (wires or bars) that conduct every time you connect them to live source. . . semiconductor needs a boost in or order for them to conduct.
It is through this process that cause them to emit heat (the nemesis of semiconductors) and therefore the result of having voltage loss.

Note:

I was hoping our Brit (Bes ) friend would join the fray--and contribute some (educational) input--to keep the topic from becoming horribly convoluted.
 
Top